Understanding the Magnetic Memory Effect in Fe-Doped NiO

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Understanding the Magnetic Memory Effect in Fe-Doped NiO Nanoparticles for the Development of Spintronic Devices Ashish Chhaganlal Gandhi, Pradeep Raja Anandan, Yu-Chen Yeh, TaiYue Li, Chi-Yuan Wang, Yasuhiro Hayakawa, and Sheng Yun Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01898 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Nano Materials

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Understanding the Magnetic Memory Effect in Fe-Doped NiO Nanoparticles for the

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Development of Spintronic Devices

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Ashish Chhaganlal Gandhi,† R. Pradeep,‡ Yu-Chen Yeh,† Tai-Yue Li,† Chi-Yuan Wang,† Y.

4

Hayakawa,‡ and Sheng Yun Wu†,*

5 6

†Department

‡ Research

of Physics, National Dong Hwa University, Hualien 97401, Taiwan

Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan

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ABSTRACT: Uniform hexagonal single phase Ni1-xFexO (x = 0, 0.01, 0.05, and 0.1)

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nanoparticles synthesized by a standard hydrothermal method are characterized with an

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enhanced lattice expansion along with a decrease in the microstrain, crystal size, and Ni

10

occupancy as a function of the Fe-concentration. The observed anomalous temperature and

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field dependent magnetic properties as a function of the Fe-content were explained using a

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core-shell type structure of Ni1-xFexO nanoparticles such that the effect of Fe-doping has led to

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a decrease of disordered surface spins and an increase of uncompensated-core spins. Perfect

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incorporation of Fe3+ ions at the octahedral site of NiO was observed from the low Fe-

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concentration; however, at a higher Fe-content, 4:1 defect clusters (4 octahedral Ni2+ vacancies

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surrounding an Fe3+ tetrahedral interstitial) are formed in the core of the nanoparticles resulting

17

in the transition of spin-glassy to the cluster-glassy system. An enhanced thermal magnetic

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memory effect is noted from the cluster-glassy system possibly because of increased intra-

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particle interactions. The outcome of this study is important for the future development of

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diluted magnetic semiconductor spintronic devices and the understanding of their fundamental

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physics.

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Keywords: Fe-doped NiO; 4:1 defect cluster; exchange bias; memory effect; spin-glassy,

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cluster-glassy 1

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1.

INTRODUCTION

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Diluted magnetic semiconductors (DMSs) at the nanoscale have attracted enhanced attention

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in the scientific community because of having both charges and a spin degree of freedom in a

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single substance in order to realize a new class of spintronic devices.1 According to recent

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findings, the magnetic transition metal (TM) 3d ion doped oxide-DMS has shown room

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temperature (RT) ferromagnetism which opens up their potential applications in advanced

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spintronic, optoelectronic, and magneto-optoelectronic devices.2, 3 Most of the oxide-DMSs

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reported in the literature are of n-type and possessing non-cubic crystal symmetry. However,

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for potential applications such as the integration of spintronic devices with advanced silicon-

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based microelectronic devices, a p-type oxide-DMSs with a cubic crystal symmetry is of prime

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interest.

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Among the different oxide-DMSs, the TM doped transition metal oxides (TMOs) such as

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nickel oxide (NiO) have an advantage of having very similar atomic radii for both dopants and

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host ions.4 NiO is a wide-bandgap (Eg ~ 4 eV) antiferromagnetic (AF) insulator with a Neel

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transition temperature TN of 523 K and has the lowest deviation from stoichiometry with a

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value below 0.1 %.5 At the nanoscale, it shows anomalous magnetic, electrical, and optical

40

properties because of the small size effect and enhanced cation nickel vacancy (VNi) defects.6-

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8

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According to our recent findings and from the literature below, a critical size of ~ 30 nm, single-

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domain NiO particles can be formed with a high amount of VNi residing on the surface rather

44

than in the core of the nanoparticle (NP).7, 9, 10 The effect of VNi results in the breaking of long-

45

range ordered Ni2+─O2-─Ni2+ superexchange interaction, leading to the formation of an

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uncompensated-core with strongly pinned spins at the interface and frustrated surface spins

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giving rise to a net magnetic moment.10,

The intrinsic p-type conductivity from NiO NPs is attributed to hole states induced by VNi.8

11

However, below ~10 nm, because of the high 2


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surface-to-volume ratio and enhanced VNi, the effect of surface spins become more dominant

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resulting in spin-glass (SG), super-spin-glass (SSG) (system of interacting particles) and

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superparamagnetic (SPM) (system of non-interacting particles) like systems.7, 12, 13 Therefore,

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depending upon grain size (d) and the non-stoichiometry, anomalous magnetic properties

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emerge at the expense of the AF property of the host NiO.

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Interestingly, non-stoichiometric properties in NiO can be enhanced further by doping

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with TMs such as Cr14, Mn15-19, Fe15, 20-35, Co14, 16, 36, Cu37, and Zn15, 38, which mostly depends

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on the method of sample preparation, and experimental variables such as thermal treatment

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(i.e., grain size), oxygen activity and the fraction of dopant.39 The above-mentioned factors

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affect the magnetic, structural and electronic properties which are of great importance for the

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basic understanding of TMO-DMSs. For example, in the case of Ni0.9Zn0.1O nanocrystals, a

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slight symmetry change in NiO structure type induced by size and/or strain effect was

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reported.40 From an application point of view, it’s necessary to have a profound knowledge of

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these properties, especially how these properties can be controlled systematically by the above

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experimental variables. However, in the literature, even though most of the TM-doped NiO

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was prepared by co-precipitation method, the RT magnetic results are not consistent. For

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example, Co-doped NiO NPs with a grain size of 8 to 10 nm have shown FM properties,36

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whereas 50 nm NPs reveal PM properties.16 In a similar way, ~ 15 nm size Mn-doped NiO NPs

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have shown FM properties,16 21 to 28 nm shows SPM/AF properties,19 and 50 nm becomes

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PM.16 The above discrepancies in the magnetic properties have been attributed to the

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contribution from the magnetization resulting from (i) the finite size resulting in enhanced

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surface effect,15 (ii) the segregation of secondary TM related impurity phases,31 (iii) double

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exchange interactions through substitutional TM ions at octahedral Ni site and charge carriers

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from defects,34 (iv) interstitially doped TM ions at tetrahedral site resulting in the formation of 3



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4:1 defect cluster5,

39, 41-43

and (v) intra-particle interactions.22 Apart from the above, the

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reported discrepancies in the magnetic properties could also be a consequence of a lack of a

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standard methodology for defining the RT magnetic properties, since simply having a non-zero

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value of coercivity and remanence at RT could possibly originate from spurious phases and

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point defects or could be related to SPM properties that may have a Curie temperature below

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RT.

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In the present work, we have used the hydrothermal method to synthesize iron (Fe)-doped

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NiO NPs with different Fe-concentrations. The hydrothermal route utilizes heat and pressure

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to initiate reactions of reagents dissolved in an aqueous medium. This opens up reaction

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chemistry not accessible under ambient conditions, and crystalline NPs can be produced

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without the requirement for post-reaction calcination. Whereas soft-chemical route usually

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involves synthesizing a precursor gel, followed by decomposing the gel or precursor into the

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designed crystalline oxide phase at an elevated temperature, but this calcination process also

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promotes agglomeration of the primary particles. The main advantage of the hydrothermal

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technique over other soft-chemical routes is the high reaction efficiency, ability to control the

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nanostructures morphology, which is able to produce superfine particles with good crystallinity

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and narrow particle size distribution etc.44 In Fe-doped NiO, ions of Fe either could replace the

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Ni ions or occupy an interstitial site which can result in complex magnetic properties.43 The

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coexistence of FM and AF spin-interactions can further enhance the magnetic anisotropy

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originating from surface spin frustration, exchange-coupling, intra-particle interactions, and

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SG to cluster-glassy like states, etc. Note that in cluster-glass groups of spins are locally ordered

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creating small domains which interact between each other similar to single spins in the simple

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spin glass. Magnetic anisotropy, which plays an important role in shaping the magnetic

95

properties, was investigated by measuring the magnetic memory effect and time relaxation 4


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dynamics from Fe-doped NiO NPs. The observation of the magnetic memory effect in the DMS

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system will be useful for the further understanding of fundamental physics and future potential

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applications of TMO-DMS nanostructures. By means of a detailed literature survey, as well as

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structural and magnetic investigations, our findings suggest that RT FM properties can be

100

achieved simply by controlling the grain size of the Fe-doped NiO NPs.

101 102

2.

EXPERIMENTAL DETAILS

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The Ni1-xFexO (x = 0, 0.01, 0.05, and 0.1) NPs were synthesized by a standard hydrothermal

104

method described in our previous work.45 As shown in Figure 1a, the precursor compound

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used in the synthesis procedure was nickel nitrate hexahydrate (Ni(NO3)·6H2O), iron nitrate

106

(Fe3(NO3)3·3H2O), and sodium hydroxide (NaOH). The obtained NaOH solution was then

107

added to the salt solution to precipitate and maintain a pH value of 12. The mixture sample was

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transferred into a Teflon-lined autoclave and heated at 150 °C for 17 h in an air flow electric

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oven, as shown in Figure 1b. The resulting precipitate was washed several times with

110

deionized (DI) water and ethanol (C2H5OH). The obtained intermediate Ni(OH)2 nanocrystal

111

was initially dried at 80 °C for 12 h and further annealed at 350 °C for 4 h, as shown in Figure

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1c. Fe ion was introduced into NiO nanoparticles, resulting in a various Fe-doped concentration

113

hexagonal shaped Ni1-xFexO nanoparticles (Figure 1d). For the detailed investigation of

114

crystalline properties of Ni1-xFexO nanoparticles through the synchrotron radiation X-ray

115

diffraction technique, as shown in Figure 1e, which will be discussed further in the text. The

116

XPS measurements have confirmed the presence of Fe 2p3/2 and Fe 2p1/2 peaks along with their

117

satellite structures and without any metallic Fe0 (binding energy ~ 706 eV) peak.45 The obtained

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strong Fe ion signal from XPS spectra suggested the presence of an Fe-cluster in the 10 %

119

sample. We note that Fe is found in the +3 oxidation state as a dopant in NiO. To reconfirm 5



120

the surface hydroxylation upon prolonged exposure to the ambient atmosphere deconvolution

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was performed for oxygen 1s peak centered with two peaks 529.3 and 530 eV binding energies.

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The peak centered at 529.3 eV corresponded to the oxygen are attached to either of Ni or Fe,

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as these binding energies corresponded to NiO and FeO formation. The other peak centered at

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530.5 eV differ from the reported surface hydroxyl groups at 531.6 eV and can be assigned to

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oxygen deficiency in the NiO matrix.46 Morphological analysis of all the samples was carried

126

out using the field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6500F

127

microscope, Tokyo, Japan). Transmission electron microscopy (TEM), high-resolution (HR-

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TEM) images and the selected area electron diffraction (SAED) patterns were obtained by an

129

analytical transmission electron microscopy (JEOL TEM 2100F microscope, Japan). The

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synchrotron X-ray powder diffraction (XRPD) data were collected at SPring-8 BL12B2 which

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is part of Taiwan X-ray facilities at SPring-8. The 18 keV (λ = 0.68889Å) X-ray source is

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delivered from the bending magnet. The 2D powder diffraction patterns were collected with

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RayonixMS225 (CCD) approximately 188 mm from the sample to detector and typical

134

exposure duration 60 secs. The powder sample was filled into a borosilicate glass capillary by

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0.5 mm diameter. The one-dimensional powder diffraction profile was converted with program

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GSAS II, and the calibration coefficients were calibrated by NIST SRM 600c LaB6. The zero-

137

field cooled (ZFC) and field-cooled (FC) temperature M(T) and applied magnetic field M(H)

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dependency of static magnetization, time-dependence of dynamic magnetic properties M(t) and

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the thermal magnetic memory effect measurements of all of the powdered samples was carried

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out using a superconducting quantum interference device (SQUID) magnetometer (MPMS

141

VSM, Quantum Design, USA). The ac susceptibility measurements were performed using a

142

physical property measurement system (PPMS-9T, Quantum Design, USA).

143 6


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144 145


3.

RESULTS AND DISCUSSION 3.1.

Morphology and Structural Properties. A hexagonal-shaped morphology was

146

observed from both the SEM and TEM images of Ni1-xFexO samples with a diameter ~ 70 to

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80 nm and width ~ 10 to 13 nm. Well-separated hexagonal shaped NPs were formed from both

148

the undoped and Fe-doped NiO. Figure 2a shows the SEM image of NiO NPs where the inset

149

of it depicts a single hexagonal-shaped NP. The SEM images of 1, 5 and 10 % samples are as

150

given in Figure S1a-c (Supporting Information), respectively. The mean diameter of the

151

NPs was estimated by fitting the log-normal distribution function: 𝑓(𝑑) =

152

[―

153

(Supporting Information), where σ is the standard deviation of the fitted function. The fitted

154

values of (〈𝑑〉,𝜎) of 0, 1, 5, and 10 % samples are (68±4 nm, 0.43±0.07 nm), (60±2 nm,

155

0.28±0.04 nm), (62±1 nm, 0.27±0.02 nm), and (70±1 nm, 0.27±0.02 nm), respectively. Figure

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2b-d shows the TEM, HR-TEM images and the SAED pattern of the 1 % sample and the results

157

from the 0, 5, and 10 % of samples are given in Figure S2 (Supporting Information). From

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TEM images of Fe-doped samples, aggregated hexagonal-shaped NPs can be seen which is

159

consistent with the SEM imaging. The HR-TEM images of all the Fe-doped samples retain a

160

crystalline nature. The interplanar distance d = 0.191 nm obtained from the 1 % sample

161

correspond to the (200) plane of NiO with the 𝐹𝑚3𝑚 space group (similarly, d values

162

corresponding to the (200) plane were also obtained from all other samples). Unlike the

163

previous result29, the SAED pattern of the 1 to 10 % samples retained the same NiO crystal

164

structure without any spurious phase. The corresponding SAED pattern of the 1 % sample

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displayed on the left side of Figure 2d can be ascribed to the existence of the polycrystalline

166

nature of NiO. A related structural model of the crystalline phase has been successfully worked

167

out using a PCED3i software package (UNL, USA) and the obtained image is shown on the

(ln𝑑 ― ln〈𝑑〉) 2𝜎2

]

1

exp

2𝜋𝑑𝜎

to the histogram obtained from the SEM images shown in Figure S1d

7



168

right side of Figure 2d.47 The simulated pattern matches very well with the observed diffraction

169

rings assigned to (111), (200), (220), (311), and (222) planes consistent with the XRPD

170

results as discussed below.

171

For a further detailed investigation of the crystalline phases in Ni1-xFexO NPs, a high

172

energy XRPD technique must be employed, which is otherwise quite difficult using usual XRD

173

techniques. Figure 3a depicts a 2D plot of the XRPD pattern of Ni1-xFexO samples over a

174

narrow scattering range of 2θ, where the vertical axis represents the value of Fe-concentration

175

x varying from 0 to 10 %. The peak intensities of the diffraction patterns were presented using

176

different colors. From the above 2D plot, two main diffraction peaks (111) and (200) indexed

177

based on Fm-3m (No.255), become visible indicating the formation of crystalline phase from

178

all of the NPs, having the same crystal structure as that of NiO. The intensity of diffraction

179

peaks decreases with the increase of Fe-concentration signaling Fe-doping has led to an

180

increase of disorders, which could be possibly originated due to defects such as substitutional

181

Fe3+ at the Ni site (Fe3Ni+ ), VNi, and the 4:1 defect cluster with interstitial Fe4i + in the

182

crystalline structure.39 Significant broader diffraction peaks are visible from the 2D XRPD plot,

183

and the effect of Fe-doping leads to a slight variation in the line-width which could be caused

184

by an effect of finite size. To know if a sample will be a correct standard, we have plotted the

185

full-width at half maximum (FWHM) of the reflections of NPs, NiO bulk and a standard LaB6

186

powder against the peak positions 2θ (deg.). This curve is called Instrument Resolution

187

Function (IRF)48 given in Figure S3 (Supporting Information). We observed that the NiO and

188

Fe-doped NiO NPs displays a greater broadening than the NiO bulk and the standard LaB6

189

powders implying that in this samples size and strain effects are present and cannot be

190

neglected. The most intense (200) and (111) diffraction peaks of XRPD spectra can be fitted

191

with Lorentzian distribution function, and its full-width at half maximum (FWHM) β could be 8


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192

determined. Using Scherrer’s formula: 𝑑 = 𝑘𝜆 𝛽cos 𝜃 (where k = 0.94 is a Scherrer’s constant

193

and θ is diffraction angle), the calculated grain size (d(111), d(200)) of 0, 1, 5, and 10 % of samples

194

is (8.3 nm, 7.1 nm), (8.4 nm, 7.2 nm), (7.4 nm, 6.5 nm) and (7.6 nm, 6.3 nm), respectively. The

195

obtained grain size is around ~ 8 times smaller than the mean diameter estimated from the SEM

196

analysis indicating the occurrence of aggregations of grains and particles which can interfere

197

with the effect of finite size.34 However, when NPs are not perfect crystals, diffraction peak

198

line-broadening could also be aroused from the microstrain effect,49 which could be caused by

199

dislocations, anti-phase domain boundaries, non-uniform lattice distortions, grain surface

200

relaxation50, etc. To estimate the effect of the strain, we used the well-known Williamson-Hall

201

(W-H) plot given by 𝛽 = 𝛽𝑠𝑖𝑧𝑒 + 𝛽𝑠𝑡𝑟𝑎𝑖𝑛 = cos 𝜃

202

distortion parameter corresponding to the strain. The intercept and slope of a linear fit in the

203

W-H plot (Figure 3b) gives the inverse of crystalline size 〈𝑑𝑋𝑅𝐷〉 and the strain η. The

204

obtained values (, η) from 0, 1, 5, and 10 % samples are (19 ± 6 nm, 0.8 ± 0.1 %), (18

205

± 5 nm, 0.7 ± 0.1 %), (14 ± 4 nm, 0.7 ± 0.1 %), and (11 ± 2 nm, 0.5 ± 0.1 %), respectively,

206

whereas very low value of strain ~ 0.016 % was obtained from both NiO bulk and LaB6

207

standard material. The estimated values of , d(200), d(111) and η versus Fe-concentration

208

x are plotted in Figure 3c. A significant decrease in the grain size and η with the

209

increase of Fe-concentration is observed suggesting an effect of Fe-doping has hampered the

210

crystal growth and has released the micro strain in NiO NPs consistent with the observed

211

decreases of XRD intensity. The decrease of grain size with the increase of Fe-concentration

212

is consistent with previous findings. However, the observed microstrain from the Fe-doped

213

NiO contradict previous findings in which samples prepared by a co-precipitation method have

214

shown an increase of the microstrain with an increase of Fe-concentration.23, 29 Since, the ionic

215

radii of Ni (0.69 Å) and Fe (0.64 Å) are very close to each other, therefore the observed

1

(〈

𝑘𝜆 𝑑𝑋𝑅𝐷〉

)

+ 4𝜂sin 𝜃 , where η is the local lattice

9



216

discrepancy indicate that the method of sample preparation could play an important role in

217

defining structural properties and therefore it can further influence the magnetic and optical

218

properties. The solid curve in Figure 3c represents a theoretical fit to the experimental data

219

〈𝑑𝑋𝑅𝐷〉 = 𝑑𝑜 +𝛼exp ( ― 𝑥𝑜), where do = 1.6 nm, α = 17±7 nm, and xo = 16±9 % are the initial

220

constants and the fitted parameters, respectively.

𝑥

221

To obtained the lattice parameters and occupancy of the constituent elements, a Rietveld

222

refinement51 of the XRPD spectra was undertaken using a general structural analysis system

223

(GSAS) software package.52 Figure 4a shows the XRPD spectra obtained from 0, 1, 5, and 10

224

% of samples (bottom to top), where the red line represents the Rietveld fit to the diffraction

225

pattern (crosses), confirming the existence of a pure NiO phase in all of the samples and the

226

corresponding fitting parameters are summarized in Table 1. Figure 4b depicts the Fe-

227

concentration x dependence of the lattice constants determined from the XRPD and SAED

228

pattern showing a good agreement. As compared to the known lattice constant of bulk NiO

229

(dash line, a = 4.175 Å)53, a lattice expansion is obtained from both the undoped and Fe-doped

230

NiO NPs. The lattice expansion from the undoped NiO NPs could be mainly due to the VNi.7,

231

54

232

indicates a perfect incorporation of Fe3+ ions into the NiO lattice at the octahedral site such that

233

for every two Fe3+, a divalent cation vacancy is present in the lattice which resulted in a further

234

increase of VNi. However, above 1 %, the percentage of substitutional Fe ions at the octahedral

235

site in NiO remains fixed, whereas the amount of Fe-ions at the interstitial site, which could

236

result in the formation of 4:1 defect cluster consisting of tetravalent interstitial iron, Fe4i + , and

237

four VNi, in total being four times negatively charged, should increase with the increase of the

238

doping concentration. A structure of NiO with substitutional Fe ions located on an octahedral

239

site and an interstitial Fe ion surrounded tetrahedrally by four cation vacancies and four oxygen

The observed drastic increase in the value of the lattice parameter from the 1 % sample

10


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240

ions in a defect free NiO matrix is as shown in Figure 1e.43 Such a complex structure at a

241

higher Fe-concentration could result in interesting magnetic properties because of the intra-

242

particle interactions which will be discussed further in the text. Moreover, the above findings

243

are consistent with the observed decrease of Ni occupancy (Nio) from 0.907±0.005 to

244

0.810±0.006 with the increase of Fe-concentration x from 0 to 10 % as shown in Figure 4c.

245

On the other hand, oxygen occupancy with a value very close to the stoichiometric value of ~

246

1 is obtained from NiO, 5 % and 10 % samples and 0.977±0.005 from the 1 % sample (open

247

symbol in Figure 4c). A solid line represents a theoretical fit 𝑁𝑖𝑥 = 𝑁𝑖𝑜 ―𝛽exp (𝑥1), where

248

Nio = 0.915±0.007, β = 0.010±0.005, and x1 = 4.3 ± 0.8 % represents the initial constant and

249

the fitted parameters, respectively.

𝑥

250 251

3.2.

Temperature Dependence of Magnetization. In the case of finite size, NiO NPs,

252

with a high amount of VNi residing on the surface rather than in the core, results in the formation

253

of an uncompensated-core and the shell of disordered surface spins due to broken Ni2+ ─ O2-

254

─ Ni2+ superexchange interactions. Figure 5a-d shows the ZFC-FC M(T) of 0 , 1 , 5 and 10 %

255

samples measured from 12 to 400 K under a small external applied field Ha = 200 Oe. The ZFC

256

M(T) of all samples have shown a sharp narrow peak at freezing temperature TF = 16±1 K

257

assigned to the collective freezing of disordered surface spins of the NPs giving rise to the

258

formation of an SG like state.27 As the temperature increases, both FC and ZFC M(T) curves

259

were characterized by an abrupt drop in the value of magnetization followed by a broad

260

maximum in ZFC M(T) at blocking temperature (TB) assigned to uncompensated spins in the

261

core of the NPs. The broadening of the ZFC curve is associated with an anisotropic distribution

262

of NPs which narrow down with the increase of Fe-concentration in agreement with the

263

observed decreasing behavior of the size distribution from the SEM analysis. The bifurcation

264

of the ZFC-FC curve from the Fe-doped NiO NPs indicates the presence of relaxing spin 11



265

moments. However, as compared to the NiO NPs, almost ~ 10 times smaller value of

266

magnetization is obtained at TF, whereas the value of the magnetization at TB increases with

267

the Fe-concentration. The above findings differ from the previous work in which the Ni1-xFexO

268

NPs prepared by the co-precipitation method have shown an increasing behavior of

269

magnetization at TF, whereas the value of magnetization at TB remains constant with the

270

increase of Fe-concentration.23 The observed variation in the magnetization with Fe-

271

concentration can be explained using a core-shell type structure of Ni1-xFexO NPs such that the

272

effect of Fe-doping leads to a decrease of the disordered surface spins and an increase of

273

uncompensated-core spins. That means at higher Fe-concentrations, 4:1 defect clusters will be

274

formed in the core of the NPs such that a high amount of VNi will be residing in the core rather

275

than on the surface of the NPs.

276

The value of TB increases from 90±8 K to 170±3 K with the increase of Fe-concentration

277

x from 0 to 10 % as shown in Figure 5e, which can be described using 𝑇𝐵 = 𝛼𝑥𝛽, where α =

278

134±1 K and β = 0.103±0.004 (dashed curve). As the temperature rises above TB, the thermal

279

excitation overcomes an energy barrier EB, giving rise to SPM or paramagnetic (PM) properties

280

and magnetization decreases linearly with the increase of temperature. The irreversibility

281

shown in the ZFC-FC magnetization is strongly dependent on the magnitude of the applied

282

magnetic field and is presumably associated with a slow relaxation process for an assembly of

283

interacting NPs.55 These characteristics can be attributed to a dipole-dipole interaction. In a

284

cluster- or spin-glassy system the dipole-dipole interaction could be of the same order as that

285

of the particle anisotropy energy. The relaxation of magneatization over a single- (non-

286

interacting particles) or multi- (interacting particles) energy barrier with an uniaxial anisotropy

287

(UA) is described by the Néel–Arrhenius law. Assuming a grain volume dependence of the

288

energy barriers to the reversal of the form EB = KAFV, where KAF is the magnetocrystalline

289

anisotropy of NiO particles and V is the median of the particles volume distribution, and a 12


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290

typical slow dynamic measurement, we obtain an expression of the form, KAFV =25kBTB. Then,

291

we can estimate the value of KA at TB using the approximation that the AF moment 𝑚𝐴𝐹 ~ 1 3

and

𝐾𝐴𝐹 ~ 𝑚𝐴𝐹3

[

for UA, i.e.,𝐾𝐴𝐹(𝑇𝐵) = 𝐾𝐴𝐹(0)

𝑇𝑁 ― 𝑇𝐵

],

where the

292

(𝑇𝑁 ― 𝑇)

293

magnetocrystalline anisotropy of KAF(0) is 4.96 × 106 erg/cm3 (bulk NiO).56 The effective

294

anisotropy comprises several intrinsic factors such as volume, surface, shape, exchange-

295

coupling and the magnetocrystalline anisotropies. The Fe-concentration x dependence of the

296

calculated KAF(TB) shows the opposite behavior as compared with TB as depicted on the left

297

side of Figure 5e. The solid curve represents a fit to the data point using 𝑇𝐵 = 𝛾𝑥𝛿, where γ =

298

3.69±0.02 × 106 erg/cm3 and δ = -0.041±0.003. The observed enhanced TB and reduced value

299

of KAF from Ni1-xFexO NPs could be due to a reduction of particle mean volume at higher Fe-

300

concentrations. However, an observed ~ 1 nm decrease in the grain size of Ni1-xFexO NPs is

301

not significant in having such a drastic change in the magnetic properties. Furthermore, the

302

number of particles per unit voume increases with the decrease of particle size leading to a

303

decrease of inter-particle distance. Therefore, the observed anomalous suppression of the

304

effective anisotropy with the increase of Fe-concentration indicates that the maximum in the

305

ZFC magnetization does not correspond to the typical TB of the non-interacting particles, where

306

intra-particle interaction, including dipole-dipole and exchange, plays a significant role in the

307

temperature dependence of the magnetization. Furthermore, an observed decrease in the value

308

of magnetization at TF and the saturation behavior of the FC M(T) curve at higher Fe-

309

concentrations suggest a possible transition of SG to a cluster-glassy like system consistent

310

with the XPS analysis from which strong Fe signal indicated Fe cluster formation.45 Our AC

311

susceptibility ’(T) measurements exhibit a pronounced anomaly at around Tv ~157 and 178 K

312

(for v=10 Hz) for x=5 % and 10% sample, which are found to be frequency dependent, as

313

shown in Figure S4a-b (Supporting Information). The peak position shifts towards higher 13


𝑇𝑁


314

temperatures (Figure S4c) and the height of the peak decrease with increasing v, consistent

315

with a glassy transition with Tv.

316 317

3.3. Exchange bias. The ZFC M(H) measurements over ± 40 kOe field were carried out

318

at various temperatures from 300 to 40 K for all the samples as shown in Figure S5a-d

319

(Supporting Information). Figure 6a shows the magnified ZFC M(Ha) loops measured at 100

320

K for 0, 1, 5 and 10 % samples with a non-zero value of coercivity (HC) and remanence (Mr)

321

ascribed to the FM component originating from the uncompensated-core of the NPs that tend

322

to get aligned towards the applied field. The inset of Figure 6a-b depicts the full ZFC-M(Ha)

323

loops with a linear increasing behavior of magnetization in the high-field region ascribed to the

324

AF component of the surface spins of NPs that remains in a disordered state at RT even after

325

the application of the magnetic field. The observed two-component FM and AF behavior from

326

Ni1-xFexO NPs with non-saturating behavior even up to 40 kOe field are consistent with the

327

previous findings.23 However, as compared to undoped NiO NPs, a slightly lower value of

328

magnetization with a tendency towards saturation is obtained from Fe-doped NPs suggesting a

329

decrease of surface contribution to the magnetization consistent with the M(T) data. A similar

330

low value of magnetization from Fe-doped NiO has been reported previously and attributed to

331

either reduced particle size or the exchange anisotropy between rich- and poor-Fe regions

332

coexisting within the sample.15 The latter could be due to the composition inhomogeneity

333

resulting from the limitations of the preparation technique. However, in this set of samples, the

334

value of the magnetization decreases up to 5 % Fe-doping, whereas a slight increase in the

335

value of magnetization is noted at the higher field from the 10 % sample. The obtained results

336

differ from most of the previous work, in which an increase of magnetization with Fe-doping

337

has been reported and was ascribed to double exchange and particle-particle interactions. A

338

similar low value of magnetization from the hydrothermally synthesized magnetite NPs as 14


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339

compared to that of co-precipitation method was also reported previously.57 Therefore, to

340

resolve this anomalous magnetic behavior, further detail study needs to be carried out.

341

Moreover, the coexistence of FM and AF interaction within a system can give rise to complex

342

magnetic properties. As can be noticed from Figure 6a, the ZFC M(Ha) loop reveals an

343

asymmetry from Ni1-xFexO NPs along both the applied field and the magnetization axis which

344

can be quantified as a spontaneous exchange bias (EB) field 𝐻𝐸𝐵 = (⌊𝐻𝐶+ ⌋ ― ⌊𝐻𝐶― ⌋) 2, and

345

vertical loop shift 𝑀𝑉𝐿𝑆 = (𝑀𝑟+ + 𝑀𝑟― ) 2, where 𝐻𝐶― (𝑀𝑟+ ) and 𝐻𝐶+ (𝑀𝑟― ) are the fields

346

corresponding to coercivity (remanence) in the first and second curve of the hysteresis loop at

347

which the magnetization (field) is zero, respectively. The observed results from undoped NiO

348

are consistent with our previous findings in which (i) the EB field was attributed to the setup

349

of UA during the first field of the hysteresis loop measurement and (ii) MVLS moments were

350

attributed to the presence of strongly pinned spins at the interface such that magnetic field

351

cannot reverse.10 Interestingly, a drastic increase in the value of all the ZFC M(Ha) magnetic

352

parameters is seen from the 1 % sample, and the effect of field cooling with 10 kOe (see Figure

353

6b) leads to further enhancement confirming the irreversibility of frozen pinned spins. The net

354

moment of these interfacial pinned spins can be quantified as 𝑚𝑝 = 1 2𝛿𝑀, where 𝛿𝑀 =

355

[𝑀( +40 kOe) ― 𝑀( ― 40 kOe)]. The obtained values of the coercivity 𝐻𝐶 = (𝐻𝐶+ ― 𝐻𝐶― ) 2,

356

remanence 𝑀𝑟 = (𝑀𝑟+ ― 𝑀𝑟― ) 2, HEB, MVLS and mp from (ZFC, FC) M(Ha) loop at 100 K

357

from 1 % sample is (1622 Oe, 1842 Oe), (-1033 Oe, -1668 Oe), (0.093 emu/g, 0.099 emu/g),

358

(0.044 emu/g, 0.078 emu/g) and (0.027 emu/g, 0.036 emu/g), respectively, which is much

359

higher than those of the reported values in the literature. The obtained enhancement in all the

360

magnetic parameters after 1 % Fe-doping in NiO NPs confirms the incorporation of Fe3+ ions

361

into a NiO lattice in the octahedral site which may have enhanced the anisotropy in this system.

362

Furthermore, as the Fe-concentration increase further, the low-field M(Ha) loop narrowed and 15



363

there is a drop in the values of various magnetic parameters with values smaller than undoped

364

NiO, confirming the presence of 4:1 defect clusters resulting in the formation of

365

uncompensated-core in the NPs. Figure 6c-d shows the temperature dependency of the

366

calculated values of -HEB and mp and the inset in the respective figure gives the x dependency

367

of HC and MVLS, respectively, where the star data points represent the values obtained from the

368

FC M(Ha) loop. The temperature dependency of Mr is shown in Figure S5c (Supporting

369

Information). As the temperature increases, thermal energy overcomes the magnetic anisotropy

370

and all the physical parameters show an exponential decreasing behavior. The obtained values

371

of (Hc, Mr) at 300 K from 0, 1, 5 and 10 % samples are (403 Oe, 0.026 emu/g), (585 Oe, 0.031

372

emu/g), (148 Oe, 0.008 emu/g) and (96 Oe, 4.5×10-5 emu/g), suggesting the transition of weak-

373

FM like properties to SPM with the increase of Fe-concentrations possibly originated from the

374

size effect. Interestingly, a pronounced enhancement in the values of HC, Mr, -HEB and MVLS

375

can be noticed below TB from the 10 % sample, whereas the value of mp does not show much

376

enhancement. The above findings suggest that the amount of frozen pinned spins at the

377

interface of FM and AF decrease with the increase of the Fe-concentration. To get further

378

insight on the effect of Fe-doping on the magnetic anisotropy, we have carried out the FC

379

M(Ha) measurement at 100 K with various Ha from all samples as shown in the Figure S6

380

(Supporting Information). Figure 7a depicts the magnified ZFC and FC M(H) loops measured

381

with different Ha from 0.5 to 10 kOe at 100 K for the 1 % samples, showing highly asymmetric

382

behavior. As can be noted from the figure, with the increase of Ha from 0 to 1 kOe, the value

383

of HC+ remain close to ~ 600 Oe, whereas the HC― shifts from -2655 to -2081 Oe. However,

384

on further increase of Ha from 3 to 10 kOe, the FC M(H) loop shifts towards the negative field

385

axis resulting in a shift of both (HC+ , HC― ) from (470 Oe, -2551 Oe) to (174 Oe, -3510 Oe).

386

Interestingly, a similar kind of Ha dependency of HC+ and HC― but with lower values is also

387

observed from undoped NiO NPs, whereas the 5 and 10 % samples show distinct behavior as 16


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388

shown in Figure 7b. Figure 7c-f shows the Ha dependency of HC, -HEB, MVLS and mp with a

389

shallow depth around ~ 1 kOe from the 0 and 1 % samples followed by increasing behavior,

390

whereas values obtained from the 5 and 10 % samples does not show much variation with the

391

increase of Ha. HC, -HEB, MVLS and mp increases monotonously with increasing cooling field

392

particular from 0 and 1 % samples without any tendency of saturation up to 10 kOe. Figure 7g

393

displays the linear increasing behavior of HEB with MVLS, and the linear fitting further proves

394

a relation of HEB proportional to MVLS signaling that magnitude of HEB is determined by the

395

number of frozen pinned spins.58 Since, both HEB and MVLS are closely connected with

396

uncompensated pinned spins at the interface of FM and AF, the above findings further

397

confirmed the decrease of frozen pinned spins with the increase of Fe-concentration.

398 399

3.4. Magnetic Memory Effect. To test the magnetic memory effect from Ni1-xFexO NPs, in

400

the present work, the FC cooling procedure suggested by Sun et al. was used.59 Initially,

401

samples were cooled down in an applied magnetic field of Ha = 200 Oe from 400 K down to

402

15 K, where sporadic stops with a zero field have been provided for a duration of 1 hr at a few

403

selected temperatures TS = 350, 300, 250, 200, 150, 50, 30, and 20 K. For a self-assembled

404

array, a step like magnetization curve named as “cooling” is seen for 0, 1, 5, and 10 % samples

405

as shown in Figure 8a-d, respectively, where the four insets show the full range of temperature

406

dependent magnetization plots. After reaching the lowest stabilized temperature, the sample

407

temperature is raised continuously in the same low applied magnetic field Ha, and the

408

magnetization is measured again and named as “warming.” The system, which exhibits a

409

memory effect, retains a distinct uptrend while warming the sample around each stopping

410

temperature TS (below TB) and regains its FC cooling value above it. In general, it is as if the

411

system “remembers” its thermal history. The step-like time dynamic magnetization

412

measurement is reproduced upon re-warming from all Fe-doped NiO NPs, whereas pure NiO 17



413

does not retain any sign of the memory effect. Furthermore, above TB, since the system is in

414

the SPM state, the absence of magnetic anisotropy energy won't retain any memory effect. The

415

unique part of discernible steps observed from Fe-doped samples, particularly from the 5 and

416

10 % samples is the decrease in the magnetization of the FC curve with the decrease of

417

temperature, below ~ 100 and ~ 150 K, respectively. This behavior is commonly assigned to

418

the interacting SG system, whereas a non-interacting SPM system shows an increasing

419

magnetization of the FC curve with the decrease in the temperature.60 In the literature, it has

420

been well documented that in an assembly of interacting NPs or an SG-like system, the length

421

of the spin-spin correlation grows during the stop, even in zero magnetic fields, and a memory

422

dip typically occurs upon heating called the ZFC aging effect.61 Therefore, to resolve the above

423

discrepancy, we have also performed a ZFC aging effect test. However, we did not see any

424

sign of the ZFC memory effect indicating that the intra-particle interactions in a system of Fe-

425

doped NiO NPs could be very weak in order to grow the spin-spin correlation length.

426

Understanding this phenomenon and the overall time-dependent magnetic response of these

427

Fe-doped NiO NPs assembly is a goal of this research. The aging magnetization at various

428

temperatures is cumulative with time relaxation. Figure 8e gives a plot of ΔM/M(%) versus

429

TS, where ΔM represents the relaxation in the magnetization at each TS obtained from the FC

430

cooling curve, and M represents the value of magnetization 40 K, where the solid curves are

431

guided by the eye. Interestingly, a broad peak like behavior with maxima around ~ 124, 123,

432

and 119 K is obtained from ΔM/M versus TS plot for the 1, 5 and 10 % samples, respectively,

433

which is within the blocking temperature region. The inset of Figure 8e shows a plot of Fe-

434

concentration x dependency of obtained values of ΔM/M at TS = 100 K, showing an

435

exponential increasing behavior with an increase of Fe-concentration. The solid line represents

436

a fit using ∆𝑀 𝑀(%) = 𝜀𝑥𝜏, where ε = 0.04±0.01 and τ = 0.42±0.16.

437 18


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438


3.5.

Time

Dependence

of

Magnetization.

To

measure

the

time-dependent

439

magnetization, M(t) relaxation samples were initially cooled down from 400 to 100 K (within

440

the blocking temperature region) in a small applied field Ha = 200 Oe, and the decay in the

441

value of the magnetization was recorded with time t in the zero field. Figure 9a-d shows the

442

obtained M(t) curves from the 0, 1, 5, and 10 % samples at 100 K for a duration of 1 hr.

443

Assuming that the distribution in the time relaxation is due to the size distribution; the M(t)

444

curves were fitted using a stretched exponential function, given as 𝑀(𝑡) = 𝑀𝑜 ― 𝑀𝑔𝑒𝑥𝑝 ( ―

445

𝑡 𝛽 ( 𝜏) , where Mo and Mg are the FM and glassy components, respectively.62 Figure 9e shows

446

a plot of fitted values of exponent β with respect to Fe-concentration x, where different colors

447

are used to represent the spin-glassy state (0 ≤ β ≤ 0.5), cluster-glassy state (0.5 ≤ β ≤ 1) and

448

PM state (1 ≤ β ≤ 1.5) from bottom to top. The value of β obtained from Ni1-xFexO NPs

449

increases linearly with the increase of x from 0.32 to 0.56 with a slope of 0.025±0.002 as

450

represented by a solid line in the figure. The above findings point towards the activation against

451

multiple anisotropy energy barriers and transition from spin-glassy to the cluster-glassy state

452

with the increase of Fe-concentration, which are in excellent agreement with the observation

453

from the M(T) measurements. The above findings suggest that the effect of Fe-doping at and

454

above 5 % in NiO NPs leads to the formation of the cluster-glassy system which gives rise to

455

an enhanced thermal magnetic memory effect. Similarly, an enhancement in the magnetic

456

memory effect up to room temperature has also been reported from exchange coupled NiFe2O4-

457

cluster and spin-glassy-NiO nanocomposites.63-64 However, as compared to reported memory

458

effect from the exchange-coupled nanocomposites, the observed memory effect from Fe-doped

459

NiO NPs is an intrinsic property and it shows enhancement with the increase of Fe-

460

concentration.

19



461

Furthermore, studied Ni1-xFexO (x = 0.01, 0.05, and 0.1) NPs with grain size varying from

462

8 to 9 nm are characterized with low-temperature FM and RT SPM properties. However, as

463

discussed in the introduction, there are reports of both RT FM and SPM properties from TM

464

doped NiO NPs. Therefore, a question remains unanswered, what could be the possible

465

mechanism for RT FM properties from TM doped TMOs? To answer this question, we

466

summarize the grain size (d) dependency of the reported RT magnetic properties from Ni1-

467

xMxO

468

Ni1-xFexO, in Figure 10a and 10b respectively, where the vertical axis represents the

469

concentration x of the dopant. Here different colors are used to represent the grain size-

470

dependent magnetic properties of undoped NiO NPs as discussed in the introduction, which

471

can be split into three regions (Re), namely Re-I (d ≤ 10 nm) SPM/SG/SSG, Re-II (10 nm ≤ d

472

≤ 30 nm) FM and Re-II (d ≥ 30 nm) AF (no finite size effect induced magnetism), respectively

473

from left to right. From Figure 10a, it can be seen that depending on the grain size, dopant

474

type, and its concentration, the TM and heavy element doped NiO mostly retains RT PM

475

properties in the Re-III region and the SPM/FM properties in the Re-II and Re-I regions. On

476

the other hand, as compared to different TMs and heavy element doped NiO, the Fe-doped NiO

477

mostly retains the RT FM properties in Re-II and III along with the FM impurity phases and

478

AF properties. However, in Re-I, along with a current set of samples most of the Fe-doped NiO

479

have retained SPM properties similar to that of undoped NiO nanostructures. Therefore, the

480

above analogy between undoped and doped-NiO suggests that the reported discrepancy in the

481

magnetic properties is related to the grain size and one can defiantly obtain RT FM properties

482

simply by controlling the grain size.

483

4.

(M = Cr14, Mn15-16, 18-19, Fe15, 20-21, 23-32, 34, Co14, 16, 36, Cu37, Zn15, 38, Sn65, and Ce66) and

CONCLUSIONS

20


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484

Single phase Ni1-xFexO NPs with various Fe-concentrations (x=0 to 10 %) were synthesized

485

by the hydrothermal method. A hexagonally shaped morphology with well separated NPs was

486

observed from the SEM and TEM analysis. The grain size and the microstrain estimated using

487

the W-H plot show decreasing behavior with the increase of Fe-concentration. The Rietveld

488

refinement of synchrotron radiation XRD measurements reveals an enhanced lattice expansion

489

from the Fe-doped sample and the decrease of Ni-occupancy with the increase of Fe-

490

concentration. The obtained values of (HC, Mr) at 300 K from the 0, 1, 5 and 10 % samples are

491

(403 Oe, 0.026 emu/g), (585 Oe, 0.031 emu/g), (148 Oe, 0.008 emu/g) and (96 Oe, 4.5×10-5

492

emu/g), suggesting a transition of weak-FM like properties to SPM with the increase of the Fe-

493

concentration. From the observed structural and magnetic properties, we argue that, initially 1

494

% Fe-doping in NiO has led to the perfect incorporation of Fe3+ ions into the NiO lattice at the

495

octahedral site, and above 1 %, the amount of Fe-ions at the interstitial site which results in the

496

formation of 4:1 defect cluster and should increase with the increase of doping concentration.

497

This results in the formation of a core-shell type structure of Ni1-xFexO NPs such that the effect

498

of Fe-doping has led to a decrease in the disordered surface spins and the increase of

499

uncompensated-core spins with Fe-concertation resulting in the transition of a spin-glassy

500

system to a cluster-glassy system. An enhanced thermal magnetic memory effect is noted from

501

the cluster-glassy system possibly because of increased intra-particle interactions.

502 503

ASSOCIATED CONTENT

504

Supporting information

505

The Supporting Information is available free of charge on the ACS Publications website at

506

DOI:

21



507

SEM and TEM analysis, crystal structure and X-ray refinement analysis, the temperature

508

dependence of magnetization, hysteresis loops took at various temperatures, and temperature

509

dependence of magnetization measured at various applied magnetic fields of Fe-doped NiO

510

nanoparticles.

511

Access Codes

512

These materials can be obtained free of charge by emailing [email protected] or

513

contacting Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan.

514

Tel: +886-3-8903717, Fax: +886-3-8900166.

515 516

AUTHOR INFORMATION

517

Corresponding Author

518

*E-mail: [email protected].

519

ORCID

520

Sheng Yun Wu: 0000-0003-2750-5431

521

Author Contributions

522

S.Y.W. and A.C.G. wrote, conceived, and designed the experiments. R. P. and Y. H.

523

synthesized the samples. Y.-C. Y, T. -Y. L., and C. –Y. W. contributed to the data analysis. All

524

authors discussed the results, contributed to the text in the manuscript, commented on the

525

manuscript, and approved the final version.

526

Notes

527

The authors declare no competing financial interest.

528 22


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529

ACKNOWLEDGMENTS

530

We would like to thank the Ministry of Science and Technology (MOST) of the Republic of

531

China for their financial support of this research through project numbers MOST-107-2112-

532

M-259-005-MY3 and MOST-107-2811-M-259-005.

533

REFERENCES

534

(1) Ohno, H., Making Nonmagnetic Semiconductors Ferromagnetic. Science 1998, 281, 951.

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(2) Radovanovic, P. V.; Gamelin, D. R., High-Temperature Ferromagnetism in Ni2+-Doped

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ZnO Aggregates Prepared from Colloidal Diluted Magnetic Semiconductor Quantum Dots.

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(3) Hoa, N. T. Q.; Lee, Z.; Kang, S.-H.; Radmilovic, V.; Kim, E.-T., Synthesis and

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Ferromagnetism of Co-Doped TiO2−δ Nanobelts by Metallorganic Chemical Vapor Deposition.

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Appl. Phys. Lett. 2008, 92, 122508.

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(6) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E., Finite Size Effects in

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Antiferromagnetic NiO Nanoparticles. Phys. Rev. Lett. 1997, 79, 1393-1396.

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(7) Gandhi, A. C.; Pant, J.; Pandit, S. D.; Dalimbkar, S. K.; Chan, T.-S.; Cheng, C.-L.; Ma,

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Y.-R.; Wu, S. Y., Short-Range Magnon Excitation in NiO Nanoparticles. J. Phys. Chem. C

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Nanoparticles. Nanomaterials 2017, 7, 231.

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(9) Li, L.; Chen, L.; Qihe, R.; Li, G., Magnetic Crossover of NiO Nanocrystals at Room

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Temperature. Appl. Phys. Lett. 2006, 89, 134102.

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(10) Gandhi, A. C.; Pant, J.; Wu, S. Y., Dense Inter-Particle Interaction Mediated Spontaneous

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Exchange Bias in NiO Nanoparticles. RSC Adv. 2016, 6, 2079-2086.

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(11) Mandal, S.; Banerjee, S.; Menon, K. S. R., Core-Shell Model of the Vacancy

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Concentration and Magnetic Behavior for Antiferromagnetic Nanoparticle. Phys. Rev. B 2009,

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(12) Rinaldi-Montes, N., Gorria, P.; Martínez-Blanco, D.; Fuertes, A. B.; Fernández, Barquín

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(13) Tiwari, S. D.; Rajeev, K. P., Signatures of Spin-Glass Freezing in NiO Nanoparticles.

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(14) Ponnusamy Pothapalayam, M.; Agilan, S.; Muthukumarasamy, N.; Velauthapillai, D.,

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Effect of Chromium and Cobalt Addition on Structural, Optical and Magnetic Properties of

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NiO Nanoparticles. In Z. Phys. Chem. 2016, 230, 1185.

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24


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(19) Mallick, P.; Rath, C.; Rath, A.; Banerjee, A.; Mishra, N. C., Antiferro to

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Venkatraman, M. R.; Velauthapillai, D., Structural, Optical and Magnetic Properties of

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Undoped NiO and Fe-Doped NiO Nanoparticles Synthesized by Wet-Chemical Process.

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(23) Moura, K. O.; Lima, R. J. S.; Coelho, A. A.; Souza-Junior, E. A.; Duque, J. G. S.;

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(24) Luo, Y.-D.; Lin, Y.-H.; Zhang, X.; Liu, D.; Shen, Y.; Nan, C.-W., Ferromagnetic

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Behaviors in Fe-Doped NiO Nanofibers Synthesized by Electrospinning Method. J. Nanomat.

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(25) Kurokawa, A., Sakai, N., Zhu, L.; Takeuchi, H.; Yano, S.; Tanoh, T.; Onuma, K.; Kondo,

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Nanoparticles. J. Korean Phys. Soc. 2013, 63, 716. 25



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(26) Mishra, A. K.; Bandyopadhyay, S.; Das, D., Structural and Magnetic Properties of Pristine

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Precipitation. J. Mag. Mag. Mater. 2008, 320, 3293-3296.

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in Fe-Doped NiO. Appl. Phys. Lett. 2005, 87, 202501.

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Prepared by the Solid State Method. J. Phys. D 2009, 42, 145001.

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(37) Rahdar, A.; Aliahmad, M.; Azizi, Y., Synthesis of Cu Doped NiO Nanoparticles by

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Chemical Method. J. Nanostruct. 2014, 4, 145-152.

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(38) Peck, M. A.; Huh, Y.; Skomski, R.; Zhang, R.; Kharel, P.; Allison, M. D.; Sellmyer, D.

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(40) Kremenović, A.; B. Antić, M. Vucinic-Vasic, Ph. Colomban, C. Jovalekić, N. Bibić, V.

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Iron Doped Nickel Oxide (Ni0.955Fe0.045)1−δO by in Situ X-Ray Absorption Spectroscopy. Phys.

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(44) Byrappa, K.; Adschiri, T., Hydrothermal Technology for Nanotechnology. Prog. Cryst.

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Growth Charact. Mater 2007, 53, 117-166.

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Environmental Characteristics by XRD, XAFS, and XPS, Chem. Mater., 2012, 24(23), 4483-

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Particle Size Reduction. J. Phys. Chem. C, 2012, 116, 4356–4364.

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(55) Salgueiriño-Maceira, V.; Correa-Duarte, M. A.; Bañobre-López, M.; Grzelczak, M.;

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Farle, M.; Liz-Marzán, L. M.; Rivas, J., Magnetic Properties of Ni/NiO Nanowires Deposited

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onto Cnt/Pt Nanocomposites. Adv. Funct. Mater. 2008, 18, 616-621.

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(56) Kondoh, H., Antiferromagnetic Resonance in NiO in Far-Infrared Region. J. Phys. Soc.

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Japan 1960, 15, 1970-1975.

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comparative study on heat dissipation, morphological and magnetic properties of hyperthermia

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suitable nanoparticles prepared by co-precipitation and hydrothermal methods. Bull. Mater.

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Sci., 2012, 35, 1047-1053

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(58) Zheng, R. K.; Wen, G. H.; Fung, K. K.; Zhang, X. X., Giant Exchange Bias and the

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Vertical Shifts of Hysteresis Loops in γ-Fe2O3-coated Fe Nanoparticles. J. Appl. Phys. 2004,

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Magnetic Nanoparticle System. Phys. Rev. Lett. 2003, 91, 167206.

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(60) Gandhi, A. C.; Reddy, P. M.; Chan, T.-S.; Ho, Y. -P.; Wu, S. Y., Memory Effect in

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Weakly-Interacting Fe3O4 Nanoparticles. RSC Adv. 2015, 5, 84782-84789.

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Superparamagnets and Superspin Glasses. Phys. Rev. B 2005, 71, 104405.

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(62) Khan, N.; Mandal, P.; Prabhakaran, D., Memory Effects and Magnetic Relaxation in

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Single-Crystalline La0.9Sr0.1CoO3. Phys. Rev. B 2014, 90, 024421.

29



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(63) Tian, Z. M.; Xu, L. M.; Gao, Y. X.; Yuan, S. L.; Xia, Z. C., Magnetic Memory Effect at

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Room Temperature in Exchange Coupled NiFe2O4-NiO Nanogranular System. Appl. Phys.

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(64) Xu, L. M.; Gao, Y. X.; Malik, A.; Liu, Y.; Gong, G. S.; Wang, Y. Q.; Tian, Z. M.; Yuan,

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S. L., Field Pulse Induced Magnetic Memory Effect at Room Temperature in Exchange

700

Coupled NiFe2O4/NiO Nanocomposites. J. Mag. Mag. Mater. 2019, 469, 504-509.

701

(65) Islam, I.; Dwivedi, S.; Dar, H. A.; Dar, M. A.; Varshney, D., Synthesis, Structural and

702

Paramagnetic Properties of SnO2 Doped NiO Nanoparticles. AIP Conference Pro. 2016, 1728,

703

020289.

704

(66) Anandha Babu, G.; Ravi, G.; Mahalingam, T.; Navaneethan, M.; Arivanandhan, M.;

705

Hayakawa, Y., Size and Surface Effects of Ce-Doped Nio and Co3O4 Nanostructures on

706

Ferromagnetism Behavior Prepared by the Microwave Route. J. Phys. Chem. C 2014, 118,

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23335-23348.

30


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Page 31 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


708

Figure captions

709

Figure 1 (a)-(d) Schematic representation of the synthesis scheme of Ni1-xFexO samples using

710

a standard hydrothermal method. (e) shows a structure of perfect NiO with Fe located on the

711

octahedral site and in 4:1 defect cluster (light gray: oxygen; red: nickel; blue: iron, white:

712

vacancies)

713

Figure 2 (a) SEM image of undoped NiO NPs and its inset depicts a single hexagonal shaped

714

NP. (b)-(d) TEM, HRTEM images and SAED pattern of 1 % Fe-doped NiO NPs, respectively.

715

A simulated SAED pattern is shown on the right side of Figure 2(d).

716

Figure 3 (a) A 2D plot of the XRPD pattern of Ni1-xFexO samples presenting two main

717

diffraction peaks (111) and (200) indexed based on Fm-3m. (b) A W-H plot, where the straight

718

line represents a linear fit. (c) A plot of Fe-concentration x dependency of the estimated values

719

of , d(200), d(111) and η, where the solid curve represents a fit discussed in the text.

720

Figure 4 (a) Rietveld refined (red curve) XRPD spectra (crosses) obtained from 0, 1, 5, and 10

721

% of samples (bottom to top), where the green and blue curve represents the background and

722

the difference in the fitted and obtained diffraction pattern. (b) Fe-concentration x dependence

723

of (a) the lattice constants determined from XRPD (filled symbol) and SAED (open symbol)

724

pattern and (c) nickel (filled symbol) and oxygen (open symbol) occupancy.

725

Figure 5 (a)-(d) ZFC and FC M(T) curve of 0, 1, 5, and 10 % samples measured with an Ha =

726

200 Oe. (e) Fe-concentration x dependency of TB (filled symbol) and KAF (open symbol), where

727

lines represent the fit.

728

Figure 6 (a) Magnified ZFC M(Ha) loops measured at 100 K for 0, 1, 5 and 10 % samples and

729

the inset shows full ZFC M(Ha) loops. (b) Magnified ZFC-FC M(Ha) loop measured at 100 K

730

for 1 % samples and the inset shows full ZFC-FC M(Ha) loops. Temperature dependence of 31



731

(c) –HEB field and (d) mp moments, where the inset of the respective figure shows the

732

temperature dependency of HC and MVLS.

733

Figure 7 (a) Magnified FC M(Ha) loops measured at 100 K for 1 % samples at different cooling

734

fields. Cooling field Ha dependency of (b) HC+ and HC― (bottom to top); (c) HC; (d) –HEB; (e)

735

MVLS; and (f) mp. (g) MVLS dependency of HEB where solid lines represent the linear fit.

736

Figure 8 (a)-(d) FC memory effect measured from 0, 1, 5 and 10 % samples, where the inset

737

of the respective figure shows the full range temperature dependent FC magnetization. (e) The

738

plot of temperature TS dependency of ΔM/M(%), where the solid lines are guided for the eye.

739

Inset of the figure shows a plot of Fe-concentration x dependency of the obtained value of

740

ΔM/M(%) at 100 K, where the solid line represents the fit.

741

Figure 9 (a)-(d) Time-dependent relaxation magnetization M(t) curve obtained at 100 K from

742

0, 1, 5 and 10 % samples, where the solid line represents a fit using a stretched-exponential

743

function. (e) Fe-concentration x dependency of the fitted value of β, where the solid line

744

represents a linear fit. Here different colors are used to differentiate the  values, with  ≥1

745

(PM state marked as the green region), 1<  < 0.5 (cluster-glassy state marked as the white

746

region), and  < 0.5 (SG state marked as the blue region).

747

Figure 10 Grain size d dependency of reported room temperature magnetic properties from (a)

748

Ni1-xMxO (M = Cr, Mn, Fe, Co, Cu, Zn, Sn, and Ce) and (b) Ni1-xFexO NPs. The different color

749

corresponds to green: undoped NiO; black: Cr-doped NiO; red: Mn-doped NiO; blue: Fe-doped

750

NiO; orange: Co-doped NiO; magenta: Cu-doped NiO; dark yellow: Zn-doped NiO; light

751

magenta: Sn-doped NiO; wine: Ce-doped NiO. The different symbols correspond to: solid:

752

FM; open: SPM; center-dot: PM; half-filled: AF magnetic properties. The center-cross symbols

753

with gray color represent the impure samples. Here different colors are used to represent the

754

size-dependent magnetic properties of undoped NiO NPs as Light-red: Re-I (d ≤ 10 nm) 32


Page 32 of 45

Page 33 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


755

SPM/SG/SSG; light-blue: Re-II (10 nm ≤ d ≤ 30 nm) FM; light-green: Re-II (d ≥ 30 nm) AF

756

(no finite size effect induced magnetism).

33



757

Page 34 of 45

Table 1. A summary of Rietveld refined XRPD parameters. Occupancy Sample

a = b = c (Å) Ni

wRp

Rp

χ2

O

NiO

4.1867±0.0001

0.907±0.005

0.993±0.005

0.0235

0.0185

0.7538

1%

4.1925±0.0001

0.900±0.004

0.977±0.005

0.0235

0.0185

0.7316

5%

4.1928±0.0005

0.883±0.004

1.003±0.004

0.0264

0.0206

0.8859

0.810±0.006

1.000±0.001

0.0339

0.0257

1.347

1

1

…..

……

…..

10 %

Bulk53

4.1883±0.0001

4.175

758 759 760

34


Page 35 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

761


Table of Contents

762 763 764 765 766 767 768 769 770 771 772 773

An enhanced thermal magnetic memory effect is noted from the cluster-glassy system possibly because of increased intra-particle interactions. The outcome of this study is important for the future development of diluted magnetic semiconductor spintronic devices and the understanding of their fundamental physics.

774 775

35


ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

(b)

(a)

Page 36 of 45

(c)

Hydrothermal Reactor

Ni(NO3)2·6H2O

+

H 2O

NaOH

150 °C

pH = 12

17 h

Fe(NO3)3·3H2O Air flow electric oven

Metal hydroxide (dried at 80 °C for 12 h)

Fe (d)

(e) Synchrotron radiation

Annealed at 350 °C

X-ray diffraction

Ni

4h Hexagonal shaped Ni1-xFexO NPs

O

Schematic illustration of the synthesis process of Ni1-xFexO nanoparticles Figure 1 Gandhi et.al. ACS Paragon Plus Environment

Page 37 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


(d) 1 %

(a) 0% (a) NiO

200 nm (b) 1 %

(c) 1 % 0.193 Å (200)

1 nm

Figure 2 Gandhi et.al. ACS Paragon Plus Environment


0.25

(a) Counts 500.0

5

x (%)

1500 2300

cos()/0.94(nm-1)

10

0.20 0.15

0.05 0.00

Grain size (nm)

5000

7100

(200)

(111)

17

18

19

2 (deg.)

20

5

21

10

15

20

25

4sin()/0.94(nm-1)

(c)

d(200)

0.8

d(111)

20

 0.6

15

10

5

16

0

25

6000

0

 NiO 0.76(10) 1% 0.69(9) 5% 0.70(11) 10 % 0.45(11) NiO bulk 0.016(10) LaB6 0.016(2)

0.10

3500

1

(b)

Strain  (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 38 of 45

0.4

0

2

4

x (%)

6

8

10


0 6

Counts (103)

(b)

SAED XRD

(331) (420)

(400)

10 % (311) (222)

3

(220)

(a)

(200)

6

4.195

4.190

5%

4.185

3 4.180

0

1%

6

Bulk NiO 4.175 1.00

3

0%

Data Fit Background Difference

6 3 20

30

2 (deg.)

40

0.95 0.90 0.85

(c) 0

2

4

x (%)

6

8

10

Occupancy

Ni O

0

0

Lattice constant (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


(111)

Page 39 of 45

0.80


(c)

0%

5% 0.01

0.1

0.00

(b)

(d) 1%

0.03

10 %

0.02

0.02

0.01 0.01

0

200

Temperature (K)

300

4000

100

200

0.00 400

300

Temperature (K)

4.2

(e)

160

4.0

140

3.8

120

3.6

100

3.4

80

0

2

4

x (%)

6

8

10

3.2

Anisotropy KAF (106 erg/cm3)

180

100

Magnetization (emu/g)


FC ZFC

TF ~ 16 ± 1 K

0.2

0.0 0.03

TB (K)

0.02

(a)

0.3

Page 40 of 45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




Page 41 of 45

(a)

1.5

𝐻

𝑀 -0.2 -3

-1

0

50

100

150

200

250

Temperature (K)

300

0.5

0

NiO 1% 5% 10 %

-1

-40

0

-20

0

20

Magnetic field (kOe)

1

30

2


0.0

40

50

100

150

200

250

300

Temperature (K)

(b)

6

8

(d)

T = 100 K 1%

6

4

4 2

0 0


1.5

-0.1

-1

0

100

150

200

250

300

Temperature (K)

2

0.5 0.0 -0.5

ZFC FC (Ha = 10 KOe)

-1.0 -1.5

-2

50

1.0

mp(10-2 emu/g)


-2

0.0

-0.2 -3

0.0

1.0

1

-2

0.1

0.5

2

-0.1

0.2

1.0

𝐻

0.0

1.5

HC(kOe)

(c)

T = 100 K

MVLS(10-2 emu/g)

0.1

2.0

𝑀



0.2

-HSEB(kOe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


-40

-20

0

20


1


2

0

40

30

50

100

150

200

250

300

Temperature (K) Figure 6 Gandhi et.al.


𝐻 0

-3

3

-2

(b)

1.5

0 kOe 0.5 kOe 1 kOe 3 kOe 6 kOe 10 kOe

𝐻

-4

(d)

0


2

1.0 0.5

0.0 0.08

(e)

0.06 0.04 0.02

2

0.00 0.04

1

0.6

H+C(kOe)

0% 1% 5% 10 %

0.4

0.01

0

2.0

2

4

6

Ha (kOe)

8

10

(g)

(c)

1.5 1.0

1.0 0.5

0.5 0

2

4

6

Ha (kOe)

8

10

0.00

0.02

0.04

MVLS (emu/g)

0.06

0.08

-HEB (kOe)

HC (kOe)

0.02

0.00

0.2

1.5

0.03

mp (emu/g)

(f)

MVLS (emu/g)

-H-C (kOe)

(a)

3

Page 42 of 45

-HEB (kOe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Magnetization (10-3emu/g)


0.0


0.3

0.2

0.1

0.0

0.020


0%

Warming Cooling

0.4

5%

50

100

150

200

Temperature (K)

250

0.012

0.008

1.2

0.004

0

300

50

100 150 200 250 300 350 400 450

Temperature (K)

(b)

2.0

1.4 1.3

Warming Cooling

0.016

0.000 0

1.5

(d)

1.1 2.6

10 % 2.4

1%

1.8

1.4



1.6

Warming Cooling

0.030 0.025 0.020 0.015 0.010 0.005 0

50

Temperature (K)

50

100

0.03

0.02

0.01

0.00

100 150 200 250 300 350 400 450

150

Temperature (K)

200

2.2

Warming Cooling

0.04

2.0 0

50

100 150 200 250 300 350 400 450

Temperature (K)

50

100

150

Temperature (K)

200

(e)

10

NiO 1% 5% 10 %



10

8

6

8

4

2 0

2

4

6

x (%)

8

10

6

4

M/M(%)


4

2

(c)

Cooling Warming Wt = 1 hr Ha = 200 Oe

M/M(%)

(a)

Magnetization (10-2 emu/g) Magnetization (10-2 emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Magnetization (10-2 emu/g) Magnetization (10-2 emu/g)

Page 43 of 45

2

0

100

200

TS (K)

300

0


(a)

9.0

4.0

5%

0% 8.5

3.5

8.0

3.0

(d)

1%

10 %

7

4.0 5

0.2

0.4

0.6

Time (hr)

0.8

1.0 0.0

0.2

0.4

0.6

Time (hr)

0.8

1.0

0.5< 

Understanding the Magnetic Memory Effect in Fe-Doped NiO

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