Enhancement of Magnetization through Interface Exchange

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Enhancement of Magnetization through Interface Exchange Interactions of Confined NiO Nanoparticles within the Mesopores of CoFeO 2

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Bharati Debnath, Aprajita Bansal, Hemant G. Salunke, Anustup Sadhu, and Sayan Bhattacharyya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12332 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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The Journal of Physical Chemistry

Enhancement of Magnetization through Interface Exchange Interactions of Confined NiO Nanoparticles within the Mesopores of CoFe2O4 Bharati Debnath,a Aprajita Bansal,a Hemant G. Salunke,b Anustup Sadhu,a and Sayan Bhattacharyya*a a

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India b Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India * Email for correspondence: [email protected] Abstract: A bimagnetic nanostructure was designed where the antiferromagnetic (AFM) NiO nanoparticles (NPs) are confined within the pores of a mesoporous ferrimagnetic (FiM) CoFe2O4 matrix. 3.4 wt% of 9 ± 1 nm NiO NPs was inserted into pores of 35 ± 5 nm clustered CoFe2O4 NPs when the -NH3+ groups of cysteamine on NiO NPs surface electrostatically bind to the –OSO3- of sodium dodecyl sulphate (SDS) attached to CoFe2O4 NPs. The role of in situ embedded NiO NPs is threefold: (i) to nearly double the saturation magnetization (MS) and coercivity (HC) by suppressing the frozen disordered spins on the surface of CoFe2O4 NPs surrounding the NiO NPs at the pores at the cost of enhanced FiM ordering, (ii) to introduce AFM/FiM exchange coupling by breaking the spin glass surface layer to provide EB of 233.0 ± 0.2 Oe at 5 K with a cooling field of 2 T, and (iii) to provide symmetry to the asymmetric nature of the hysteresis loop of CoFe2O4. In the absence of cooling field, the pristine CoFe2O4 NP porous matrix shows hysteresis loop shifts of >1000 Oe and asymmetric magnetization reversal which are uncommon in spinel oxides. Keywords: Magnetization, Exchange bias, Spin glass, Nanoparticle, Mesopores.

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1. Introduction Although magnetic NPs are well known for a range of biomedical, catalytic, electronic and magnetic applications, the unstable ordering of the electron spins at temperatures of practical interest, the so-called superparamagnetism, limits their applications.1 One of the most researched solution to this problem is to create an exchange anisotropy by interfacing ferromagnetic (FM)/FiM NPs with AFM phases, such that the anisotropy created by pinning the FM/FiM spins by compensated AFM spins can overcome the thermal energy.2 The EB effect, first discovered in 1956 by Meiklejohn and Bean,3 typically refers to the shift of the hysteresis loop along the magnetic field axis in materials with AFM-FM interfaces when field cooled through Néel temperature, TN of the AFM phase such that the FM/FiM Curie temperature, TC is greater than TN.4 The loop shift takes place due to an induced unidirectional anisotropy which also increases the coercivity (HC). The EB effect is especially utilized in multilayer films for applications in magnetic random access memories, spin valves, or magnetic tunnel junctions.5 Instead of the usual AFM-FM interfaces, EB was also observed in unconventional systems such as interfaces involving a spin glass phase.6 Most of the NPs demonstrating EB involves systems with FM core and AFM shell, where TC > TN according to the Meiklejohn and Bean paradigm.3,4,7 A majority of the AFM phases are oxides of FM transition metals and the disordered oxide shell is due to uncontrolled oxidation of the FM core.8,9 In addition to that, nanocomposites have been designed with randomly distributed counterparts to demonstrate the EB effect.10-13 However, nanostructures suffer from disordered AFM-FM interfaces which result in a large fraction of uncompensated spins at the interface, thus reducing the EB and hence


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lowers the induced anisotropy as compared to thin films and multilayers.4,14-16 In the past decade, there have been efforts to create inverted core-shell NPs where the AFM core was shelled with FM or FiM materials.

1,8,17-24

The basic idea was to develop biphasic

materials where EB will show non-monotonic dependence on the core diameter. These materials also exhibit large HC and spontaneous FM/FiM order. Another criterion for exhibiting large EB is the presence of ≥ 50 weight% AFM material as compared to that of FM or FiM. With lesser AFM wt%, there is incomplete pinning of the FM spins at the interface. However, in majority of these reports,18,21,24 the loop shifts along the magnetic field axis were observed in unsaturated hysteresis loops at low sweeping fields, which indicates interplay of magnetization reversal of the spins at the AFM-FM/FiM interface and results in minor unsaturated loops.25-27 In this work, a bimagnetic nanostructure was designed as per the inverted Meiklejohn and Bean paradigm, where AFM NiO NPs are confined within the pores of a mesoporous FiM CoFe2O4 matrix. The morphology can be visualized as AFM nm-scale inclusions within FiM phase to create multiple AFM-FiM three-dimensional interfaces. The weight% of the AFM NPs was kept far lower than the FiM phase, in order to ensure that all the NPs are trapped inside the pores and none are present on the surface similar to in situ embedded porous morphologies reported earlier.28-30 To the best of our knowledge, this inverted and NP embedded biphasic morphology has never been tried before. In most reports on EB, the nanocomposites consist of AFM matrices with dispersed FM/FiM NPs on the surface of the matrix.21,31-34 In this work, the NiO NP filled CoFe2O4 porous material was investigated for their structural and morphological characteristics and the inbuilt anisotropy was studied by vibrating sample magnetometer (VSM). Firstly we



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show that highly anisotropic frozen spin disorder at the CoFe2O4 NP surface can induce loop shift under zero field cooling, lowering the MS simultaneously. Secondly, after filling the CoFe2O4 pores with a miniscule 3.4 weight% of NiO NPs and cooling the system from below TN of NiO, loop shift could be observed at 5 K limited by the spin disorder at the NiO-CoFe2O4 multiple interfaces and related to the magnetization reversal mechanism with two or more metastable orientations of the spin axis.27 One of the fascinating aspects of this work is the increase in MS after incorporation of AFM NiO NPs within the pores of FiM CoFe2O4. 2. Experimental Section 2.1. Materials. Nickel(II) acetate tetrahydrate (Ni(CH3COO)2.4H2O; Sigma Aldrich, 99%), oleylamine (Sigma Aldrich, technical grade 70%), oleic acid (Sigma Aldrich, technical grade 90%), trioctyl phosphine (TOP, Sigma Aldrich, 90%), cysteamine hydrochloride (Sigma Aldrich, ≥ 98%), cobalt (II) chloride hexahydrate (CoCl2.6H2O; Merck, ≥ 98%), iron (II) chloride tetrahydrate (FeCl2.4H2O; Loba Chemie, 98% ), SDS (Merck, ≥ 90%), ammonia solution (Merck, ≥ 25%) were used without further purification. 2.2. Synthesis of NiO NPs. NiO NPs were synthesized by air oxidation of freshly prepared Ni NPs. 0.41 g of Ni(CH3COO)2.4H2O was dissolved in a solution containing 13.8 ml oleylamine, 1.3 ml oleic acid and 3.6 ml TOP. The reaction mixture was then heated at 220°C for 2 h, yielding a black colloidal solution of Ni NPs which was subsequently isolated by centrifugation with ethanol and redispersed in hexane. After exposing the black suspension to air for more than 24 h, NiO NPs were obtained.


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2.3. Synthesis of Porous CoFe2O4. Porous CoFe2O4 was prepared using a previously applied micellar method.35 0.95 g CoCl2.6H2O and 1.58 g FeCl2.4H2O were added to 5 ml aqueous solution of 1M SDS and stirred at room temperature (RT). The solution temperature was increased to 80°C under continuous stirring and an excess quantity of hot NH4OH solution was added to the mixed metal chloride solution till the the color turned green. Within few minutes, the slurry turned brown. Stirring was continued for another 3 h, then cooled to RT and isolated by centrifugation. 2.4. Synthesis of NiO-CoFe2O4. The synthesized NiO NPs were capped with oleic acid and oleylamine. Oleic acid and oleylamine were replaced with cysteamine hydrochloride (capping agent) through a ligand exchange reaction in H2O. Cysteamine hydrochloride in H2O was added to the hexane solution of colloidal NiO NPs in the molar ratio of 10:1. The mixture was stirred for 6 h, after which the hexane phase became colourless and the aqueous phase turned black. The latter was separated by centrifugation. An aqueous solution of CoCl2.6H2O and FeCl2.4H2O in molar ratio of 1:2 was stirred continuously for 1 h. Ligand exchanged NiO NPs were added to the metal chloride mixture and additonally stirred for 1 h at RT. Thereafter 1.5 g SDS was added under continuous stirring. After 30 min, the solution temperature was raised to 80°C for about 20 min. NH4OH heated up to 80°C was added to the above solution till the solution turned green and soon after to dark brown. The slurry was continuously stirred for 3 h, cooled to RT and the precipitate was isolated by centrifugation and washed with ethanol. 2.5. Calcination. Both the as-prepared porous CoFe2O4 and NiO-CoFe2O4 were calcined in N2 at 550oC for 4 h with a heating rate of 6oC/min followed by normal cooling.



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2.6. Characterization. The calcination of the as-synthesized products was carried out in a Carbolite wire-wound tube furnace - single zone, model MTF 12/38/400. The particle size of the colloidal solution and zeta-potential was measured by Malvern Zetasizer Nano ZS diffuse light scattering (DLS) instrument. The inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out in a Thermo Scientific X-series with Plasma lab software. The X-ray diffraction (XRD) measurements were carried out with a Rigaku (mini flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation. Rietveld refinements were performed by General Structure Analysis System (GSAS) software (Los Alamos National Laboratory Report, 2004). The GSAS was run by least square refinement condition. Transmission electron microscopy (TEM) images were obtained by UHR-FEG-TEM, JEOL, JEM 2100 F model using 200 kV electron source. Field emission scanning electron microscopy (FESEM) images were recorded in Carl Zeiss SUPRA 55VP FESEM. EDAX measurements were performed with the Oxford Instruments X-Max with INCA software coupled to the FESEM. The surface area and porosity measurements were carried out with a Micromeritics Gemini VII surface area analyzer. The nitrogen adsorption/desorption isotherms are reported by BJH (BarretJoyner-Halenda) surface/volume mesopore analysis. The pore volume was calculated using the Frenkel-Halsey-Hill isotherm equation. Each sample was degassed at 300oC for 2 h. The Fourier transform infrared (FTIR) measurements were carried out with a Perkin Elmer spectrum RX1 with KBr pellets. The surfactant content in the samples was determined by estimating the carbon, hydrogen and nitrogen percentages using 2400 series II CHNS/O Analyzer PerkinElmer, USA. Magnetic properties were studied using the Cryogenics - Physical Property Measurements System with the VSM probe in the


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temperature range 5-300 K and applied fields up to 4 T. The temperature-dependent zerofield-cooled (ZFC) susceptibility was measured by cooling the samples to 5 K under zero magnetic field after which 0.1- 1.0 T field was applied and data were collected from 5 to 300 K. In case of field-cooled (FC) measurements, the samples were cooled in the presence of varying applied fields. 3. Results and Discussion 3.1. Structural Analysis. Figure 1 shows the XRD patterns. The as-prepared NiO NPs crystallized in the face-centered cubic lattice with space group Fm3m according to JCPDS card No. 47-1049 and the calculated lattice parameter was a = 4.089 Å. The crystallite size obtained from Debye Scherrer equation was ~6 nm, after subtracting the instrumental broadening. The as-prepared CoFe2O4 had a spinel structure with space

group Fd3m according to JCPDS card No. 22-1086. The calculated lattice parameter was

a = 8.392 Å. The slight shift in the lattice parameters from the reported values are due to the strain involved in nanocrystalline samples. The XRD reflections of the as-prepared NiO NP embedded CoFe2O4 combined sample were of semi-crystalline nature as compared to when they were calcined. Since the weight% of NiO was much lower compared to CoFe2O4, only the CoFe2O4 reflections were observed in the embedded NiO-CoFe2O4 system. Rietveld refinement of the XRD patterns of as-prepared CoFe2O4 and NiO-CoFe2O4 systems was performed to examine the phase purity of the samples. With NiO-CoFe2O4 system the best fitting was obtained with 3.4 wt% NiO (Figure 1). Also, no additional crystalline phases such as CoO/α-Fe2O3 were found in CoFe2O4. The fitted parameters are presented in Table 1. ICP-MS analyses revealed Ni/Co = 0.1, which



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also estimates to ~3.4 wt% NiO with respect to CoFe2O4. Such an agreement between XRD Rietveld analysis and ICP-MS data is exemplary. 3.2. Morphology. The 3-dimensional morphology of the nanostructures was probed by FESEM imaging and analyses. The highly interacting and closely spaced spherical NiO NPs have a mean diameter of 9 ± 1 nm (Figure 2a,b). On the other hand, CoFe2O4 had a porous morphology with 35 ± 5 nm diameter NP clusters (Figure 2c). When the NiO NPs were embedded in CoFe2O4, the cluster-like appearance was no longer visible (Figure 2d). A typical EDAX spectrum is presented in Figure 2e. EDAX spectra were recorded at more than 10 locations on the sample, and NiO was found to be nearly homogeneous within the CoFe2O4 matrix. The average weight% of NiO matched very well with the ICP-MS data. The homogeneous distribution of NiO over an area of ~1.5 µm was confirmed from elemental line scan and mapping as presented in Figure 2f,g, where the local undulations in the distribution of NiO were not observed. The NPs were covered with surfactants which was evident from the high intensity carbon map. 3.3. Formation of NiO-CoFe2O4 Nanostructure. The formation of the in situ embedded porous nanostructure is schematically represented in Figure 3a. The synthesized NiO NPs with organic surfactants are insoluble in polar solvents like H2O and a ligand exchange process is required to make the NiO NPs dispersable in H2O which is also the dispersing medium of post synthesized porous CoFe2O4. After ligand exchange with cystamine, the organic surfactant capped NiO NPs transform to water soluble cystamine capped NiO, which when mixed with the precursors of mesoporous CoFe2O4, bind with the sulfate groups on CoFe2O4 to obtain NiO-CoFe2O4 in situ. However, due to a lower percentage of NiO with respect to CoFe2O4, several pores of CoFe2O4 remain unfilled as shown in 8 ACS Paragon Plus Environment

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the high resolution TEM image of a calcined NiO-CoFe2O4 ensemble (Figure 3b), also represented by a geometric model. 15 ± 1 nm CoFe2O4 particles are held together by organic moieties to create pores of 14 ± 1 nm in diameter (indicated by solid arrows). 3.4 weight% NiO NPs could block only few pores (indicated by dashed arrows) in the in situ embedded system, where more than one NP sits in one pore. The magnified view in Figure 3c shows a NiO NP within the CoFe2O4 matrix. Lattice fringes show the (111) interplanar spacing of NiO to be 0.24 nm and CoFe2O4 to be 0.48 nm. The Fast Fourier transform (FFT) patterns show the corresponding reflections and thus confirming high crystallinity of the calcined sample. The embedded mesoporous nanostructure was experimentally verified by nitrogen adsorption/desorption isotherms (Figure 4). Without NiO, the as-prepared and calcined CoFe2O4 had surface area of 73 ± 1 and 37 ± 2 m2/g, respectively. The decrease of surface area after calcination was due to the decomposition of the surfactant template at 550oC that hold the CoFe2O4 particles together to form the pores. According to IUPAC classification, the isotherms appear as type III physisorption with H3 type hysteresis loop, indicative of large and slit-shaped pores from aggregates of plate-like particles. The asprepared and calcined CoFe2O4 had mean pore diameters of 21 ± 1 and 38 ± 1 nm and the pore volumes were 0.510 ± 0.004 and 0.436 ± 0.002 cm3/g, respectively. The increase of pore diameter and decrease of pore volume in the calcined CoFe2O4 suggests the closure of a majority of pores and opening up of few larger pores. After embedding NiO NPs, the surface area increased to 123 ± 1 and 64 ± 3 m2/g for the as-prepared and calcined NiOCoFe2O4, respectively. The as-prepared NiO-CoFe2O4 had a mean pore diameter of 20 ± 1 nm and pore volume of 0.999 ± 0.001 cm3/g, whereas the calcined NiO-CoFe2O4 had 9 ACS Paragon Plus Environment


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37 ± 1 nm pores with an average pore volume of 0.857 ± 0.002 cm3/g. Due to the adapted synthesis protocol, NiO NPs should be surrounded by CoFe2O4 NPs in analogy to NiO NPs being trapped at the pores of porous CoFe2O4. The near doubling of surface area and increase in the pore diameter distribution after incorporation of NiO NPs is possible since the NiO NP can elbow out the surrounding CoFe2O4 NPs thus increasing the pore diameter and surface area. This would not have taken place if the 3.4 weight% NiO NPs immobilized on the surface of the CoFe2O4 matrix. FTIR studies provided evidence of organic functional groups retained in the asprepared and calcined samples (Figure 5). Since calcination was performed in N2, the surfactant moieties were not entirely removed at 550oC. The surfactants are responsible to stabilize the small size of NiO and CoFe2O4 NPs. Based on the FTIR results, reaction schematics leading to the NiO NP trapped porous CoFe2O4 system is presented in Figure 6. In the FTIR spectra, the νs,

OH

(3400-3427 cm-1) mainly arises from the absorbed

moisture in the KBr pellets, which was also checked with blank KBr. In the as-prepared NiO NPs, stretching frequency bands of the oleic acid groups attached to the NP surface are observed.36,37 The –CH2 and –COOH groups of oleic acid were retained in smaller fractions after calcination. Also, TOP is observed to be coordinated to the surface of the NiO NPs.38 The bands below 620 cm-1 correspond to the Ni-O stretching of the oxide core.39 After calcination, a majority of the frequency bands of oleic acid decreased in intensity, however TOP was still coordinated. The oleic acid and TOP capped NiO NPs (Figure 6A) are not dispersible in aqueous solvents. Hence ligand exchange reactions were performed with cysteamine hydrochloride and when the NiO NPs were capped with the sulfur groups of cysteamine moieties (Figure 6B), the NPs could be dispersed in 10 ACS Paragon Plus Environment

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water. Zeta potential measurement showed a surface charge of +8.5 mV, which confirmed the replacement of oleic acid by cysteamine molecules. This was an important step in order to homogenize the NiO NPs with the aqueous solutions of Co- and Fe-salts. In the as-prepared CoFe2O4 NPs, νs of OSO3- group and C-H bending vibration of the alkyl chain confirms capping of the CoFe2O4 NP surface by SDS molecules.40 The band at 594-597 cm-1 corresponds to νs (Fe3+ - O2-) of the Fe3+ ions in the tetrahedral voids of the spinel structure.41 The intensity of this band increases after calcining the CoFe2O4 NPs due to better crystallinity. The 21± 1 nm pores were created at the end of the nonionic alkyl chains attached to the sulfate groups coordinated to CoFe2O4 NPs (Figure 6C). When the cysteamine capped NiO NPs were added to the precursors of CoFe2O4 in presence of SDS, the zeta potential was -20.7 mV. Thereafter pH of the solution was maintained at ~14, and the final surface charge was observed to be -13.1 mV. The -NH3+ groups of cysteamine electrostatically bind to the sulfate ion already attached to the CoFe2O4 NPs. Thus, NiO NPs could be trapped at the center of the CoFe2O4 NPs and the pore volume increases to 0.999 ± 0.001 cm3/g (Figure 6D). From the FTIR spectra in Figure 5, νs, N-H (2925 cm-1) and νs, C-N (1326 and 985 cm-1) appears as shoulder peaks due to the lesser content of NiO NPs in the ensemble.42,43 These peaks were absent in the calcined sample. 3.4. Magnetism. Magnetic measurements were performed on the as-prepared CoFe2O4 and NiO-CoFe2O4 systems and the results are presented in Figure 7. The surfactant content in CoFe2O4 and NiO-CoFe2O4 were determined to be 2.4 and 3.7 wt%, respectively from the carbon, hydrogen and nitrogen estimation in the samples.



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Accordingly, the surfactant contribution has been eliminated from the magnetization values. 3.4.1. Magnetic Properties of Porous CoFe2O4. From the unsaturated ZFC magnetization loop at 5 K with a maximum applied field of 4 T in Figure 7a, pristine CoFe2O4 shows MS, HC and remanent magnetization (MR) of 25.5 ± 0.6 emu/g, 5190.0 ± 0.2 Oe and 17.2 ± 0.4 emu/g, respectively. MS did not reach the bulk saturation magnetization of CoFe2O4 (93.9 emu/g) due to factors common to NPs such as canted spin structure, surface spin non-collinearity and presence of spin glass ordering.44-46 The reduced MS can also be corroborated to the partially oxidized CoFe2O4 NP surface leading to the amorphous AFM CoO/α-Fe2O3 surface layer, which however could not be detected by XRD Rietveld and TEM analyses. Figure 7a shows that under ZFC conditions, the increase of magnetization in the first quadrant of the loop is slower as compared to those under FC. The ascending and descending branches of the hysteresis loops can be correlated to the asymmetric magnetization reversal which is an uncommon magnetic phenomenon and occurs in highly anisotropic systems but rarely observed in CoFe2O4.45 The ascending branch of the hysteresis loop from the highest negative field to highest positive field is governed by coherent rotation of the spins.47,48 In contrast, the reversal of the spins via domain wall formation and propagation determines the descending branch of the hysteresis loop i.e. from the highest positive field to highest negative field. Interestingly, a loop shift of 1078.0 ± 0.4 Oe is observed in the negative direction of the magnetic field axis under ZFC conditions. ZFC loop shift was recently observed in


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manganites,16 anti-perovskite PdNCr3,49 and Ni-Mn-In alloy,50 however this is the first observation in a spinel system. The magnetization values ±25.5 emu/g are the same at the highest positive and negative magnetic fields and the origin of loop shifts is related to the development of unidirectional anisotropy below the blocking temperature.49,50 After field cooling with 2 and 4 T applied fields from 300 to 5 K, the hysteresis loop shifts are lowered to 394.0 ± 0.8 and -70.0 ± 0.2 Oe, respectively (Figure 7a). The negative sign denotes the shift towards positive side of the field axis. HC also decreases correspondingly to 1260.0 ± 0.1 and 873.0 ± 0.68 Oe with 2 and 4 T cooling fields, and MS increases as 26.6 ± 0.6 and 26.9 ± 0.7 emu/g, respectively. MR is 18.3 ± 0.4 and 17.9 ± 0.5 emu/g with 2 and 4 T cooling fields, respectively. Thus the built-in anisotropy of the system decreased with increasing cooling fields. The observed loop shift cannot be correlated to the earlier explanations on the origin of EB under ZFC conditions.16, 50 On the contrary, our results correlate to a disordered spin state similar to the spin glass phase at the surface of the CoFe2O4 NPs.51,52 After 2 and 4 T field cooling, requirement of lesser negative field for spin reversal indicates easy coherent rotation. The cooling field induced formation of larger domains and easier coherent rotation gives rise to steeper ascending and descending FC curves.47 Because of similar reasons, the loop is shifted with lower HC and this shift decreases in the negative side and moves to the positive side of the field axis with increasing cooling fields. Although magnetization reversal becomes difficult towards the negative field direction, the same is not true when the applied field is changed to the positive side of the axis. This observation points towards a pinning effect between the disordered surface layer and FiM phase. The more the disordered spins are aligned with the cooling field the anisotropy induced by this random glassy state reduces,



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lowering both HC and the observed loop shift. The fragility of this anisotropy towards moderately high fields shows its origin in a frozen spin glass state, which has also a trend of decreasing Hc and HEB with increasing cooling fields, unlike the conventional AFM/FM interfaces. 3.4.2. Magnetic Properties of NiO-CoFe2O4. 3.4.2.1. Temperature Dependent Magnetization of NiO-CoFe2O4. The NiO-CoFe2O4 embedded system demonstrates a large bifurcation of the temperature dependent ZFC and FC magnetization curves which suggests the presence of strong inter-particle interactions (Figure 7b).53 The FC magnetization reached a plateau below 40 K instead of a steady increase with decrease in temperature, which was due to the magnetic interactions of the FiM NPs.32,54 The plateau in the FC magnetization can also point towards the freezing of localized disordered spins.55 With the increase in applied field, the bifurcation point between ZFC and FC curves shifts towards lower temperatures (Figure 7b).56 Moreover, the ZFC magnetization approaches zero at ≤ 16 K with 0.01 T applied field, which signifies the existence of magnetic moments with short relaxation times.57 The disordered spins are likely to be localized at the interface between NiO and CoFe2O4 NPs and can coexist with the dominant long-range FiM ordering at the CoFe2O4 NP core. 3.4.2.2. Field Dependent Magnetization of NiO-CoFe2O4. At 300 K the magnetization did not saturate even at 3 T field due to the presence of superparamagnetic fractions corroborated to ZFC-FC loops above 260 K (Figure 7c). Using Langevin function,56 MS at 300 K was found to be 28.1 ± 0.0 emu/g, accompanied by a negligible HC of 19.0 ± 0.0 Oe and MR = 0.6 ± 0.0 emu/g. It is possible that due to NP size distribution and varying extent of clustering of CoFe2O4 NPs, few larger NPs remain beyond the 14 ACS Paragon Plus Environment

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superparamagnetic limit to give a finite HC and MR at 300 K. At 5 K, large hysteresis was observed in the unsaturated loops, accompanied by the increase of MS, HC and MR to 41.6 ± 0.1 emu/g, 10610.0 ± 0.4 Oe and 24.2 ± 0.1 emu/g, respectively. It is interesting to note the near doubling of MS and HC after addition of NiO to CoFe2O4 NPs. The AFM inclusions nullify the spin disorder of the CoFe2O4 NPs surrounding the filled pore to result in a tremendous increase in the long range magnetic ordering of the embedded NiO-CoFe2O4 system as compared to CoFe2O4 NPs synthesized by similar protocol. This observation is noteworthy since AFM NiO has a low magnetic moment, Ms ~ 1.8 emu/g (Figure 7c inset) and thus the increase in Ms of the embedded system is not possible without an effective compensated coupling between the filled NiO NPs inside the CoFe2O4 NP pores.27 In this case, 3.4 wt% NiO is not enough to lead to unsaturation of the loops. Rather the inclusion of NiO NPs results in coupling of the NiO-CoFe2O4 spins and reduces the fraction of surface disordered spins of CoFe2O4 NPs at the circumference of the filled pores thus increasing MS of the ensemble. To confirm that the increase of MS after incorporation of NiO NPs inside the CoFe2O4 pores is not due to any FM Ni inclusions in the NiO bulk, the field dependent magnetization of NiO was measured at 300 and 5 K (Figure 7c inset). The magnetization of NiO NPs reached ~1.8 emu/g at 5 K with continuous unsaturation up to 4 T, which is a clear signature of AFM behavior in agreement with other NiO systems.58,59 3.4.2.3. Exchange Bias in NiO-CoFe2O4. When the sample was field cooled with 2 T applied field from 300 to 5 K and measured in between ±4 T, the hysteresis loop was shifted by 233.0 ± 0.2 Oe in the negative direction of the field axis (Figure 7d). It is to be noted here that the TN of bulk NiO is 525 K and the sample was cooled only from 300 K



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< TN, which is not as per the typical measurement protocol. Under 2 T field cooling, HC and MR are 10280.0 ± 0.4 Oe and 30.2 ± 0.1 emu/g, respectively. The increased MR suggests that the sample retains a higher magnetic moment in the presence of cooling field. The ZFC MS (= 41.6 ± 0.1 emu/g) was shifted to 47.6 ± 0.1 emu/g under 2 T FC. This is a clear indication that major fraction of the total magnetization originates from the volume of the FiM matrix and a part of it results due to the freezing of moments at the AFM-FiM multiple interfaces. The loop shifts in the unsaturated hysteresis loops of as-prepared CoFe2O4 and NiO-CoFe2O4 NPs (Figure 7a,c) can be argued to be either the EB effect or signature of minor loops. With low sweeping fields, the magnetization consisting of random distribution of easy axes cannot be completely reversed and so the loop shifts in the unsaturated loops were considered as minor loops, where the displacement along the field and magnetization axes is a natural phenomena.26,60 On the contrary, in this case the equal magnetization values in the highest positive and negative magnetic fields proclaim that the shifted hysteresis loops are obtained from an unmagnetized state at the same temperature and they are not asymmetric minor loops, thus the loop shifts can be considered as due to the EB effect.43,61 Moreover, in our case the maximum field applied was 4 T instead of a small sweeping field and reproducible results were obtained from measurements performed in two different magnetometers. In the absence of cooling field from 300 to 5 K, the CoFe2O4 moments freeze in random directions and the net exchange anisotropy is averaged out to provide a spontaneous loop shift of 41 Oe under ZFC conditions. After the external magnetic field is applied, it aligns the CoFe2O4 moments and the NiO spins also freeze below a certain


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temperature. A cooling field of 2 T generates aligned moments both in the FiM and AFM NPs with frozen magnetic moments. The tendency to exchange couple the surface spins of CoFe2O4 and NiO NPs reduces the disordered spins on CoFe2O4 NP surface, increasing the MS. This results in pinning the FiM spins by the AFM NiO phase. The weak pinning of the core FiM moments is due to the random distribution of the magnetization easy axes from the ‘still misaligned’ spins in addition to the compensated AFM spins parallelly coupled to the FiM spins at the interface.12,34 When the cooling field was increased to 4 T, the corresponding loop shift decreased. In this case, the loop shift was 194 ± 0.2 Oe, HC decreased to 10260.0 ± 0.4 Oe, MS slightly increased to 48.1 ± 0.1 emu/g and MR remained nearly unchanged i.e. 30.5 ± 0.1 emu/g. The decrease of loop shift with increase in cooling field was previously attributed to the interaction of frozen disordered spins with the larger applied cooling field.1,62 The decrease of loop shift with 4 T cooling field clearly indicates the presence of a competing force against the interfacial spin coupling of NiO and CoFe2O4. Similar to the Fe/Fe-oxide system,60 a field of 4 T can induce Zeeman coupling between the magnetic field and the spin glass layer on the surface of the NiO and CoFe2O4 NPs. 3.4.2.4. Thermo-remanent magnetization (TRM) Studies. The existence of spin glass type ordering in the NiO-CoFe2O4 system was verified by the TRM studies (Figure 8a,b), which fits well with stretched exponential decay equation:63 M(t) = M1 + M0 × exp[˗ (t/τ)1˗n], where M1 is the residual magnetization, (M0 + M1) is the initial magnetization, τ is the characteristic time constant and n is an exponent. The data were fitted well with the stretched exponential which is clear indication of a glassy phase. The fitted parameters are presented in Table 2. The exponent n decreases whereas the initial and residual



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magnetization increases from 100 to 5 K. The higher τ at 5 K also indicates slower magnetic relaxation and the prominent existence of spin glass state towards lower temperatures. The proposed spin structure in the NiO-CoFe2O4 embedded system in Figure 8c shows that although the frozen disordered spins are omnipresent at the surface of all the CoFe2O4 NPs, the pores filled by NiO NPs can partially eliminate those spins though mutual exchange coupling. Although the inclusion of NiO NPs inside the pores of CoFe2O4 does not represent a conventional EB system,4 it minimizes the disordered spins at the CoFe2O4 NP surface and increases the MS by 1.8 times from that of porous CoFe2O4, at the same time performs nanoscale tuning of the magnetization reversal processes by exchange coupling with the FiM CoFe2O4 spins. 4. Conclusions In summary, multiple AFM-FiM interfaces were created in the inverted geometry of AFM NPs confined inside a porous FiM material. 9 ± 1 nm NiO NPs were in situ embedded inside the pores of mesoporous CoFe2O4 with 35 ± 5 nm diameter particle clusters. 3.4 wt% NiO NPs could block only few pores in the embedded system and increase the surface area of as-prepared CoFe2O4 by 50 m2/g. The chemical binding of the NiO NPs and porous CoFe2O4 was achieved by ligand exchange whereby cysteamine moieties attached to NiO NPs could bind to the dangling sulfate groups on CoFe2O4. The incorporation of AFM NiO NPs within the FiM CoFe2O4 pores has a surprisingly positive effect in increasing the MS from 25 ± 0.6 for CoFe2O4 to 41.6 ± 0.1 emu/g at 5 K by minimizing the disordered spins at the surface of the CoFe2O4 NPs. The filled NiO NPs also introduce AFM/FiM exchange coupling by breaking the spin glass surface layer to provide EB of 233.0 ± 0.2 Oe at 5 K with a cooling field of 2 T. A cooling field of 4 T 18 ACS Paragon Plus Environment

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resulted in Zeeman coupling between the NiO NP spins and the magnetic field at 5 K, which reduces the overall loop shift from 233.0 ± 0.2 Oe to 194.0 ± 0.2 Oe. The designed in situ embedded morphology can be extended to other AFM/FiM or AFM/FM NP composites to increase the effective magnetization at the same time introduce EB assisted by the spin glass ordering for potential applications in random access memory, spintronics, spin valves etc. Acknowledgement This work is supported by the Council of Scientific and Industrial Research (CSIR), India under sanction no. 01(2689)/12/EMR-II. BD and AS thank DST-INSPIRE programme and University Grants Commission (UGC), New Delhi for their fellowships, respectively. References (1) Salazar-Alvarez, G.; Sort, J.; Suriñach, S.; Baro, M. D.; Nogués, J. Synthesis and Size-Dependent Exchange Bias in Inverted Core-Shell MnO|Mn3O4 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 9102-9108. (2) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogués, J. Beating the Superparamagnetic Limit with Exchange Bias. Nature 2003, 423, 850-853. (3) Meiklejohn, W. H.; Bean, C. P. New Magnetic Anisotropy. Phys. Rev. 1957, 105, 904-913. (4) Nogués, J.; Schuller, I. K. Exchange bias. J. Magn. Magn. Mater. 1999, 192, 203-232. (5) Wang, J.; Omi, T.; Sannomiya, T.; Muraishi, S.; Shi, J.; Nakamura, Y. Strong Perpendicular Exchange Bias in Sputter-Deposited CoPt/CoO Multilayers. Appl. Phys. Lett. 2013, 103, 042401. (6) Baaziz, W.; Pichon, B. P.; Fleutot, S.; Liu, Y.; Lefevre, C.; Greneche, J.-M.; Toumi, M.; Mhiri, T.; Begin-Colin, S. Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size and Nanosized-Dependent Composition, Defects, and Spin Canting. J. Phys. Chem. C 2014, 118, 3795-3810.



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∑ (

∑[ ]

Table 1: XRD-Rietveld refinement parameters. Here Rwp = χ2 =

, RObs = ∑

!"#

] [$

, wi=

,

, N = number of data points, Iobs = observed intensity, Ical =

calculated intensity, P = number of parameters.

Sample [space group]

Lattice parameters (Å), Angles (°)

Atomic positions (x,y,z)

CoFe2O4 [Fd3m]

a = b = c = 8.3749 ± 0.0004 Å α = β = γ = 90°

Co1 (0.125, 0.125, 0.125) Co2 (0.5, 0.5, 0.5) Fe1 (0.125, 0.125, 0.125) Fe2 (0.5, 0.5, 0.5) O (0.25027, 0.25027, 0.25027)

3.4% NiO @ CoFe2O4

CoFe2O4 (Phase1) a = b = c = 8.3474 ± 0.0012 Å α = β = γ = 90°

Co1 (0.125, 0.125, 0.125) Co2 (0.5, 0.5, 0.5) Fe1 (0.125, 0.125, 0.125) Fe2 (0.5, 0.5, 0.5) O (0.25027, 0.25027, 0.25027)

[CoFe2O4 Fd3m NiO Fm3m ]

NiO (Phase2) a = b = c = 4.1764 ± 0.0001 Å α = β = γ = 90°

Ni (0, 0, 0) O (0.5, 0.5, 0.5)


Occupation Goodness Weighted number of fit profile (reduced (RWP) χ2) Co1 = 0.5 1.061 Co2 = 0.5 Fe1 = 0.5 0.72 % Fe2 = 0.5 O = 1.0

Co1 = 0.5 Co2 = 0.5 Fe1 = 0.5 Fe2 = 0.5 O = 1.0 Ni = 1 O=1

1.124 0.99 %


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Table 2: TRM fitting parameters for NiO-CoFe2O4 system according to the equation: M(t) = M1 + M0 × exp[˗ (t/τ)1˗n]. M(t) and t are variables, M0 , M1 and τ are parameters, n (&"') ∑ where O = observed value, C = is constant and χ2 = reduced chi-square = (!"#")

(

calculated value, σ2 = variance, N = number of data points and P = number of fitted parameters. The equation, M(t) = M1 + M0 × exp[˗ (t/τ)1˗n] was fitted with different values of n and the best fit was obtained for n = 0.3 at 5 K and 0.52 at 100K. The values of M0, M1, τ were obtained from the fit.

System

NiO@CoFe2O4 -AP

100 K M1 = 0.52 ± 0.003 emu/g M0 = 0.01 ± 0.0002 emu/g τ = 1500 ± 20 s n = 0.52 χ2 = 3.8 x 10-8


5K M1 = 0.59 ± 0.001 emu/g M0 = 0.017 ± 0.0002 emu/g τ = 10100 ± 300 s n = 0.3 χ2 = 1.2 x 10-7

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Figure 1: XRD patterns of (a) NiO NPs, (b) porous CoFe2O4 and (c) as-prepared NiOCoFe2O4 and (d) calcined NiO-CoFe2O4. XRD-Rietveld analyzed patterns of (e) CoFe2O4 and (f) NiO-CoFe2O4. The legends: diff (difference plot between observed and calculated patterns; Obs (observed pattern); Calc (calculated pattern); and Bckgr (background plot).



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Figure 2: (a) FESEM image of NiO NPs and (b) corresponding diameter histogram. FESEM images of (c) porous CoFe2O4 and (d) NiO-CoFe2O4 matrix. (e) EDAX spectrum of (d) where the peaks of Al, Si, P, S and Cl are from the substrate. (f) Elemental line scan and (g) Elemental mapping showing the homogeneous distribution of the elements.


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Figure 3: (a) Schematic representation of the formation of in situ embedded NiO NPs inside the mesopores of CoFe2O4. (b) TEM image of calcined NiO-CoFe2O4 correlated to a geometric model. The solid and dotted arrows show the empty and filled pores, respectively. (c) High resolution TEM image and the FFT patterns of the NiO NP trapped at the pore and the CoFe2O4 porous matrix.



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Figure 4: N2 adsorption-desorption isotherms and (inset) the pore size distribution profiles. AP and Cal represent the as-prepared and calcined samples, respectively.


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Figure 5: FTIR spectra of (a) as-prepared NiO NPs, (b) calcined NiO NPs, (c) asprepared porous CoFe2O4, (d) calcined porous CoFe2O4, (e) as-prepared NiO-CoFe2O4 and (f) calcined NiO-CoFe2O4. The bands: 1) O-H stretching, 2, 3) asymmetric and symmetric -CH2 stretching (oleic acid), 4, 5) asymmetric and symmetric –COOH stretching (oleic acid), 6) (C8H17)3P-NiO stretching, 7) -HC=CH- stretching (oleic acid), 8) Ni-O stretching, 9) O-H bending, 10) C-H bending (SDS alkyl chain), 11) –OSO3stretching, 12) O-H out plane vibration, 13) Fe3+-O2- stretching, 14) N-H, 15, 16) C-N and 17) M-O stretching.



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Figure 6: Synthetic scheme depicting the formation of (i) NiO NPs, (ii) NiO NPs after ligand exchange, (iii) porous CoFe2O4 and (iv) in situ embedding of NiO NPs inside the pores of CoFe2O4.


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Figure 7: (a) Magnetization (M) as a function of field (H) plots of CoFe2O4 NPs under ZFC and FC (2 and 4 T) conditions at 5 K. NiO-CoFe2O4: (b) ZFC (open squares) and FC (filled squares) curves at different applied fields. (c) M-H loops under ZFC condition at 300 and 5 K and FC conditions at 5 K. Inset: M-H loops of NiO NPs at 300 and 5 K. (d) Enlarged portion of (c) showing the hysteresis loop shift and coercivity at 5 K.



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Figure 8: (a,b) TRM plots for NiO-CoFe2O4 system. (c) Schematic showing the spin arrangement after embedding of NiO NPs inside the pores of CoFe2O4. Outlined maroon spheres indicate the CoFe2O4 NPs, blue spheres are the NiO NPs and blue stars show the empty pores. The disordered spins responsible for the spin glass ordering are shown by short black arrows and the reduction of spin disorder around the NiO NPs are shown by replacing the black misaligned arrows with green FiM spins.


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Enhancement of Magnetization through Interface Exchange

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