Silica-Coated and Bare Akaganeite Nanorods: Structural and

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Silica-Coated and Bare Akaganeite Nanorods: Structural and Magnetic Properties Marin Marko Tadi#, Irena Milosevic, Slavko Kralj, Mamadou Mbodji, and Laurence Motte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01547 • Publication Date (Web): 21 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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The Journal of Physical Chemistry

Silica-Coated and Bare Akaganeite Nanorods: Structural and Magnetic Properties

Marin Tadic1,*, Irena Milosevic2, Slavko Kralj3, Mamadou Mbodji2, Laurence Motte2 1

Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Science, University of

Belgrade, POB 522, 11001 Belgrade, Serbia 2

Université Paris 13, Sorbonne Paris Cité, Laboratoire CSPBAT, CNRS, UMR 7244, 93017

Bobigny , France 3

Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

ACS Paragon Plus Environment

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ABSTRACT: We report on structural and magnetic properties of uniform silica coated akaganeite nanorods with length of L ~ 80 ± 15 nm and diameter D ~ 15 ± 5 nm as well as silica shell thickness of about 5 nm. Unexpected negative difference between field-cooled (FC) and zero-field-cooled (ZFC) magnetization ∆M= MFC - MZFC < 0, room temperature ferromagnetism and exchange bias effect have been found. The nanorods are investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) measurements. The magnetic measurements were also performed on bare akaganeite nanorods in order to discriminate the effects of silica coating on the magnetic properties. The measured coercivity and exchange bias effect of bare β-FeOOH nanorods are much lower compared with same properties of SiO2@β-FeOOH nanorods emphasizing effect of silica coating on the magnetic properties. These results are discussed considering the core-shell structure of akaganeite nanorods i.e. the inner part of the akaganeite nanorod has antiferromagnetic ordering, whereas the nanorod surface exhibits some disorder spin state.

KEYWORDS: surface effects, exchange bias effect, coercivity, magnetism.


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1. INTRODUCTION During the past decade iron-based nanomaterials have been at the focus of a research interest due to a variety of interesting physical properties and their huge potential for applications, such as magnetic seals and inks, magnetic recording media, catalyses, pigments, ferrofluids, contrast agents for magnetic resonance imaging, therapeutic agents for cancer treatment, immunoassays, targeted drug delivery vehicles and magnetic hyperthermia.1-11 Among various iron-based nanomaterials, the antiferromagnetic nanosized iron oxide and hydroxide systems, together with their preparation and characterization, have attracted special attention in materials science. These antiferromagnetic nanomaterials exhibit interesting magnetic properties that are significantly different from those of their corresponding bulk counterparts. Indeed, recent reported experimental results have shown large magnetic moment of antiferromagnetic particles, lowtemperature and high-temperature hysteresis behavior, superparamagnetism, thermo-induced magnetic moment, enhancement of the susceptibility and exchange bias effect.12-19 It has been realized that these interesting properties of the antiferromagnetic nanomaterials are a consequence of disordered surface spins and partially non-compensated internal spins i.e. a spin imbalance due to a lack of particles' structural perfection.19-21 Among antiferromagnetic iron oxides and hydroxides, akaganeite is less well known, possibly due to the inherent difficulties in preparation of a single crystalline phase: such a preparation requires special conditions owing to the akaganeite's structural instability.20 Akaganeite is interesting due to its structural, magnetic and catalytic properties, as well as for practical applications such as for the removal of pollutants, sorption, ion-exchange, lithium-air batteries.22-27 Moreover, the β-FeOOH phase is important and widely used as a precursor in the


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preparation of iron, hematite, maghemite and magnetite nanosized materials.16,28-32 Akaganeite is an antiferromagnetic material with a Néel temperature in the range of TN ≈ 240-299 K.12,20,33 A substantial reduction of the Néel temperature, down to TN = 15 K, has been observed in amorphous β-FeOOH nanowires.19 It has also been observed that the Curie-Weiss temperature varies within a wide range for differently synthesized β-FeOOH materials.12,33 It has been reported in the literature that the wide ranges of the two temperatures, TN and θ, are influenced by interstitial water molecules, vacancies and chlorine anions.12,20,33 Moreover, the size of particles and their morphology have a large influence on the magnetic properties of β-FeOOH. In particular, some researchers have focused on the ferromagnetism of the antiferromagnetic βFeOOH nanoparticle systems below the Neel temperature and its superparamagnetism at room temperature. Luna et al. reported magnetic properties of akaganeite nanocrystals with mean diameter of (3.3 ± 0.5) nm and with a small amount of rodlike akaganeite particles with (23 ± 5) nm in length and (5 ± 1) nm in diameter.20 These studies revealed complex magnetic relaxation processes, high values of coercivity and an exchange bias effect, both at low temperatures. The conclusion was that these results arise from the frustration of the antiferromagnetic order owing to surface effects and an insufficient filling of akaganeite channels with Cl- anions.20 Mashkour et al. synthesized cellulose akaganeite hybrid nanocrystals in situ from a ferrous chloride aqueous solution in the presence of pre-oxidized cellulose nanocrystals.34 They found that the synthesized nanoparticles were composed of rod-like cellulose nanocrystal cores coated by shells of spherical chloride-containing akaganeite nanoparticles with an average diameter of about 4–6 nm. The as-synthesized sample was a superparamagnet that offered a good magnetic response, which is interesting for practical applications.34 Several papers, reported nanosized akaganeite coated by silica shell, but these papers lacked detailed magnetic characterization.23,29,31 Silica


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coated nanosized materials provide additional possibilities, such as enhanced reactivity of the surface, better dispersibility and higher magnetic susceptibility, as compared with bare nanomaterials.23,29,34 The silica shell around magnetic nanoparticles prevents agglomeration and improves colloidal stability. In recent years, a new application of silica as drug vehicles in drug delivery systems has also been explored, as silica is nontoxic and highly biocompatible.23,29,35 Based on the reported results we think that it is very important to further improve the magnetic properties, as well as surface stability and biocompatibility of akaganeite nanoparticles, that may lead to practical applications in future technologies and devices. In this paper, we reported novel magnetic properties of akaganeite materials. The silica coated akaganeite (SiO2@β-FeOOH) nanorods show ferromagnetic behavior at room temperature, negative value of ∆M = MFC - MZFC and exchange bias effect even at 250 K, which distinguishes them from other akaganeite materials. To our knowledge, similar magnetic properties in akaganeite materials, ferromagnetism and exchange bias effect, were observed at much lower temperatures (below 20 K) and in the case of heterogeneous mixture of rather equiaxial akaganeite nanocrystals and small amount of rodlike particles.20 Due to sample heterogeneity, it was rather difficult to conclude what kind of particles (equiaxial or rodlike) or their specific mixture and size distribution lead to these interesting magnetic effects at low temperature (ferromagnetism and exchange bias effect).20 Recently, we have reported a study of bare akaganeite nanorods (β-FeOOH) prepared by forced hydrolysis of aqueous FeCl3/HCl solutions in presence of dopamine as chemical shape-agent.36 The reported results revealed the existence of ferromagnetism and exchange bias effect at 50 K.36 In this work, we have focused investigations on structure and magnetic properties of silica-coated akaganeite nanorods (SiO2@β-FeOOH) with comparison of the properties of bare akaganeite nanorods. The magnetic


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properties of silica-coated nanorods are significantly enhanced as compared with the magnetic properties of bare nanorods.36 The measured coercivity of silica-coated akaganeite nanorods at room temperature is about two times higher, and the exchange bias effect was observed up to much higher temperature ~ 250 K.

2. EXPERIMENTAL METHODS 2.1. Preparation of silica coated akaganeite nanorods (SiO2@β-FeOOH). The bare akaganeite nanorods were synthetized, as described previously, by forced hydrolysis of aqueous FeCl3/HCl solutions in presence of dopamine as chemical shape-control agent.16 The assynthesized akaganeite nanorods were further coated with a 5-nm-thick silica shell. In brief, 20 mL of the suspension containing as-synthesized akaganeite nanorods (159 mM, Fe ions) were transferred into 50 mL of ethanol solution containing 0.8 mL of aqueous ammonia (25 %) and 150 mg of polyvinylpyrrolidone (PVP). Then, the mixture of 0.6 mL of tetraethoxysilane (TEOS) and 2 mL of ethanol were added drop-by-drop into the above suspension over a period of 10 minutes, while vigorously stirring. The silica coated akaganeite nanorods (SiO2@βFeOOH) were obtained after 8 hours of stirring, followed by washing with distilled water using centrifuge (20 minutes, 15 000 g). 2.2. Characterization Techniques. The X-ray powder diffractometer (Phillips PW1710) employing CuKα (λ=1.5406 Å, 2θ=15-70º) radiation was used to characterize the crystal structure of the nanocomposite. The size, morphology and nanostructure of the nanorods were observed by TEM measurements (transmission electron microscopy, JEOL JEM 2100). The chemical composition of the sample was analyzed using an Oxford Instrument’s INCA energy-


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dispersive spectrometer (EDS) provided in the TEM. Magnetic hysteresis properties and magnetization's dependence on the temperature were measured using a VSM magnetometer (vibration sample magnetometer) in a wide range of temperatures (50–400 K) and applied DC fields (up to 3 T). For ZFC magnetization, the sample was cooled from room temperature to 50 K in the absence of magnetic field and then a finite magnetic field was applied. The magnetization was measured while the sample was heated up to 400 K. The FC magnetization was performed by cooling the sample from 400 K down to 50 K in the presence of the same field as in the ZFC mode and the data were taken while heating the sample up to 400 K.

3. RESULTS AND DISCUSSION 3.1. Microstructural and Morphological Studies. The XRPD (X-ray powder diffraction) pattern of as-synthesized nanostructure confirms akaganeite crystal structure and the intensity of the corresponding peaks is similar to that of Ref. 16, which indicates the nanorod structure. Thereafter, the chemical composition of the SiO2@β-FeOOH nanorods was analyzed by EDS. The EDS spectrum is shown in the inset of Fig. 1, where peaks for O, Fe and Si can be found in the spectra. A quantitative analysis yields a composition of 66.48% O, 26.39% Fe and 7.13% Si (all in atom percent). The signals for Cu and C peaks are due to the carbon foil and the copper-supported grid.


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Figure 1. The XRPD diffraction pattern of the SiO2@β-FeOOH nanorods. The inset shows the EDS spectrum of the nanorods. Transmission electron microscopy (TEM) studies of the sample shows formation of uniform nanorods coated with thin silica shell (Fig. 2(a)-(f)). The nanorods have a length of about 80 ± 15 nm and a diameter of about 15 ± 5 nm, with aspect ratio ~5. The high resolution TEM images (Fig. 2(e)) show both lattice fringes with inter layer spacing of 0.53 nm which is consistent with the (200) plane of β-FeOOH, and the amorphous silica shell around nanorods. The thickness of amorphous silica shell is about 5 nm (Fig. 2(d) and (e)). The ellipsoidal morphology of the nanoparticles is attained through the observation of few nanorods oriented parallel to the electron beam of TEM microscopy (Fig. 2 (f)).


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Figure 2. (a)-(f) Transmission electron micrographs (TEM) of silica coated akaganeite nanorods. (f) Arrows show nanorods oriented parallel to the electron beam of TEM microscopy.

3.2. Magnetic Studies. Magnetic properties of the synthesized SiO2@β-FeOOH nanorods were investigated by a VSM magnetometer. The field-cooled (FC) and zero-fieldcooled (ZFC) magnetization measurements were performed from 50 to 400 K. In this study the temperature dependence of magnetization M(T) in ZFC and FC modes were measured with


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applied magnetic fields H = 1000 Oe and H = 10 kOe. The ZFC-FC magnetization measurements are shown in Fig. 3(a) (H = 1000 Oe) and 3(b) (H = 10 kOe).

Figure 3. (a) and (b) Temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measured in the fields of 1000 Oe and 10 kOe. (c) and (d) The difference between the FC and ZFC magnetization as a function of temperature for the same applied fields.

It can be seen that the magnetizations decrease gradually with the increases of the temperature from the low temperature up to 400 K in both ZFC and FC curves. Moreover, the intersection of the ZFC and FC curves and their bifurcation can be seen. In order to further investigate the FC and ZFC magnetization properties, the temperature dependence of the difference magnetization ∆M = MFC − MZFC are plotted in Fig. 3(c) and (d). The ∆M shows negative values above 203 ± 10 K (H = 1000 Oe, Fig. 3(c)) and 190 ± 10 K (H = 10 kOe, Fig.


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3(d)). This unusual feature was related to magnetostriction.37 The origin of such an anomalous behavior in the β-FeOOH nanorods still presents a strong challenge to our understanding of this material and demands further investigation. The Néel temperature TN = 252 ± 5 K is determined from the inverse susceptibility and its deviation from the linear fit to the high temperature data (1/χ vs. T, Fig. 4(a)). This value is in agreement with literature.12,20,33 The plot of 1/χ vs. T (Fig. 4(b)) above 300 K obeys the Curie–Weiss law χ=C/(T-θ) (Fig. 4(b), full line). The values of Curie constant C and Curie-Weiss temperature θ were obtained as a result of the best fit. The slope of the fitting line provides the value of C, whereas the intercept with temperature axis provides the value of θ. We determined a positive Curie-Weiss temperature θ = 9 ± 1 K. This positive Curie-Weiss temperature value should be attributed to ferromagnetic exchange interactions between Fe3+ ions at the surface of particles. This value of θ agrees with values reported in the literature.12,32 The value of Curie constant C = 1.85 ± 0.08 emu·K/mol·Oe is also determined from the fit of the Curie-Weiss law. The effective magnetic moment per β-FeOOH unit in the nanorods is estimated to be µeff = 2.83·(C)1/2= 3.85 ± 0.18 µB, which is smaller than in the bulk (µeff = 4.4 µB).12 Using the equation µeff = [g2·µB2·S·(S+1)]1/2, where g is gyromagnetic ratio (g = 2) and S spin, we obtained S = 1.49 ± 0.07 for the Fe3+ ions, which is smaller than observed in other iron oxide structures. The observed lower value of effective spin S can be explained by the compression of the coordination octahedra toward the Fe3+ ions by interstitial ions in the akaganeite structure.33 The exchange field HE is determined from the obtained Néel temperature TN and the spin S for the Fe3+ ions using mean-field theory and the equation TN = HE·g·µB·(S+1)/3·kB. The obtained value is HE = (2.26 ± 0.12)·106 Oe which was expected for nanosized β-FeOOH.12 The sublattice magnetization is estimated as MS = 1/2·N·g·µB·S = 345 ± 19 emu/cm3, where N = 2.5·1022 at.Fe/cm3 for akaganeite. This value is lower than the observed


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one in bulk akaganeite (MS = 413.6 emu/cm3).12 This lowering of the sublattice magnetization MS can be associated with the surface effect, crystal lattice defects and interstitial molecules and ions (H2O and Cl anions).

Figure 4. (a) The inverse ZFC magnetic susceptibility as a function of temperature (H=1000 Oe). The red line is a linear fit of the linear part of the data for high temperatures. (b) The inverse ZFC susceptibility with Curie-Weiss law fit.

To obtain additional insight into magnetic properties of the sample, measurements of field dependence magnetization M(H) at different temperatures were performed. Hysteresis has been observed in nanosized antiferromagnetic systems, whereas in bulk antiferromagnetic materials hysteretic behavior is not expected.38-43 It has been reported that the surface effects and crystal lattice defects determine these properties.42,43 In this work we measured hysteresis behavior in the range from 300 K up to 50 K (Fig. 5). Fig. 5 shows the field dependence of the isothermal magnetizations in the −30 kOe to 30 kOe range.


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Figure 5. Field dependence of magnetization recorded at 50 K (a), 100 K (b), 200 K (c) and 300 K (d). The insets show law field magnetization behavior. Inset of Fig. 5(d) (upper left corner) shows temperature dependence of coercivity for the sample (50 K, 100 K, 150 K, 200 K, 250 K and 300 K).

As shown in Fig. 5, the β-FeOOH nanorods show ferromagnetic-like behavior (hysteresis loops). The values of the coercivity, remanent magnetization and saturation magnetization for the various temperatures are shown in Table 1 .


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Table 1. Hysteresis characteristics of the SiO2@β-FeOOH nanorods at various temperatures.

T [K]

50

100

150

200

250

300

HC [Oe]

125 ± 5

115 ± 5

90 ± 5

85 ± 5

25 ± 3

21 ± 3

Mr

0.032 ±

0.019 ±

0.011 ±

0.0088 ±

0.0014 ±

0.0011 ±

[emu/g]

0.002

0.001

0.0008

0.0005

0.0001

0.0001

4.20 ± 0.31

3.74 ± 0.27

3.39 ± 0.25

3.03 ± 0.22

MS 5.83 ± 0.32 4.84 ± 0.31 [emu/g]

The values of MS were determined by extrapolating 1/H to zero-field in the M vs. 1/H plot based on the high field data. Existence of the hysteresis loops as well as the absence of the magnetization saturation should be noticed (Fig. 5). Inset of Fig. 5(d) (upper left corner) shows the variation of the coercivity with temperature of the sample (50 K, 100 K, 150 K, 200 K, 250 K and 300 K). It is apparent from this figure that the coercivity strongly depends on the temperature (HC increases as the temperature decreases from 300 to 50 K), as expected for nanoparticle systems. Therefore, our measurements of the magnetic hysteresis properties indicate a contribution of ferromagnetic-like and antiferromagnetic components in the akaganeite nanorods. The existence of hysteresis loops below and above TN is a novel property of akaganeite nanorods, quite distinct from the bulk akaganeite. These results are corroborated by uncompensated-spin ordering, either on the surface, or in the core of antiferromagnetic materials.43,44 Sugiyama et al. demonstrated that dislocations in antiferromagnetic crystals imply unique magnetic properties.43 Furthermore, they showed that ferromagnetic ordering of


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individual dislocations in antiferromagnetic material originates from the local non-stoichiometry. We believe that hysteresis properties in the akaganeite nanorods are a consequence of the disorder structure and non-stoichiometry (due to interstitial water molecules, vacancies and chlorine ions). We propose that the hysteresis properties above the Néel temperature are due to the short-range exchange interactions between surface spins which interactions persist at room temperature. This room temperature ferromagnetism presents a novel effect in akaganeite nanorods. Surface uncompensated magnetic moment µunc in akaganeite nanorods can be estimated using the relation µunc = nunc·S·g·µB.12 The number of uncompensated spins is determined by relation nunc = nsurf1/2. In elongated nanoparticles, the relation nsurf = N·d·A holds, where N is the number of atoms per unit volume and d is the thickness of an atomic layer. The surface area of an ellipsoid is A = π/2·(D2+(D·L·α/sinα)), where D is diameter, L is length, and α = arccos(D/L). For akaganeite N = 2.5·1022 at.Fe/cm3 and d = 2.4 Å for the iron atom.12 Hence nanorods with an average length L = 80 nm and average diameter D = 15 nm represent nsurf = 18316 ± 860 Fe atoms. This gives µunc = 402 ± 28 µB suggesting that the surface spins have a high impact on the magnetic properties of the SiO2@β-FeOOH nanorods. The unusual magnetism in SiO2@β-FeOOH nanorods is also given by hysteretic behavior where initial magnetization curve lies below the hysteresis loop (Fig. 5). Similar behavior of M vs. H have been reported in NiO and Fe2O3 nanoparticles.21,45,46 Our work shows that SiO2@βFeOOH nanorods exhibit similar behavior, confirming this effect for nanosized materials. The origin of this anomalous behavior in the magnetization process is still controversial.21,45,46 We may conclude that the spins in the surface layers of the nanorods play a central role in such hysteretic behavior in our work.


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To gain further insight into the origin of ferromagnetic-like properties of the nanorods, a field-cooled (FC) hysteresis loops have been recorded at different temperatures (Fig. 6). In an antiferromagnetic material with exchange bias effect, the hysteresis loop is displaced along the field axis so that it is no longer centered on zero field and the coercivity increases substantially. 44,47-49

The exchange bias field HEB and coercivity field HC are calculated using HEB = -

(HC1FC+HC2FC)/2 and HC = |(-HC1FC+HC2FC)|/2 where HC1 and HC2 are the values of the magnetic field at which magnetization vanishes.20,44 Up to now, the exchange bias effect and hysteresis loop shift have only been observed in the β-FeOOH nanoparticles at low temperatures.20 The FC hysteresis loops in this study were measured after cooling the sample in an applied field of 30 kOe from 400 K temperature down to 50, 100, 150, 200, 250 and 300 K. The measured FC magnetization curves exhibit the typical features of an exchange bias system (Fig. 6), i.e., a shift of the hysteresis loop along the magnetic field axis and an increase of coercivity compared to the ZFC hysteresis values. These features occur in nanoparticle systems due to the exchange coupling between the different magnetic structures.44 We conjecture that because of the disorder surface spin layer of the nanorod and its coupling with antiferromagnetic core; the exchange bias effect is observed in the sample. Fig. 6(f) shows the variation of the exchange bias field HEB and coercivity HC as a function of temperature. This represents the first reported measurement of temperature dependence of HEB in akaganeite nanorods and the influence of silica shell on such behavior. In nanorods at temperatures higher than TN, exchange bias field vanishes due to strong decrease of coupling between the surface and core spins. It can be seen (Fig. 6(f)) that HEB first increases as a function of temperature and then reaches a maximum followed by a decreasing trend. Similar behavior of HEB has been explained by spinflip coupling in the interfacial interactions.50-52 This model suggests that the interfacial


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uncompensated spins and their coupling with surface spins play an important role in the exchange field mechanism. It is known that all exchange bias models require the presence of uncompensated magnetization in the antiferromagnetic materials, thus indicating that the proposed core-shell model for the nanorods might be appropriate. The distinct magnetic characteristics of the SiO2@β-FeOOH nanorods are summarized in Table 2.

Table 2. Magnetic characteristics of the SiO2@β-FeOOH nanorods: TN-Néel temperature, θCurie Weiss temperature, C-Curie Weiss constant, µeff-effective magnetic moment, S-effective spin, µunc-surface uncompensated magnetic moment, HC-coercivity, Mr-remanent magnetization and HEB-exchange bias field. TN

Θ

C

µeff

[K]

[K]

[emu·K/mol·Oe]

[µB]

S

µunc

HC

Mr

HEB

HEB

[µB]

[Oe]

[emu/g]

[Oe]

[Oe]

(300 K)

(300 K)

(100 K)

(250 K)

252

9

1.85

3.85

1.49

402

21

0.0011

497

92

±

±

±

±

±

±

±

±

±

±

5

1

0.08

0.18

0.07

28

3

0.0001

10

7


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Figure 6. Hysteresis curves at 50 K (a), 100 K (b), 150 K (c), 200 K (d) and 250 K (e) after FC in 30 kOe. The insets show law field magnetization behavior. Fig. 6(f) shows temperature dependence of coercivity HC and exchange bias field HEB for the sample (50 K, 100 K, 150 K, 200 K, 250 K and 300 K).


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To discriminate the effects of silica coating on the magnetic properties, field dependence of magnetization M(H) measurements at different temperatures were performed for the bare akaganeite nanorods in both ZFC and FC modes (see Figures S1, S2 and S3 in the Supporting Information (SI)). The ferromagnetic-like behavior and exchange bias effect were also observed for the bare akaganeite nanorods (Fig. S1 and S2). In comparison with the magnetic properties of silica coated akaganeite nanorods, much lower values of coercivity and exchange bias field were observed in the bare akaganeite nanorods (Fig. S3). The measured coercivity of bare β-FeOOH nanorods at room temperature is about two times lower compared with the coercivity of silica coated nanorods (Fig. S3(a)). The exchange bias effect for the bare akaganeite nanorods was observed up to temperature 150 K (HEB = 22 Oe, Fig. S2 and S3) whereas with silica coating, such HEB value is still measured at 250 K (HEB = 92 Oe). These comparisons show the influence of silica shell on the magnetic properties of akaganeite nanorods. After silica coating, it is not expected to find any modifications of nanorod’s core structure because soft silica coating process at room temperature is used. Thus, silica shell has an impact only on the nanorods' surface and some modifications and changes can occur only on the surface of the akaganeite nanorods after coating. We may conclude that silica coating increase the surface effects which play dominant role in the magnetic properties of the akaganeite nanorods.

4. CONCLUSIONS The silica coated β-FeOOH nanorods of high uniformity were presented, together with their structure and magnetic properties. We found novel magnetic properties (MFC - MZFC < 0, room temperature ferromagnetism and exchange bias effect) of SiO2@β-FeOOH nanostructure, that


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might be important in future practical applications and fundamental research. The magnetization measurements of the sample reveal a Curie-Weiss temperature θ = 9 ± 1 K, an exchange field HE = (2.26 ± 0.12)·106 Oe, a sublattice magnetization MS = 345 emu/cm3, an effective magnetic moment µeff = 3.85 ± 0.18 µB for the Fe3+ ion, an effective spin S = 1.49 ± 0.07, surface uncompensated magnetic moment µunc = 402 ± 28 µB, hysteresis loops (ferromagnetic-like properties) at room temperature (HC = 21 ± 3 Oe and Mr = 0.0011 ± 0.0001 emu/g) and exchange bias field HEB = 497 ±10 Oe. Based on these results, we have conjectured that the inner part of the nanorod has an antiferromagnetic ordering, whereas the surface shell of the akaganeite nanorod exhibits some disordered spin state. The magnetic properties of silica coated and bare akaganeite nanorods have been compared. We found much stronger coercivity and exchange bias effects for the coated nanorods indicating an enhancement of surface effects in the coated ones as compared to the bare nanorods.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Marin Tadic)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources This study was financially supported by a Serbia-France bilateral project for 2013-2014, and by the Serbian Ministry of Science, under Grant no. III 45015.

ACKNOWLEDGMENT The authors are grateful to the CNRS (France) and the MSTD (Serbia, Ministry of Science, Technology and Development) for having support this project.

ASSOCIATED CONTENT Supporting Information Figure S1: Field dependence of magnetization recorded at 50 K (a), 100 K (b), 150 K (c), 200 K (d), 250 K (e) and 300 K (f) for the bare akaganeite nanorods. The insets show law field magnetization behavior. Figure S2: Hysteresis curves at 50 K (a), 100 K (b), 200 K (c) and 250 K (d) after FC in 30 kOe. The insets show law field magnetization behavior. Figure S3: Temperature dependence of coercivity HC and exchange bias field HEB for the for bare akaganeite nanorods and after coating with silica. This information is available free of charge via the Internet at http://pubs.acs.org.


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Silica-Coated and Bare Akaganeite Nanorods: Structural and

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