Magnetic Properties and Charge Transfer Transition Induced by Jahn

Article pubs.acs.org/JPCC

Magnetic Properties and Charge Transfer Transition Induced by Jahn−Teller Effect in FeGa2O4 Nanoparticles I. S. Lyubutin,† S. S. Starchikov,*,† N. E. Gervits,† Chun-Rong Lin,*,‡ Yaw-Teng Tseng,‡ Kun-Yauh Shih,§ Jiann-Shing Lee,‡ Yu. L. Ogarkova,† A. O. Baskakov,† and K. V. Frolov† †

FSRC “Crystallography and Photonics” of Russian Academy of Sciences, Moscow 119333, Russia Department of Applied Physics and §Department of Applied Chemistry, National Pingtung University, Pingtung County 90003, Taiwan

‡

ABSTRACT: Complex iron−gallium oxide nanoparticles FeGa2O4/FeGaO3 with a cubic spinel structure and the size of about 30 nm were synthesized by the combustion method. In the process of the synthesis, a new phase γ-FeGaO3 created at the surface of the FeGa2O4 core adopts the spinel structure of the core. In the pure FeGa2O4 compound, the cation distribution obtained from the Mössbauer data is (Fe2+0.76Ga3+0.24)tet [Fe2+0.24Ga3+1.76]oct O4 at room temperature. With decreasing temperature, the Jahn−Teller distortions of tetrahedral A-sites initiate the charge transfer transition at 140−90 K leading to the redistribution of Fe2+ and Ga3+ cations over the A- and B-sites. A fine correlation and connection between the critical temperature and critical concentration of the J-T ions associated with transition from the dynamic to cooperative J-T effect was observed. At low temperatures, the magnetic and Mössbauer measurements reveal a highly frustrated magnetic structure of spinglass type with the spin-freezing temperature of TSG = 26 K. The positive value of the Curie temperature θC = 106 K indicates the ferromagnetic interaction between iron ions. The estimated value of anisotropy energy ⟨Eanis⟩/kB is about 181.3 K. The Fe3+ and Fe2+ ions present in the FeGaO3 and FeGa2O4 phases dominantly contribute to the exchange anisotropy Eex and magnetocrystalline anisotropy Ecryst, respectively. The total anisotropy energy (Eanis = Eex + Ecryst) can be tuned by the variation of compounds in the FeGa2O4/FeGaO3 complex composites. but form short-ordered clusters 1. However, the clusterization can be eliminated by adding approximately 0.2 mol of ferric ions Fe3+.4 Depending on the degree of inversion, the migration of Fe3+ into B-site causes a strong magnetic interaction between the sublattices. The presence of the Jahn−Teller ions Fe2+ in FeGa2O4 can initiate local crystal distortions, which is expected to be observed by Raman vibrational and Mö s sbauer spectroscopy. Moreover, the cooperative Jahn−Teller distortions can change crystal symmetry, which leads to modification of physical-chemical properties such as an unit cell volume, electroconductivity, heat capacity, and so forth.5−9 In our study, Fe1+xGa2−xO4 nanoparticles were synthesized by the combustion method. Several complementary methods (TEM, EDX, XRD, Raman and Mossbauer spectroscopy, and magnetic measurements) were applied for investigation structural, magnetic and electronic properties of the Fe1+xGa2−xO4 nanoparticles, and interesting effects specific of the nanosize samples were observed.

1. INTRODUCTION Among the vast variety of materials considered to be potentially applicable in the field of biomedicine being produced in the nano size, the iron gallate FeGa2O4 arouses special and outstanding interest. It can possess the inverse spinel structure inherited from magnetite Fe3O4,1 and the magnetic properties of FeGa2O4 are tunable so that at the stage of synthesis the future nanoparticles can be surely adjusted for certain purposes, such as magnetic drug delivery or hyperthermia. Moreover, the magneto-optical and cathodoluminiscent features of the FeGa2O4 nanoparticles make it possible to use them as cathodoluminescent agents and magnetic-resonance (MR) contrast materials.2 There are two important crucial parameters for such contrast materials: the spin−lattice (T1) and spin−spin (T2) relaxation times which also depend on the type of cations in the spinel structure. Adding different ions allows tuning these parameters to improve the local contrast in the MRI method.2 In addition, the electronic and magnetic properties of FeGa2O4 can be useful in magnetic-tunnel junctions (MTJ).3 Determination of cation distribution in FeGa2 O 4 is interesting and relevant topic, because it affects the magnetic features of the sample and the fields of its application. The distribution of Fe2+ and Ga3+ ions in octahedral [B] and tetrahedral (A) sublattices depends on the process of synthesis. In general, Fe2+ ions are not distributed randomly in the lattice, © 2016 American Chemical Society

Received: August 5, 2016 Revised: October 17, 2016 Published: October 26, 2016 25596

DOI: 10.1021/acs.jpcc.6b07928 J. Phys. Chem. C 2016, 120, 25596−25603

Article

The Journal of Physical Chemistry C

Figure 1. TEM image of Fe1+xGa2−xO4 nanoparticles (a) and the size histogram of the nanoparticles. The solid line is the Gaussian shape of the size distribution.

2. SAMPLE PREPARATION AND CHARACTERIZATION Fe1+xGa2−xO4 nanoparticles were prepared by the combustion method. Gallium was dissolved in diluted nitric acid, while iron nitrate hydrate, citric acid monohydrate, and glycine were dissolved in distilled water. These compounds were mixed to form the precursor solution which was further concentrated by heating until excess free water evaporated and spontaneous ignition occurred. Combustion finished with the formation of greenish-black residue which served as a precursor of the iron− gallium oxide compound. The precursor was heat treated at temperature of 900 °C in a stream of 10% H2/Ar mixed gas for 4 h. Transmission electron microscopy (TEM) was applied to characterize the morphology and microstructure of the particles. A Tecnai G2 F20 instrument equipped with an energy dispersive X-ray detector (EDS) and field emission scanning electron microscopy (FESEM, XL-40FEG, Philips Co. Ltd.) were used for TEM analysis which was performed at the accelerating voltage of 200 kV. The phase purity and crystal structure of the samples were controlled by powder X-ray diffraction (XRD) using a Multiflex MF2100 instrument, Rigaku Co. Ltd., with Cu Kα radiation (λ = 1.5418 Å). To record the Raman spectra, we used a Princeton Instruments Acton SP2500 monochromator/spectrograph equipped with Spec-10 system with a nitrogen cooled CCD detector. SpectraPhysics Beamlock 2080 Krypton laser with 647.1 nm line was used as the excitation source for Raman signal. Standard MS-1104Em spectrometer operating in the constant acceleration mode. was used to collect Mössbauer absorption spectra of 57Fe nuclei. Low temperature Mössbauer measurements were performed using a closed cycle helium cryostat. A gamma-ray source 57Co(Rh) was at room temperature, and a standard α-Fe absorber was used for calibration. Magnetization measurements were conducted in applied fields up to 20 kOe using a superconducting quantum interference device (SQUID-VSM, MPMS, Quantum Design). A representative TEM image shown in Figure 1a indicates that the Fe1+xGa2−xO4 nanoparticles are uniform with the similar size and spherical shape. From TEM images we calculated an average size of nanoparticles and plotted the histogram for the size distribution Figure 1b. The approx-

imation of the distribution by Gaussian shape gives an average particle size around 30 nm, which correlates with XRD data. As shown in Figure 2, the XRD pattern of the Fe1+xGa2−xO4 nanoparticles consists of well resolved reflections which

Figure 2. Comparative X-ray diffraction patterns of Fe1+xGa2−xO4 nanoparticles (bottom panel) and a bulk FeGaO3 sample (top panel). The indicated reflection indexes correspond to the cubic spinel-type structure (space group Fd3̅ m ) for Fe 1+xGa 2−x O4 and chiral orthorhombic structure (space group Pna21) for FeGaO3 sample.

correspond to the cubic symmetry of spinel-type crystal structure with a space group Fd3̅m.10 An average particle size, calculated by the method of Scherer11 from the width of (311) peak, was found to be about d = 30.2 nm. We calculated the lattice parameter of the sample using the DicVol program and obtained the value of a = 8.343(1) Å. Figure 3 shows the energy-dispersive X-ray spectra of Fe1+xGa2−xO4 nanoparticles. Radiation lines of the corresponding chemical elements are indicated above the appropriate peaks. The obtained atomic compositions Fe(11%), Ga(25%), and O(64%) indicate a deviation from the exact formula FeGa2O4, implying the presence of some extra phase in the 25597


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Figure 3. Energy-dispersive X-ray spectra of Fe1+xGa2−xO4 nanoparticles. Radiation lines of the corresponding chemical elements are indicated above the appropriate peaks. Extra (unlabeled) peaks correspond to C Kα, Cu Kα, and Cu Kβ appeared from the carbon coated Cu grid which was used to support the sample in the electron microscopy studies.

ferrous Fe2+ ions in tetrahedral (A) and octahedral [B] oxygen sites of FeGa2O4,12−15 and the relative content of Fe2+ ions in these sites is about 56(1) and 18(1)%, respectively. On the basis of the Mössbauer data we can define the distribution of the Fe2+ and Ga3+ cations over A-and B-sites in FeGa2O4 sample (at 295 K) as (Fe2+0.76Ga3+0.24)tet [Fe2+0.24Ga3+1.76]oct O4. The cause of the ferric Fe3+ ions in the nanoparticles should be specifically examined. An assumed presence of the Fe3+Ga3+O3 phase is not obvious, since the crystal structure of FeGaO3 can be either orthorhombic (Pna21) or rhombohedral (R3c).16 This is not consistent with a cubic spinel-like structure (Fd3̅m) of nanoparticles observed in the present XRD measurement. To identify the origin of ferric Fe3+ ions in nanoparticles, the bulk compound FeGaO3 was prepared by the similar combustion method that was applied for the synthesis of Fe1+xGa2−xO4 nanoparticles. It was established from the XRD pattern (Figure 2) that the crystal structure of our bulk FeGaO3 sample corresponds to the chiral orthorhombic structure with space group Pna21. In addition, we found that the frequencies of the main peaks in the Raman spectrum of our FeGaO3 sample (see below Figure 7) are very close to those observed in the spectrum of the orthorhombic nanoparticle sample FeGaO3.16 As shown in Figure 5, the room-temperature Mössbauer spectrum of our bulk FeGaO3 sample can be well approximated by three quadrupole doublets with narrow lines. The grid lines on the top of the spectrum (Figure 5) correspond to three nonequivalent sites of iron Fe3+ ions in orthorhombic FeGaO3 compound, which is consistent with known crystallographic data.17,18 For more reliable spectra processing, we computed the distribution function of the quadrupole splitting P(Δ), which displays the probability P of existence of the quadrupole component with the Δ splitting value.19 Three maxima clearly observed in the P(Δ) function (inset in Figure 5) unambiguously support the presence of three quadrupole doublets, thus conforming the spectra fitting. The isomer shift values for three doublets are very close of about δ = (0.34−0.35) mm/s but the quadrupole splitting values are different Δ = 0.40(1), 0.64(1) and 1.07(1), mm/s, respectively. These hyperfine parameters δ and Δ correspond

sample, while an excess of oxygen can be related to the oxygen absorbed at the particle surface.

3. STRUCTURAL AND ELECTRONIC PROPERTIES 3.1. The Room-Temperature Mö ssbauer Spectra. The room-temperature Mö ssbauer spectrum of Fe1+xGa2−xO4 (Figure 4) has no magnetic splitting and consists of three

Figure 4. Room temperature Mössbauer spectrum of Fe1+xGa2−xO4 nanoparticles. Solid lines are calculated quadrupole doublets fitted to the experimental spectrum demonstrating the presence of three nonequivalent states of iron ions. Red and green doublets (and grid lines on the top) correspond to the Fe2+ ions in octahedral B-site and tetrahedral A-site of spinel FeGa2O4, respectively. A purple doublet corresponds to the Fe3+ ions in the γ-FeGaO3 spinel nanocompound. The purple line at the bottom is a fitting disparity. A vertical bar (1%) indicates the value of the Mössbauer absorption.

quadrupole doublets indicating the paramagnetic state of iron ions. The hyperfine parameters of doublet D1 are: the isomer shift δ(1) = 0.333(5) and quadrupole splitting Δ(1) = 0.790(5) mm/s), while the relative fraction of these ions is about 26(1)% of total iron content in the sample. The values δ(1) and Δ(1) are typical of a high spin ferric state Fe3+. Two additional doublets D2 and D3 observed in the spectrum have the hyperfine parameters of δ(2) = 0.933(5) and δ(3) = 1.034(5) mm/s; Δ(2) = 1.240(5) and Δ(3) = 2.725(5) mm/s, respectively. The parameters of D2 and D3 are typical of 25598


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doublets D2 and D3 corresponding to the Fe2+ ions in the Aand B-sites of FeGa2O4 (Figure 6). This indicates that the site

Figure 5. Room temperature Mö s sbauer spectrum of bulk orthorhombic FeGaO3. The spectrum fit to three doublets (shown by solid lines and grid lines on the top) corresponds to three nonequivalent sites of the Fe3+ ions in accordance with a known crystallographic structure.17,18 The inset shows the distribution function of quadrupole splitting P(Δ). Three maxima, clearly observed in the distribution, unambiguously support the presence of three quadrupole components, thus conforming the spectra fitting. The purple line at the bottom is a fitting error. Figure 6. Temperature evolution of the Mössbauer spectra between 295 and 35 K in the Fe1+xGa2−xO4 sample. The dashed lines are shown to trace the decrease of the red doublet intensity (due to rearrangement of Fe2+ and Ga3+ ions over tetrahedral and octahedral sites) in the FeGa2O4 compound and changes in the value of quadruple splitting.

to Fe3+ ions in nonequivalent sites with the relative site occupations S = 44(3)%, 25(4)%, and 31(2)%, respectively. According to neutron diffraction data,17,18 two nonequivalent sites Fe1 and Fe2 for iron ions and two sites Ga1 and Ga2 for gallium ions are present in the orthorhombic structure of FeGaO3. The iron ions may occupy two Fe1 and Fe2 sites and one Ga2 site.17 Mössbauer parameters δ and Δ obtained from our spectra of bulk FeGaO3 are in good agreement with previously reported for the orthorhombic FeGaO3;18 however, the values of these parameters do not coincide with the δ and Δ values observed for D1 Mössbauer component in Fe1+xGa2−xO4 nanoparticles. This indicates that the D1 component is associated with a new iron−oxygen phase of Fe 3+ ions present in the Fe1+xGa2−xO4 nanoparticles, and in addition, this phase should have the cubic spinel structure as it follows from the XRD data. Besides magnetite Fe3O4, the cubic spinel structure (Fd3m ̅ ) with Fe3+ ions is the most probable in maghemite γ-Fe2O3,20 and also cubic cation-deficient spinel structure of γ-Ga2O3 is well-known in the literature.21−23 Thus, it can be concluded that the new phase like γ-FeGaO3 similar to cubic γ-Fe2O3 and γ-Ga2O3 can be created in the combustion method of synthesis of the Fe1+xGa2−xO4 nanoparticles. Taken into account the EDS data indicating the excess of oxygen at the particle surface, it is most likely that the γFeGaO3 phase appears on the particle surface. From this analysis, we can conclude that the Fe1+xGa2−xO4 nanoparticle samples consist of a FeGa2O4 core covered with a FeGaO3 shell. The iron content in the FeGa2O4 and FeGaO3 phases of the composite is in the ratio of about 74/26 (%) relative to the total content of iron in the sample. Taking into account the molar mass of FeGa2O4 (259.2886 g/mol) and FeGaO3 (173.5662 g/mol), we get the mass ratio FeGa2O4/FeGaO3 as 81/19 (%). 3.2. Jahn−Teller Effect and the Charge Transfer Transition in the FeGa2O4 Nanoparticles. The evolution of Mössbauer spectra in FeGa2O4 / FeGaO3 with decreasing temperature from 295 to 30 K (before magnetic ordering) reveals changes in the intensity of the resonance lines of

occupation varies with temperature. As shown in Figure 7a, the site occupation is stable between 295 and 140 K but it dramatically changes in the region of 140−90 K, and then it stabilizes again at temperatures (90−30 K) before the magnetic transition. It is important that these changes in the D2 and D3 line areas correlate with the behavior of the quadrupole splitting Δ of doublet D2 corresponding to the tetrahedral A-sites. As shown in Figure 7b, the Δ(D2) value gradually increases with lowering temperature from 295 to 30 K revealing a plateau in the range of 140−90 K. However, the Δ(D3) value for the Bsites does not essentially change in all temperature interval 295−30 K (Figure 7b). The isomer shifts in both sites decrease smoothly with temperature in agreement with the behavior of second-order Doppler shift. The behavior of quadrupole splitting Δ(T) in the A- and B-sites well correlates with calculations and Mössbauer data obtained for spinel Fe2TiO4.24 Ion Fe2+ (3d6, e3g t32g) in the A-sites of FeGa2O4 spinel structure is the Jahn−Teller (J-T) ion,25 and the observed behavior of Δ(D2) vs temperature is the signature of the J-T effect associated with distortion of the tetrahedral sites.26−28 Ion Fe2+ in the A-sites has a dx2−y2 ground state, and at tetragonal distortion the energy separation between dz2 and dx2−y2 levels in the lower doublet assigns the Δ value. The Δ(T) dependence for the A-sites in Figure 7 demonstrates the temperature dependence of the J-T distortions in the FeGa2O4 nanoparticles. The strong temperature dependence of the quadrupole splitting parameter Δ(D2) and its essential increasing at low temperatures indicates that the main contribution to the Δ value (i.e., to the electric field gradient EFG at the A-sites) originates from the electronic contribution of the 3d6 shell in Fe2+ ions at the A-site, while the lattice contribution is almost independent of temperature. As follows from Figure 7a, in (Fe2+t Ga3+1−t) [Fe2+1−t Ga3+1+t] O4 nanoparticles there are two critical concentration (tcr1 and 25599


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Figure 7. Temperature dependences of occupation of the octahedral [B] and tetrahedral (A) sites by Fe2+ ions obtained from the areas of Mössbauer components (a) and the values of quadrupole splitting Δ parameters (b) in FeGa2O4 nanoparticles. The dashed lines are guide for the eye.

tcr2) of Fe 2+ in the A-sites, which stabilizes the J-T distortions.25,29 At tcr1 = 0.77(2) the structure is stable at temperatures between 295 and 140 K, and at tcr2 = 0.52(2) the structure persists to be stable from 90 K down to the magnetic transition temperature of about 30 K. Apparently, in the range of 295−140 K, we observe the dynamic J-T distortions which start to transform to the cooperative distortion below 140 K. In the transition region of 140−90 K, the dramatic charge redistribution takes place between the A- and B-sites, which results in stabilization of the cooperative J-T distortions below 90 K. Thus, the critical concentration of Fe2+ ions in the A- sites of FeGa2O4 nanoparticles, which stabilizes the cooperative J-T distortions at low temperatures is about tcr2 = 0.53(2). In these experiments we observed a fine correlation and connection between the critical temperature and critical concentration of the J-T ions in the A-sites associated with the transition from the dynamic to cooperative J-T effect. Normally, at low temperatures, the cooperative J-T effect results in a macroscopic distortion of the crystals.29 In spinels, this leads to the appearance of a tetragonal or orthorhombic phase. If one raises the temperature or changes the concentration of the J-T ions, these macroscopic distortions disappear, and the crystal acquires the symmetry which it had without the J-T ions (in this case, the cubic spinel). At the presence of J-T ions, the electron makes fluctuations correlated with anions. Local distortions, averaged in time, lead to the dynamic Jahn−Teller effect.29 It was supposed5 that the dynamic J-T effect in bulk materials should be associated with small concentration of J-T ions and/ or relatively high temperatures. In a certain sense, we have observed an opposite tendency in the FeGa2O4 nanoparticles. The cooperative J-T distortion was developed with decreasing the concentration t value in A-sites. Several reasons for such behavior can be taken into account: (i) Under redistribution of Fe2+ and Ga3+ cations over the A- and B-sites, the total amount of J-T ions Fe2+ remains the same; (ii) the J-T ions are located in both A- and B-sites, and A−B interactions should affect the lattice distortions;5 (iii) the size and surface effects in nanoparticles can influence the behavior of J-T distortions; (iv) our FeGa2O4 nanoparticles are covered by the FeGaO3 shell which can also affect the distortions of the core lattice. In summary, we conclude that with decreasing temperature the local crystal symmetry of the A-sites in FeGa 2 O 4 nanoparticles became lower thus changing the electron structure of cations in these sites. This initiates a redistribution

of Fe2+ and Ga3+ cations over the A- and B-sites of FeGa2O4, and this effect can be considered as a charge transfer transition. This leads to the transition from the dynamic to cooperative JT distortion. The cation distribution in FeGa2O4 obtained at room temperature as (Fe2+0.76Ga3+0.24)tet [Fe2+0.24Ga3+1.76]oct O4 is valid in the range of 295−140 K, and it changes to (Fe2+0.53Ga3+0.47)tet [Fe2+0.47Ga3+1.53]oct O4 at temperatures 90− 30 K. At 295−140 K, the average charge values in the (A) and [B] sites are (2.24) [5.76], and they change to (2.47) [5.53] at 90−30 K. Between 140 and 90 K, the site occupation in FeGa2O4 nanoparticles depends on temperature due to structural transformations initiated by the J-T effect. 3.3. Raman Spectroscopy Data. As shown in Figure 8, Raman spectrum of Fe1+xGa2−xO4 nanoparticles consists of

Figure 8. Room temperature Raman spectra of Fe1+xGa2−xO4 nanoparticles (upper panel). The spectrum of the bulk FeGaO3 sample is shown at the bottom for comparison.

several broadened peaks with frequencies at about 282, 325, 420, 695, 763, and 980 cm−1. To our knowledge, there are no Raman measurements on bulk cubic spinel FeGa2O4, and only some general properties of our spectrum can be treated taking into account the data available for other cubic spinels.30 It is predicted from the group theory analysis that five Raman active modes (A1g + Eg + 3T2g) are expected for the space group Fd3̅m of spinels. The presence of vacancies and/or defects in the crystal structure can activate other phonon modes which can be observed in the Raman spectrum. High intensive 25600


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The Journal of Physical Chemistry C peak in the region of 650−800 cm−1 may correspond to internal vibrations of the MO4 (M = Fe, Ga) tetrahedra, which is the A1g mode in cubic spinels.31 The shape of the peak reveals a shoulder at about 763 cm−1 (Figure 8), which can be associated with cation redistribution.32 The strong line appearing at 980 cm−1 can be associated with a vibron level appearing in the dynamic J-T effect due to strong bonding of d electrons and ligands (oxygen) in tetrahedrons containing J-T ions.33 According to our Mössbauer measurements (discussed above), an additional phase of cubic FeGaO3 was found in the Fe1+xGa2−xO4 sample. As shown in Figure 8, the Raman spectrum of a bulk FeGaO3 sample with orthorhombic structure is absolutely different from the pattern of the nanoparticle sample with cubic spinel structure.

temperature dependence of reciprocal magnetization 1/M(T) can be well approximated by the Curie−Weiss law. We have found that the effective Curie temperature θC estimated from a linear plot (inset in Figure 9b) is rather high (106 K) and positive. Formally, this indicates that the interaction between the iron ions is of ferromagnetic nature. Apparently, the interaction between the core FeGa2O4 and the shell FeGaO3 compounds affects the magnetic properties of this complicated composite. Spin-glass behavior can be expected in compounds with spinel structure in the case of random distribution of magnetic and nonmagnetic cations between the tetrahedral (A) and octahedral [B] sublattices.39 Magnetic frustrations can be produced due to the random magnetic dilution in the two sublattices.40 In our FeGa2O4/FeGaO3 complex oxides, the A− A, B−B, and A−B interactions are engaged in the competition when the Fe and Ga ions are distributed between the A- and Bsublattices. Disordered magnetic structure caused by the random distribution of Fe and Ga ions in the A- and B-sites becomes apparent in the temperature behavior of the FC and ZFC magnetization curves and in a large distribution of Mössbauer hyperfine parameters Hhf, as it is shown below in subsection 4.2. The parameter of magnetic frustration f = ΘC/ TN (TSG) estimated for our compound is about 4.1(1), which indicates a high level of frustration.41 4.2. Low Temperature Mö ssbauer Data. Low-temperature Mössbauer spectra exhibit Zeeman splitting indicating the magnetic ordering of all iron ions (Figure 10). Spectral lines are

4. MAGNETIC PROPERTIES 4.1. The Data of Magnetic Measurements and SpinGlass Behavior. The field dependences of magnetization M(H) reveal hysteresis loops only at temperature of 5 K, impaling a ferromagnetic behavior (Figure 9a). Magnetization

Figure 9. Field (a) and temperature (b) dependences of the magnetization for FeGa2O4/FeGaO3 nanoparticles. FC and ZFC dependences of the magnetization shown in (b) were obtained in an applied field of 100 Oe. Inset in (a) shows a part of the hysteresis curves in an enlarged scale. Straight line in the inset in (b) shows Curie−Weiss approximation of the experimental data for the inverse susceptibility.

gradually increases with grows of the applied field, but it does not saturate in the field of 20 kOe. At 5 K, the coercivity HC value is about 2.7 kOe (inset in Figure 9a). The measurement of the M(T) dependences in zero-field-cooled (ZFC) and fieldcooled (FC) regimes (Figure 9b) revealed splitting of the FC and ZFC curves at low temperature, and the maximum of magnetization was observed in the ZFC curve at about 25 K. Such an anomaly is a characteristic feature of a spin-blocking temperature in superparamagnetic nanoparticles, and the similar behavior is typical of the spin-freezing effect expected in spin-glass compounds at temperature TSG of magnetic ordering.34,35 The particle-size distribution has an influence on the sharpness of the maximum in the ZFC curve, and the maximum is narrow in monodisperse particles.36 In addition, in our nanoparticles, the temperature where the FC and ZFC curves start splitting coincides with the maximum of ZFC magnetization, which indicates that the particles are rather monodisperse.37 As temperature decreases below TSG, the FC magnetization is saturated or even slightly decreases (Figure 9a). This indicates appearing of strong magnetic interaction,38 apparently between the core-FeGa2O4 and shell-FeGaO3 particles. Above TSG, the

Figure 10. Representative Mö ssbauer spectra of Fe1+xGa2−xO4 nanoparticles at temperatures around the magnetic transition. Solid lines are the calculated spectra. Grid lines on the top indicate the line position for FeGaO3 and FeGa2O4 phases.

significantly broadened at the lowest temperatures, and a pronounced background is present in the spectrum center along with the six-line pattern. This is an indication of the nonhomogeneous magnetic system, in which some of iron magnetic moments fluctuate on the boundary between the superparamagnetic and spin-freezing states.42 The magnetic splitting of the spectra gradually decreases as temperature rises, 25601


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can be suggested that the Ecryst contribution in FeGaO3 is negligible, but the exchange anisotropy Eex mainly contributes to the total anisotropy energy Eanis. In FeGa2O4 compound, Fe2+ ions (3d6, S = 2) have a large value of orbital momentum providing energy of magnetocrystalline anisotropy Ecryst, which contributes considerably to the total energy Eanis.44 Thereby, magnetic anisotropy of nanocomposites can be adjusted by variation of the fraction of FeGaO3 and FeGa2O4 phases in the process of the sample synthesis.

and a paramagnetic doublet develops at the expense of the magnetic sextet (Figure 10). The mean values of the magnetic hyperfine field ⟨Hhf⟩ obtained from the broaden spectra are shown in Figure 11, and

5. CONCLUSION The complex iron−gallium oxide nanoparticles FeGa2O4/ FeGaO3 with cubic spinel structure were synthesized by the combustion method. In the process of the synthesis, a new phase γ-FeGaO3, most probably creating at the surface of the FeGa2O4 core, adopts the spinel structure of the core. The cation (charge) distribution in the pure FeGa2O4 compound obtained from Mössbauer data at room temperature can be given as (Fe2+0.76Ga3+0.24)tet [Fe2+0.24Ga3+1.76]oct O4. With decreasing temperature, the local crystal symmetry of the tetrahedral A-sites in FeGa2O4 becomes lower due to Jahn− Teller distortions, which initiates the charge transfer transition leading to the redistribution of Fe2+ and Ga3+ cations over the A- and B-sites. There are two critical concentration tcr of Fe2+ in the A-sites 0.76 and 0.53 which stabilize the dynamic and cooperative J-T distortions at temperatures 295−140 K and 90−30 K, respectively. Between 140 and 90 K, the site occupation in FeGa2O4 nanoparticles depends on temperature due to the structural transformations initiated by the J-T effect. At low temperatures, the magnetic and Mössbauer measurements reveal the highly frustrated magnetic structure with the magnetic transition (spin-freezing) temperature of about TSG = 26 K, which is related to a spin-glass magnetic system. A positive value of the Curie temperature θC = 106 K indicates ferromagnetic interactions in the nanocomposites. The linear dependence of the magnetic hyperfine field ⟨Hhf⟩(T) observed supports the model of collective magnetic excitation specific of the spin-glass properties rather than superparamagnetic relaxation. The estimated value of anisotropy energy ⟨Eanis⟩/kB is about 181.3 K. The Fe3+ and Fe2+ ions present in the FeGaO3 and FeGa2O4 phases dominantly contribute to the exchange anisotropy energy Eex and magnetocrystalline anisotropy energy Ecryst, respectively. The total anisotropy energy (Eanis = Eex + Ecryst) can be tuned by the variation of compounds in the FeGa2O4/ FeGaO3 composites.

Figure 11. Dependence of the magnetic hyperfine field ⟨Hhf⟩ on temperature in the Fe1+xGa2−xO4 nanoparticles. The straight line is approximation of the experimental data by eq 1. Dashed line is the guide for the eye. The inset shows the temperature behavior of the width of the total Mössbauer spectrum near the magnetic transition. The magnetic transition becomes apparent from the steep increase of the width at TSG.

the spin freezing temperature TSG can be roughly estimated from the temperature dependence of the magnetic field ⟨Hhf⟩(T). Meanwhile, the TSG value can be estimated more precisely from the behavior of the total width of Mössbauer spectra in the temperature region near the magnetic transition. As shown in the inset in Figure 11, sharp increasing of the width at 26 K indicates the magnetic transition. Thus, obtained TSG value coincides with that found from magnetic measurements (Figure 9b). The highest value of ⟨Hhf⟩ extrapolated to 0 K is about 48.0 T which is less than the field in the bulk iron oxides. Probably, this is associated with delocalization of the 3d electrons of Fe or/and with fluctuations of the magnetic moments near the easy magnetization axis, which results in decreasing of Hhf. We found that the ⟨Hhf⟩(T) curve does not reach saturation even at the lowest temperatures (Figure 11), meanwhile at elevated temperatures, this curve can be well approximated by a linear law. Mørup et al.43 proposed to consider such behavior in terms of collective magnetic excitations consisting of small spin fluctuations around an easy direction of magnetization. This effect should be distinguished from superparamagnetic fluctuation of the magnetic moment among the various easy directions. The linear dependence of Hhf vs temperature was predicted in this model43 at T ≪ TSG Hhf H0

≈1−

TkB ⟨Eanis⟩

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

(1)

Notes

Here Eanis is the anisotropy energy and H0 is the value of ⟨Hhf⟩ extrapolated to zero temperature. From the fit of the experimental ⟨Hhf⟩(T) curve to the straight line (Figure 11), the ⟨Eanis⟩/kB value was obtained to be 181.3 K. Apparently, the anisotropy energy Eanis includes two main contributions: the exchange anisotropy Eex and the magnetocrystalline anisotropy Ecryst. In our nanocomposites, the Fe3+ and Fe2+ ions are present in the FeGaO3 and FeGa2O4 compounds, respectively. The electron structure of Fe3+ ion is 3d5 with spin S = 5/2, and the orbital momentum is frozen. It

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the Russian Scientific Foundation under Project #14-12-00848 is acknowledged.

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