Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe2

Article pubs.acs.org/IC

Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe2−xCrxO4 Nanoparticles Synthesized by Chemical Combustion

Igor S. Lyubutin,† Chun-Rong Lin,*,‡ Sergey S. Starchikov,† Arseniy O. Baskakov,† Natalia E. Gervits,† Konstantin O. Funtov,† Yaw-Teng Tseng,‡ Wen-Jen Lee,‡ Kun-Yauh Shih,§ and Jiann-Shing Lee‡ †

Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, Moscow 119333, Russia Department of Applied Physics, National Pingtung University, Pingtung County 90003, Taiwan § Department of Applied Chemistry, National Pingtung University, Pingtung County 90003, Taiwan ‡

ABSTRACT: A series of nickel−chromium−ferrite NiFe2−xCrxO4 (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (TN is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4. The saturation magnetization MS in the largest NPs is about (0.98−0.95) μB, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O4. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about Tcom = 360−365 K. This value significantly exceeds the point Tcom in bulk ferrites NiFexCr2−xO4 (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe0.75Cr1.25O4 with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.

1. INTRODUCTION Due to interesting electromagnetic characteristics, ferrites can be used in various fields,1 such as in moisture sensors,2 in microwave electronic devices,3 in power devices,4 and also as hypothermic agents in biomedicine.5 High resistance, the possibility of tuning of magnetic and electric properties, and simple and cheap production makes them very useful for magnetic recording, ferro-liquids, biosensors, and other applications.6 The location of magnetic ions in octahedral [B] and tetrahedral (A) crystal sites of ferrites can be varied by introducing different metal ions,7 size reduction,8 and changing the condition of synthesis.9 Thus, the information on such compounds tends to be vital and highly appreciated for theoretical and practical studies. Nickel ferrite NiFe2O4, possessing inverse spinel structure (Fe)[FeNi]O4,10 demonstrates ferrimagnetism, produced by antiferromagnetic (AFM) ordering of magnetic moments of Fe3+ in A-sites and Ni2+ and Fe3+ moments in octahedral Bsites. The system has a collinear magnetic structure and obeys the Neel model.11 Nickel ferrites are characterized by superior magnetic permeability at high frequencies, high electric resistance,12,13 chemical stability, mechanical hardness, and low cost, and they can find wide application in the highfrequency region of electronics. Magnetic parameters, such as saturation magnetization and coercive force, which are of © 2017 American Chemical Society

extreme technological importance, can be changed and tuned by chromium doping. The introduction of Cr3+ ions leads to significant variations of cation distribution over the A- and Bsites depending on the method of synthesis and Cr concentration. The structure of NiFe(2−x)CrxO4 (0 ≤ x ≤ 1) is a cubic spinel, and the lattice parameter decreases slightly due to Cr-substitution. The bulk sample with chromium content of x = 1.0 is advised to be used as a permanent magnet, since it exhibits high coercivity.14−16 Another interesting property of the system NiFe (2−x) Cr x O 4 is the occurrence of the compensation point Tcomp at some Cr concentrations.17−19 At this point the temperature-dependent magnetization decreases to zero even below the Neel temperature due to compensation of the magnetic moments of the A- and B-sublattices, ordered antiferromagnetically in the ground state at low temperatures. According to the Neel approach, Tcomp is not a point of the magnetic phase transition; however, very many anomalous effects were observed at this point, such as magnetoresistance, the magneto-optical Faraday effect, the Hall effect, coercitivity, and the magnetocalorimetric effect.20−22 The aim of the work is to synthesize and study nanoscale materials based on nickel ferrites, which have anomalous Received: July 29, 2017 Published: October 2, 2017 12469

DOI: 10.1021/acs.inorgchem.7b01935 Inorg. Chem. 2017, 56, 12469−12475

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Inorganic Chemistry magnetic properties that are important for technical applications. A series of nanocrystalline particles NiFe0.75Cr1.25O4 with different size was synthesized by the method of chemical combustion. The morphology, phase composition, crystal structure, and magnetic and electronic properties of the nanoparticles were investigated by several complementary methods, including XRD, HRTEM, Mössbauer, and Raman spectroscopy. Several magnetic anomalies related to the particle size effects were observed and analyzed.

2. SAMPLES PREPARATION AND METHODS OF CHARACTERIZATION

Figure 1. XRD pattern of NiFe0.75Cr1.25O4 nanoparticles annealed at different temperatures TA between 600 and 900 °C. Indicated reflexes correspond to the cubic spinel crystal structure with space group Fd3̅m.

All chemicals: Ni(NO3)3·6H2O, Fe(NO3)3·9H2O, Cr(NO3)3·9H2O, citric acid monohydrate (C6H8O7·H2O), nitric acid (HNO3), and glycine (NH 2 CH2 COOH) are of analytical grade (Merck.). NiFe0.75Cr1.25O4 nanoparticles (NPs) were prepared by the solution combustion method. In a typical process, glycine and citric acid were used as fuels. The reagent materials were susceptible to spontaneous combustion due to contact with nitric acid, which is a strong oxidizer. The initial molar ratio was Ni:Fe:Cr = 1:0.75:1.25. First, 6.3345 g of Ni(NO3)3·6H2O, 6.60 g of Fe(NO3)3·9H2O, and 10.8952 g of Cr(NO3)3·9H2O were dissolved in 30 mL of distilled water mixed with 10 mL of HNO3, and then 5.0 g of citric acid and glycine in a weight ratio of 1:4 was added into the solution. This precursor solution was concentrated by heating it in a cylindrical Pyrex dish until the excess free water evaporated and spontaneous ignition occurred. To remove the organic impurities involved in the residue and acquire various crystallite sizes of NiFe0.75Cr1.25O4 NPs, the precursor was heat-treated in air at temperatures ranging from 500 to 900 °C for 1 h followed by natural furnace cooling to room temperature. The series of NPs was studied by an exhaustive set of methods. The crystal structure and phase purity of the samples were examined by Xray powder diffraction (XRD, Mutiflex MF2100, Rigaku Co. Ltd.) with Cu Kα radiation (λ = 1.5418 Å). The morphology and microstructure of the particles were characterized by transmission electron microscopy (TEM, Tecnai G2 F20, FEG-TEM, Philips Co. Ltd.) with an accelerating voltage of 200 kV, and field emission scanning electron microscopy (FESEM, XL-40FEG, Philips Co. Ltd.). Raman spectra were obtained with a Princeton Instruments Acton SP2500 monochromator/spectrograph equipped with a Spec-10 system with a nitrogen cooled CCD detector. A Spectra-Physics Beamlock 2080 Krypton laser with a 647.1 nm line was used as an excitation source for a Raman signal. The Mössbauer absorption spectra of 57Fe nuclei were recorded in the temperature range between 5 and 600 K with a standard MS-1104Em spectrometer operating in the constant accelerations regime. The gamma-ray source 57Co(Rh) was at room temperature. The calibration was performed with a metal α-Fe standard absorber. Magnetic properties were analyzed using a commercial superconducting quantum interference device magnetometer (SQUID, Lakeshore 7404 Co. Ltd.) at temperatures between 5 and 390 K and in an applied field of 0 ≤ H ≤ 50 kOe.

by the larger number of cation vacancies in samples annealed at low temperatures. The absence of cations in the interstitial sites of the oxygen frame of the spinel structure could lead to enhanced repulsion of O2− − O2− ions, which increases the distance between these ions.25 The representative SEM images shown in Figure 2a,b for the 47.7 nm sample NiFe0.75Cr1.25O4 demonstrate a uniform rounded shape of the agglomerated nanoparticles. As seen in Figure 2b many particles are aggregated into spherical composites with a size of about 150 nm. In larger particles a couple of balls creates dumbbell shape pairs, meanwhile in small particles the balls are separated (Figure 2b). As seen in the TEM images in Figure 3a, the 47.7 nm nanoparticles form transparent plates and 3D units with a characteristic size of about 50−100 nm. The electron diffraction pattern in Figure 3c,d indicates the single crystalline structure of the particles. Obviously, the hexagonal arrangement of the ED reflexes in Figure 3c corresponds to the [114 ] axes of the spinel cubic structure, while the square net in Figure 3d obtained in the conical beam indicates the [100] type of axes. The HRTEM patterns in Figure 3e,f reveal a layered atomic structure indicating the single crystal properties of the particles, which is also supported by electron diffraction (inset in Figure 3e).

4. RAMAN SPECTROSCOPY Room-temperature Raman spectra of all samples were obtained using a 647.1 nm line of a krypton laser as excitation (Figure 4). Each sample was tested at several points, and no changes were found in the spectra, which proves the uniformity of the sample composition. It is known26 that five vibration modes (3T2g + Eg + A1g) are expected in the Raman spectrum of a spinel type compound with cubic structure (sp. gr. Fd3̅m). As a rule, the frequencies of the modes are in the ratio T2g(1) < Eg < T2g(2) < T2g(3) < A1g.26 In our spinel NiFe0.75Cr1.25O4, the most intensive peak should belong to the symmetric breathing mode A1g of oxygen ions in the (FeNi)O4 tetrahedrons. The pronounced shoulder observed at the left side of the main peak (inset in Figure 4) indicates the doublet structure of the A1g mode. Most probably, the doublet structure of A1g appears due to two kinds of metal present in tetrahedral sites. As is shown below, the cation distribution over tetrahedral (A) and octahedral [B] sites obtained from our Mössbauer and magnetic measurements is given as (Fe0.75Ni0.25)[Cr1.25Ni0.75]O4. The different electronic structures of Fe and Ni ions (3d5, 3d8) and ionic radii [r(Fe3+) = 0.67 Å, r(Ni2+) = 0.69 Å] lead to different metal−oxygen bond distances, thus changing the local symmetry of NiO4 and FeO4 tetrahedrons.27 Then, it can be concluded that the most

3. CHARACTERIZATION OF SAMPLES BY XRD, TEM, HRTEM, AND ELECTRON DIFFRACTION XRD analysis of the nanoparticles NiFe0.75Cr1.25O4 (Figure 1) revealed a cubic spinel-type crystal structure (space group Fd3̅m). Using the Scherrer formula, an average size of coherent scattering regions was calculated from the (311) reflex. We found that the average particle size strongly depends on the final annealing temperature TA of the precursor, and the size reduces from 47.7 to 1.6 nm with TA decreasing from 900 to 500 °C. The unit cell parameter a was calculated using the DicVol program.23 The lattice parameter decreases with increasing particle size, and the a value for the larger particles (about 8.305 Å) is consistent with the data for the bulk NiFe0.75Cr1.25O4 compound.24 That behavior can be explained 12470


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Figure 2. SEM mages of NiFe0.75Cr1.25O4 nanoparticles annealed at the temperature TA = 900 °C (d = 47.7 nm).

Figure 3. TEM images of NiFe0.75Cr1.25O4 nanoparticles annealed at 900 °C (d = 47.7 nm) (a,b,c,d) and HRTEM images of NPs annealed at 600 C (4.5 nm) (e,f).

intensive A1g(1) peak at 703.5 cm−1 originates from FeO4, whereas the A1g (2) peak at 662.3 cm −1 is from NiO 4 tetrahedrons (inset in Figure 4). As suggested in ref 28 a shoulder near 670 cm−1 can appear in the Raman spectrum of the bulk NiFe2O4 spinel due to the short-range ordering of Ni2+ and Fe3+ at the B sites with tetragonal P4122/P4322 symmetry. Such short-range ordering exists in randomly oriented twins, while the symmetry of a bulk crystal is cubic. This shows that Raman spectroscopy detects the changes in local symmetry very effectively. In our NPs, the frequency and line width of the main peak at about 704 cm−1 only slightly change with a particle size of d ≥ 11.7 nm. However, the intensity of this line strongly decreases with NPs size reduction (Figure 4). In the smallest particles, this line is shifted to a higher frequency by 2 cm−1 and broadened by 6 cm−1 as compared to large NPs of d = 47.7 nm. In addition, the intensity and line width of the A1g mode strongly change in the 4.5 nm particles (Figure 4). This behavior can be explained by the quantum size effect of the phonon confinement.29 The lower-intense T2g(2) mode corresponds to translation along one direction of the lattice with cations and oxygen atoms moving in opposite directions.26 The less-intense T2g(1) and T2g(3) modes are barely visible at the background level. These peaks are broadened, having a dual structure, which is due to

different kinds of metal cations present in the lattice sites of NiFe0.75Cr1.25O4. With decreasing particle size, the position and intensity of these peaks remain practically unchanged, which indicate the local crystalline identity of the samples. In addition, a new peak appears in all NPs samples at about 786 cm−1, whose intensity varies as the particle size decreases (Figure 4). The most intensive and sharp peak was observed in the spectrum of the 20.8 nm sample. A similar high frequency mode was observed in the spectra of bulk chromium spinels NiCr2O4, ZnCr2O4, and MgCr2O4,30 and it was attributed to the vibrations of the chromium(VI) oxide species due to Cr−O stretching modes.31,32 It can be assumed that in our NPs this mode is associated with phonons of octahedral CrO6 sites.

5. MÖ SSBAUER SPECTROSCOPY The room-temperature (RT) Mössbauer spectra of the NPs samples with d ≥ 11.7 nm exhibit magnetic hyperfine splitting, indicating that the iron ions are in a magnetically ordered state (Figure 5a). With decreasing particle size, the line broadening in the inner part of the spectrum increases and a paramagnetic doublet appears in the spectrum center of small particles. This behavior is evidence of the superparamagnetic properties of NPs. The doublet shape in the spectra of the smallest particles 12471


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hyperfine fields Hhf at iron nuclei (Figure 5b) using the SpectrRelax program.33 Computer fitting to this model showed good agreement between the experimental and calculated spectra (Figure 5b), and the obtained values of the Mössbauer parameter Hhf are shown in Table 1. Table 1. Values of Hyperfine Parameters Calculated from the Mössbauer Spectra of NiFe0.75Cr1.25O4 Nanoparticles at Temperatures of 90 and 297 Ka Sample

d, nm

δ, mm/s at 90 K

Hhf, kOe at 90 K

δ, mm/s at 297 K

Hhf, kOe at 297 K

NFC-900 NFC-800 NFC-700 NFC-650 NFC-600

47.7 33.9 20.8 11.7 4.5

0.370(5) 0.372(5) 0.371(5) 0.37(1) 0.37(2)

498(1) 498(1) 494(1) 486(2) 467(3)

0.26(1) 0.26(1) 0.26(1) 0.28(2) 0.27(1)

429(2) 426(2) 419(3) 400(5) 0

a d is the nanoparticle size, δ is the isomer shift, and Hhf is the magnetic hyperfine field at iron nuclei, which value corresponds to the maximum in the distribution function P(H) shown in the right panel of Figure 5.

It was found that the values of isomer shift δ and quadrupole shift ε are the same for all samples, whereas Hhf values decrease as the particle size is reduced. The isomer shift value of δ = 0.37(1) mm/s at 90 K reduces to 0.26(1) mm/s at room temperature. These δ values correspond to iron ions Fe3+ in the high-spin (HS) state. Ferrous ions Fe2+ are absent in these samples. The single Mössbauer component with the obtained δ and Hhf parameters (Table 1) indicates that all Fe3+ ions in our NPs are in the tetrahedral sites of NiFe0.75Cr1.25O4. For the iron ions in octahedral sites the values of δ and Hhf parameters must be higher due to the lower degree of covalency of the Fe−O bonds.34,35 This observation leads to two possible variants of the cation distribution (Fe0.75Cr0.25)[NiCr]O4 or (Fe0.75Ni0.25)[Cr1.25Ni0.75]O4, where parentheses denote tetrahedral (A) sites and square brackets denote octahedral [B] sites.

Figure 4. Room-temperature Raman spectra of NiFe0.75Cr1.25O4 nanoparticles with different particles sizes. The inset shows an enlarged view of the doublet shape of the A1g mode for the d = 47.7 nm particles, where arrows indicate components presumably corresponding to phonon modes of the FeO4 (703.5 cm−1) and NiO4 (662.3 cm−1) tetrahedrons.

6. MAGNETIC MEASUREMENTS Temperature dependences of the magnetization in a strong applied field of 5 T are shown in Figure 6. In larger particles

Figure 5. Mössbauer spectra of NiFe0.75Cr1.25O4 nanoparticles with different sizes at room (a) and 90 K (b) temperatures. The distribution P(H) of the magnetic hyperfine field values Hhf calculated for corresponding spectra at 90 K is shown in the right panel.

4.5 and 1.6 nm indicates the paramagnetic state of these samples at RT. For a quantitative analysis, low temperature spectra (at 90 K), in which thermal fluctuations of the magnetic moments are substantially suppressed, were selected. The initial spectra processing in the model of one magnetic component (Zeeman sextet) gave a good fit (Figure 5b). Furthermore, taking into account the size distribution of NPs, we have built the distribution function P(H) for the values of the magnetic

Figure 6. Temperature dependences of magnetization of NiFe0.75Cr1.25O4 NPs with different sizes in a strong magnetic field of H = 5 T. The inset shows the magnetization behavior in the compensation-point region. 12472


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Inorganic Chemistry NiFe0.75Cr1.25O4 the magnetization takes near zero at a temperature Tcom below the Neel point (which is about 575 K20,35) and then increases with further temperature rise. Obviously, this effect occurs as a result of compensation of the magnetic moments of the A- and B-sublattices ordered antiferromagnetically in the ground state of low temperatures. The value of the compensation point Tcom for NPs with d = 47.7, 33.9, and 20.8 nm is about 360−365 K. This significantly exceeds the Tcom value in bulk ferrites NiFexCr2−xO4 (about 315 K) with a similar Cr concentration.20,35 The effect of total compensation was observed in a weak applied field of 100 Oe in larger NPs with d ≥ 20.8 nm (Figure 7). However, in smaller NPs with d ≤ 11.7 nm the compensation does not occur (see Figure 7 and inset in Figure 6).

Figure 8. Field dependences of magnetization of NiFe0.75Cr1.25O4 nanoparticles with different sizes at low and high temperatures.

were determined by extrapolating the M(H) curves to H = O, and the magnetic moments calculated per the (NiFe0.75Cr1.25O4) formula unit are given in Table 2. Table 2. Saturation Magnetization MS at 5 K, Coercitivity Hc, and Magnetic Moments Calculated per Formula Unit of (NiFe0.75Cr1.25O4) for NPs with Different Sizes

Figure 7. Temperature dependences of magnetization in field cooled (FC) and zero field cooled (ZFC) regimes for NiFe0.75Cr1.25O4 NPs with different sizes in a weak magnetic field of H = 100 Oe.

We found that the applied magnetic field essentially influences the magnetization of small NPs. As seen in Figure 6, the magnetization of NPs with d = 4.7 nm subject to a field of 5 T is not zero even at temperatures above 400 K; meanwhile, the Mössbauer measurements without an external field indicate a paramagnetic state of these NPs above 240 K (Figure 5). The temperature dependences of the field-cooled (FC) and zero-field-cooled (ZFC) magnetization in a weak applied field of 100 Oe (Figure 7) reveal a maximum on the ZFC curve typical of superparamagnetic behavior. The spin blocking temperatures obtained from the position of the maximum (Figure 7) are 115 and 125 K for 4.5 and 11.7 nm samples, respectively. The ZFC and FC curves are split at about 230 and 280 K for 4.5 and 11.7 nm samples, respectively, which can be associated with magnetic coupling appearing between the particles. The values of these temperatures correlated with Mössbauer measurements indicated the appearance of magnetic ordering in superparamagnetic NPs. Field dependences of magnetization M(H) reveal hysteresis loops in all samples at 5 K, whereas, at RT, the loops are evident only in larger NPs with d ≥ 20 nm (Figure 8). The magnetization is not saturated in a high field of 5 T in all NPs, which, obviously, is an indication of the canted magnetic structure. The values of saturation magnetization MS at 5 K

Nanoparticle size, nm

MS, emu/g

MS, μB per f.u.

HC, kOe

47.7 33.9 20.8 11.7 4.5

23.8 23.1 16.9 17.3 18.7

0.98 0.95 0.70 0.71 0.77

3.7 4.8 6.6 7.7 7.9

In the largest particles, d = 47.7 and 33.9 nm, the moments are about MS = 0.98 and 0.95 μB, respectively, and the MS value is reduced to 0.77- 0.70 μB as the particle size decreases to 4.5 nm. We note here that the MS value in NiFe0.75Cr1.25O4 NPs is more than twice that in bulk ferrite (Fe)[CrNi]O4 where MS = 0.4 μB.22,36 The total magnetic moment of NPs (at T = 5 K) decreases remarkably with decreasing particle size, whereas the coercivity increases significantly (from 3.7 to 7.9 kOe). This can be associated with a frustrated magnetic structure developing in the surface layer of NPs.

7. DISCUSSION As shown above in Section 5, the Mössbauer data revealed two possible variants of the cation distribution in NiFe0.75Cr1.25O4 NPs over tetrahedral (A) and octahedral [B] sites: (Fe0.75Cr0.25)[NiCr]O4 and/or (Fe0.75Ni0.25)[Cr1.25Ni0.75]O4. Taking the spin magnetic moments of Fe3+, Cr3+, and Ni2+ as 5, 3, and 2 μB , respectively, in the Neel mean-field approximation of collinear antiferromagnetic ordering in the A- and B-sublattices, the calculated values of the total magnetic moment Mtot are M tot = 0.5μB for (Fe0.75Cr0.25)[NiCr]O4 12473

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Inorganic Chemistry M tot = 1.0μB for (Fe0.75Ni 0.25)[Ni 0.75Cr1.25]O4 and

(2)

M tot = 5.5μB for (Ni)[Fe0.75Cr1.25]O4

(3)

sublattice can be easily changed under the influence of temperature and/or applied magnetic field. Magnetic compensation in (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4 NPs occurs as a result of an increase of the φ angle value with the temperature. At this point, the resultant moment of the B-sublattice decreases to the value of the A-sublattice moment, which is ordered in the opposite direction.

Considering the value of MS = 0.98−0.70 μB, obtained from magnetic measurements for NPs with different sizes, we conclude that the real formula is (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4. This indicates that the magnetic structure of NPs in the ground state of low temperature is close to the collinear structure, and the direction of the total magnetic moment Mtot of the ferrite must coincide with the direction of the moment in the octahedral B-sublattice. The deviation of Mtot from its value in a pure collinear magnetic structure (1.0 μB) can be explained by the presence of a canted spin structure in the octahedral Bsublattice. These results well correlate with data for bulk compounds of ref 35. The variety of magnetic ions in this ferrite leads to several types of exchange interactions between and within the A- and B-sublattices, which differ in sign and strength. The competition of antiferromagnetic (AFM) and ferromagnetic (FM) interactions leads to a highly frustrated magnetic ordering in octahedral B-sites, which results in a canted magnetic structure in this sublattice. The canted angle φ between the magnetic moments of Cr and Ni ions in the Bsublattice is very flexible, and its value is significantly subjected to temperature and applied field. The occurrence of magnetic compensation in the (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4 NPs is associated with an increase in the value of the angle φ with temperature rise until the resultant moments of the B-sublattice became equal to the moment of the A-sublattice. The existence of the compensation point Tcom has a technological importance37,38 because at this point only a small magnetic field is required and enough to change the sign of the net magnetization. In addition, the compounds with a compensation point can be used as magneto-optic recording materials (including direct overwrite capability) with a highspeed switching in a magnetic field.37

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chun-Rong Lin: 0000-0003-4880-6196 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Russian Scientific Foundation (Project #14-12-00848P) and Ministry of Science and Technology of Taiwan (MOST 105-2112-M-153−003) for financial support.

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REFERENCES

(1) Goldman, A. Modern Ferrite Technology, 2nd ed.; Springer: New York, 2006. (2) Bagum, N.; Gafur, M. A.; Bhuiyan, A. H.; Saha, D. K. MgCl2 doped CuxZn1−xFe2O4 ferrite humidity sensors, Phys. Phys. Status Solidi A 2010, 207, 986−992. (3) Vladikova, D.; Ilkov, L.; Karbanov, S. Optimal prefiring conditions of substituted nickel ferrites for microwave applications. Phys. Status Solidi A 1990, 121, 249−255. (4) Ji, H.; Lan, Z.; Yu, Z.; Xu, Z. Microstructure and temperature dependence of magnetic properties of MnZnTiSn ferrites for power applications. IEEE Trans. Magn. 2010, 46, 974−978. (5) Bae, S.; Lee, S. W.; Hirukawa, A.; Takemura, Y.; Jo, Y. H.; Lee, S. G. AC magnetic-field-induced heating and physical properties of ferrite nanoparticles for a hyperthermia agent in medicine. IEEE Trans. Nanotechnol. 2009, 8, 86−94. (6) Pardavi-Horvath, M. Microwave applications of soft ferrites. J. Magn. Magn. Mater. 2000, 215−216, 171−183. (7) Seshan, K.; Shashimohan, A. L.; Chakrabarty, D. K.; Biswas, A. B. Effect of cation distribution on the properties of some magnesiumnickel ferrites. Phys. Stat. Sol. (a) 1981, 68, 97−101. (8) Epelak, V. Nanocrystalline materials prepared by homogeneous and heterogeneous mechanochemical reactions. Ann. Chim. 2002, 27, 61−76. (9) O’Neill, H. S. C.; Annersten, H.; Virgo, D. The temperature dependence of the cation distribution in magnesioferrite (MgFe2O4) from powder XRD structural refinements and Mössbauer spectroscopy. American mineralogist 1992, 77, 725−740. (10) Winell, S.; Amcoff, Ö .; Ericsson, T. Cation ordering in NiFe2‑xCrxO4 -spinels studied by Mössbauer spectroscopy in external fields. Phys. Status Solidi B 2008, 245, 1635−1640. (11) Patange, S. M.; Shirsath, S. E.; Toksha, B. G.; Jadhav, S. S.; Shukla, S. J.; Jadhav, K. M. Cation distribution by Rietveld, spectral and magnetic studies of chromium-substituted nickel ferrites. Appl. Phys. A: Mater. Sci. Process. 2009, 95, 429−434. (12) Shirsath, S. E.; Toksha, B. G.; Jadhav, K. M. Structural and magnetic properties of In 3+ substituted NiFe2O4. Mater. Chem. Phys. 2009, 117, 163−168. (13) Mitra, S.; Mandal, K.; Choi, E. S. Dynamic Magnetic Properties of NiFe2O4 Nanoparticles Embedded in SiO Matrix. IEEE Trans. Magn. 2008, 44, 2974. (14) Gabal, M. A.; Angari, Y. A. Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite. Mater. Chem. Phys. 2009, 118, 153−160.

8. CONCLUSION A series of nickel−chromium−ferrite nanoparticles NiFe(2−x)CrxO4 with an average size d from 1.6 to 47.7 nm was synthesized by the solution combustion method. Mössbauer spectroscopy and magnetic measurements revealed a series of magnetic anomalies which are strongly dependent on the NPs size. With d decreasing, the value of saturation magnetization MS is remarkably decreased (from 0.98 to 0.70 μB), whereas the coercivity is essentially increased (from 3.7 to 7.9 kOe). Small NPs with d < 11 nm demonstrate superparamagnetic behavior in the Mössbauer spectra, while larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. The effect of magnetic compensation was discovered in the (Fe0.75 Ni0.25)[Ni0.75 Cr1.25]O4 NP-samples with d > 20 nm. At the compensation point Tcomp the oppositely directed magnetic moments of the A- and B-sublattices become equal, and the total magnetization is zero. However, the effect of compensation does not occur in smaller nanoparticles. The observed deviation of Mtot from its value in a pure collinear magnetic structure (1.0 μB) is explained by a canted spin ordering in the octahedral B-sublattice, which appears due to the competition between AFM and FM exchange interactions in the A- and Bsublattices. Due to magnetic frustrations, the canted angle φ between the magnetic moments of Cr and Ni ions in the B12474


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Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe2

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