Nanosized powders of NiZn ferrite: Synthesis

Nanosized powders of NiZn ferrite: Synthesis, structure, and magnetism Adriana S. Albuquerque, José D. Ardisson, Waldemar A. A. Macedo, and Maria C. M. Alves Citation: Journal of Applied Physics 87, 4352 (2000); doi: 10.1063/1.373077 View online: http://dx.doi.org/10.1063/1.373077 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/87/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of inhomogeneous cation distribution in magnetic enhancement of nanosized Ni0.35Zn0.65Fe2O4: A structural, magnetic, and hyperfine study J. Appl. Phys. 114, 093901 (2013); 10.1063/1.4819809 Spin glasslike behavior and magnetic enhancement in nanosized Ni–Zn ferrite system J. Appl. Phys. 108, 034307 (2010); 10.1063/1.3456174 Cation distribution in nanosized Ni–Zn ferrites J. Appl. Phys. 95, 5746 (2004); 10.1063/1.1699501 Mössbauer spectroscopic and x-ray diffraction studies of structural and magnetic properties of heat-treated ( Ni 0.5 Zn 0.5 ) Fe 2 O 4 nanoparticles J. Appl. Phys. 93, 7492 (2003); 10.1063/1.1540146 Magnetic and structural properties of ultrafine Ni–Zn–Cu ferrite grown by a sol–gel method J. Appl. Phys. 87, 6241 (2000); 10.1063/1.372667

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JOURNAL OF APPLIED PHYSICS

VOLUME 87, NUMBER 9

1 MAY 2000

Nanosized powders of NiZn ferrite: Synthesis, structure, and magnetism Adriana S. Albuquerque, Jose´ D. Ardisson, and Waldemar A. A. Macedoa) Laborato´rio de Fı´sica Aplicada, Centro de Desenvolvimento da Tecnologia Nuclear-CDTN, CP 941, 30123-970 Belo Horizonte, MG, Brazil

Maria C. M. Alves Laborato´rio Nacional de Luz Sı´ncrotron-LNLS, CP 6192, 13083-970 Campinas, SP, Brazil

共Received 27 September 1999; accepted for publication 2 February 2000兲 The structure and magnetic behavior of nanostructured powders of stoichiometric NiZn ferrite, Ni0.5Zn0.5Fe2O4, synthesized by coprecipitation, are investigated by extended x-ray-absorption fine structure spectroscopy 共EXAFS兲, x-ray diffraction, Mo¨ssbauer spectroscopy, and vibrating sample magnetometry. Samples of high purity and high homogeneity were obtained by annealing at relatively low temperatures (300– 800 °C) resulting in nanoparticles with average diameter between 9 and 90 nm, as determined by x-ray diffraction. EXAFS was applied to follow Ni, Zn, and Fe cations distribution and the evolution of the short range order of the samples with increasing annealing temperature. Our results show ferrimagnetic NiZn ferrite nanosized powders with high purity, 1:1 Ni to Zn stoichiometric ratio and superparamagnetic behavior. Moreover, the samples exhibit good structural ordering already after heat treatment at 400 °C. Analysis by vibrating sample magnetometry indicated a critical particle diameter for the transition from monodomain to multidomain behavior close to 40 nm. © 2000 American Institute of Physics. 关S0021-8979共00兲08309-2兴

I. INTRODUCTION

The capability of producing ultrafine nanosized ferrite powders is important for tailoring the geometry and properties of magnets produced by powder metallurgical processes, and is expected to result in new applications as the properties of these unique magnetic materials improve.1 While conventional methods to obtain ferrite magnets involve high temperatures, chemical routes like coprecipitation,2 sol–gel3 and hydrothermal processing4 can provide convenient nonconventional ways to prepare ultrafine powders of high purity and high homogeneity at relatively low temperatures.5 NiZn ferrite is a soft magnetic ceramic that has spinel configuration based on a face-centered cubic lattice of the oxygen ions, with the unit cell consisting of 8 f.u. of the type (Znx Fe1⫺x ) 关 Ni1⫺x Fe1⫹x 兴 O4. In this formula the metallic cations in ( ) occupy the tetrahedral A sites and the metallic cations in 关兴 occupy the octahedral B sites.6 NiZn ferrites are well known and have been used for many years in the electrical and electronic industries. Nowadays, these materials are largely studied in the search for improved properties7 and new applications,8 especially in the nanometric scale as ultrafine powders9 and thin films.10,11 For nanometric Ni1⫺x Znx ferrites, it is possible to obtain good dielectric properties12 and high performance13 at relatively low sintering temperature. The magnetic characteristics of the material are strongly affected when the particle size becomes very small, due to the influence of thermal energy over the magnetic moment ordering, originating the superparamagnetic relaxation phenomenon.14 In this work, we have studied the a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

synthesis of nanosized ferrimagnetic Ni0.5Zn0.5Fe2O4 powders by coprecipitation followed by low temperature annealing and investigated the structural and magnetic properties of the obtained samples. We have applied extended x-rayabsorption fine structure spectroscopy 共EXAFS兲, x-ray diffraction, Mo¨ssbauer spectroscopy and vibrating sample magnetometry. II. EXPERIMENT

The samples were synthesized using Fe, Ni and Zn nitrates as precursors, by dissolving them in de-ionized water in the required mole proportion. NaOH 共2.5 M兲 was used as precipitating agent, and the resulting powder was submitted to annealing for 2 h at relatively low temperatures 共between 300 and 800 °C兲 in ordinary atmosphere.15 A high temperature reference sample was also obtained by annealing at 1350 °C. The precipitation of the NiZn ferrite occurred according to the following reaction: 0.5Ni共NO3兲2.6H2O⫹0.5Zn共NO3兲2.6H2O ⫹2Fe共NO3兲3.9H2O⫹8NaOH ⇒Ni0.5Zn0.5Fe2O4⫹8NaNO3⫹28H2O. The chemical composition and the purity of the samples were checked by x-ray fluorescence, and structural characterization was carried out by x-ray diffraction, employing Cu K␣ radiation. EXAFS was applied to follow the site distribution of metal cations and the structural evolution obtained at the different annealing temperatures. EXAFS measurements were performed by using the XAS beamline16 of the National Synchrotron Light Laboratory 共LNLS兲 at Campinas, with the storage ring running at a typical current

0021-8979/2000/87(9)/4352/6/$17.00 4352 © 2000 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 200.131.27.181 On: Mon, 01 Jun 2015 11:51:15

Albuquerque et al.

J. Appl. Phys., Vol. 87, No. 9, 1 May 2000

of 100 mA. Transmission spectra encompassing the K absorption edges of Ni 共8333 eV兲, Zn 共9569 eV兲 and Fe 共7111 eV兲 were measured at room temperature. The energy of the photons was selected and scanned with a Si共111兲 channel cut double crystal monochromator. Entrance monochromator slits were set to 1 and 7 mm for the vertical and horizontal directions, respectively. The total resolution, including the core hole broadening, was 2.4 eV@Fe K edge, 3.0 eV@Ni K edge and 3.9 eV@Zn K edge. Two ion chambers were used to monitor the intensity of the beam before and after the samples. The energy scans were done at steps of 2 eV and 2 s/point. EXAFS spectra were analyzed by using the WINXAS program17 and following standard fit procedures18 that involved: background subtraction, determination of absorption edge, normalization, simulation of atomic absorption, extraction of the pure EXAFS signal and determination, by Fourier transform 共FT兲, of the contribution of the different neighbor shells to the absorption spectrum. The EXAFS oscillations corresponding to only one neighbor shell were then obtained by inverse Fourier transform into k space and the resulting structural parameters 关number of neighbors (N), atomic distances (R), and Debye–Waller factors ( ␴ )兴 were determined by least-squares fitting by using experimental phase and amplitude functions deduced from model compounds. Spectra collected from standards of Fe3O4, NiO, ZnO, NiFe2O4, ZnFe2O4 and Ni0.5Zn0.5Fe2O4 共high temperature reference sample兲 were also measured, analyzed and compared to our samples. The FT of the EXAFS signals were obtained from 3.4– 14.4 Å ⫺1 for Ni and Fe, and from 3.6 to 12 Å ⫺1 for Zn, by using a Bessel window with a coefficient equal to 4. Transmission 57Fe Mo¨ssbauer spectra 共from 20 K to room temperature兲 have been obtained on a constant acceleration transducer with a 57Co/Rh source. The Normos leastsquares-fit program was employed to calculate the spectral hyperfine parameters. Coercivity and saturation magnetization were determined by vibrating sample magnetometry at 300, 100, and 4.2 K. III. RESULTS AND DISCUSSION

Figure 1 shows x-ray diffraction patterns of the NiZn– ferrite samples. The powder, both as-obtained and after heat treatment at 300 °C shows poor crystallization, with less defined diffraction lines, and the structural evolution with increasing annealing temperature can be observed. Samples heated up to 400 °C exhibit diffraction peaks attributable to stoichiometric NiZn ferrite, Ni0.5Zn0.5Fe2O4, and a lattice parameter equal to 8.39 共1兲 Å, as calculated by an extrapolation method.19 Average particle diameter, as estimated by using the Scherrer’s equation, range from 9 to 90 nm and are indicated in Table I as a function of treatment temperatures. The 1:1 Ni to Zn stoichiometry, within a maximal variation of 0.02 mol, was indicated by x-ray fluorescence for all annealed samples.20 Figure 2 shows the measured x-ray-absorption spectra encompassing the K edges of Fe, Ni and Zn for the different NiZn ferrite samples. Experimental spectra of NiO, and ZnO and a theoretical spectrum for Fe3O4 are also shown for comparison. The Fe3O4 spectrum was calculated using the FEFF7

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FIG. 1. X-ray diffraction patterns of the nanosized Ni0.5Zn0.5Fe2O4 samples as-obtained 共a兲 and after annealing at 300 °C 共b兲, 400 °C 共c兲, and 700 °C 共d兲, together with the reference sample annealed at 1350 °C 共e兲.

code.21 Even without a satisfactory resolution at the nearedge regions, it is possible to distinguish the presence of some key features in the spectra, which may suggest the local environment of the metal centers, since the near-edge features are strongly influenced by the electronic and geometric properties of the absorbing atoms. The EXAFS spectrum of NiO has a small prepeak feature and a strong absorption at 8340 eV 共1s→4 p transition兲 characteristic of octahedral coordination of Ni2⫹ ions.22 The prepeak is assigned to a formally forbidden electron transition 1s→3d that becomes possible by the 4p ligand orbitals mixing with the 3d metal orbitals as a result of symmetry distortions. The intensity of this feature is directly correlated to the distortion of the octahedral symmetry of the metal site. The spectra of all NiZn ferrite samples show the same feature present in the NiO spectrum, indicating that Ni has octahedral coordination. ZnO has the wurtzite structure and the coordination of Zn by oxygen atoms is tetrahedral. The Zn 3d orbital is completely filled so there is no pre-edge feature at the nearedge region. Only the main edge corresponding to the 1s →4 p transition is present. Initially, the spectra of the NiZn ferrite samples annealed at lower temperatures are similar to that of ZnO and, after annealing at 400 °C, a splitting of the main edge appears. Nevertheless, since the near-edge region is strongly affected by multiple scattering effects coming from higher neighboring shells, as mentioned before, it is

TABLE I. Average particle diameter D (⫾3 nm) of the Ni0.5Zn0.5Fe2O4 samples as a function of the annealing temperature T. T(°C)

as-obtained

300

400

600

700

750

800

D 共nm兲

⬍6

9

20

30

39

50

90


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FIG. 3. EXAFS Fourier transform of the Ni 共a兲, Zn 共b兲, and Fe 共c兲 absorption K edges, obtained from Ni0.5Zn0.5Fe2O4 samples as obtained 共.兲, annealed at 300 °C 共⫹兲, 400 °C 共o兲, 700 °C 共⫻兲, and 1350 °C 共—兲. FIG. 2. X-ray absorption spectra at the Ni, Zn, and Fe absorption K edges, obtained from Ni0.5Zn0.5Fe2O4 samples as-obtained 共a兲 and annealed at 300 °C 共b兲, 400 °C 共c兲, 700 °C 共d兲, and 1350 °C 共e兲. The spectra of NiO and ZnO and the FEFF calculated spectrum of Fe3O4 are also presented for reference.

hard to identify the origin of the observed splitting. To explain this point, some calculations with the FEFF code are underway. Fe3O4 has an inverse cubic spinel structure. In this compound, the oxygen atoms are in an almost perfect cubic close packing, with the Fe atoms lying in tetrahedral and octahedral interstices. In this structure, Fe3⫹ ions at A sites have tetrahedral coordination by oxygen, and Fe2⫹ and Fe3⫹ ions at B sites have octahedral coordination. Our theoretical spectrum for Fe3O4 shown in Fig. 2 is very similar to a result previously reported by Pizzini et al.23 Nevertheless, this simulation does not reproduce the pre-edge feature existing in Fe3O4. In spite of this, it can be observed in Fig. 2 that the Fe K edge absorption spectra of the NiZn ferrite, Ni0.5Zn0.5Fe2O4, are very similar to that calculated for Fe3O4, corresponding to iron atoms in tetrahedral A and octahedral B sites. The Fourier transformed EXAFS data are shown in Fig. 3. The different FT peaks have been identified to correspond to the coordination of metal cations by oxygen and by metal ions on the characteristic tetrahedral A and octahedral B sites of the spinel configuration of the NiZn ferrite24,25 共MA and MB , respectively兲. The results of the quantitative analysis done for the first FT peak of the Ni and Zn K edges are listed in Table II.

The first peak in the FTs of the Ni K edge 关Fig. 3共a兲兴 corresponds to Ni–O bonds, the second peak corresponds to Ni–MB , and the third one to Ni–MA . From the increase in the peak amplitude 共especially the second and third FT peaks兲, it can be observed that the higher the temperature, the better the crystallization of the structure, as expected. We notice that the long-range order obtained after annealing at 700 °C lies very close to that obtained after treatment at high temperature (1350 °C). The inverse FT of the first peak, at a radial coordinate R equal to 2.1 Å 共appearing in the Fig. 3 at 1.6 Å due to an electron phase shift兲 indicates, in all samples, the octahedral coordination of the Ni by the oxygen (N ⬇6), in agreement with the near-edge features of the EXAFS data and in accordance with the results of other authors.25,26 As can be observed in Table II there is no big influence of the temperature on the Debye–Waller factors, i.e., a reasonably good structural ordering is achieved even in the as-obtained sample. For the Zn K edge 关Fig. 3共b兲兴, the first peak corresponds to Zn–O bonds, the second peak corresponds to the Zn–MB and Zn–O, and the third one refers to Zn–MA . The inverse FT analysis of the first peak indicates tetrahedral coordination of the Zn by the oxygen (N⬇4). These results confirm the inference obtained from the inspection of the near-edge region. The fast evolution of the structural order even at low temperatures (⭓400 °C) is indicated by the clear decrease of the Debye–Waller factor with heat treatment. The analysis of the Fe K edge spectra is more complex since Fe atoms are present at the A and B sites. From Fig.


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TABLE II. Structural parameters of the nanosized Ni0.5Zn0.5Fe2O4 samples, as obtained from the first FT peak of the Ni and Zn x-ray-absorption K edges. T is the annealing temperature, N is the number of neighbors in the first shell, R is the interatomic distance, and ␴ is the Debye–Waller factor. The accuracy is ⫾0.02 Å for R and ⫾15% for N. Ni K edge

Zn K edge

T(°C)

as-obtained

300

400

700

1350

as-obtained

300

400

700

1350

N R 共Å兲 ␴2

5.7 2.10 0.0014

5.7 2.10 0.0012

5.7 2.10 0.0013

6.3 2.10 0.0012

6.0 2.10 —

3.6 1.89 0.0007

3.7 1.88 0.0005

3.8 1.89 0.0004

4.0 1.89 0.0001

4.0 1.90 —

3共c兲, the higher structural disorder is clear for the samples annealed below 400 °C, as observed for Ni and Zn. The FT peak centered near 1.5 Å corresponds to FeA – O and FeB – O bonds, while the peak near 2.6 Å corresponds to FeB – MB , and the one near 3.2 Å 共all values are uncorrected from the phase shift兲 includes contributions from FeB – MA , FeA – MB , FeA – O, FeA – MA , and FeB – O. 24 From the fittings, two Fe–O distances were found: 1.90 and 2.05 Å. These values are intermediary between those expected for stoichiometric NiZn ferrite 共1.87 and 2.06 Å for tetrahedral and octahedral sites, respectively兲.24,25 Nevertheless, these results are not presented in Table II because we have found that N and ␴ 2 are strongly correlated in our fits, i.e., it is possible to obtain the same fit quality for different values of neighbors and next neighbors and Debye–Waller factor. This effect becomes stronger if two coordination shells are allowed to be fitted at the same run. Figure 4 shows room temperature Mo¨ssbauer spectra of nanosized NiZn ferrite powder samples, as-obtained and after annealing at 400 and 800 °C, together with the spectrum of the reference sample, annealed at 1350 °C. All samples present superparamagnetic behavior characterized by a significant reduction of the magnetic hyperfine field or, for annealing below 400 °C, by the collapse of the six lines of the ferrimagnetic spectra. Low-temperature measurements 共20 K兲 are shown in Fig. 5. These resolved spectra were fitted

using one sextet corresponding to Fe3⫹ ions at A sites (FeA ), and a magnetic hyperfine field distribution relative to Fe3⫹ at B sites (FeB ). The use of a B HF distribution for FeB is justified by the presence of different magnetic neighbors affecting the iron atoms in distinct ways, since nonmagnetic Zn2⫹ and magnetic Fe3⫹ are distributed at random on A sites.27,28 The Mo¨ssbauer parameters obtained at low temperature 共20 K兲 were isomer shifts ISA ⫽0.41(2) mm/s and ISB ⫽0.47 (2) mm/s 共relative to ␣-Fe兲, quadrupole splitting close to zero for both sites and magnetic hyperfine fields (B HF) varying from 50.4共3兲 to 51.3共3兲 T for FeA and from 49.7共3兲 to 51.6共3兲 T for FeB 共average B HF兲, for annealing temperatures increasing from 300 to 800 °C. The asprepared sample presented B HF of 47.5共3兲 and 42.9共3兲 T for FeA and FeB , respectively. The obtained B HF values are in agreement with those reported by Leung et al.29 for conventional well-crystallized Ni0.5Zn0.5Fe2O4. The relative concentration of Fe atoms at A and B sites was obtained from the Mo¨ssbauer spectral areas assuming identical recoilless fractions for FeA and FeB , and the resulting AreaB /AreaA ratio was always equal to 3.0共1兲. This is exactly the expected value for a distribution of Fe cations in stoichiometric Ni0.5Zn0.5Fe2O4, where Ni2⫹ ions occupy octahedral B sites only, and Zn2⫹ ions occupy tetrahedral A sites only.6 In this way, Mo¨ssbauer results confirm the cations distribution indicated by the EXAFS measurements.

FIG. 4. Room temperature Mo¨ssbauer spectra of the Ni0.5Zn0.5Fe2O4 FIG. 5. Low temperature 共20 K兲 Mo¨ssbauer spectra of the Ni0.5Zn0.5Fe2O4 samples as-obtained 共a兲, after annealing at 400 °C 共b兲 and 800 °C 共c兲, tosamples as-obtained 共a兲, after annealing at 400 °C 共b兲, and 800 °C 共c兲, together with the reference sample, annealed at 1350 °C 共d兲. gether with the reference sample, annealed at 1350 °C 共d兲. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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the superparamagnetic nature of these samples and/or to the noncolinearity of the magnetic moments at the surface of the nanoparticles.31,32 IV. CONCLUSIONS

FIG. 6. Saturation magnetization 共䉭兲 and coercivity 共䊊兲 at 300 K vs annealing temperature of the nanosized Ni0.5Zn0.5Fe2O4 powder samples. The curves present also data points of a sample annealed at 900 °C, prepared to confirm the trend in H C , and of the reference sample annealed at 1350 °C.

For the investigated material, size effects can also be observed in the results of vibrating sample magnetometry. This is shown in Fig. 6, which presents the saturation magnetization and coercivity measured at 300 K as a function of the annealing temperature. The coercivity reaches a maximum after annealing at 700 °C, corresponding to an average particle diameter near 40 nm 共Table I兲, a value that represents the critical diameter (D C) for multidomain/ monodomain behavior30 of the NiZn ferrite samples. In the multidomain range, above D C , the magnetization changes by domain wall motion, and the coercivity decreases as the particle size increases. In the single domain region 共below D C兲, the coercivity decreases when the particle size is reduced, since the thermal energy can affect the alignment of the magnetic moments inside the domain, characterizing superparamagnetic behavior. The plot of the magnetization (M ) as a function of applied magnetic field per temperature (H/T) for a sample annealed at 400 °C and with particles of 20 nm average diameter is shown in Fig. 7. The superposition of the M vs H/T curves at 300 and 100 K is clear evidence of superparamagnetic behavior,30 which is removed at 4.2 K. The saturation magnetization of our samples ranges from 13 to 63 emu/g at 300 K, from 29 to 78 emu/g at 100 K, and from 31 to 80 emu/g at 4.2 K, as D increases from about 9 to 90 nm. The observed decrease in the saturation magnetization of the small NiZn ferrite particles can be attributed to

In this work, we have investigated the formation of stoichiometric nanocrystalline Ni0.5Zn0.5Fe2O4 powders obtained by coprecipitation followed by heat treatment at relatively low temperatures (⭐800 °C). EXAFS measurements were applied to follow the initial crystallization of the samples and the results indicated the formation of stoichiometric NiZn ferrite with good structural order after annealing for 2 h at temperatures from 400 to 800 °C. Ni atoms occupy octahedral B sites only, whereas Zn atoms occupy tetrahedral A sites only. As shown by Mo¨ssbauer spectroscopy at room temperature, ferrite powders with average particle diameters smaller than 30 nm exhibit strong superparamagnetic relaxation. Larger particles showed resolved ferrimagnetic spectra with reduced magnetic hyperfine fields. These studies revealed a critical diameter for the transition from monodomain to multidomain behavior near 40 nm. ACKNOWLEDGMENTS

The financial support of the CNEN, the CNPq, and the FAPEMIG 共Brazilian Agencies兲 are gratefully acknowledged. The authors thank the LNLS staff for the beamtime. Research project partially performed at LNLS National Synchrotron Light Laboratory, Campinas, Brazil.

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