2858 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Structural and Magnetic Characterizationsof Coprecipitated Ni–Zn and Mn–Zn

Ferrite NanoparticlesB. Parvatheeswara Rao1;2, Chong-Oh Kim1, CheolGi Kim1, I. Dumitru3, L. Spinu4, and O. F. Caltun3

Research Center for Advanced Magnetic Materials and Department ofMaterials Engineering, Chungnam National University, Daejeon 305-764, Korea

Physics Department, Andhra University, Visakhapatnam 530003, IndiaDepartment of Solid State and Theoretical Physics, A. I. Cuza University, Iasi 700506, Romania

AMRE, University of New Orleans, New Orleans, LA 70148, USA

Two mixed ferrite systems, namely Ni0 65Zn0 35Fe2O4 (Ni–Zn) and Mn0 75 Zn0 18 Fe2 07 O4 (Mn–Zn) have been prepared by copre-cipitation method, and then the resulting ultrafine powders were heat treated at different temperatures from 200 to 800 C for improvedcrystallinity and magnetic properties. The samples were characterized by X-ray diffraction, vibrating sample magnetometry, and ferro-magnetic resonance spectrometry. As a result of the heat treatment, the average particle size has been found to increase from 9.9 to 15.7nm for Ni–Zn ferrites and from 2.4 to 10.2 nm for Mn–Zn ferrites, and the corresponding magnetization values have increased from9.1 to 23 emu/g for Ni–Zn ferrites and from 7.9 to 11.7 emu/g for Mn–Zn ferrites, respectively. The results are discussed in the light ofchanges in particle size and inversion degree parameter for cationic distribution at nanoscales.

Index Terms—Coercivity, coprecipitation, ferromagnetic resonance linewidth and magnetic materials, magnetization, Ni–Zn andMn–Zn ferrite nanoparticles.

I. INTRODUCTION

FERRITES still continue to be an intense area of researchmainly due to their versatility in finding a large number

of applications over a wide frequency range due to their lowcost and high electromagnetic performance [1]. Recently, manyferrite research groups have shifted toward developing these ma-terials at nanoscales as the performance in their ceramic prepa-ration routes is reaching their limits due to their higher elec-trical conductivity and domain-wall resonance [2], [3]. Further,in order to meet the demands made by the industry for cores ca-pable of working at higher frequencies [4], production of ferriteparticles at nanoscales forms an important step before makingthem into miniaturized compacts. When the size of the mag-netic particle is smaller than the critical size for multidomainformation, the particle exists in a single-domain state and do-main-wall resonance is avoided; thus, the material can work athigher frequencies.

Several chemical methods were reported for preparation offerrite nanoparticles in the literature. These include coprecipi-tation [5], sol–gel in different matrixes [6], [7], hydrothermal[8], citrate precursor [9], etc. Though different routes producedifferent microstructures and particle sizes, based on the easeand reproducibility, the coprecipitation and sol–gel methods arewidely investigated. However, a close review of the literaturereveals that many reports so far have concentrated on simplerather than complex ferrite systems perhaps due to having betterinsight into the behavior of these materials at nanoscales forsimple systems at first. In the present paper, we focus on the

Digital Object Identifier 10.1109/TMAG.2006.879901

preparation of Mn–Zn and Ni–Zn ferrite nanoparticles by copre-cipitation method, and the results of the structural and magneticcharacterizations of the prepared powders are reported here.

II. EXPERIMENTAL DETAILS

The ferrite systems of Ni Zn Fe O (Ni–Zn) andMn Zn Fe O (Mn–Zn) have been chosen for thepresent study as these materials exhibit high saturation magne-tizations in their respective series and are good candidates forpower applications beyond 1 MHz. Synthesis of these systemswas carried out by taking the high-purity starting materials aschloride salts for cationic solutions and the sodium hydroxidesolution as the base. The preheated NaOH solution is pouredinto the cationic solution in a thin flow while maintaining thestirring at 500 rpm and the temperature at 100 C until theprecipitation occurs. The precipitate was washed and filteredrepeatedly before drying, and this ultimately resulted in veryfine particles which were subsequently heat treated at 200, 400,600, and 800 C for crystallization and for better performance.All the samples were subjected to X-ray diffraction, VSM, andferromagnetic resonance spectrometry characterizations.

III. RESULTS AND DISCUSSION

Typical XRD patterns of Ni–Zn (NZ600) and Mn–Zn(MZ400 & MZ600) ferrite nanoparticles are shown in Fig. 1.All the peaks for Ni–Zn ferrite indicate single-phase spinelcrystal structure. However, Mn–Zn ferrites up to 400 C showpeaks related to spinel only though the structure is partly crys-talline and partly amorphous, whereas the samples at 600 and800 C show additional peaks corresponding to Mn O [10]and Fe O [11] indicating that the oxidation states of Mn limit

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RAO et al.: STRUCTURAL AND MAGNETIC CHARACTERIZATIONS OF COPRECIPITATED NI–ZN AND MN–ZN FERRITE NANOPARTICLES 2859

Fig. 1. Typical XRD patterns of Mn–Zn and Ni–Zn ferrite nanoparticles.

Fig. 2. 311 peak of Ni–Zn ferrite sample heat treated at 400 C. The solidlines represent the best fitted Gaussian and Voigt curves.

the degree of ferrite formation at these temperatures. It is wellknown that the Mn is more sensitive to atmosphere between600 and 1180 C and is prone to change oxidation states whenfired in air atmosphere [12]. The samples are observed toexhibit higher peak intensities with the increase in heat treatingtemperature, and this was attributed to increased crystallinity.

Average particle size (D) of the samples in each case wascalculated from the broadening of the respective high-intensity311 peak using the Scherrer equation [13],where is the X-ray wavelength (1.541 84 ) and is thebroadening of the peak at angle, . The is measured usingthe equation [14], where B is the measuredfull-width at half-maximum (FWHM) of the experimental pro-file and is the instrumental broadening.

For the estimations, the peaks have been considered asGaussian in shape though the best fitted Gaussian and Voigtcurves look alike, as shown in Fig. 2 for NZ400 sample. Theestimated sizes of the average ferrite particles for all the sam-ples are listed in Table I along with other magnetic parametersdeduced from the room temperature hysteresis loops, shown inFig. 3 for typical samples, and ferromagnetic resonance spectrataken on the samples. The average particle sizes are found toincrease with increase in annealing temperature in both Mn–Zn

Fig. 3. Typical hysteresis loops of Mn–Zn and Ni–Zn ferrite nanoparticles.

TABLE IAVERAGE PARTICLE SIZE AND OTHER MAGNETIC DATA OF Ni–Zn

AND Mn–Zn FERRITE NANOPARTICLES

and Ni–Zn ferrite systems. An increase in average particle sizefrom 9.9 to 15.7 nm has been observed for Ni–Zn samples, andfrom 2.4 to 10.2 nm was seen for Mn–Zn samples as the heattreating temperature increases from 200 to 800 C. However,relatively smaller average particle sizes of Mn–Zn ferritescompared to Ni–Zn ferrites may be due to more amorphousnature of those samples.

At the same time, the magnetization values have increasedfrom 9.1 to 23 emu/g for Ni–Zn ferrites and from 7.9 to 11.7emu/g for Mn–Zn ferrites with the increase in heat treating tem-perature from 200 to 800 C. The observed values of magne-tization for all the samples are smaller compared to the mag-netization values of their bulk samples. Interestingly, as the av-erage particle size increases the coercivity also finds an increasein Ni–Zn ferrites whereas it decreased from a higher value inMn–Zn ferrites, as depicted in Fig. 4. Variations in ferromag-netic resonance field and linewidth follow closely with that ofthe variations of magnetic data in Ni–Zn ferrite system, whilethey showed no such correlation in Mn–Zn ferrites probably dueto additional phases observed in this system.

Magnetic parameters, particularly magnetization and coer-civity, of ferrite nanoparticles are reported to be different fromthose prepared by ceramic methods due to the speculated dif-ferences in inversion degree of distribution of cations betweenA- and B-sites of the spinel lattice [15], [16] and also due to thespin disorder in the shell around the core [17], [18]. In otherwords, ionic distribution in nanoferrites may be differed by a

2860 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Fig. 4. Coercivity versus particle size of Mn–Zn and Ni–Zn ferritenanoparticles.

certain degree against their preferences in bulk materials and thisdegree (of inversion) is dependent on particle size. Within thesame composition, since the magnetic properties are determinedby microstructural aspects, there have been attempts to modifythe particle size by an appropriate heat treatment and thereby theinversion degree and the magnetic properties [19]. In the presentstudy, the increase in magnetization with the increase in heattreating temperature in both Ni-Zn and Mn-Zn ferrite systemscould be understood as a result of the increase in particle size andthereby the change in degree of inversion parameter. However,this change in inversion parameter would be examined furtherby conducting a Mössbauer study on these samples.

The coercivity variations in both the systems resemble a typ-ical particle-size-dependent behavior [6]. As the particle sizedecreases, the coercivity increases to reach a maximum at athreshold particle size, which could be characteristically de-scribed as transformation from multi domain nature to singledomain nature, and then decreases. The observed increase in co-ercivity in Ni–Zn samples and decrease in coercivity in Mn–Znsamples with the increase in average particle size indicate thatdifferent threshold particle sizes exist for these two systems. Inthe Ni–Zn ferrite system, if the particle size further decreases,this may well lead to zero coercivity and a transition from ferri-magnetic to superparamagnetic state.

The ferromagnetic resonance spectra of all the samplesshowed single-resonance peaks, but each of them is slightlyasymmetric. The asymmetric nature, in general, could bedue to the contribution of nonuniform resonance modes apartfrom the main mode of resonance. Since the compositionsunder investigation are the same and only the heat treatmentis different, the gyromagnetic ratio is supposed to be the samefor all the samples, and then the particle size and the resultingmagnetization obviously would be expected to influence theFMR parameters [20]. The observed values of resonance fieldand linewidth are in accordance with the above.

In summary, X-ray diffraction patterns of Ni–Zn samples dis-played single-phase spinel structure and a better degree of crys-tallinity while Mn–Zn samples showed additional phases andare slightly amorphous. The average particle sizes of the sam-ples have been observed to lie in the range from 2–16 nm. Thesmaller magnetizations of the samples were attributed to lowerparticle sizes and to deviations in inversion parameter.

ACKNOWLEDGMENT

This work was supported in part by the DRDO, India, andReCAMM, Chungnam National University, Korea, under theBrain Pool program.

REFERENCES

[1] A. Razzitte and S. Jacobo, “Magnetic properties of MnZn ferrites pre-pared by soft chemical routes,” J. Appl. Phys., vol. 87, pp. 6232–6234,2000.

[2] Z. X. Tang, C. M. Sorensen, K. J. Klabunde, and G. C. Hadjipanayis,“Size dependent Curie temperature in nanoscale MnFe O particles,”Phys. Rev. Lett., vol. 67, pp. 3602–3605, 1991.

[3] J. Smit and H. P. J. Wijn, Ferrites. Eindhoven, The Netherlands:Philips Technical Library, 1959, p. 73.

[4] R. Lebourgeois, J. P. Ganne, and B. Lloret, “High frequency Mn-Znpower ferrites,” J. Phys. IV France, vol. 7, pp. 105–108, 1997. Suppl.C1.

[5] A. S. Albuquerque, J. D. Ardisson, W. A. A. Macedo, and M. C. M.Alves, “Nanosized powders of NiZn ferrite: Synthesis, structure, andmagnetism,” J. Appl. Phys., vol. 87, pp. 4352–4357, 2000.

[6] M. George, S. S. Nair, A. M. John, P. A. Joy, and M. R. A. Raman,“Structural, magnetic and electrical properties of the sol–gel preparedLi Fe O fine particles,” J. Phys. D: Appl. Phys., vol. 39, pp.900–910, 2006.

[7] Z. Yue, J. Zhou, L. Li, H. Zhang, and Z. Gui, “Synthesis of nanocrys-talline NiCuZn ferrite powders by sol–gel auto-combustion method,” J.Magn. Magn. Mater., vol. 208, pp. 55–60, 2000.

[8] C. Rath, N. C. Mishra, S. Anand, R. P. Das, K. K. Sahu, C. Upad-hyay, and H. C. Verma, “Appearance of superpara-magnetism on heatingnanosize Mn Zn Fe O ,” Appl. Phys. Lett., vol. 76, pp. 475–477,2000.

[9] A. Singh, A. Verma, O. Thakur, C. Prakash, T. Goel, and R. G.Mendiratta, “Electrical and magnetic properties of Mn–Ni–Zn ferritesprocessed by citrate precursor method,” Mater. Lett., vol. 57, pp.1040–1044, 2003.

[10] J. A. Lee, C. Newnham, F. Stone, and F. Tye, “Thermal decompositionof manganese oxyhydroxide,” J. Solid State Chem., vol. 31, pp. 81–93,1980.

[11] P. Osmokrovic, C. Jovalekic, D. Manojlovic, and M. B. Pavlovic, “Syn-thesis of MnFe O nanoparticles by mechanochemical reaction,” J. Op-toelectro. Adv. Mater., vol. 8, pp. 312–314, 2006.

[12] H. Zhong and H. Zhang, “Effects of different sintering temperature andMn content on magnetic properties of NiZn ferrite,” J. Magn. Magn.Mater., vol. 283, pp. 247–250, 2004.

[13] B. D. Cullity, Elements of X-ray Diffraction, 2nd ed. Reading, MA:Addison-Wesley, 1978.

[14] X. Zeng, Y. Liu, X. Wang, W. Yin, L. Wang, and H. Guo, “Preparation ofnanocrystalline PbTiO3 by accelerated sol–gel process,” Mater. Chem.Phys., vol. 77, pp. 209–214, 2002.

[15] J. P. Chen, C. M. Sorensen, K. Klabunde, G. Hadjipanayis, E. Devlin,and A. Kostikas, “Size dependent magnetic properties of MnFe Ofine particles synthesized by coprecipitation,” Phys. Rev. B, vol. 54, pp.9288–96, 1996.

[16] D. J. Fatemi, V. G. Harris, V. M. Browning, and J. P. Kirkland, “Pro-cessing and cation distribution of MnZn ferrites via high-energy ballmilling,” J. Appl. Phys., vol. 83, pp. 6867–6869, 1998.

[17] J. H. Liu, L. Wang, and F. Li, “Magnetic properties and Mössbauerstudies on nanosized NiFe O particles,” J. Mater. Sci., vol. 40, pp.2573–2575, 2005.

[18] S. Calvin, E. E. Carpenter, V. G. Harris, and S. A. Morrison, “Multiedgerefinement of extended X-ray absorption fine structure of manganesezinc ferrite nanoparticles,” Phys. Rev. B, vol. 66, pp. 224 405–224 413,2002.

[19] D. Yang, L. K. Lavoie, Y. Zhang, Z. Zhang, and S. Ge, “Mössbauerspectroscopic and X-ray diffraction studies of structural and magneticproperties of heat treated (Ni Zn ) Fe O nanoparticles,” J. Appl.Phys., vol. 93, pp. 7492–7494, 2003.

[20] W. A. Kaczmareck, A. Calka, and B. W. Ninham, “Magnetic proper-ties of aerosol synthesized Co-substituted spinel ferrites,” IEEE Trans.Magn., vol. 29, pp. 2649–2651, 1993.

Manuscript received March 13, 2005 (e-mail: cgkim@cnu.ac.kr ).