Structure and magnetic properties of nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 powders prepared by ball milling

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1 Structure and magnetic properties of nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 powders prepared by ball milling K. Gheisari* 1, S. Shahriari 2, A. Rezvanpour 3 and S. Javadpour 4 In this study, nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 powders were prepared using a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling were studied by X- ray diffraction technique, scanning electron microscopy, transmission electron microscopy and vibrating sample magnetometre. It is revealed from the results of the phase analysis that nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite with average crystallite size of 6?18 nm and non-uniform lattice strain of 0?33% has been formed after 60 h of milling time. A progressive increase of saturation magnetisation and a dramatic decrease in coercivity were also observed with increasing milling time. Keywords: Ni Zn ferrite, Nanocrystalline, Magnetic properties, Ball milling Introduction The Ni Zn ferrite (Ni 1-x Zn x Fe 2 O 4 ) is the most versatile ferrite from the viewpoint of its large number of applications, due to its high value of magnetic permeability, high electrical resistivity, high Curie temperature and low power loss at high frequencies. 1,2 Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite is one of the Ni Zn ferrite compositions, well known for high electrical resistivity (r>6200 V cm) 3 and high saturation magnetisation (M s >78 emu g 21 ). 4 Recently, more and more attentions have been paid to the magnetic nanostructured materials due to their unique magnetic properties compared to those obtained for the same conventional microstructured materials. 5 Several methods, such as coprecipitation, 6 reverse micelle, 7 sol gel, 8 autocombustion synthesis 9 and ball milling are used to synthesise nanostructured Ni Zn ferrite. Among these methods, ball milling method is a relatively simple technique, which allows the preparation of nanostructured Ni Zn ferrite at a relatively low temperature Microstructure characterisation and phase transformation kinetics of Ni 0?5 Zn 0?5 Fe 2 O 4 ferrite powders prepared via ball milling were studied in detail by Bid and Pradhan. 10 Magnetic properties of the ball milled prepared Ni 0?5 Zn 0?5 Fe 2 O 4 ferrite powders also were investigate by Jalaly et al. 11 and Ye et al. 12 However, there are only a few reports on the 1 Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University, Ahvaz, Iran 2 Department of Materials Science and Engineering, Islamic Azad University of Ahvaz, Ahvaz, Iran 3 School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran, Iran 4 Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran *Corresponding author, khalil.gheisari@yahoo.com characterisation of Ni Zn ferrite powders synthesised by ball milling and structural evaluations and magnetic properties of ball milled prepared Ni 1-x Zn x Fe 2 O 4 ferrite powders in some technologically important compositions (x,0?7) in terms of milling time have not been investigated to date. In our previous work, 13 the effect of heat treatment on the structure and magnetic properties of ball mill prepared Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite powders has been investigated. The aim of this work is to study the structure and magnetic properties of nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 powder prepared by ball milling as a function of milling time. It would be worth mentioning that as compared with the literature in this field, this paper has paid special attention to Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite composition. Experimental method The initial high purity (.99%) powders of 12?3 wt-%zno, 20?2 wt-%nio and 67?5 wt-%fe 2 O 3 were weighed and mixed according to the composition of Ni 0?64 Zn 0?36 Fe 2 O 4, then introduced into a cylindrical tempered steel vial of the capacity of 100 ml. The ballto-powder weight ratio was 10 : 1. The milling was carried out at room temperature by using a planetary ball mill at the vial rotation speed (v) of 350 rev min 21 and the disc rotation speed (V) of 212 rev min 21 (v/ V 5 1?65) for different milling times varying from 0?5 to 60 h. For comparison purposes, Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite was also prepared by conventional calcination of the initial mixed powders at 1250uC for 2 h. The phase analysis and the structural properties were characterised by the use of X-ray diffraction (XRD, Shimadzu XRD-6000 diffractometer with Cu K a1 radiation). Structural parameters including crystallite size, lattice parameter and lattice strain of Ni Zn ferrite were 216 ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 19 October 2012; accepted 5 January 2013 DOI / Y Powder Metallurgy 2013 VOL 56 NO 3

2 1 X-ray diffraction patterns of ball milled powders for various milling time: formation of Ni Zn ferrite is observed after 60 h of milling obtained from Rietveld s powder structure refinement analysis of X-ray powder diffraction data. The Rietveld calculations were performed by the TOPAS 3 software (from Bruker AXS). In the TOPAS software, the Double Voigt approach 14 is used for obtaining crystallite size and strain components. The crystallographic models of Ni Zn ferrite (Cubic, Fd3m, COD ID: ), ZnO (Hexagonal, P6 3 mc, COD ID: ), NiO (Cubic, Fm3m, COD ID: ) and Fe 2 O 3 (Rhombohedral, R-3c, COD ID: ) were used as starting models for the refinements. The models were selected from the official website of the crystallography open database (COD). 15 For each refinement, the following variations were applied: the background parameter, scale factor, cell parameter, zero point correction, Lorentzian crystal size and Gaussian lattice strain. In order to judge the quality of the fitting on the structure model, the Bragg reliability factor R Bragg was used. The refinements resulted in proper fits to the experimental data and average R Bragg value was less than 4%. Transmission electron microscope (TEM, JEOL-JEM 2010) was utilised to study the microstructure of the powder particles. Magnetic measurements were obtained at room temperature with a vibrating sample magnetometer (VSM, Lakeshore 7404) with a saturating field of 10 koe, field increment of 404 Oe and a field ramp rate of 40?4 Oes 21. Results and discussion Structure Figure 1 shows the XRD patterns of the ball milled powders for different milling times. From these patterns following points can be concluded: (i) for milling time of 0?5 h, XRD pattern presents Fe 2 O 3, NiO, ZnO compounds simultaneously. It can be observed from this figure that the diffraction line intensities of the Fe 2 O 3 and ZnO compounds gradually decreased with increasing milling time. Fig. 2a shows that the most intense reflection of Fe 2 O 3 (2h533?28u) and the strongest isolated reflection of ZnO (2h531?94u) are almost vanished after 60 and 36 h of milling time respectively. In addition, all diffraction lines of NiO compound broadened and shifted to the left with increasing milling 2 Magnified view of XRD patterns of ball milled powders extracted from Fig. 1 (ii) (iii) time. Diffraction line shifting and broadening for the most intense reflection of NiO (2h543?3u) is shown in Fig. 2b. At milling time of 60 h, all diffraction lines of the starting material are vanished or changed to the reflection angles of Ni Zn ferrite in such a way the characteristic diffraction lines of Ni Zn ferrite phase are appeared distinctly the XRD results presented in Figs. 1 and 2 show that the ZnO diffraction lines are first vanished during ball milling in comparison to the characteristic diffraction lines of NiO and F 2 O 3 compounds. This finding, which is consistent to the Bid et al. 10 and Jalaly et al. 11 reported data, indicates higher solid state diffusion rate of ZnO phase diffraction line broadening of Ni Zn ferrite phase indicates a decrease in the crystallite size and the introduction of lattice strain during the high energy ball milling. The Rietveld analysis of the diffraction pattern exhibits that the average crystallite size, the lattice strain and the lattice parameter values of Ni Zn ferrite phase are 6?18 0?30 nm, 0?33 0?05% and 0? ?00042 nm respectively. It is interesting to note that Bid and Pradhan 10 obtained a crystallite size and lattice parameter of 4?6 and 0?8393 nm after 11 h of milling using a high energy planetary ball mill. Figure 3 displays the XRD patterns of the as synthesised ferrite powders prepared by the ball milling and the solid state reaction method. It is evident from this figure that not only diffraction lines of the mechanically alloyed powder are broadened, but also its broadened diffraction lines are shifted to lower angles as compared to those of the calcined powders. Diffraction line broadening is attributed to the second order internal stress (non-uniform or microstress) while angular diffraction line shift is resulted from the first order internal stress (uniform or macrostress). 16 Angular shift of diffraction lines of the mechanically alloyed powders to the lower angles indicates a tensile residual stress within the ferrite crystal lattice arising from high energy ball milling. The Powder Metallurgy 2013 VOL 56 NO 3 217

3 3 X-ray diffraction patterns of Ni Zn ferrite powders obtained after 60 h milling time and same powders prepared using solid state reaction method (calcined at 1250uC for 2 h) mean value of uniform lattice strain can be estimated roughly by the following equation: e n ~ d n{d 0 (1) d 0 where d n is the spacing of the (hkl) planes of the as milled powders parallel to surface of the specimen holder of the X-ray diffractometer, d 0 is the spacing of the same planes in the absent of the stress (i.e. calcined powders) and e n is the mean value of uniform lattice strain normal to the crystalline planes. Assuming that the material is elastically isotropic, the mean value of uniform lattice strain of the 60 h ball milled powder is y0?40%. The nanocrystalline structure of the as synthesised powder was also confirmed by high resolution transmission electron microscopy (HRTEM) micrographs. Figure 4 shows the HRTEM bright field image and the corresponding selected area electron diffraction (SAED) of the Ni Zn ferrite powders formed within 60 h of ball milling. The corresponding diffraction pattern and the lattice fringes indicate an almost completely random orientation of the resultant nanocrystalline structure. The average grain size estimated from the HRTEM images is between 5 and 10 nm, which is relatively consistent with the value estimated by the analysis of the XRD data. Magnetic properties Figure 5 shows the hysteresis curves (magnetization vs. external field) for 2 different samples that undergo mechanical alloying at different milling times (36 and 60 h). Magnetic properties such as saturation magnetization, remanent magnetisation, coercivity and susceptibility can be determined from these curves. The variation of saturation magnetisation and coercivity as a function of milling time are illustrated in Fig. 6. Referring to this figure, the saturation magnetisation increases and coercivity decreases with increasing the milling time. The increase in saturation magnetisation can be attributed to the Ni Zn ferrite phase formation at the high milling time. The progress of mechanochemical reaction between initial oxide powders (i.e. ZnO, NiO and Fe 2 O 3 ) during the ball milling can be monitored by means of saturation magnetization variations. The coercivity variations in the nanocrystalline mechanically alloyed soft magnetic powders are affected by several factors, which in some cases make it difficult to interpret coercivity as a function of milling variables. Zeng et al. 17 summarised various contributions to 4 Image (HRTEM) and corresponding SAED pattern of Ni Zn ferrite powders obtained after 60 h milling time 5 Hysteresis curves for samples milled at 30 and 60 h 218 Powder Metallurgy 2013 VOL 56 NO 3

4 6 Saturation magnetisation and coercivity as function of milling time coercivity, including residual internal stress, crystallite size, contamination (oxidation, inclusions), various defects (dislocations), non-spherical shape, surface irregularities, surface anisotropy, interparticle interaction, etc. Nevertheless, it seems that the gradual increase in Ni Zn ferrite with milling time is the dominant factor controlling coercivity variations during the ball milling. Figure 6 also shows that coercivity varies inversely with saturation magnetisation. The variation of remanence ratio (M r /M s ) versus milling time is shown in Fig. 7. The variation of the remanence ratio with milling time is very similar to that observed for the coercivity. The remanence ratio decreases from 0?106 at 0?5 h to 0?039 at 60 h. These values are much less than the theoretical value for an isotropic (0?5) or cubic Stoner Wohlfarth particle (0?83). 18 In the theoretical calculations, the effect of free poles formed on the most boundaries is ignored. These free poles set up demagnetising fields which can cause the actual value of M r /M s to be substantially lower than the calculated value. 18 The variation of magnetic susceptibility (dm/dh) with applied magnetic field (Fig. 8) shows peaks separated by magnetic field 2H m, indicating the maximum susceptibility. The maximum susceptibility shows an upward trend with increasing milling time (Fig. 7), 7 Remanence ratio and maximum susceptibility as function of milling time 8 Field dependence of magnetic susceptibility of ball milled powders for various milling time: separation of peaks is represented by field 2H m which is approximately similar to that observed for the magnetisation variations (Fig. 6). The overall trend of the milling process shows a progressive narrowing of the susceptibility peaks with increasing the milling time, which can be ascribe to the coercivity variation with milling time. As shown in Fig. 6, the coercivity also decreases with increasing the milling time. The narrowing of the susceptibility peaks has been also observed by Muthuselvam et al. 19 for Co 2 FeO 4 ferrite prepared by mechanical alloying and subsequent annealing. They have found that a single phase structure of Co 2 FeO 4 ferrite exhibits a narrow susceptibility peak due to the homogeneous structure of ferrimagnetic domains. The results of our mechanical alloyed samples are also consistent with their findings. As illustrated in Fig. 1, a single phase structure of Ni Zn ferrite gradually develops with milling time, which in turn leads to the narrowing of the susceptibility peaks. In conclusion, all of the measured magnetic properties confirm that soft magnetic properties of the synthesised powders have been improved with the increase in milling time. Conclusions The structure and magnetic properties of ball mill prepared nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite have been carefully studied as a function of milling time. The important observations can be summarised as follows. Powder Metallurgy 2013 VOL 56 NO 3 219

5 1. Nanocrystalline Ni 0?64 Zn 0?36 Fe 2 O 4 ferrite powders with the average crystallite size of 6?18 nm, non-uniform lattice strain of 0?33% and lattice parameter of 0?83977 nm has been formed after 60 h of ball milling. High resolution transmission electron microscopy micrographs also confirmed the nanocrystalline nature of the prepared powders. 2. In addition to the non-uniform lattice strain, a uniform lattice strain was also observed in the as milled powders. The mean value of uniform lattice strain for 60 h ball milled powders was found to be 0?40%. 3. Saturation magnetisation increased from 0?37 to 18?5 emu g 21 and coercivity decreased from 333?1 to 60?7 Oe when milling time increased from 0?5 to 60 h. Improvement of soft magnetic properties can be ascribed to the gradual increase in Ni Zn ferrite phase with increasing milling time. Acknowledgement The authors would like to thank the Shiraz University for providing support to this research. References 1. A. Goldman: Modern ferrite technology ; 1990, New York, Van Nostrand Reinhold. 2. E. C. Snelling: Ferrites for inductors and transformers ; 1983, New York, Research Studies Press. 3. A. K. M. Akther Hossain, S. T. Mahmud, M. Seki, T. Kawai and H. Tabata: Structural, electrical transport, and magnetic properties of Ni 1-x Zn x Fe 2 O 4, J. Magn. Magn. Mater., 2007, 312, O. Mirzaee, M. A. Golozar and A. Shafyei: Influence of V 2 O 5 as an effective dopant on the microstructure development and magnetic properties of Ni 0?64 Zn 0?36 Fe 2 O 4 soft ferrites, Mater. Char., 2008, 59, K. H. J. Buschow and F. R. de Boer: Physics of magnetism and magnetic materials ; 2004, New York, Kluwer Academic Publishers. 6. I. H. Gul, W. Ahmed and A. Maqsood: Electrical and magnetic characterization of nanocrystalline Ni Zn ferrite synthesis by coprecipitation route, J. Magn. Magn. Mater., 2008, 320, S. Thakur, S. C. Katyal and M. Singh: Structural and magnetic properties of nano nickel zinc ferrite synthesized by reverse micelle technique, J. Magn. Magn. Mater., 2009, 321, J. Azadmanjiri: Structural and electromagnetic properties of Ni Zn ferrites prepared by sol gel combustion method, Mater. Chem. Phys., 2008, 109, S. Deka and P. A. Joy: Characterization of nanosized Ni-Zn ferrite powders synthesized by an autocombustion method, Mater. Chem. Phys., 2006, 100, S. Bid and S. K. Pradhan: Characterization of nanosized Ni-Zn ferrite powders synthesized by an autocombustion method, Mater. Chem. Phys., 2004, 84, M. Jalaly, M. H. Enayati, F. Karimzadeh and P. Kameli: Mechanosynthesis of nanostructured magnetic Ni Zn ferrite, Powder Technol., 2009, 193, L. Yu, J. Zhang, Y. Liu, C. Jing and S. Cao: Fabrication, structure and magnetic properties of nanocrystalline NiZn-ferrite by highenergy milling, J. Magn. Magn. Mater., 2005, 288, Kh. Gheisari, Sh. Shahriari and S. Javadpour, Structure and magnetic properties of ball-mill prepared nanocrystalline Ni Zn ferrite powders at elevated temperatures, J. Alloys Compd, 2013, 552, K. S. Choo, Kh. Gheisari, J. T. Oh and S. Javadpour: Structure and magnetic properties of nanostructured Ni 0?77 Fe 0?16 Cu 0?05 Cr 0?02 (Mumetal) powders prepared by mechanical alloying, Mater. Sci. Eng. B, 2009, B157, Crystallography Open Database, (accessed 11 December 2012). 16. B. D. Cullity: Elements of X-ray diffraction, 2nd edn, 286; 1978, Reading, MA, Addison-Wesley Publishing Company. 17. Q. Zeng, I. Baker, V. McCreary and Z. Yan: Soft ferromagnetism in nanostructured mechanical alloying FeCo-based powders, J. Magn. Magn. Mater., 2007, 318, B. D. Cullity and C. D. Graham: Introduction to magnetic materials, 2nd edn, 328; 2009, Hoboken, NJ, John Wiley & Sons. 19. I. P. Muthuselvam and R. N. Bhowmik: Structural phase stability and magnetism in Co 2 FeO 4 spinel oxide, Solid State Sci., 2009, 11, Powder Metallurgy 2013 VOL 56 NO 3