Effect of domain transformation on the magnetic properties of Cu x Ni 1-x Fe 2 O 4 ferrites Justice Msomi *, ** and Thomas Moyo * * School of Physics, Westville Campus, University of KwaZulu-Natal, Durban 4000, South Africa, moyo@ukzn.ac.za ** Department of Physics, Qwaqwa Campus, University of the Free State, Phuthaditjhaba 9866, South Africa, msomijz@qwa.uovs.ac.za ABSTRACT We report the effect of crossover from multi- to single- domain particles on the magnetic properties of Cu x Ni 1-x Fe 2 O 4 (x = 0, and 0.5) oxides due to grain size reduction to nanometer scale. The Mössbauer spectra for milled nanosized powder show a combination of ordered and paramagnetic behavior. The coercive fields ( C H ) is found to increase with reduction in grain size ( G ) according to the equation G b a H m m C + = for multi- domain particles. For single-domain particles C H decreases with G according to 2 G b a H s s C - = . 1 INTRODUCTION The magnetic properties of ferrites are dependent on the type of cations involved and their distribution amongst the tetrahedral (A) and octahedral (B) sites. Cation distribution changes when the particle size is reduced to nanometer scale. This results in nanosized oxides having properties that are drastically different from their bulk counterparts. As an example Zn atoms which are known to have preference for A sites in bulk compounds were found to migrate to B sites with reduction in grain size of ZnFe 2 O 4 oxide. This led to the enhancement of magnetic ordering temperature from 10 K to about 115 K [1]. In general, upon reduction in grain size G , ferromagnetic materials become magnetically harder and exhibit increased coercivity. However, below a certain grain size, the coercivity is observed to decrease and soft magnetization begins to prevail. This appears to be a common feature for ferrites [2-4] and Fe-based alloys [5]. In Fe-based mechanically alloyed nanocrystalline materials, the coercive field C H is found to follow a 6 G -power law relation [5]. The analysis of C H with reduction in grain size in ferrites has not been exhaustive [2-4]. The coercive field is a useful parameter in providing information about the internal microstructure of a material and its relation to domain wall movements. In this paper we investigate the effects of grain size reduction on properties of Cu x Ni 1- x Fe 2 O 4 ferrites and establish quantitatively the evolution of such properties with G . We made the NiFe 2 O 4 (x = 0) and Cu 0.5 Ni 0.5 Fe 2 O 4 (x = 0.5) by solid-state reaction [6-8] and systematically reduced grain sizes to nanometer scale using a Restch high-energy planetary ball mill. 2 RESULTS AND DISCUSSIONS The typical variation of XRD with milling time is shown in Figure 1. The most significant lines were successfully indexed to cubic spinel structure. The broadening of the peaks indicates reduction in grain size with milling. The grain sizes were calculated from the width of the (311) XRD peak using the Scherrer formula [1, 8]. 222 220 440 511 422 400 311 0 h 440 511 422 400 222 311 220 30 h 440 511 422 400 222 311 220 40 h 40 50 60 70 2 θ Relative intensity 440 511 422 400 222 311 220 50 h Figure 1. XRD pattern with milling time for NiFe 2 O 4 . Figure 2 shows the variation of grain sizes with milling time for both NiFe 2 O 4 and Cu 0.5 Ni 0.5 Fe 2 O 4 compounds. The saturation of grain sizes is observed after milling for about 30 hours. Larger grain sizes are obtained for NiFe 2 O 4 after milling for at least 10 hours. We attribute this to the atomic size difference between Ni (0.069 nm) and Cu (0.072 nm) NSTI-Nanotech 2007, www.nsti.org, ISBN 1420063766 Vol. 4, 2007 397