Enhancement of microstructure and initial permeability due to Cu substitution in Ni 0.50  x Cu x Zn 0.50 Fe 2 O 4 ferrites A.K.M. Akther Hossain a,n , M.L. Rahman b a Department of Physics, Bangladesh University of Engineering and Technology, Bangladesh b Department of Mathematics and Natural Sciences, BRAC University, Dhaka-1212, Bangladesh article info Article history: Received 25 December 2010 Received in revised form 24 February 2011 Available online 4 March 2011 Keywords: Nanoparticle Auto combustion Permeability Ne ´ el temperature Saturation magnetization Hopping length abstract Structural and magnetic properties of Cu substituted Ni 0.50x Cu x Zn 0.50 Fe 2 O 4 ferrites (where x ¼ 0.0–0.25) prepared by an auto combustion method have been investigated. The X-ray diffraction patterns of these compositions confirmed the formation of the single phase spinel structure. The lattice parameter increases with the increase in Cu 2 þ content obeying Vegard’s law. The particle size of the starting powder compositions varied from 22 to 72 nm. The theoretical density increases with increase in copper content whereas the Ne ´ el temperature decreases. The bulk density, grain size and permeability increases up to a certain level of Cu 2 þ substitution, beyond that all these properties decrease with increase in Cu 2 þ content. The bulk density increases with increase in sintering temperatures up to 1250 1C for the parent composition, while for substituted compositions it increases up to 1200 1C. Due to substitution of Cu 2 þ , the real part of the initial permeability increases from 97 to 390 for the sample sintered at 1100 1C and from 450 to 920 for the sample sintered at 1300 1C. The ferrites with higher initial permeability have a relatively lower resonance frequency, which obey Snoek’s law. The initial permeability strongly depends on average grain size and intragranular porosity. The saturation magnetization, M s , and the number of Bohr magneton, n(m B ) , decreases up to x ¼0.15 due to the reduction of the A–B interaction in the AB 2 O 4 spinel type ferrites. Beyond that value of x, the M s and the n(m B ) values are enhanced. The substitution of Cu 2 þ influences the magnetic parameters due to modification of the cation distribution. & 2011 Elsevier B.V. All rights reserved. 1. Introduction For the last few years the soft ferrites having fine particle size have been extensively studied due to their wide range of technolo- gical applications. According to their structure spinel ferrite has tetrahedral A-site and octahedral B-sites in AB 2 O 4 crystal structure. It shows various magnetic properties depending on the composition and cation distribution. Various cations can be placed in A- and B-sites to tune their magnetic properties [1]. The NiCuZn spinel ferrites are soft magnetic materials, which have wide application in advanced technologies such as multilayer chip inductors (MLCIs) [2], multilayer LC filters [3], magnetic temperature sensors [4] and humidity sensors [5]. Soft magnetic materials with initial particle size in the nanometer scale are now of interest because of their unique magnetic properties, which differ considerably from those of bulk materials and become technologically very important. Magnetic nanoparticles exhibiting superparamagnetic behavior display little or no remnant magnetization and coercivity while keeping very high saturation magnetization and high permeability. They have potential applications in biomedicine [6], magnetic drug delivery and cell- sorting system [7–9]. As more and more attention has been devoted to the nano-sized magnetic materials for their unique properties compared to their bulk counterparts the scientific interest on the nano-sized soft NiCuZn ferrite is on the rising. Several investigations have focused their attention on NiCuZn spinel ferrites because copper containing ferrites have interesting electrical and magnetic proper- ties. Copper ferrite is distinguished among other spinel ferrites by the fact that it undergoes a structural phase transition accompanied by a reduction in the crystal symmetry to the tetrahedral due to the Cooperative Jahn–Teller effect [10]. Various methods have been developed to fabricate NiCuZn ferrite nanoparticles. The crystal structure and particle size of the materials control their physical as well as magnetic properties, which depend entirely on the method of preparation, nature and concentration of dopant [11, 12]. In this study we report the synthesis of NiCuZn ferrites adopting the auto combustion method, which is simple and has low production cost. 2. Experimental The various compositions of Ni 0.50–x Cu x Zn 0.50 Fe 2 O 4 (where x ¼ 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) were prepared by the auto Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.031 n Corresponding author. Tel.: þ88 02 966 5613; fax: þ88 02 861 3046. E-mail address: akmhossain@phy.buet.ac.bd (A.K.M. Akther Hossain). Journal of Magnetism and Magnetic Materials 323 (2011) 1954–1962