Research Article Estimating the Cation Distributions in Ni 0.65− Zn 0.35 Co  Fe 2 O 4 Ferrites Using X-Ray, FT-IR, and Magnetization Measurements M. Chaitanya Varma, 1 GSVRK Choudary, 2 A. Mahesh Kumar, 3 and K. H. Rao 4 1 Department of Physics, College of Natural and Computational Sciences, Wollega University, P.O. Box 395, Nekemte, Ethiopia 2 Department of Physics, Bhavan’s Vivekananda College, Sainikpuri, Secunderabad 500094, India 3 Department of Physics, GITAM University, Visakhapatnam, Andhra Pradesh 530045, India 4 Department of Physics, RGUKT, IIIT Nuzvid, Andhra Pradesh 521201, India Correspondence should be addressed to GSVRK Choudary; gsvrk.choudary@gmail.com Received 20 October 2013; Accepted 15 January 2014; Published 4 March 2014 Academic Editor: Ali Hussain Reshak Copyright © 2014 M. Chaitanya Varma et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. he fundamental requirements for the shit of critical frequency to microwave frequencies are smaller grains with single domain, high resistivity, high saturation magnetization, moderate permeability, moderate magnetic anisotropy, and low spin relaxation time. With these guidelines an attempt to produce high performance ferrite for high frequency applications the present work aimed to synthesize cobalt substituted Ni-Zn ferrites using sol-gel method. Investigation of efects of cobalt on crystallite size, saturation magnetization, initial permeability, magnetic anisotropy, and spin relaxation time reveals the suitability of these materials for high frequency applications. Further in this paper cat ion distribution was proposed from the basis of variations in these properties. he results of this paper are thus useful to tailor the properties apt for high frequency applications. 1. Introduction Ferrites are the most widely used cores in high frequency power electronics, broadband transformers, pulse transform- ers, antennas, and sensors because of their excellent magnetic and electrical properties. he extensive study on mixed ferrite systems revealed that Ni-Zn ferrite was the only core material suitable for high frequency applications up to 100 MHz due to its high value of saturation magnetization, moderate permeability, and high resistivity. he increasing demand for materials with improved eiciency and reduced size and cost necessitates high operational frequencies beyond 100 MHz. However, at higher frequencies, magnetic cores with improved resistivity, high saturation magnetization, and high permeability are inevitable to minimize eddy current losses and magnetic losses [1–4]. he high frequency limit for ferrites is governed by Snoek’s law according to which there exists an efective limit for the product of critical frequency and permeability [5]. he limiting frequency is controlled by spin rotation of domains and as such the use of ferrite core in the GHz range is considered to be a complicated task. he recent developments in nanotechnology signify that the electrical resistivity can be increased manyfold with the gen- eration of a vast number of grains and grain boundaries if the material is produced in nanoform [6, 7]. he improvement in saturation magnetization can be achieved by modifying the crystallite size of the material comparable to characteristic length, a responsible parameter for exchange coupling to take place among the nanograins [8, 9]. he eicient per- formance of the core at higher frequencies is expected to be synchronized with the rotation of domains under a small AC ield and the process of rotation is known to be critically afected by magnetic anisotropy [10]. he magneto crystalline anisotropy determines the direction and stability of the magnetization within the material. Critical size below which a spherical particle exists as a single-magnetic domain depends on magnetic anisotropy. Also, the coercivity and the permeability are controlled by the magnetic anisotropy constant. hus in order to shit the critical frequency of operation to GHz range, it is necessary to achieve the best Hindawi Publishing Corporation Physics Research International Volume 2014, Article ID 579745, 9 pages http://dx.doi.org/10.1155/2014/579745