Magnetic properties of nanocrystalline Co–Ni ferrite

Abstract

A spinel structure which is formed by a nearly closepacked fcc array of anions with holes partly filled by the cations can be represented by the formula AB2O4 [1], A represents metallic ions located in A interstitial (tetrahedral) sites and B metallic ions located in B (octahedral) sites. Due to the large electronegativity of oxygen, the ionic type of bonds prevails in almost all oxide spinels [2]. Magnetic properties of nanoparticles have been of great interest in recent years partly because of the development of high-density magnetic storage media with nanosized constituent particles or crystallites [3, 4]. Much attention have been attracted by the investigation of nanophase spinel ferrite particles owing to their technological importance in the application areas, such as microwave devices, high speed digital tape and disk recording, ferrofluids, catalysis, and magnetic refrigeration systems [5]. Magneto-optical properties of CoFe2O4 and NiFe2O4 have been investigated in references [6–9]. The investigation of ferromagnetic resonance properties in CoxNi1)xFe2O4 with low cobalt concentration (X £ 0.1) were performed many years ago [10–13]. Cobalt ferrite, CoFe2O4, is a wellknown hard magnetic material [14]. Recently, Ni ions were added to Co ferrite films to improve the magnetic recording properties of cobalt ferrite films, which are promising as high-density perpendicular recording media [15, 16]. To our knowledge, the systematic investigation of the magnetic properties of nanocrystalline Co–Ni ferrite CoxNi1)xFe2O4 with x variated from 0 to 1 has not been reported. In this article, the crystal structure and the magnetic properties of nanosized Co–Ni ferrite prepared by the polyvinyl alcohol (PVA) sol-gel method are reported. CoxNi1)xFe2O4 (0.0 £ x £ 1.0) ferrite powders were produced by the PVA sol-gel method [17]. Cobalt nitrate, nickel nitrate and ferric nitrate were mixed at a Co:Ni:Fe ion ratio of x:1–x:2 and dissolved in deionized water, this mixture was then added to the aqueous PVA solution with continuous stirring for 30 min. After that, the mixed solution was dehydrated at 80 C until the dry gel-type precursor was obtained. Portion of this precursor was calcined at 800 C for 2 h, yielding the materials examined in this work. The crystallographic properties of the samples were examined by powder X-ray-diffraction (XRD) measurements with CuKa radiation (Philips x’ pert, Holland). The macromagnetism measurements were performed using a vibrating sample magnetometer (VSM) (Lakeshore 7304, USA). Figure 1 shows X-ray diffraction patterns of CoxNi1)x Fe2O4. All samples were found to be single phase spinel. The average particle sizes as determined from the X-ray diffraction line breadths are about 34 – 2 nm. The lattice constant a0 of each sample listed in Table 1 is obtained by plotting the lattice constant versus the Nelson–Riley function and extrapolating the result to h = 90 . A comparison between the lattice constant of the nanosized Cox Ni1)xFe2O4 and that of the bulk material [1] is shown in Fig. 2. The lattice constant a0 increases almost linearly with the increasing cobalt concentration (x). This can be explained by the fact that the ionic radius of Co (0.78 A) is larger than that of Ni (0.69 A). The larger value of the lattice constant for the nanosized CoxNi1)xFe2O4 than for the bulk material with x ranging from 0 to 1.0 may be due to the lattice expansion induced by the reduced particle size Z. P. Niu (&) AE Y. Wang AE F. S. Li Key Lab for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China e-mail: niuzp@lzu.edu.cn J Mater Sci (2006) 41:5726–5730 DOI 10.1007/s10853-006-0099-3