Room temperature A and B-site magnetic contributions in ferrimagnetic ZnFe₂O₄ thin film and nanoparticles studied using XMCD

Full text: To investigate the source of magnetism in nanostructured zinc ferrite, we present a study using X-ray absorption (XAS) and magnetic circular dichroism (XMCD) at the L2,3 edges of Fe on zinc ferrite samples of different morphology (films and nanoparticles), all showing a relatively large magnetization at room temperature. The samples consist of: a) a zinc ferrite thin film grown at low O₂2 pressure of 10⁻⁵ mbar, the film thickness being 57 nm (ZFO1), b) nanostructured ZnFe₂O₄ particles obtained by mechanically milling hydrothermally prepared nanoparticles (sample 2ZF10H, average grain size D = 13 nm) and c) nanostructured ferrite particles obtained by mechanical grinding bulk ZnFe₂O₄ (BZF10H, D = 14 nm). In all cases, the x-ray diffraction patterns indicate that all the samples have the spinel structure with cubic symmetry. Our results show that the XMCD signals at room temperature display the typical features observed for ferrites, i.e., a positive peak A1 assigned to magnetic contributions from Fe³⁺ at tetrahedral A-sites of the spinel structure, and two negative peaks B1 and B2 related to Fe³⁺ ions at octahedral B-sites. The dichroic signal of film ZFO1 is markedly higher than those of nanoparticles 2ZF10H and BZF10H. Particularly, XMCD of ZFO1 shows markedly intense B1 and B2 peaks. This fact results in a larger magnetic moment m per iron ion, which is about 40% higher than the m found for nanoparticles. The results are interpreted in terms of differences observed on the distribution of cations, the absence of nonmagnetic ions at B sites added to overpopulation of A sites that took place in ZFO1 film and the role of oxygen vacancies. (author)