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

C.E. RODRÍGUEZ TORRES; P. MENDOZA ZÉLIS; G.A. PASQUEVICH; S. J. STEWART

Campinas

Conferencia; 23ª Reunião Anual de Usuários do LNLS; 2013

LNLS

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 diferent 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 difraction 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 diferences 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. X-ray magnetic circular dichroism (XMCD) is one of the most useful types of measurement to investigate magnetic properties using synchrotron radiation X-rays. XMCD is measured at the X-ray absorption edge of a specified element in a sample, which enables element-selective measurement. The intensity of XMCD is proportional to the mean magnetic moment projected onto the direction of the incident X-ray, and the sign reflects the direction of the probed moment relative to that of the total magnetization. Due to contributions of Fe3+ occupying octahedral sites and Fe3+ occupying tetrahedral sites in iron oxides are clearly distinguishable in XMCD signal it is possible to obtain hysteresis loops for each iron site. In this work we present XMCD hysteresis loops taken at Fe L-edge on maghemite and Zn ferrites nanoparticles and thin films in total electron yield at LNLS (PGM beamline). The results will be compared with hysteresis loops taken by a vibrating sample magnetometer in the same conditions.