Magnetic ZnFe2O4 nanoferrites studied by X-ray absorption and Mössbauer spectroscopies

S. J. STEWART, S. J. A. FIGUEROA, M. B. STURLA, F. SIVES, J. RAMALLO-LÓPEZ, S. G.MARCHETTI, J. F. BENGOA, F. GARCÍA, F. G. REQUEJO

La Plata, Argentina

Workshop; Workshop 35th Anniversary of Hyperfine interactions at La Plata; 2005

The cation site occupancy and, thus, the physical properties of spinel ferrites MFe2O4 can be modified by reducing their characteristic lengths down to a nanometric scale. Particularly, when the normal spinel ZnFe2O4 is in a nanostructured state, the altered cation distribution amongst the octahedral (B) and tetrahedral (A) interstitial sites of its structure changes its long-range magnetic ordering from antiferromagnetic to ferrimagnetic one. To study the magnetic behaviour of zinc nanoferrites having different particle sizes and degree of inversion, we have synthesized ZnFe2O4 nanoparticles (5 to 13 nm) by hydrothermal methods and mechanical treatments. The samples were characterized by X-ray diffraction, X-ray absorption spectroscopy (XANES and EXAFS) at the Zn and Fe K edges, X-ray magnetic circular dichroism (XMCD) at the Fe K edge, 57Fe Mössbauer spectroscopy and magnetic measurements. The Fe K XANES results indicate that the pre-edge peak intensity increases with the grain size as a consequence that more Fe ions occupies sites of a lower symmetry (A). Further, the Zn K EXAFS analysis shows that the B occupancy by Zn ions increases with the grain size. On the other hand, the observed characteristic features of the XMCD spectra reflect the magnetic character of the compound. The Mössbauer results show that all the samples behave as a superparamagnet at room temperature. At 4.2 K the Mössbauer spectra are composed by two magnetic components, which corresponds to Fe3+ ions occupying both A and B sites. All theseresults provide consistent evidence of the high degree of inversion achieved by this method, which modifies the long range ordering and causes an important increment of the magnetic response of the material (Ms 70 emu/g).