Structural and magnetic properties of ordered inverse spinel LixFe5O8

Abstract

The inverse spinel lithium ferrite compound LixFe5O8 has attracted a lot of attention because of its highest magnetoelectric coupling temperature. Here, we have investigated the effect of Lithium substitution on the structural and magnetic properties of ordered LixFe5O8−y (x = 0.91 and 1.02) using the X-ray diffraction, Mössbauer spectroscopy, dc magnetization, and neutron diffraction techniques. The Rietveld refinement of X-ray diffraction patterns confirms single phase formation of these compounds in cubic structure (space group P4332). Lithium composition for both compounds was determined using the atomic emission spectrometry (ICP-AES) measurements. The Rietveld refinement of neutron diffraction patterns reveal that both compounds order in a ferrimagnetic structure, where the magnetic moments of iron at the octahedral (12d) and tetrahedral (8c) sites are aligned anti-parallel. The refined values of the ordered moments for Fe3+ ions at 12d (octahedral) and 8c (tetrahedral) sites are 4.185 µB and 4.545 µB, respectively, for x = 0.91; and the corresponding values for x=1.02 are 4.950 µB and 5.274 µB, respectively. An increase in the value of saturation magnetization (in the M vs H study) has been observed with the increasing concentration of the Li. The observed values of the saturation magnetization at 5, 50, 100, 200 and 300 K are 3.91, 3.88, 3.85, 3.76, and 3.51 μB per f.u. for x=0.91 whereas for x=1.02 the corresponding values are 4.76, 4.70, 4.68, 4.56 and 4.33 μB per f.u. The observed increase in the saturation magnetization and ordered moment with increasing the Li concentration has been ascribed to the presence of iron (~9%) at the lithium site (4b) and similar amount of iron vacancies at 12d (octahedral) and 8c (tetrahedral) sites, for the x = 0.91 compound. Our results show that the magnetic properties of these compounds are tunable. The ferrimagnetic order determined using the microscopic neutron diffraction technique in the present study will provide a new pathway to build theoretical models for the magnetoelectric coupling.