Structural and magnetic properties of ordered inverse spinel Li x Fe5O8

Highlights: • Effect of Lithium substitution on the structural and magnetic properties of ordered spinel ferrite Li_xFe₅O_8−y (x = 0.91 and 1.02). • The ferrimagnetic order determined using the microscopic neutron diffraction technique in LiFe₅O_8. • Tunable magnetic properties of these compounds Li_xFe₅O_8−y (x = 0.91 and 1.02). -- Abstract: The inverse spinel lithium ferrite compound Li_xFe₅O₈ 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 Li_xFe₅O_8−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 µ_B and 4.545 µ_B, respectively, for x = 0.91; and the corresponding values for x=1.02 are 4.950 µ_B and 5.274 µ_B, 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 μ_B 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 μ_B 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.