Exchange-coupling of hard and soft magnetic sublattices and magnetic anomalies in mixed spinel NiFe_(0.75)Cr_(1.25)O_4 nanoparticles

摘要

A set of single-crystalline nanoparticles (NPs) of nickel-chromium ferrite NiFe0.75Cr1.25O4with a cubic spinel structure were synthesized and investigated. The NPs size can be varied from about 5 to 50 nm by the final annealing of the precursor at different temperatures. The distribution of cations over the tetrahedral (A) and the octahedral [B] sites (Fe0.75Ni0.25) [Ni0.75Cr1.25] O4was established from the magnetic and Mössbauer measurements. In large NPs, the magnetic structure at low temperatures is close to the collinear antiferromagnetic (AFM) structure of the Neel type; and the total magnetic momentMtotof the ferrite coincides with the direction of the B-sublattice moment. Several size-dependent magnetic anomalies were revealed. Three types of magnetic ions present in the A- and B- sublattices cause the competition of AFM and FM exchange interactions resulting in the highly frustrated magnetic ordering and the occurrence of canted magnetic structure in the octahedral B-sublattice. The frustrated structure is very flexible and significantly subjected to temperature and applied field. It results in several magnetic anomalies observed, including the occurrence of magnetic compensation, abnormal behavior of ZFC and FC magnetization curves and hysteresis loops. It was shown that magnetic anomalies can be explained in terms of exchange coupling of “soft” and “hard” magnetic B- and A-sublattices. This effect in the (Fe0.75Ni0.25) [Ni0.75Cr1.25] O4NPs can be considered as anatomic-scale analogof a similar effect observed in two-phase exchange-coupled alloys developed for permanent magnets and for the perpendicular recoding media.