Surface Effects on Morin Transition, Exchange Bias, and Enchanced Spin Reorientation in Chemically Synthesized DyFeO₃ Nanoparticles

Abstract

A giant linear magnetoelectric effect was observed by Y. Tokura’s group recently in multiferroic DyFeO 3, which demands a detailed investigation of its magnetic properties. Additionally, there is little information on the changes of chemical and physical properties of these materials with the reduction in particle size in spite of the potential applications of these materials in nanoscale devices. As the wet-chemical synthesis of these materials in nanosize and getting a control over crystallinity and stoichiometry is nontrivial and poses a serious challenge prohibiting the study of their size-dependent properties. Here, we report the synthesis of DyFeO 3 nanoparticles using a surfactantless hydrothermal method with a detailed magnetic property measurement. The as-synthesized DyFeO 3 nanoparticles showed excellent crystallinity with average particle size in the range 50−60 nm. The structural analysis indicated that they are of a distorted orthorhombic pervoskite crystal structure. Detailed dc magnetization measurements in the temperature range of 3−350 K could isolate the presence of Dy 3+ −Fe 3+ and Dy 3+ −Dy 3+ superexchange interactions, which showed up as spin reorientation transitions in various temperature regions due to the differing magnitude of their interactions resulting in continuous rotation of antiferromagnetic component of Fe 3+ spins with cooling of the sample. Nanosized DyFeO 3 showed spin-reorientation transitions near 315 and 70 K due to the Dy 3+ −Fe 3+ interaction accompanied with an opening up of the hysteresis loop followed by antiferromagnetic ordering around 4 K due to a possible Dy 3+ −Dy 3+ interaction. We also observed significant effect of the particle size reduction on the magnetic properties. The main effects seen by us were in terms of (1) pronounced spontaneous spin reorientation transitions, (2) the absence of Morin transition, and (3) presence of temperature-dependent exchange bias in the DyFeO 3 nanoparticles. We present a detailed mechanism to explain these features based on the interplay of Dy 3+ and Fe 3+ spins as well surface disorder, anisotropy, canting, and so forth.