Surface driven exchange bias in nanocrystalline CoCr2O4

Abstract

This article reports exchange bias (EB) in nanocrystalline cobalt chromite CoCr₂O₄, - a phenomenon manifested by loop shift followed by increase in coercivity after cooling the sample in static magnetic field. The average particle size of the nanocrystalline CoCr₂O₄ is 30 nm, confirmed from transmission electron microscopy (TEM) images. Thermal variation of magnetization data M(T), in zero field cooling (ZFC) and field cooling (FC) modes depict long range order below the lock-in transition at T_L~16 K, spin-spiral transition at T_S~28 K and a collinear ferrimagnetic (FiM) state at T_C~100 K. Magnetic hysteresis loops are recorded in ZFC and FC modes at different cooling fields (H_cool). In addition, hysteresis loops are recorded in FC mode for a wide temperature range. In both the cases, a clear signature of conventional exchange bias effect is observed. Analysis of the training effect in magnetic hysteresis loop and temperature dependent ac susceptibility (χ_ac) at different frequencies suggest that the surface spins of CoCr₂O₄ nanoparticles play an important role in controlling the magnetic properties, and enhance the EB effect in collaboration with competition between non-collinear spiral spin state and collinear long range ferrimagnetic order.