Surface driven exchange bias in nanocrystalline CoCr <sub>2</sub> O <sub>4</sub>
S. Goswami, P. K. Manna, Subhankar Bedanta, Sunita Dey, Manashi Chakraborty, D. De
Abstract
Abstract This article reports exchange bias (EB) in nanocrystalline cobalt chromite CoCr 2 O 4 , - 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 CoCr 2 O 4 is 30 nm, confirmed from transmission electron microscopy 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 state at T C ≈100 K. Magnetic hysteresis loops are recorded in ZFC and FC modes at different cooling fields ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>H</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> <mml:mi mathvariant="normal">o</mml:mi> <mml:mi mathvariant="normal">o</mml:mi> <mml:mi mathvariant="normal">l</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> ). In addition, hysteresis loops are recorded in FC mode for a wide temperature range. In both 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 CoCr 2 O 4 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.