Constricted double loop hysteresis and exchange bias attributed to the surface anisotropy in nanocrystalline La1/3Sr2/3Fe1−xCrxO3

We investigate magnetic properties of nanocrystalline ferrites with composition La1/3Sr2/3Fe1−xCrxO3 (LSFCO) for x=0, 0.02, 0.04 and 0.06. Thermal variation of zero field cooled magnetization displays a weak signature close to the charge (TCO) as well as antiferromagnetic ordering for x=0, 0.02 and 0.04. In addition, another disordered glassy magnetic transition (Tg) is noticed for all the compositions. Tg is predominantly observed, although signature of charge ordering almost disappears for x=0.06. Interestingly, a constricted double loop type magnetic hysteresis loop is observed for x=0.02. This structure in the hysteresis loop disappears with further increase in Cr substitution. A systematic shift in the magnetic hysteresis loop is observed for all the compositions, while samples are cooled in a static magnetic field, which is the manifestation of exchange bias (EB) effect. The EB field (HE) and magnetization (ME) decrease remarkably due to minor Cr substitution at x=0.02 and then it reveals a sluggish increase with increasing x. For x=0.04 the EB effect emerges below ∼TCO, increases sluggishly with lowering of temperature and below ∼Tg, it increases rapidly. The cooling field dependence exhibits significant increase of HE and ME for x=0.04, which is associated with the considerable increase of coercivity. Possible origin of EB correlated with the magnetic phase coexistence has been argued for nanocrystalline LSFCO.