Asymmetric shift of exchange bias loop in Ni-Ni(OH)₂ core-shell nanoparticles

We report the observation of the asymmetric shift of exchange bias loop in Ni-Ni(OH)₂ core-shell nanoparticles where the average size of the ferromagnetic (FM) Ni nanoparticles is ∼30 nm and the thickness of antiferromagnetic (AFM) Ni(OH)₂ shell is ∼5 nm. The exchange bias (EB) found below Néel temperature (T_N ∼ 22 K) of Ni(OH)₂ is path dependent, while the coercivity (H_C) increases and decreases for positive and negative bias field respectively. In the present case, we found that the inversion symmetry of hysteresis loop is broken and the shift in EB loop is only observed in descending part of the hysteresis loop, which is conspicuous. We demonstrate that the asymmetric shift of EBs in these core-shell nanoparticles is due to the presence of frustrated super spin glass (SSG) at the interface which influences the reversal mechanism of the hysteresis loop. It is argued that the net interface moment from the SSG at the interface of core-shell nanoparticles sets a unidirectional anisotropy after field cooling, which is thought to be the origin of this path dependency of the EB and observed via descending part of the hysteresis loop, ushering potential for novel spin based applications.