Prof. Qingyu Xu
Shuang Zhou, Ji Wang, Xiaofeng Chang, Shuangbao Wang, Bin Qian, Zhida Han, Qingyu Xu*, Jun Du*, Peng Wang & Shuai Dong*
The single multiferroic materials are very rare, due to the conflicting requirements of the out-shell electronic structure in oxide perovskites, e.g., empty shell for ferroelectricity and partially filled outer-shell for magnetism. This is the well-known d0 criterion for ferroelectricity in oxide perovskite, which is quite valid. However, for fluoride, the interaction between transition metal ion M with F is much more ionic than M-O, leading to weak hybridization. Together with the higher degree of
Coulombic interactions, the d0 criterion might be broken in fluorides.
BaMnF4 powders were prepared by hydrothermal method. Hysteretic field dependent magnetization curve at 5 K confirms the weak ferromagnetism aroused from the canted antiferromagnetic spins by magnetoelectric coupling. The blocking temperature of 65 K for exchange bias coincides well with the peak at 65 K in the zero-field cooled temperature-dependent magnetization curve, which has been assigned to the onset temperature of two-dimensional antiferromagnetism. An upturn kink of exchange field and coercivity with decreasing temperature was observed from 40 K to 20 K, which is consistent with the two-dimensional to three-dimensional antiferromagnetic transition at Néel temperature (~26 K). In contrast to the conventional mechanism of magnetization pinned by interfacial exchange coupling in multiphases, the exchange bias in BaMnF4 is argued to be a bulk effect in single phase, due to the magnetization pinned by the polarization through magnetoelectric coupling.
The main contributions of this work are:
1. The weak ferromagnetism due tomagnetoelectric coupling induced canting between antifferomagnetic aligned spins has been confirmed;
2. We provide a novel mechanism of exchange bias, in contrast to the convential case of interfacial exchange coupling. The magnetization here is pinned by the polarization through magnetoelectric coupling, which is a bulk effect.
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