Dynamic properties of spin cluster glass and the exchange bias effect in BiFeO₃ nanocrystals

A spin cluster glass behavior and a complicated exchange bias effect are observed in high quality BiFeO₃ nanocrystals grown by a hydrothermal method. The dynamic properties of the spin clusters investigated by measuring the frequency dependences of ac susceptibility show that the relaxation process can be described using a power law with the glass transition temperature T _g = 57K, relaxation time constant τ₀ = 4.4 × 10^{- 10}s, and critical exponent zv = 10.3 ± 1.9, consistent with a three-dimensional Ising spin glass. The exchange bias field (H _EB) varies non-monotonically with temperature and achieves a minimum at T_g. The abnormal shift of hysteresis loops above T_g may be interpreted in terms of a Malozemoff's random-field model with a framework of antiferromagnetic core/spin-cluster shell structure and a two-dimensional diluted antiferromagnet in a field (2D-DAFF) model, respectively. The exchange anisotropy of the BiFeO₃ nanocrystals will shed light on a possible application for magnetism related nanosized devices.