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 = 57 K, relaxation time constant tau(0) = 4.4 x 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.