Competing exchange bias and field-induced ferromagnetism in La-doped BaFeO3

An exchange bias (EB) effect was observed in mixed valent LaxBa1−xFeO3 (x=0.125, 0.25, 0.33) perovskites exhibiting the antiferromagnetic (AFM) helical order among Fe4+ ions coexisting with the ferromagnetic (FM) cluster phase in the ground state. The La3+ ions for Ba2+ site substitution, associated with increase in number of the AFM coupled Fe3+ - Fe4+ pairs as well as some Fe3+ - Fe3+ pairs, leads to strengthening of the AFM phase and consequently to the alteration of the EB characteristics, which depend on level of the La doping x. At low doping x≤0.25, an abnormal dependence of the EB field, HEB, on the cooling field, Hcool, was found. The HEB increases rapidly with increasing cooling field at low Hcool, but it falls suddenly at cooling fields higher than 20 kOe, reducing by an order of magnitude at 90 kOe. The suppression of EB is caused by the field-induced increased volume of the FM phase, due to the transformation of the AFM helical spin structure into the FM one. Thus, low-doped LaxBa1−xFeO3 demonstrates a competition of two alternate cooling-field-induced effects, one of which leads to the EB anisotropy and another one to the enhanced ferromagnetism. In contrast, the x=0.33 sample, having a strong AFM constituent, shows no field-induced FM and no drop in the EB field. Accordingly, the HEB vs Hcool dependence was found to be well explained in the framework of a model describing phase-separated AFM-FM systems, namely, the model assuming isolated FM clusters of size ∼4 nm embedded in the AFM matrix.