Abstract
An exchange bias (EB) effect was observed in mixed valent (, 0.25, 0.33) perovskites exhibiting the antiferromagnetic (AFM) helical order among ions coexisting with the ferromagnetic (FM) cluster phase in the ground state. The ions for site substitution, associated with increase in number of the AFM coupled - pairs as well as some - 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 . At low doping , an abnormal dependence of the EB field, , on the cooling field, , was found. The increases rapidly with increasing cooling field at low , 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 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 sample, having a strong AFM constituent, shows no field-induced FM and no drop in the EB field. Accordingly, the vs 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.
- Received 9 January 2017
- Revised 15 March 2017
DOI:https://doi.org/10.1103/PhysRevB.95.134428
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