Role of spin disorder and crystallinity in the exchange bias effect of the Fe3O4@MnxOy nanocomposites

Abstract

In this study, we report a correlation between structural crystallinity and the exchange bias effect in a bimagnetic nanocomposite system. Three batches of Fe₃O₄@Mn_xO_y nanocomposites, labelled A1, A2, and A3, were prepared using a one-step hydrothermal synthesis method at different synthesis temperatures. Their structural and morphological properties were characterized using XRD, FESEM, and TEM analysis. For the amorphous sample A1, a strong exchange bias effect (H_EB ∼ 4.3 kOe) was observed, which reduced to a comparatively smaller H_EB value (∼1.2 kOe) in the improved crystalline sample A2. Interestingly, for the crystalline A3 sample, the exchange bias behaviour disappeared or became negligible. This reduction in exchange bias behaviour from amorphous to crystalline state highlights the role of spin disorder associated with the glassy magnetic state. Moreover, the analysis of the training effect and magnetic relaxation studies using rotating and freezing spins confirmed the presence of spin glass behaviour and surface defects, which progressively evolved with the transitions from amorphous to crystalline phases.