Structural, optical, and magnetic properties with finite-size and interface effects of noncollinear ferrimagnetic ACr2O4 (A = Ni, Co, and Mn) single-domain nanocrystals

Abstract

Ferrimagnetic (FIM) spinel chromites with competing magnetic nearest-neighbour and next-nearest-neighbour interactions usually exhibit a noncollinear magnetic ground state with possible interesting interface phenomena in their single-domain nanocrystals (NCs). Herein, we reveal the intriguing finite-size and core–shell interface effects of FIM spinel chromite ACr₂O₄ (A = Ni, Co, and Mn) NCs with particle sizes barely larger than the critical size limit of their noncollinear FIM order. Structural, optical, and magnetic properties of the single-domain NCs are comprehensively studied in comparison to their typical bulk properties. In a normal spinel structure, the single-domain NCs exhibit bulk optical absorption characteristics with a wide optical energy gap. Magnetization anomalies corresponding to the bulk collinear and noncollinear (canted and spiral) FIM transitions are maintained, with additional coexisting spin-glassy transitions, in the NCs’ magnetic core that exhibits magnetization volume fractions of ∼0.2, 0.35 and 0.3 for the Ni, Co and Mn chromite NCs, respectively. The nonmonotonic temperature dependence of the magnetic coercivity that decreases with the decrease in temperature below T_s for the noncollinear FIM spinel chromite NCs is attributed to the bulk magnetocrystalline anisotropy. Furthermore, a large exchange bias effect below T_s indicates a strong interface coupling between the NCs’ noncollinear FIM core and their surface shell with frozen uncompensated spins. Our results shed light on the exotic magnetic properties due to finite-size and core–shell interface effects, in the functional insulating FIM spinel chromite NCs with a single-domain size.