Structural, optical, and magnetic properties with finite-size and interface effects of noncollinear ferrimagnetic ACr2O4 (A = Ni, Co, 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). Here, we reveal intriguing finite-size and core-shell interface effects in FIM spinel chromites ACr2O4 (A= Ni, Co, Mn) NCs with particle sizes barely larger than a 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 the bulk optical absorption characteristics with a wide optical energy gap. Magnetization anomalies corresponding to the bulk collinear and noncollinear (canted and spiral) FIM transitions, with extra coexisting spin-glassy transitions, are demonstrated to be maintained in the NCs' magnetic core of volume fractions of ~0.2, 0.35 and 0.3 for the Ni, Co and Mn chromite NCs, respectively. A nonmonotonic temperature dependence of the magnetic coercivity that decreases with decreasing temperature below Ts for the noncollinear FIM spinel chromite NCs is attributed to the bulk magnetocrystalline anisotropy. Furthermore, a large exchange bias effect, unusually in opposite behavior of the coercivity with temperature, below Ts indicates a strong interface coupling between the NCs' noncollinear FIM core and their surface shell with frozen uncompensated spins. Our results spotlight on exotic variations from the typical bulk magnetic properties due to finite-size and core-shell interface effects in the functional insulating FIM spinel chromite NCs at a single-domain size.