Synthesis and Size-Dependent Exchange Bias in Inverted Core−Shell MnO|Mn₃O₄ Nanoparticles

Core−shell nanoparticles of MnO|Mn₃O₄ with average particle sizes of 5−60 nm, composed of an antiferromagnetic (AFM) core and a ferrimagnetic (FiM) shell, have been synthesized and their magnetic properties investigated. The core−shell structure has been generated by the passivation of the MnO cores, yielding an inverted AFM-core|FiM-shell system, as opposed to the typical FM-core|AFM-shell. The exchange-coupling between AFM and FiM gives rise to an enhanced coercivity of ∼8 kOe and a loop shift of ∼2 kOe at 10 K, i.e., exchange bias. The coercivity and loop shift show a non-monotonic variation with the core diameter. The large coercivity and the loop shift are ascribed to the highly anisotropic Mn₃O₄ and size effects of the AFM (i.e., uncompensated spins, AFM domains, and size-dependent transition temperature).