Glassy magnetic freezing and exchange bias effect in NiCo2O4/(Co-Ni)O nanoparticles composite

An anomalous magnetic behavior and exchange bias effect are systematically studied for NiCo2O4/(Co-Ni)O nanoparticles synthesized by a microwave combustion method. Based on a complementary characterization by powder X-ray diffraction and high-resolution transmission electron microscopy, a composite of spinel Ni-cobaltite, NiCo2O4, and rock-salt-structured Co-Ni monoxide, (Co0.65Ni0.35)O, nanoparticles was synthesized in almost uniform elemental distribution. Optical absorbance measurements indicated multiple optical energy gaps with values of 2.02, 3.63 eV and 1.54, 4.015 eV attributed to the cobaltite and monoxide phases, respectively. A magnetic cluster-glass-like behavior with a blocking temperature of -75 K and irreversibility between the ZFC and FC magnetizations below -254 K were observed at low fields. An inherent exchange bias effect with sig-nificant shifts in both coercive field and remanent magnetization at temperatures up to room temperature is explored.