Exchange interaction in metal oxides core-shell nanoparticles \(\hbox {Co}_3\hbox {O}_4\)–CoO

This article reports exchange bias (EB) effect in \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ –CoO nanoparticles despite having a ferromagnetic (FM) metal and antiferromagnetic (AFM) metal oxide interface. Pining mechanism in the overlapping domain of AFM spinel \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ and AFM CoO leads to a shift in magnetic hysteresis ( \(M-H\) $M-H$ ) loop towards the field axis followed by an increase in coercivity, when the system is cooled under static magnetic field − a typical signature of EB effect. Samples are prepared via sol–gel route followed by a controlled oxidation-reduction procedure; characterized via X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) analysis. Detailed studies of two samples of \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ –CoO show strong dependence of EB with a variation in crystallite size/phase fraction. Exchange interaction is maximum at lowest temperature and gradually vanishes above the AFM Néel temperature ( \(T_N\) $T_N$ ) of \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ . Another double interface ternary sample Co– \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ –CoO with FM Co at the center produces higher EB field than \(\hbox {Co}_3\hbox {O}_4\) ${\text{Co}}_3{\text{O}}_4$ –CoO nanostructures.