Nanostructured CuO with antiferromagnetic core and weakly ferromagnetic shell

Abstract

Nanostructured CuO sample with average crystallite size ~ 11 nm was prepared by the thermal decomposition of carbonate precursor. The variation of inverse susceptibility with temperature shows that sample is inherently antiferromagnetic. Temperature dependent magnetization studies in the range 20–300 K at an applied magnetic field of 500 Oe show two cusps at ~ 221 K (TN1) and ~ 181 K (TN2). The reduction in Néel temperature in comparison with that of bulk CuO can be attributed to the weaker super-exchange Cu–O–Cu paths due to the presence of O2− vacancies. An increase in magnetization with fall in temperature below the blocking temperature (~ 85 K) is also observed which is due to the oxygen vacancies in the surface of nanoparticles. Analysis of O1s X-ray photoelectron spectrum proves the presence of copper and oxygen vacancies in the sample. The exchange bias effects observed in the hysteresis curves leads to the conclusion that more than one kind of ordering is present in the sample. A core-shell magnetic structure with antiferromagnetic core and weakly ferromagnetic shell is proposed for sample. Ferromagnetic interaction is found to persist even at room temperature and is most probably originating from the presence of O2− vacancies.