Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co3O4 nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co3O4 nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.

► The magnetic behavior of Co3O4 nanoparticles is unique among antiferromagnetic nanoparticles, since the system enters into a blocked state above TN.
► Strangely, the temperature of the zero-field-cooled (ZFC) peak, TP, is also the bifurcation temperature, Tbf, and is independent of the magnetic field.
► Aging and memory are observed only in the field-cooled (FC) protocol and not in the ZFC protocol, which rules out spin-glass behavior.
► We report TN using specific heat measurements for this system.