Magnetic properties of Co-impregnated zeolites

The structure and magnetic properties of Co-containing zeolites prepared by wet impregnation were investigated. The samples were calcined and then reduced in flowing H2. The samples studied have large saturation magnetization due to the presence of cubic Co particles over a wide range of sizes. The zero field cooling-field cooling curves show a sharp magnetization peak with a blocking temperature around 7 K followed by an exponential decay and two other peaks—of much lower amplitude—around 160 and 280 K. The low temperature peak is analyzed considering first, at Tcrit, thermal relaxation toward equilibrium over an energy barrier, with increasing viscosity S with T. Above Tcrit relaxation does not occur and viscosity abruptly goes to zero. The behavior of the smallest Co particles is unusual above the blocking temperature.