Magnetic properties and their dependence on deposition parameters of Co/Al_2O_3 multilayers grown by pulsed laser deposition

摘要

Co/Al_2O_3 multilayered thin films were grown on Si (111) substrates by pulsed laser deposition (PLD) at temperatures from room temperature (RT) to 600 deg C. The Co/Al_2O_3 multilayered thin film grown at RT contains continuous cobalt layers in alumina matrices, with no evidence of island formation. On the other hand, cobalt showed a tendency to form islands in alumina matrices for growth temperatures in the range of 300-600 deg C. All the Co/Al_2O_3 multilayered thin films showed ferromagnetic behavior up to RT. It was observed that variations in the deposition parameters can significantly influence the magnetic properties of Co/Al_2O_3 multilayers. Depending on the temperature and pulse rate, RT coercivities in the 50-300 Oe range were observed. Films deposited at 600 deg C using a laser pulse rate of 10 Hz exhibited a decrease of coercivity with increasing measurement temperature. On the other hand, films deposited at 600 deg C using a reduced pulse rate of 2 Hz demonstrated an "anomalous" relationship between low-temperature coercivity and temperature. In these films, coercivity exhibited a weak tendency to increase with temperature. Squareness (M_r/M_s) of the hysteresis loops and its dependence on the temperature was also shown to be strongly affected by the deposition parameters. These observations have been rationalized on the basis of two competing magnetic anisotropies that act along different directions in the material.