Magnetic behaviour of Co-AlN thin films with various Co concentrations

We have investigated the magnetic behaviour of Co-AlN (Al_1-xCo_xN) thin films with various Co concentrations (x) of 0.06-0.25. For all Co concentrations, the hysteresis are not observed in any magnetization curves at the temperatures of 10-300 K. For the Co concentration x below 0.10, the magnetization is almost zero at the temperature above 50 K and increases slightly with decreasing temperature below 50 K. In contrasts, for the Co concentration x above 0.17, the peak temperature in zero-field-cooled magnetization can be observed and its value increases from around 5 to 10 K with increasing Co concentration. On the basis of these results, it is found that the room-temperature ferromagnetism is not observed at any Co concentrations. It is suggested that the magnetic behaviour at the temperature more than 10 K changes from paramagnetism or spin-glass state to superparamagnetism or spin-glass state with increasing Co concentration.