Calibration and measurement of the thermal reflection coefficient of heat assisted magnetic recording media

With increased interest in heat assisted magnetic recording (HAMR), the thermal reflection coefficient of HAMR media becomes more important, as it is related to the change of optical parameters of the media at different temperatures and can potentially be used for non-contact temperature measurement. In this report, we introduce a method to calibrate the thermal reflection coefficient of magnetic thin films by in situ measurement of the thermal reflectance as well as the magneto-optic Kerr effect (MOKE) signal from the media. In the measurement, we use one beam to locally heat up the media, while using a second beam, whose diameter and intensity is much smaller, to measure in situ the MOKE and thermal reflectance signal of the heated media. We characterize the media temperature by heating up the magnetic media with prewritten magnetic patterns in an ultra-high vacuum system and the resulting magnetic remanence in the prewritten area is measured by magnetic force microscopy. Thus the thermal reflection coefficient is measured by performing a pump-probe experiment, with the temperature calibrated at the zero thermoremanence temperature of the HAMR media, at which temperature all grains under test have reached Curie temperature. This method can be extended to comparative studies of the thermo-optical properties of magnetic thin films, whose magnetic properties are sensitive to temperature.