Measuring Thermal Gradient in HAMR Using Pseudorandom Bit Sequences

The downtrack thermal gradient (DTTG) is a key metric determining the performance of heat-assisted magnetic recording heads. It describes the change in temperature with respect to the position at the recording point, which determines the transition width and consequently the linear density capability. Here, we present a method for measuring the DTTG by stepping the laser power while writing a series of pseudorandom bit sequences (PRBSs). The sharp change in laser power changes the diameter of the thermal spot produced in the media by the laser and near-field transducer, which in turn produces a shift in the location of the written transitions. The magnitude of this shift is measured from the readback signal and may be used to calculate the thermal gradient. The PRBS-based technique described here has the additional advantage of enabling corrections for a number of errors in the calculation of the thermal gradient, including the nonlinear dependence of the transition shift on the magnitude of the laser power modulation and changes in the curvature due to the change in the size of the thermal spot.