To increase data density and smaller bit size on magnetic storage devices such as hard drives, the material’s coercivity must be high enough to protect the data from unwanted magnetic disturbances. However, a high coercivity would also spell difficulty for read/write heads to record and erase data.

Heat-assisted magnetic recording (HAMR) addresses the dilemma by introducing a laser above the read/write head to locally heat the material, temporarily lowering its coercivity when data is being written. Unfortunately, the laser creates unwanted thermal energy diffusion inside the slider, causing new problems such as material protrusion, which could decrease the device efficiency or even lead to device failure.

To investigate angstrom-level laser-induced protrusion (LIP) at the read/write head surface, Cheng et al. used a touchdown-based scheme and focused on the effects of laser-on time, laser current, and operating linear velocity.

With the laser turned on, the LIP forms within microseconds, while the fly height change (FHC) happens over milliseconds. The researchers focused on the long timescale of laser heating as the FHC takes place, and isolated the LIP using the time constants.

While the laser is on, the touchdown power decreases due to spacing loss from laser heating. This touchdown power change was measured to determine LIP formation and the FHC. Results showed that the FHC reaches a steady state in about 28 milliseconds, with the protrusion size presenting a two-stage linear relation with the laser current. As the operating linear velocity increases from 12 to 24 meters per second, the LIP is reduced by about half.

The researchers plan to next investigate the wear from over-push touchdowns and the smear from laser heating.

Source: “Measurement of angstrom-level laser induced protrusion using touchdown in heat-assisted magnetic recording,” by Qilong Cheng, Haoyu Wang, Siddhesh V. Sakhalkar and David B. Bogy, Applied Physics Letters (2020). The article can be accessed at