Radio-frequency (RF) fly height monitoring system for Heat-Assisted Magnetic Recording (HAMR)

In HDDs, during write operation, the transducer head flies over the media with no continuous monitoring on the adequacy of its flying height against sudden incidents. If the head hits an asperity or a lubricant spill drops from the head, the head momentarily oscillates around its intended fly height. This can compromise the data being written. Only by issuing a read command can the drive evaluate the quality of the data just written. This verification after write is not normally performed because it impacts drive performance. Heat-Assisted Magnetic Recording (HAMR) holds promises for higher data densities in magnetic media into the future and an “always-on” fly height monitoring system is desired. In HAMR, maintaining adequate fly height during write operation is a requirement from both the magnetic writing and the near-field heating antenna operation. The Radio-Frequency (RF) based system described in this work for fly height monitoring is non-magnetic in nature and, since the gap between head and media is smaller than the mean distance between air molecules, this system is also immune to temperature changes in the head or in the ambient. As shown in figure 1, the air gap between the flying head and the media is represented in electrical domain by capacitors (C1 and C2 in fig. 1). An RF signal of frequency significantly above the frequencies taken by the data signal's spectrum (cf. fig. 1) is injected onto the write lines. This RF signal capacitively couples onto all the elements on the head and the head-media interface (cf. fig. 1). When the head changes its fly height, the corresponding capacitance for the air gap changes its value. This varying capacitance behaves as a charge pump, which amplitude modulates (AM) the RF signal and injects this AM-modulated RF signal back into the head wiring. This signal travels back through the suspension and is detected at the pre-amplifier end. The amplitude of the sidebands carry information on fly height changes. The closer the head flies to the media, the stronger the AM sideband tones for the same disturbance in fly height.