Short pulse laser heating for HAMR: Density potential and implementation issues

Abstract

Heat assisted magnetic recording is the most promising technology for high density recording . Due to thermal diffusion of the media, its density potential is limited for continuous wave laser heating . Short pulse laser heating can greatly increase the density potential [1][2][3] . This heating strategy also leads to a low transducer temperature increase [4] . Therefore, short pulse laser heating is a must approach to reach the full potential of HAMR technology . In this talk, the recording performance of short pulse laser heating at density of 4-5 Tb/in 2 is analyzed . The results showed that the media magnetic switching time becomes a key factor to determine recording performance for pulse width of less than 200 ps . A relatively large damping constant a is desired to speed up its switching time . For a higher density recording, a relatively high bit density is preferable because a lower track density could be recorded by applying a higher magnetic field H which can relax the demand for a . Fig . 1 shows the dependences of the required minimal a and magnetic field H on desired recording density for bit length of 6 .5nm . Considered the measured FePt damping constant [5], a recording density of 4 .5 Tb/in 2 is achievable for FePt media with pulse width of 100ps . Density of 5 T/in 2 is also possible to be achieved by using the media with a larger saturation magnetization M s , which will lead to a fast switching speed . In real implementation of short pulse laser heating for HAMR, there are four factors for short pulse laser series to affect the recording performance: i) . Phase shift between laser pulse and magnetic field for the uniform pulse laser series [3, 6]; ii) . Frequency jitter of the pulse laser series; iii) . Laser pulse width variation; and iv) . Laser pulse power variation . The effects of these factors on the recording performance were analyzed . Fig . 2 shows the dependence of SNR on phase shift at laser pulse width of 100ps . Unlike the wide pulse heating, short pulse heating gives a narrower deterioration gap because of the short heating period . Fig . 3 shows the effects of the media peak temperature variation on SNR at pulse width of 100ps . SNR decrease with the increase of peak temperature deviation . At sigma of 6%, SNR decrease about 2dB . Pulse laser generation with pulse width of ≤ 100ps is a key issue for short pulse laser heating . In this talk, the different technologies for semiconductor laser to generate short pulse laser output are discussed . It is indicated that gain-switching technology is a better way to generate short pulse output (≤100ps) for HAMR application . Fig . 4 shows the pulse laser waveforms with pulse frequency of 2 GHz and pulse width of 97ps generated by gain-switched semiconductor laser . The frequency standard deviation (s) is 9 .23MHz and the pulse width standard deviation is 5 .4ps . Finally, some of other concerns for short pulse laser heating are also discussed .