Heat Assisted Magnetic Recording (HAMR)

1        Heat Assisted Magnetic Recording

1.1       Literature

In view of superparamagnetism limit, the magnetic recording system has to deal with the tradeoff among the signal to noise ratio, thermal stability, and the head write ability. Just few years ago, magnetic recording is switched from longitudinal recording system to perpendicular magnetic recording (PMR) system to detain the superparamagnetism limit. The perpendicular recording system successfully delays the superparamagnetism limit by significantly improve the write ability of magnetic recording. This allowed using higher anisotropy media, which is to maintain the thermal stability of the magnetization of the smaller grains. Current product by PMR, areal density is achieved at about 350Gb/in². There are some works [1, 2, 3] reported that the PMR is able to push the areal density up to 1 Tb/in². But, however, above this areal density, PMR would again come to the superparamagnetism limit.

To further increase the areal density of magnetic recording above 1 Tb/in², energy assisted to write is required. Two major examples for energy assisted schemes have been proposed are Heat Assisted Magnetic Recording (HAMR), and Microwave Assisted Magnetic Recording (MAMR).

Cited from: M. H. Kryder, ”Future Magnetic Recording Technologies”, 2002

Figure 1

Figure 1 shows the schematic diagram of an HAMR recording system. In principal, HAMR is to use an external heating source to locally heat up the recording medium before write by the recording head. This heating up the medium is able to reduce the anisotropy (Ku) of the medium. The reduction of medium coercivity with medium temperature is shown in figure 2. The normal operation temperature in hard disk drive without the extra heating source is around 77^oC. For HAMR, the medium need to further heat up to the temperature about the Curie temperature (Tc) of the medium in order able to write. For example, Curie temperature for FePt L1_o phase thin-film is at about ~650K. Curie temperature is the critical temperature at which the medium magnetism and the medium anisotropy field vanish. Therefore, at medium Curie temperature, high anisotropy media able to be write by the inductive head. Then, the medium need to rapid cooling down to retain the magnetization orientation had been written.

Cited from: M. H. Kryder, Proceedings of the IEEE, vol 96, issue 11, 2008

Figure 2

            Another attraction of the HAMR is that a very high effective writing field gradient can be achieved with no required contribution from the magnetic character of the recording head.

1.2       Latest Status of HAMR

           

In APMRC Jan 2009, Seagate had reported HAMR work successfully in experiment which can record at a track width of ~50 nm and an areal densities of ~240 Gb/in2 on high coercivity FePt media [4]. The experiment is carry out in a system that using the high recording medium (FePt), and a recording head integrating a pole head and a heating source to heat the medium to its Curie point at where the data is desired to be recorded. The head was flown over the FePt media with a Curie point of 650 K rotating as 2700 RPM with a head-medium-spacing of ~10 nm. Laser with 830 nm wavelength focused to a spot size of ~120 nm. The data recorded by HAMR is shown by MFM image in figure 3.

Cited from: W.A. Challener, FB-2, APMRC 2009, Grand Hyatt Hotel, Singapore, Jan. 14-16, 2009.

Figure 3

1.3       Issues on HAMR

                    

                     The issues on HAMR can be classified into two major categories, optical issues and thermal issues. Optical issues for example, how to produce a nano-sized optical spot (< 20 nm) and with an efficiency high enough to heat the media to high temperature (300 ^oC). Furthermore, the way to even reduces the spot size to achieve even higher areal densities. Since, the media is heat up to high temperature, this bring out thermal issues. For example, whether the lubricant layer it is able to withstand the high temperature. Other than these two major issues, other issues like how to integrating the writing pole head and the laser source also needed to solve. Certainly, same with other energy assisted recording approach, high Ku magnetic media with small grain size is demanded to achieve high areal densities. All of these issues need to be solve in order for HAMR come into product state. Current research on FePt has Ku about 6-10 x 10⁷ erg/cc with Ms of about 1100 emu/cc and H_k of around 116 kOe and able achieves grain size of ~3 nm.

Optical Issues

            In HAMR, the diffraction limit is the major obstacle. In conventional far field optic, the diffraction limit for the full width half-maximum (FWHM) optical spot size d as estimated from scalar diffraction theory is

Where l is the wavelength and NA is the numerical aperture of the focusing lens. In order to have smaller spot size, the laser wavelength should be as short as possible and the NA should be as high as possible. The current available wavelength with laser diode is about 375 nm. The NA is the product of the sine of the half-angle of the focused cone of light and the refractive index of the medium in which the light is focused. Researcher has proposed a solid immersion lens (SIL) [5] in which the light is brought to a focus at the bottom surface of a hemispherical lens. This approach gives NA of 1.8. By SIL approach together with 375 nm available wavelength correspond to a focused spot size of 106 nm. However, this spot size is still far away from the 25 nm spot size necessary for achieving areal densities > Tb/in² in magnetic recording system.

            To achieve areal densities in Tb/in² range the spot size should reduce and in order the read-back signal is significant, the fly height (FH) should be low. Therefore, it is both necessary and natural to employ near field optic in HAMR. The near field optic makes use of apertures or antennas, or combination of both to overcome the diffraction limit. But, however it come another issue on the energy efficiency to transfer the heat to the medium with relative small spot size.

Thermal Issues

            Except optical issues, thermal issues for HAMR are also critical. Since the medium need to heat up above 300^oC, thermal effect on the medium and slider is still have a lot of uncertainty. The thermal issues on HAMR system can divide into two respects, thermal effect on slider and thermal effect on medium.

            Slider heat absorption can come from two channels: (1) laser absorption along the laser energy delivery structure built in the slider (since the HAMR recording head is an integrated head on the slider, when the laser energy is activated, part of the heat will be absorb by the slider); (2) the reflected laser heat from the medium (when the laser reach the medium, some energy is absorbed and some are reflected from medium). This laser-induced slider temperature rises will cause the sensor sensitivity changes, the slider deformation, and the magnetic head protrusion; furthermore, it will cause the fly height unstable [7]. Figure 4 shows the temperature changes of slider with different laser powers at different rotation speeds.

Cited from: B. X. Xu, et al.,JAP 99, 2006

Figure 4

            The recording disk use to record data is constructed by many layers. Instead of magnetic layer, the thermal effect on other layer for example lubricant layer, soft underlayer, and intermediate layer need to understand. These would seriously affect the recording disk robust and recording performance.

            Furthermore, the thermal gradient would cause the transition location changes along the track direction and cross-track direction. In addition, the thermal erasure on the neighboring tracks is another serious problem in HAMR. Dr. Xu Baosi has studied on the thermal profile in medium in order to study the heating effect on the medium [8]. Figure 5 shows the cross-track thermal profiles at 2000 rpm and 3000 rpm.

Cited from: B. X. Xu, et al., JMMM 320, pp. 731-735, 2008

Figure 5

1.5       Future Prospect

Combine with BPM

With current available material, HAMR alone is unable to achieve areal density above 10 Tb/in². Bit patterned media has predefined domains, as a medium it behaves as a single magnetic domain due to a strong coupling between the grains in a bit providing. Thus, BPM approach able to improve the thermal stability of the media. Current reports have shown that BPM able to achieve areal density to about 3 Tb/in² [9, 10]. Figure 6 shows the achievable areal density by BPM with considering difference thermal stability energy. Combination of HAMR with BPM is able to push areal density above 10 Tb/in².

Figure 6: Achievable areal density by BPM