Yeow Teck Toh

Thermal issues and their effects on heat-assisted magnetic recording system (invited)

Compared to the conventional magnetic recording, the big difference in the heat-assisted magnetic recording (HAMR) is the introduction of the thermal energy. It is necessary to study the thermal effects on the HAMR system which includes the head, the media, the head and disk interface (HDI) and data recording and reproducing performance. In this paper, the key thermal issues in the HAMR system and their effects on the recording performances are analyzed and discussed. In the HAMR head, the heat sources are analyzed, and their effects on the temperature increase of the HAMR head are studied. The thermal profile of the media generated by the near field transducer is a determinant for achieving high density. Based on the measured thermal parameters of the medium materials, the thermal profiles of the medium at different medium parameters are studied. The results indicate that the boundary thermal resistance and the insulation of the grains are very important factors for obtaining small thermal spot sizes. Th...

Relationship Between Near Field Optical Transducer Laser Absorption and Its Efficiency

Heat-assisted magnetic recording (HAMR) has shown its promising application for recording density beyond 1 Tb/in2 . However, the reliability/lifetime of the near field optical transducer is a big challenge. The temperature rise of the transducer is one of the reasons that lead to its failure. In this paper, the laser absorption of the transducer is investigated compared with the transducer efficiency (high efficiency is one of the main targets in the transducer design). The results show that with a fix medium and fix transducer material, high efficiency is always accompanied by high laser absorption which could be 10-25%. High efficiency and low absorption can be obtained by optimizing the medium properties and using transducer material with low refractive index. With the built HAMR head model, the transducer temperature rise is evaluated. The results show that the transducer temperature increase depends on the absorbed power, its size, and the space between the transducer and magnetic pole. The space affects the transducer temperature rise more seriously in large transducer size. Big size is preferable for the transducer to reduce its temperature increase.

Performance benefits from pulsed laser heating in heat assisted magnetic recording

Smaller cross track thermal spot size and larger down track thermal gradient are desired for increasing the density of heat assisted magnetic recording. Both parameters are affected significantly by the thermal energy accumulation and diffusion in the recording media. Pulsed laser heating is one of the ways to reduce the thermal diffusion. In this paper, we describe the benefits from the pulsed laser heating such as the dependences of the cross track thermal width, down track thermal gradient, the required laser pulse/average powers, and the transducer temperature rise on the laser pulse width at different media thermal properties. The results indicate that as the pulse width decreases, the thermal width decreases, the thermal gradient increases, the required pulse power increases and the average power decreases. For shorter pulse heating, the effects of the medium thermal properties on the thermal performances become weaker. This can greatly relax the required thermal properties of the media. The results...

HAMR media design in optical and thermal respects

Heat-assisted magnetic recording media is critical in high density recording. Except medias magnetic property, its thermal property plays a very important factor. In this paper, combining medias optical property, its thermal property is studied. Its design in optical and thermal respects is discussed.In a heat-assisted magnetic recording (HAMR) system, the thermal performances of recording medium are very critical for recording density because the track density and bit density are dominated by thermal spot size in the cross-track direction and temperature gradient in the down-track direction. The optical intensity and its distribution generated by the near field optical transducer serve as a heat source to heat the medium. Therefore, the medium optical response to the transducer is also very important for the mediums thermal response. In this paper, based on the structure required by the magnetic performance, the effects of the medium structures on its optical and thermal performances are studied. The results show that a thinner interlayer (MgO layer) is beneficial to the mediums performance. However, the seed layer (NiTa layer) deteriorates the performance seriously. Small in-plane thermal conductivity (good isolation) of the granular recording layer is also very important for the thermal performance of the medium. However, the out-plane thermal conductivity is not very critical in certain ranges.

Dynamic Thermal Responses of Heat-Assisted Magnetic Recording Head in Data Writing Process

In the data writing process of heat-assisted magnetic recording (HAMR), due to the existence of servo sectors in the magnetic recording media, the laser power has to be “off” when the near field optical transducer passes over the servo sectors so as to protect the servo sector from being erased. In this paper, HAMR head dynamic thermal responses to the laser source heating and transducer absorption in the data writing process are studied. The results show that the dynamic temperature response times of the laser source and air-bearing surface are around 1 s. The transducer temperature can almost follow the transducer heating change (caused by absorption) because of its very fast temperature response which is around 2 μs. However, the transducer protrusion response time to the transducer absorption is 2 ~ 3 ms. Owing to the long response times of the slider temperature to the laser source heating and the transducer protrusion to its laser absorption, the fly height changes caused by the slider temperature increase and the transducer protrusion may be compensated with the thermal fly height control technology.

Optical property study of FePt-C nanocomposite thin film for heat-assisted magnetic recording.

Optical properties of the FePt-C nanocomposite thin film that was synthesized by sputtering with MgO/NiTa underlayer on glass substrate have been determined by an approach combining spectroscopic ellipsometry and transmission over the wavelength range of 380 - 1700 nm. It was observed that the refractive index is larger than the extinction coefficient, indicating that free electron absorption is not the dominant optical transition in the FePt-C thin film. Compared with FePt thin film, the FePt-C thin film has smaller optical constants, which lead to better optical performance including smaller optical spot on recording media and higher transducer efficiency for heat assisted magnetic recording.

Efficiency Analysis of Near Field Optical Transducer Used in Heat-Assisted Magnetic Recording

For heat-assisted magnetic recording system, the efficiency of the optical energy delivery system is important for its application. It affects not only the requirement to laser source output power, but also the heat-assisted magnetic recording (HAMR) head performance and reliability. In this paper, the key factor caused the low efficiency of the near field optical transducer is studied. The results show that the main reason is the longitudinal polarization of the transducer radiation. Large permittivity difference between recording layer and air gap makes large electric field intensity difference within recording layer and air gap. Due to small electric field coupling from air to recording layer, large percentage of the incident power is reflected, absorbed or scattered by other components. The investigation results for power dissipation distributions show that the largest portion of power dissipation is reflection. Reusing the reflected power will be an effective way to improve whole optical system efficiency. Simulation results show that more than 30% increase of the efficiency can be obtained by reusing the reflected power.

Thermal Analysis of Heat-Assisted Magnetic Recording Optical Head with Laser Diode on Slider

For the optical head used in heat-assisted magnetic recording (HAMR), mounting a laser diode chip on the slider offers a more integrated, compact, and stable design. However, the heat generated by the laser diode will cause the head temperature to increase, which may decrease the laser output power and change the slider flying status. In this paper, the thermal analysis of the HAMR head including the laser diode and a transducer is conducted. The effects of the laser diode power, the power absorbed by the transducer, boundary thermal resistance between the laser diode chip and the slider substrate, and slider fly speed and fly height on the laser temperature increase, the transducer temperature increase, and the air-bearing surface temperature distribution are studied. The deformation of the air-bearing surface caused by its temperature change is also analyzed.

Analysis of Heat-Assisted Magnetic Recording to Density of 4 Tb/in

Thermal performance of media is a key factor limiting heat-assisted magnetic recording density. In this paper, the effects of near-field optical transducer tip size on the thermal profiles of media are studied, and the results show that even with a tip size of 10 nm, the obtained cross-track thermal spot size and down-track thermal gradient still cannot meet the requirements of 4 Tb/in2 for continuous-wave laser heating. Pulse laser heating can improve the thermal distribution significantly, and the requirements for 4 Tb/in2 can be met at a pulsewidth of 100 ps. Dynamic micromagnetic recording simulation with Landau-Lifshitz-Bloch equation is conducted for pulse laser heating recording. The results indicate that 4 Tb/in2 density is realizable for FePt recording media. It is also pointed out that, for short-pulse laser heating recording, media with large magnetic damping constant is important.

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Heat Assisted Magnetic Recording (HAMR) is a promising technology for next generation hard disk drives with significantly increased data recording capacities. In HAMR, an optical near-field transducer (NFT) is used to concentrate laser energy on a magnetic recording medium to fulfill the heat assist function. The key components of a NFT are transducer material, cladding material, and adhesion material between the cladding and the transducer materials. Since transducer materials and cladding materials have been widely reported, this paper focuses on the adhesion materials between the Au transducer and the Al2O3 cladding material. A comparative study for two kinds of adhesion material, Ta and Cr, has been conducted. We found that Ta provides better thermal stability to the whole transducer than Cr. This is because after thermal annealing, chromium forms oxide material at interfaces and chromium atoms diffuse remarkably into the Au layer and react with Au to form Au alloy. This study also provides insights o...