Heat-assisted Magnetic Recording Media Research Articles

FePt-C granular thin film for heat-assisted magnetic recording (HAMR) media

We studied a FePt-C granular film for ultra-high density perpendicular recording media towards 1 Tbits/in.2 because of strong magnetocrystalline anisotropy at its L10-phase. We deposit a Fe52Pt48-C50 % (6.7 nm) film on oxidized silicon substrates at 400 °C and 0.50 Pa Ar pressure. The perpendicular anisotropy of the film is 20 kOe, with a perfect squareness of 1. Bright-field transmission electron microscopy (TEM) images display that the FePt granular film has small and uniform grains of 6.4 ± 1.5 nm. Further work on high-resolution TEM imaging demonstrates excellent L10 ordering for this FePt granular film, which is consistent with the texture measurement by X-ray diffraction. Thus, we prove that FePt granular film is a promising candidate for high-density heat-assisted magnetic recording media.

A HAMR Media Technology Roadmap to an Areal Density of 4 Tb/in$^2$

This paper discusses heat-assisted magnetic recording (HAMR) media requirements and challenges for areal densities (AD) beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Based on recent roadmap discussions the focus is primarily on granular chemically ordered L10 FePtX-Y-perpendicular media with reduced average grain size down to 〈D〉 = 3-5 nm relative to current CoCrPt based perpendicular magnetic recording (PMR) media with average grain size 〈D〉 = 7-9 nm. In HAMR media the combination of thermal conductivity and Curie temperature TC determines the required laser power during recording. Key challenges are sigma variations of D and TC which need to be reduced to σD/D ~ 10-15% and σTC/TC ~ 2%. In addition AD is limited by switching field distribution (SFD) and thermal spot size. The key goal going forward is to optimize heads, media, head-media-spacing (HMS) and read-back channel technologies to extend AD to 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and beyond.

Laser-Heating-Induced Damage to Ultrathin Carbon Overcoat in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a technique for overcoming the superparamagnetic limit and enabling large increases in the storage density of hard disk drives. The performance of the disk carbon overcoat under the high temperature in the heating-assisted writing process is a concern. Laser heating in HAMR is quite different from conventional slow heating. Laser heating temperature and total laser heating duration over the lifetime of the drive are two dominant factors in the experimental study of laser-heating-induced damage to the carbon overcoat, which must be carefully controlled. In this study, a rough estimation of the total laser heating time for a given point on the media over the 5-year lifetime of the drive is given. It is expected to be only 0.1 ms. The methods of controlling laser heating temperature and total laser heating time in experimental studies are explained in detail. Laser-heating-induced damage to the a-C:Nx and a-C:Hx overcoats on HAMR media are studied. Surface topographical changes caused by the laser heating are evaluated with atomic force microscopy and structure changes by visible Raman spectroscopy. It is found that laser heating induces surface topographical and structure changes, especially for the a-C:Nx overcoat.

Experimental estimates of in-plane thermal conductivity in FePt-C granular thin film heat assisted magnetic recording media using a model layered system

Cross-plane thermal conductivity kth measurements of vertical stacks of FePt/C were used to estimate the in-plane thermal conductivity of Heat Assisted Magnetic Recording (HAMR) media that consist of columnar FePt grains segregated by thin C grain boundaries. FePt/C multilayers with varied repeat units and FePt layer thicknesses (chosen to represent HAMR media grain sizes) were measured using Frequency-Domain Thermoreflectance to determine kth in the direction normal to the layers. The data suggest that when FePt grains are less than 8 nm in diameter, the in-plane kth for HAMR media is below 1 W/m-K and the anisotropy of kth (cross-plane/in-plane) will exceed 10.

Experimental study of lubricant depletion in heat assisted magnetic recording: different lubricants on HAMR media

Heat assisted magnetic recording (HAMR) is a promising choice to surpass the super-paramagnetic limit in magnetic recording and to allow continued increase in the areal recording density of hard disk drive. However, lubricant depletion on disk surface might be a problem in HAMR due to high temperature during writing process. In this work, depletion of four types of commercial lubricants on our HAMR media has been evaluated under equivalent HAMR conditions. Linear relationships between the lubricant depletion depth and the logarithm of the equivalent laser heating time have been established for these lubricants. The depletion depths of the lubricants at different laser heating durations are predicated. It is found that the performance of the four evaluated lubricants is acceptable in respect of lubricant depletion depth by hypothesizing the total heating duration for a given point on the media over the lifetime of the drive is 106 ns.

L10 FePtX-Y media for heat-assisted magnetic recording (Phys. Status Solidi A 7∕2013)

Heat-assisted magnetic recording (HAMR) is a new technology that will increase the achievable areal density in magnetic hard disk drives. In order to write bits into HAMR media, a laser locally heats beyond the Curie temperature and rapid cooling in the presence of the head field produces transitions in the magnetic orientation. Chemically ordered FePt exhibiting high magnetic anisotropy, will be used for the new thermally stable recording media. As discussed in the Review Article by D. Weller et al. (pp. 1245–1260), HAMR media are optimized by growing granular L10 FePt on a relatively thin thermal resistor seed layer on top of a thicker thermal conductor or heatsink layer. In addition soft underlayers are suggested to optimize the magnitude and angle of the magnetic write field. The achievable areal density depends on the near-field transducer dimension and the thermal conductivity of the media. Besides grain size, shape and distribution requirements, the thermal properties of both the magnetic and the seed layers need to be improved and optimized to achieve the required signal-to-noise ratio for higher areal densities in future magnetic recording, which will be in the range of 1.5-5 Tb∕in2.

Power Absorption and Thermal Analysis of Head and Media for Heat-Assisted Magnetic Recording

A method from near-field optics to thermal analysis is developed for the heat-assisted magnetic recording (HAMR) head and media. The distributions of power absorptions in the system of the HAMR head and media are simulated with finite-difference time-domain (FDTD), which is coupled with transiently thermal conduction equation for thermal analysis. The study focuses on the influence of the writer pole on the efficiency of transducer and thermal performance of the HAMR head and media. The results show that the efficiency of transducer largely reduces from 3.36% to 0.55% due to the power absorption of the writer poles. However, the writer pole plays a role of thermal sink bar so that the temperature of transducer also reduces largely. The distance between the transducer and writer pole strongly affects the efficiency of transducer, temperatures of media, transducer and writer pole. It is found that without the thermal sink role of the writer pole, the temperature of transducer is very high when the media temperature is required to reach the Curie temperature of FePt media. Two shapes of the writer poles including rectangle and step-down shapes have been used for the efficiency and thermal analysis. Their size effects on efficiency and temperatures are also studied.

Thermoreflection Measurement of Magnetic Thin Films

With recent developments in heat assisted magnetic recording (HAMR), there is growing intense interest in understanding the thermal-optical properties of the HAMR media. We study the phase change of the optical beam as it reflects from a FePt thin film under strong laser irradiation by carrying out close-aperture reflection Z-scan experiments. It is found that the far field spatial distribution of the reflected beam is sensitively dependent on the thermal field distribution in the media. We also carry out a temperature dependent pump-probe experiment to study the variation of the differential thermoreflection signal with different media temperature. It is found that the differential signal is sensitively dependent on the media temperature. The present studies show potential to enable noncontact temperature measurement of HAMR media and detect the spatial distribution of the thermal field in the HAMR media.

HAMR Media Design in Optical and Thermal Aspects

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 medium's 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 medium's 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.

HAMR Thermal Modeling Including Media Hot Spot

All previous thermal modeling on heat exchange between hard disk drive (HDD) head and media treated media as a heat sink with uniform ambient temperature. However in heat-assisted magnetic recording (HAMR) system, the media temperature is no longer uniform and the temperature at the hot spot center can reach as high as 800 to 1000 K depending on recording layer's Curie temperature. In this paper, both media hot spot and the air bearing flow are included in formulating heat transfer across the head-media interface. Numerical results for a waveguide only slider and a slider with a metallic near-field transducer (NFT) are presented to illustrate the effect of HAMR media temperature.

In-Line Sputter System Prepared $L1_{0}$ Ordered FePt Granular Film for HAMR Application

The FePt-based granular films for heat-assisted magnetic recording (HAMR) application have been developed by using the in-line sputter system. The microstructure development of the FePt-based HAMR media as a function of different doping elements and film thicknesses was investigated. The microstructure of the FePtC-Ag granular film showed a nonuniform grain size distribution. FePt-C granular films with small and uniform grains were obtained at different film thicknesses. It was found that the grain size slightly increased with the increasing film thickness. The well-isolated FePt grains with main grain size of 6.4 nm ± 0.6 nm and 6.86 ± 0.8 nm were obtained for 6 and 10 nm FePt-C granular films, respectively. The developed FePt-C granular film with well-isolated FePt grains has the potential to be used as the recording media for HAMR application.

Investigation of Heat-Assisted Magnetic Recording Media Films in Four Dimensions

Heat-assisted magnetic recording is one of the most promising approaches to achieve extremely high density magnetic recording. A laser is used to temporarily heat the recording media film to render its switching field to become lower than the magnetic field of the writing head. After writing, the film material is cooled down to room temperature for data storage. In this dynamic writing process, the challenge is to tailor and optimize the time resolved structural/thermal/magnetic properties of the media films and their correlations during/after the laser heating. In this paper, the four dimensions of current progress in HAMR media research, i.e., structure, magnetic properties, thermal properties and time, are briefly overviewed and discussed. Their correlations are also emphasized.

Microstructure Control of L10-Ordered FePt Granular Film for Heat-Assisted Magnetic Recording (HAMR) Media

L10-ordered FePt nanogranular film is one of the promising candidates for the next-generation high areal density media for heat-assisted magnetic recording (HAMR). To realize a suitable microstructure for the HAMR media, the granular structure composed of uniformly dispersed L10-FePt nanoparticles with various segregants has been optimized using various combinations of segregants and seed layer materials. Although the microstructure of FePt-C came so close to the ideal one for HAMR, it would not be the final answer as the growth of the columnar grain structure is not possible due to too strong phase separation between FePt and C. In this article, we review recent investigations aiming at achieving the ideal microstructure for HAMR media.

L10 FePtX-Y media for heat-assisted magnetic recording

Highly chemically ordered L10 FePtX–Y nano-granular films with high perpendicular magnetic anisotropy are key media approaches for future heat-assisted magnetic recording (HAMR). They are sputtered at elevated temperature on glass disks coated with adhesion, heat sink, and texturing layers. Adding X = Ag reduces the required deposition temperature and X = Cu lowers the Curie temperature. Current seed layers are NiTa for adhesion and heat sink and well-oriented MgO (002) layers for highly textured FePtX(002) grains surrounded by Y = carbon and/or other segregants. Magnetic anisotropies larger than 4.5 × 107 erg cm−3 and coercivities beyond 5 Tesla have been achieved. The combination of thermal conductivity and Curie temperature determines the required laser power during recording. Key goals are to optimize media, heads, head-disk-spacing, and read-back channels to extend the areal density to 1.5–5 Tb in−2. Head and media in heat-assisted magnetic recording1. LD, laser diode; TFC, thermal fluctuation control; NFT, near field transducer. 1Lidu Huang et al., “HAMR Thermal Modeling Including Media Hot Spot”, APMRC 2012.

Multiple oxide content media for columnar grain growth in L1 FePt thin films

An approach to enhance the height-to-diameter ratio of FePt grains in heat-assisted magnetic recording media is proposed. The FePt-SiOx thin films are deposited with a decrease of the SiOx percentage along the film growth direction. When bi-layer and tri-layer media are sputtered at 410 °C, we observe discontinuities in the FePt grains at interfaces between layers, which lead to poor epitaxial growth. Due to increased atomic diffusion, the bi-layer media sputtered at 450 °C is shown to (1) grow into continuous columnar grains with similar size as single-layer media but much higher aspect ratio, (2) have better L10 ordering and larger coercivity.

Application of the grain flipping probability model to heat assisted magnetic recording

Heat assisted magnetic recording (HAMR) is often lauded as one of the key technologies poised to replace conventional granular magnetic recording (CGMR). Conventional recording is expected to eventually fail because as the information-bearing grains continue to shrink, they become thermally unstable and will spontaneously flip due to excitations from the ambient temperature. HAMR grains are smaller and have larger anisotropies making them thermally stable, but unwritable at room temperature. Heat from a laser is applied to assist during the writing. The grain flipping probability (GFP) model has been proposed to model and predict the densities achievable in conventional recording systems. In this work we modify the GFP to include a circular hot-spot of a laser as a 2 D Gaussian and predict the expected densities that might be achieved on HAMR media with 4 nm grains. In this work we examine the effect of varying the hot-spot diameter, the hot-spot peak, and the alignment of the hot-spot to the magnetic footprint profile.

Opto-Thermal Analysis of Novel Heat Assisted Magnetic Recording Media Based on Surface Plasmon Enhancement

Novel media structure based on metal/dielectric layer for the surface plasmon enhancement was designed and analyzed opto-thermally to understand the effectiveness of local heating in the magnetic media for heat assisted magnetic recording (HAMR) application. Near-field optical and thermal characteristics of this surface plasmon assisted HAMR (SPAH) media were studied with a nano-slit aperture of grating HAMR head using finite differential time domain (FDTD) method and finite volume method (FVM). From FDTD simulation results, it was confirmed that the near-field transmitted light from nano-slit aperture of HAMR head is coupled strongly with the metal/dielectric interface of SPAH media to excite the surface plasmon waves. To compare the generated heat in SPAH media with conventional continuous media, FDTD data were converted to the poynting vector to calculate thermal distribution in both media. The transmitted power of 45 mW from the head resulted in the temperature of 575 K in the recording layer with 2 ns duration time at the SPAH media while same condition showed 429 K at the continuous media. Since thermal efficiency of SPAH media is around 2 times better than that of continuous media, it is expected that the surface plasmon assisted HAMR media can be applied to the future high density magnetic recording.

Magnetic properties and microstructure of TbCo/(SiN x/Co) n films

The Tb 32Co 68/(SiN x /Co) n films ( n = 0 – 3) were prepared by magnetron sputtering. The magnetic anisotropy of all Tb 32Co 68/(SiN x /Co) n films are perpendicular to the film plane. It is found that the saturation magnetization ( M s) and perpendicular coercivity ( H c⊥) of the Tb 32Co 68/(SiN x /Co) 3 film are 263 emu/cm 3 and 3592 Oe, respectively. This film appears to be a promising material as a heat-assisted magnetic recording (HAMR) medium. The cross-sectional high resolution transmission electron microscope (HRTEM) images show that the interface roughness between the (SiN x /Co) n layers and TbCo layer increases as n is increased. The rough surface provides more obstacles and pinning sites that hinder the motion of the domain walls at interface between the (SiN x /Co) n layers and TbCo layer. Therefore, the H c⊥ values are profoundly influenced by the interface roughness.

Effect of gradient alignment in heat assisted magnetic recording

Heat assisted magnetic recording (HAMR) is one of the leading technologies to extend magnetic storage. Significant progress has been achieved in head and media fabrication [M. Seigler et al., IEEE Trans. Magn. 44, 119 (2008); Y. Peng et al., TMRC, Seagate Research, 2008], resulting in a basic technology demonstration (C. Hardie et al., ODS Conference Proceedings, 2008) of HAMR. Both field and field-gradient limitations of a conventional perpendicular recording are overcome by engineering the thermal profile (notably the gradient) and recording at a temperature near Tc (thus requiring a smaller head field). We have used a micromagnetic recording model to study the effect of thermal and field-gradient alignment in HAMR by varying the separation between the thermal spot and the leading edge of the head field. The output of the recording model includes transition jitter, which is based on Monte Carlo simulations of isolated transitions. We use a realistic granular medium with HK∼50–80 kOe and a grain size of ∼2.5–6 nm that covers a broad range of HAMR media parameters. The model indicates that HAMR can achieve &amp;gt;1 Tbyte/in.2 using a grain size of 6 nm and is scalable to &amp;gt;4 Tbyte/in.2 on a granular media by careful alignment of the thermal and magnetic field gradients.

FePt Magnetic Nanoparticles and Their Assembly for Future Magnetic Media

Magnetic nanoparticles with extremely high anisotropy such as chemically ordered L10 phase FePt nanoparticles have been considered one of the best candidates for future magnetic recording media with areal density beyond 1 Tbit/in2 for either the nanocomposite-film-type heat-assisted magnetic recording (HAMR) media or the self-organized-magnetic-array (SOMA)-type bit patterned media. However, current preparation methods via phase transformation must overcome many obstacles, including particle agglomeration, twinning, and difficulty of easy axis alignment. In this paper, the effort on and promise of the preparation of monodispersed magnetic nanoparticles with high anisotropy by a gas phase condensation method is reviewed and reported. The focus is to review recent progress on the fabrication of monodispersed highly ordered L10 phase FePt nanoparticles without phase transformation, successfully self-assembled and magnetically aligned magnetic nanoparticles for SOMA and HAMR media. The mechanisms for directly forming L10 phase FePt nanoparticles during nucleation and growth processes and magnetically aligning these particles are analyzed.