Heat Assisted Magnetic Recording Engineering Essay

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Heat-assisted magnetic recording is a magneto-optical recording technique to extend areal densities greater 1 Tb/in2. HAMR is a concept of heating a recording medium to reduce the coercivity so that the applied magnetic writing field can easily direct the magnetization of the medium during the temporary magnetic

oftening of the medium caused by the heat source . HAMR allows for the use of small grain media , which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure thermal stability.

By heating the medium, the Ku, or the Hc is reduced such that the magnetic write field is sufficient to write to the medium .Once the medium cools to ambient temperature , the medium has a high value of coercivity to assure thermal stability of the recorded information . Although HAMR permits writing on high Hc media with lower magnetic fields and can produce higher write gradients than conventional magnetic recording, Head/media spacing and the development of high Hc media with small grains remains challenging.HAMR can be applied to any type of magnetic storage media , including perpendicular media and patterned media.In this article , the latest developments of HAMR technology is unveiled.

1.(b)Latest status of Heat-Assisted Magnetic Recording (HAMR):

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The development of the read/write head and the recording media plays a vital role in increasing the storage capacity of magnetic data storage devices . HAMR requires the development of novel components. These include the light delivery system, the thermo-magnetic writer, a robust head disk interface, and rapid cooling media. Designing these components into a high-performance data storage system requires system level optimization. Some of the recent developments of HAMR read/write head and recording media are explored in this section.

1.1 Read/Write Head Developments :

a) LASER Diode :

Diode lasers have been the enabling technology for optical data storage . They are an inexpensive source of high-power coherent light. The original optical data storage systems used diode lasers operating at a wavelength of 830 nm. It is advantageous to reduce the wavelength in order to obtain smaller focused spots . The laser wavelength for CD players is 780 nm, and for DVD players is 635 or 650 nm. In DVD

Blu-ray recorders, the wavelength is 405 nm. This corresponds to a focused spot size of 240 nm, which is in fact about the smallest focused spot size allowed by the diffraction limit for visible light when focusing in air. Laser diodes are currently available at wavelengths as short as 375 nm for optical recording heads.

b) Light Condensers:

It is advantageous to focus the light through a medium with the highest possible index to obtain thesmallest focused spot. Liquid Immersion Oil (LIO) is often used between the objective lens and the sample for high-resolution optical microscopy as show in Fig.(2).A Solid immersion lens (SIL) is a lens in which the light is brought to a focus at the bottom surface of a hemispherical lens as shown in Fig.(3). The diffraction limit for the optical spot size is determined by the refractive index of the solid lens. Although the light is brought to a focus within the high index medium of the SIL, the optical energy in the focused spot can be coupled easily into a recording medium that is situated within the near field of the optical spot . For efficient and lossless coupling without too much spreading of the optical spot size, the distance between the medium and the SIL must be less than about half the diameter of the spot.

A variation on the SIL is the solid immersion mirror (SIM) shown in Fig.(4). Both the SIL and SIM can be designed for a planar waveguide geometry to be more easily integrated into the manufacturing processes of hard disk drive recording heads . Although the angular distribution of the light within an SIM is somewhat different than that of an SIL, an SIM also provides a high index solid medium in which to bring the light to a focus.

At a wavelength of 405 nm, the planar Surface Immersion Mirror has achieved a spot size of 90 nm . Unfortunately, the optical spot sizes that are obtainable by focusing light from the currently available diode lasers through SILs or SIMs are much larger than the 25 nm spot size necessary for > 1Tb / in2 storage densities in HAMR . Even if shorter wavelength lasers were to become available, it is unlikely that materials with sufficiently large refractive indexes that are transparent at those wavelengths could be found for a solid immersion-based optical system for HAMR.

c) Near-Field Optics:

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The HAMR recording medium must be located within a few nanometers of the recording head for the magnetic reader to have sufficient resolution at Tb / in2 storage densities. As a result, it is both natural and necessary to employ near field optics in a HAMR storage system to achieve optical spot sizes much smaller than the diffraction limit. Near field optics make use of apertures or antennas, or some combination there of, to overcome the diffraction limit.

Although it is relatively simple to confine light to spots much smaller than the diffraction limit, the primary difficulty in HAMR is to deliver a significant fraction of the incident light power within this small spot to the sample.

In order to enhance the coupling efficiency of the NFT to the recording medium, a variety of other aperture shapes have been used. Some of these apertures which are widely used is shown in

d) Light Coupling System:

So far, we have discussed about the laser diode light source, the condenser optics (SIMs, SILs) andthe NFOs. These parts should be efficiently coupled for a complete optical system. There are many methods for coupling the light source .The simplest approach is to place the laser directly on the recording head and the NFT directly on the output facet of the laser, as shown in Fig. 6(a). Such devices have been termed very small aperture lasers (VSALs). One advantage of VSALs is that the light that does not get emitted through the aperture is generally reflected back into the laser cavity and thereby contributes to additional stimulated emission. Nanoantennas have also been recently integrated onto the output facet of diode lasers . A diode laser on a recording head can also be coupled to the NFT by means of a planar SIL or SIM. The laser light may be coupled to the recording head via an optical fiber or free space. If the recording head incorporates an objective lens and a SIL, then a small mirror may be located above the objective to reflect laser light propagating down the suspension directly onto the lens, as shown in Fig. 6(b).

A three-dimensional SIM as shown in Fig. 6(c) has also been which can be used as the basis for a slider in a near field recording system . Finally, the laser light may be coupled into a planar waveguide by means of a diffraction grating , as shown in Fig. 6(d). Grating coupling efficiencies as large as 55% are theoretically possible for relatively small gratings that can fit easily on the end face of a slider. All of these approaches can be designed to use an optical fiber to carry the light from the laser to the recording head.

  1. The laser is mounted directly on slider.
  2. The light is focused directly on the lens and SIL in the slider.
  3. A transparent slider incorporates a SIM.
  4. A grating, prism, or other device couples light into a planar waveguide.

e) Integrated Recording Head:

The challenge of building an HAMR head is the integration of the light delivery and focusing optics with the magnetic field delivery system.Requirements of Integrated HAMR head:

  1. must be able to fly at low head-to-media spacing (HMS) ( <10 nm)
  2. able to deliver a high field (5-10 koe) at the position of the heat spot
  3. must be small and reliable
  4. read back the written signal as accurate as possible
  5. should be mass producible at low cost.

Some of the design considerations for an effective head are spot size , reliability , cost , and power mefficiency. Cost and power considerations suggest that diode lasers of less than 150 mW be used. In designs of 500 Gb/in2, near field focusing optics can be used without NFTs. The optics must be having short wavelength to approach 500 Gb/in2, and 473, 445, or 405 nm blue laser are possible options. For areal densities above 500 Gb/in2, the diffraction limits of visible light prevent the achievement of the small spot sizes required. In order to achieve densities approaching Tb/in2, spot sizes of 50 nm are needed. Seagate Technology has produced a fully integrated HAMR head with optical and magnetic field delivery and an integral reader shown .

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1.2 Recording Media Development:

HAMR media require both appropriate magnetic properties and appropriate thermal properties. Due to the utilization of thermal assistance during the recording process, an integrated HAMR medium capable of supporting high density requires not only the fabrication of thermally stable media with high anisotropy and small grain size but also proper thermal design for heat confinement and management in the media. Materials for HAMR media are chosen (or designed) to have the highest possible magnetic anisotropy Ku and largest temperature dependence of switching field.

Requirements of recording media:

  1. must have high anisotropy (Ku)
  2. small grains( 5 nm) and well isolated grains
  3. moderate Curie temperatures.( CT)

SmCo5 , FePt, and Fe14 Nd 2B have bulk Ku values around 200,70, and 46 Merg/cm3 respectively and well suited for higher density recording using HAMR. Rare earth-Co based materials have also made good progress recently, with high perpendicular anisotropy being achieved in thin films of SmCo5 deposited on Cu/Ti and Cu/Pt dual under layers.

1.(c) Key Issues to be resolved :

Introduction of HAMR in ultra-high density hard disk drives (HDD) presents many technical challenges. Perhaps,the greatest of these is the imparting of localized heating at the recording site along with a high-resolution magnetic field. The most likely physical means of achieving such heating are through optical or electron beam techniques. The key issues which has to be solved for HAMR are listed below.

HAMR Components

Key Issues

Recording Head

1) Integrated magneto-optic head design

2)Near field apertures,antennas design

3) Waveguides design for narrower spot

4) Source of LASER

5) Diffraction limit

6) Air Gap, Spot size

7) Reliability, Cost & power efficiency

Recording Media

1) Higher anisotropic constant material

2)Smaller and stable grains

3)Higher Coercivity material

4) Lubrication layer

5) Thermal Stability

Table 1. HAMR issues

1.(d) Conclusion

So far we have seen the latest developments of HAMR and the issues which has to be solved . The perpendicular recording will be limited to about 1 Tb/in2 because the write field that may be produced by a magnetic write head is limited to about 2.5 T by the maximum saturation flux density of known magnetic materials. This limitation makes it impossible to scale the head field with the media coercivity, which is required if the linear density is to be increased while maintaining thermal stability in the media. HAMR utilizes thermal energy produced by a laser incorporated into the write head to overcome this limitation. Heat generated by absorption of the laser light in the medium reduces the anisotropy of the medium during the write process, making it possible to record with available head fields. Moreover, the effective head field gradient, which determines the width and precision of the written bit , is considerably higher with HAMR than can be achieved with a magnetic head, alone. This high effective head field gradient results in both better defined written transitions and narrower and more well-defined track widths. Although HAMR involves thermal processes, which are often expected to be slow, proper design of the media with a heat sink, facilitates heating and cooling speeds much less than 1 ns, potentially enabling data rates in the Ghz range. HAMR thus appears promising as a technology to replace perpendicular recording. However, considerable work had to be done to push the technological frontiers . The need for a HAMR write head to provide both magnetic fields and optical energy to the media places new requirements on the write head design. Overcoming these challenges will require a dedicated effort, if HAMR is combined with bit-per-grain recording such as might be achieved with bit patterned media, HAMR could enable densities two orders of magnitude higher than in current products. At the present time, nothing indicates that the physics of HAMR imposes a barrier to attainment of an AD of 100 Tb/in2 or beyond. Certainly, employment of BPM and MAMR with HAMR enables an extension of storage technologies, reaching to 100 Tb/in2 based on the thermal stability of known magnetic materials .

Section 2. Difference between Heat Assisted Magnetic Recording and Microwave Assisted Magnetic Recording

2.(a) Abstract

HAMR is a promising approach for enabling large increase in the storage density of hard disk drives.A laser beam is used to heat the recording area of the medium momentarily to reduce its coercivity below that of the applied magnetic field from the recording head. In HAMR, the recording media have a very high magnetic anisotropy, which is essential for the high thermal stability of the magnetization of the extremely small grains in the medium. By temporarily heating the media during the recording process, the media coercivity can be lowered below the available applied magnetic write field, allowing higher media anisotropy and therefore smaller thermally stable grains. The heated region is then rapidly cooled in the presence of the applied head field whose orientation encodes the recorded data. With a tightly focused laser beam heating the media, the write process is similar to magneto optical recording, but in a HAMR system the readout is performed with a magneto resistive element. MAMR is an energy assisted magnetic recording technology which utilizes the effect of ferromagnetic resonance in the media. It is a mechanism by which bit recording is done at a head field significantly below the medium coercivity in a perpendicular recording geometry. By applying a localized ac field at adequate frequency to the perpendicular recording medium, saturation recording can be achieved with recording field amplitudes significantly below the medium coercivity, or the medium perpendicular anisotropy field.

Utilizing the effect of spin momentum transfer, a localized ac field at microwave frequencies is generated to assist recording in conventional perpendicular recording geometry. icrowave assisted recording scheme results in excellent recording performance at high linear densities for media with coercivity nearly three times the recording field magnitude. In this context, the differences between HAMR and MAMR are analyzed in brief.

2.(b) Heat Assisted Magnetic Recording (HAMR)

In HAMR , each recording bit is stored on a high anisotropy field medium by temporarily heating a nanometer sized region, reducing its coercive force, and rapidly cooling it in the applied head field whose direction encodes the recorded data. Because of the utilization of thermal assistance during the write process, a HAMR medium requires not only appropriate magnetic properties but also proper thermal design for heat confinement and management. The HAMR write process is similar to magneto-optical recording, but the reading is performed with a magnetic element. Achievable areal density will be dependent on Curie temperature, where reaches zero, but not . Since is usually more consistent than in media, HAMR media could have tighter switching field distribution and more potential for higher density recording.

The core component of HAMR is a recording head that integrates optics, including an efficient light delivery system from a laser diode to a near field optical transducer used to produce a heated region much smaller than the diffraction limit, with a magnetic head as shown in above Fig.2(a) HAMR technology also involves new recording physics, new media materials and lubricants which can withstand fairly high temperatures. The presence of heat in HAMR also results in different physical channel behaviors.

Modulating the laser power, for example, can provide an additional parameter which can be utilized to perform write precompensation and further optimize the overall system performance. It is likely that a fully optimized near field optical system will convey around a few % of the energy emitted by the low-cost laser diode into about 20 ` recording spot.

With such high temperatures required for HAMR, the lubricant and the carbon overcoat could degrade leading to poor performance. Other high temperature effects such as thermal pole tip protrusion as well as transient elastic thermal distortion of the media surface could further exacerbate the head-media interface stability. In HAMR, the media must be stable at much smaller grain sizes yet be writable at suitably elevated temperatures. The media can appropriately be lowered below the available write field limited by the maximum of existing magnetic materials, allowing adoption of higher anisotropy materials with smaller and better isolated grains of around sub-5 nm to achieve thermally stable and high media. Moreover, the effective write field gradient can considerably be enhanced in HAMR, if the thermal spot and the head field position are appropriately aligned. Thus, HAMR offers not only a new degree of freedom in material design but also better defined written transitions with system-level optimization to increase the areal density . Employment of HAMR with BPM enables an extension of both technologies , with projections on areal density reaching to about 100 Tb/in based on the thermal stability of known magnetic materials .

2.(c )Microwave Assisted Magnetic Recording (MAMR)

Several kinds of technology have been proposed to solve the issues between the signal-to-noise ratio (SNR), the thermal stability of a magnetic recording medium, and the writability of a writer. Heat-assisted magnetic recording (HAMR) is considered to be promising candidate to realize ultrahigh recording density over 2 Tb/in2 .However, they depart drastically from the conventional fabrication technology for writing heads and recording media. Microwave-assisted magnetic (MAMR) utilizes the ferromagnetic resonance phenomenon to dramatically reduce the magnetic switching field in the writing process . In MAMR,each recording bit is stored on a very high anisotropy field medium with a significantly lower head field than the coercive force. Here, the write field is applied along the easy magnetization axis of media, and the microwave AC field, is applied along the orthogonal direction, which activates the precessional motion of media magnetization through the ferromagnetic resonance ffect as shown in Fig 2.(b). The excitation of the precessional motion by of the order of kOe amplitude and a few tens of GHz frequency can assist magnetization reversal in the media. Anticlockwise and clockwise polarized microwave fields assist the magnetization to switch and to re-switch, respectively.

2.(b) Microwave Assisted Magnetic Recording

In MAMR, the assisted energy is provided directly to the spins as opposed to HAMR in which the energy is delivered to the atomic lattice. The core component of MAMR technology is a spin torque driven microwave oscillator , which would be easily manufactured if developed. Efficiency of the field generation layer (FGL) is also a challenge. A microwave assisted magnetic recording system includes a write pole that generates a write magnetic field, an element that generates a radio frequency to assist magnetic field, and a recording medium that moves relative to the write pole. The recording medium is exposed to the radio frequency assisted magnetic field before it is exposed to the write magnetic field.

The ac field generator drawing at the top is rotated 90o with respect to the drawing below. The schematic view of the MAMR head is shown in Fig.2(c). This technology enables the use of recording medium materials that have relatively large magnetic anisotropy constants, thus solving the superparamagnetism problem. MAMR is a potential candidate for the future technology. Researchers demonstrated that strong, stable microwave magnetic fields can be generated by injecting a polarized spin current into a soft-magnetic layer . However, many physical aspects need to be clarified before it can become a practical technology. In particular, it is essential to investigate the design of the writing head that mounts the microwave-generating device.

2.(d) Challenges:

Both HAMR and MAMR has got challenges which will revolutionize the magnetic data storage industry to the greater extent if solutions are found.

Key Components

HAMR

MAMR

Recording Head

Assistance

Near Field

Microwave

Nn Recording Media

Writability

Complex Process

Features

Low Tc

Rapid Cooling

Low

Damping Constant

Servo Mechanism

TMR/Adaptive

Extension

Air Gap (Spacing)

Protrusion

Extension

Read/Write

High Bits / inch

Dual Gradient

High Tracks / inch

Thermal Density

Field Generation Layer Width

Signal Processing Technique

Dimension (1 D)

Architecture

Optimization using

Assisted Methodology

Technology

High Temperature Reliability

Microwave Oscillator

Cost

Optics

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2.(e) Conclusion:

To achieve more than 10Tb/ in2 using Microwave Assisted Magnetic Recording, the combination of Bit Patterned Magnetic Recording with Heat Assisted Magnetic Recording can be utilized. The MAMR component and its manufacturing process are essentially the same as in conventional PMR, and almost no specific components and additional cost will be required.

Section 3: Acknowledgement and References

3.(a) Acknowledgement

I would like to thank Dr. Yihong Wu and Dr. Hyunsoo Yang, Department of ECE ,National University of Singapore for their continuous support and guidance. My sincere thanks to all esearchers who have contributed to this topic.

3.(b) References

[1].Future Options for HDD Storage Y.Shiroishi, K.Fukuda, I.Tagawa

[2].Microwave Assisted Magnetic Recording Jian-Gang Zhu Xiaochun Zhu , and Yuhui Tang

[3]. Investigation on Magnetic Fields From Field-Generating Layer in MAMR Kazuetsu Yoshida, Eisei Uda, Natsuumi Udagawa , and Yasushi Kanai

[4]. Extensions of perpendicular recording O. Heinonen , K.Z. Gao

[5]. The road to HAMR W. A. Challener, C. Peng, A. V. ltagi, D. Karns, Y. Peng, X. Yang, X. Zhu, N. J. Gokemeijer, Y.T.Hsia, G. Ju, R. E. Rottmayer, M. A. Seigler, and E. C. Gage

[6].Heat Assisted Magnetic Recording Mark H. Kryder, Edward C. Gage, Terry W. McDaniel, William A. Challener,Robert E. Rottmayer, Ganping Ju, Yiao-Tee Hsia, and M. Fatih Erden

[7]. Heat-Assisted Magnetic Recording Robert E. Rottmayer, Sharat Batra, Dorothea Buechel, William A. Challener

[8]. Prospects for Magnetic Recording over the next 10 years.R.W.Wood and H. Takano

[9]. Future Trends in Magnetic Storage Technology Mark H. Kryder

[10].Working Today on Tomorrow's Storage Technology George Lawton

[11].Numerical Investigation of Thermal Problems in HAMR Q.D.Zhiang, B.X Xu, J.Zhang

[12].Perspective of Magnetic Recording System at 10 Tb/in2 Zhi-Min Yuan, Bo Liu, Tiejun Zhou

[13]. Thermal Assisted Magnetic Recording Koji Matsumoto, Akihiro Inomata,Shin Ya Hasegawa

[14].Fabrication of a Solid Immersion Mirror and Its Optical Evaluation Takeshi Mizuno, Takatoshi Yamada, Hiroyuki Sakakibara

[15]. Issues in Heat-Assisted Perpendicular Recording Terrv W. McDaniell, William A. Challene, and Kursat Sendu

Notes Reference:

[1]. Lecture Notes by Dr Yihong Wu, Dr. Hyunsoo Yang ,Dept. of ECE , National University of Singapore