FePt Nanogranular Films for High Density Heat-assisted Magnetic Recording

Introduction

A hard disk drive (HDD) is a data storage device that stores digital information by magnetizing nanosized magnets on flat disks and retrieves data by sensing the resulting magnetic field. HDDs have been the predominant storage device in computers for the past 50 years. Although solid-state devices (SSD) are now meeting an increasing portion of the storage needs of portable computers, HDD technology are expected to continue to play a major role in handling the ever increasing digital data necessary for cloud computing. The number of HDDs sold in 2012 was approximately 800 million, nearly one tenth of the world′s population!

A HDD is comprised of one or more rigid disks with magnetic heads supported on actuator arms to write and read digital information. The disk rotates at a very fast rate (~7,000 rpm) with a small magnetoresistive head scanning the disk within a distance of a few nanometers. The unit of areal density of recording is the number of bits per square inch (bpsi) and that of a current HDD is ~700 Gbpsi. The higher the areal density, the more data stored in the same volume. In the past, the density of HDDs increased by 100% every year, but recent growth rates have slowed to 25%. Although various technological improvements have been made in both recording heads and media in the current perpendicular magnetic recording (PMR) system, ~1 Tbpsi is considered to be the limit for the PMR method. Thus, a transition to a new magnetic recording method must be made in the near future to continue the trend toward 4 Tbpsi or higher.

New Technologies

Heat assisted magnetic recording (HAMR) is a promising technology for achieving this goal. However, for this method to be successful, new media using a high magnetocrystalline anisotropy material must be developed. The current recording media are CoCrPt-SiO₂ nanogranular films, in which nanosized CoCrPt alloy particles with hcp structure are dispersed uniformly with a strong preferred orientation of [0001] as shown in Figure 1.

Figure 2. Ideal media structure for HAMR. Red and blue colors show the direction of magnetic poles, and their boundary is the magnetic domains that separate the neighboring bits. The L1₀-FePt (the upper right figure) must grow in columns separated by the nonferromagentic "segregant." The column diameter and height ratio D/h must be higher than 1.5, and a diameter of less than 4 nm is desired.

To obtain a sufficient signal-to-noise ratio for a bit density of 1 Tbpsi, more than 10 ferromagnetic particles must be contained in an area of ~25 × 25 nm. Therefore, the ferromagnetic particles must be refined further to ~4 nm to achieve an areal density above 1 Tbpsi. This requirement makes the ferromagnetic particles thermally unstable as the magnetocrystalline energy K_uV becomes comparable to the thermal energy k_BT, where K_u is the magnetocrystalline constant, V is the volume of the particle, and k_B is the Boltzman constant. In order to store recorded information for longer than 10 years, the value of K_uV/k_BT must be larger than 60. This means that a ferromagnetic material with high magnetocrystalline anisotropy must be used for high density recording as V becomes very small. While the K_u of CoCrPt alloys is ~0.3 MJ/m³, the L1₀-ordered FePt has a one order of magnitude larger magnetocrystalline anisotropy of 6.6 MJ/m³, which makes the minimum size of stable ferromagnetic particles ~4 nm for spheres and 2.4 nm for cylinders.¹ However, as the size of the hard magnetic particles is reduced, the magnetic field required to magnetize the particle increases enormously. The typical switching field for the current recording media is about 0.8 T, while the magnetic field to switch the magnetization of the nanosized hard magnet will be greater than 3 T. However, the highest magnetic field that can be generated using a write head (a small electromagnet) is limited to about 1.5 T. This means the nanosized particles of hard magnetic materials are not writable. This dilemma is known as the "trilemma" of magnetic recording: ultrahigh density magnetic recording requires nanosized ferromagnetic particles, but this requirement makes them thermally unstable. To overcome this problem, a high K_u material must be used; however, switching the field of the particles causes them to be become too large to write. Thus, the magnetization switching requires energy assistance. Heat-assisted magnetic recording (HAMR) uses heat to achieve the magnetization switching of high K_u particles with thermal energy using a well-focused laser beam.² At elevated temperature, writing can be accomplished using the magnetic field that can be generated with a write head. When temperature decreases after writing, the high K_u of the nanosized particle has sufficient thermal stability for permanent recording.

New technologies are necessary in both the head and medium to realize HAMR. The HAMR head has been previously demonstrated using a plasmonic antenna.³ However, a suitable magnetic recording medium to demonstrate the feasibility of high-density HAMR recording has not been successfully produced until recently. The desirable HAMR media should be made of densely dispersed ferromagnetic particles of high magnetocrystalline anisotropy with a uniform particle size of 4-6 nm with columnar grains of the aspect ratio D/h>1.5 as shown in Figure 2. Among various hard magnetic materials, L10-ordered FePt has been considered to be the most promising since it has excellent corrosion resistance. In addition, the film must exhibit high coercivity above 3 T, more than four times larger than that of the current recording media.

Technological Challenges of Deposition

The most critical issue for developing a HAMR recording medium is to achieve a L1₀ chemical order in FePt particles of 4–5 nm with the easy axis of magnetization [001] perpendicular to the film plane. When FePt is sputter-deposited with oxides, we can disperse nanoparticles in oxides easily; however, the FePt phase is the fcc disordered A1 phase while the structure of the equilibrium phase is L1₀. When the films are annealed after the sputter deposition, the A1 phase transforms to the L1₀ phase; at the same time, particle coarsening occurs and the desired nanogranular structure with a narrow size distribution is totally destroyed. In 2008, our group established a way to fabricate perpendicular magnetic thin films with FePt particles of a 5.5 nm mean diameter and a 2.3 nm size dispersion on thermally oxidized Si substrates (646687) using the magnetron sputtering technique as shown in Figure 3.⁴

Figure 3. TEM image of the FePt-C granular thin film grown on an MgO underlayer on a thermally oxidized Si substrate. The first encouraging media structure with ~6 nm FePt particles uniformly dispersed with a narrow size distribution.⁴

The key was to sputter-deposit FePt and C on a heated substrate so the ordering to the L1₀ structure can progress during the film growth at a much lower temperature than that required for the post-deposition annealing of A1-FePt. At the same time, C phase separates from FePt forming a thin channel of amorphous carbon. However, the coercivity of this material was only in the order of 0.8-1.5 T due to the low degree of the L10 order in the FePt particles, and a higher coercivity had to be achieved for successful HAMR media. Recently we succeeded in processing a nanoparticle-dispersed perpendicular magnetic thin film with a mean particle diameter of 6.1 nm, size dispersion of 1.8 nm, and coercivity of 3.7 T by adding Ag to the FePt-C granular films.5 To align the [001] crystal orientation perpendicular to the film, a thin layer of MgO was grown on a Si substrate, followed by sputtering a FePt-C magnetic layer. This film had the highest grade of particle dispersion and crystal alignment of L1₀-FePt particles. Subsequent static tester results measured at the HGST San Jose Research Center have shown that 550-Gbpsi HAMR recording is possible on this medium as shown in Figure 4.⁵ This recording density was the highest one achieved by HAMR and was comparable to that of conventional perpendicular magnetic recording method at that time.

Figure 4. A) TEM image of the (FePt)_0.9Ag_0.1-40 vol% C granular film; B) static magnetization curves, and the patterns of recording bits by the static HAMR head. The mean particle diameter of the FePt particles is 6.1 nm, and their size dispersion is 1.8 nm. Coercivity is 3.7 T, which is more than five times higher than that of the conventional magnetic recording media. In the recording pattern with the static HAMR head, 15-nm bits were observed with a bit width of 92 nm. Converted to a recording density, this is equivalent to 550 Gbpsi.⁶

Summary

Since (FePt)Ag-C nanogranular films can be deposited easily by sputtering on the MgO interlayer, where crystal orientations are naturally aligned to the [001] direction during the film growth, the (FePt)Ag-C films can be fabricated on substrates other than Si, including glass. From this viewpoint, the technology has high potential to be extended to industrially viable production lines in the future and can be considered to be an important advancement toward the practical application of FePt-based HAMR media. However, there are still a few challenges that need to be resolved: one is to limit the surface roughness and another is to grow the FePt grains in a columnar shape with an aspect ratio D/h of at least 1.5 as shown in Figure 2. Thorough investigations on the FePt-C system have led to the conclusion that we need to seek a new segregant material to replace C. Another issue is the slow growth rate of the MgO insulator by RF sputtering. We recently tested an electrically conductive underlayer, MgTiO, to develop a FePt [001] that can be grown with DC sputtering, and expect to eventually achieve the ideal nanostructure as shown in Figure 2. Moreover, Seage has recently announced a 1-Tbpsi HAMR recording. Thus, commercial implementation of HAMR appears to be right around the corner.