Akira Kikitsu

Reduction of ordering temperature of an FePt-ordered alloy by addition of Cu

We have found that the addition of Cu to an FePt alloy film is an effective approach for reducing the ordering temperature of FePt. The coercivity of the FePtCu film is around 5 kOe after annealing at 300 °C, whereas that of FePt shows several hundred Oe. In the FePtCu film annealed at 700 °C, the coercivity is almost the same as for the FePt films. Therefore, the FePtCu film displays a hard-magnetic property similar to that of the FePt film. The results of x-ray diffraction indicate that a ternary FePtCu alloy is formed. Thus, the formation of the ternary FePtCu alloy is considered to play an important role in reducing the ordering temperature.

Recent Progress of Patterned Media

Recent progress in the development of patterned magnetic recording media is presented, focusing on the results of patterned media fabricated by a self-assembled mask process. Several issues such as fabrication process, flyability, and signal processing have been discussed so far. However, recent progress seems to resolve these problems. A fabrication process using nano-imprint lithography is a possible solution in terms of productivity. Good results for flyability have been reported for discrete track recording media. Certain advantages with respect to signal processing have been reported. Although problems remain to be resolved, patterned media is expected to be commercialized following the introduction of the discrete track media, as a high density perpendicular magnetic recording media.

High-density recording capability of granular media composed of Co-Pt grains and SiO/sub 2/ matrix

Co-Pt and SiO/sub 2/ granular films were prepared on a Cr seed layer. Co-Pt grains dispersed in the SiO/sub 2/ matrix discretely. Conical grains grew in contact with the Cr layer and spherical grains grew between the conical ones. Coercivity exceeded 2 kOe and coercive squareness reached about 0.7 at a remanent magnetic moment of 0.4 memu/cm/sup 2/. Activating magnetic moment was 10/sup -15/ emu and the index of the thermal stability was about 100. Bit error rate was held below 10/sup -5/ up to a linear density of 280 kfci. The recording pattern at the track width of 1 /spl mu/m was clearly visible. The Co-Pt and SiO/sub 2/ granular media have potential for 5 Gb/in/sup 2/ recording.

Thermally assisted magnetic recording on a bit-patterned medium by using a near-field optical head with a beaked metallic plate

A near-field optical head with a beaked metallic plate was used for writing marks on a Co∕Pd bit-patterned medium with a diameter of 20–25nm and a pitch of 30nm. Magnetic-force-microscope images of the medium show that the magnetizations of single bits were selectively reversed by the head. The light-utilization efficiency (defined as the ratio of the absorbed power in the medium to the incident light power) was estimated from the writing condition used and thermal modeling as about 5%.

Effect of added Cu on disorder-order transformation of L1/sub 0/-FePt

The L1/sub 0/-FePt ordered phase is formed at lower annealing temperature by addition of Cu. FePtCu films start ordering at 300/spl deg/C or a lower temperature, whereas FePt and FePtAg films are still disordered alloys after annealing at 300/spl deg/C. The results of X-ray diffraction indicate that FePt and the added Cu form an FePtCu ternary alloy, whereas FePtAg decomposes into FePt and Ag phases in the disorder-order transformation. From thermodynamic considerations, it is concluded that reduction of the ordering temperature of FePt is due to the formation of an FePtCu solid solution.

Fabrication of Ridge-and-Groove Servo Pattern Consisting of Self-Assembled Dots for 2.5

Bit patterned media (BPM) is fabricated by a directed self-assembled polymer mask. CoPt magnetic dots with 17 nm pitch (2.5 Tb/in2) consists of ridge-and-groove servo patterns as well as circular data tracks. Distribution of dot size and dot pitch is 16% and 15%, respectively. The position error signal (PES) is estimated numerically employing a simulation model using estimated deviation of dot size and pitch. PES linearity was found to be fairly good in spite of large dot pitch deviation. Distribution of dot size and dot pitch causes distortion in the reproduced waveform, but it does not degrade the phase information of the servo signal. This result indicates that the ridge-and-groove servo has a high tolerance to the amplitude noise and is suitable for BPM. When the width of the read head is close to the dot pitch, the linearity error increases owing to the dot size and dot pitch deviation. Reduction of these distributions is necessary in order to apply BPM to hard disk drives.

\hbox{Tb/in}^{2}

We investigated the microscopic structure of FePtCu ternary ordered alloys from both theoretical and experimental points of view. From the theoretical viewpoint, first principles calculation was carried out based on the full-potential linear augmented plane wave method in order to obtain the electronic structure and magnetic anisotropy energy of the FePtCu ternary ordered alloy. From the experimental viewpoint, the ultraviolet photoelectron spectroscopy measurement was used to identify the Cu position in FePtCu ternary ordered alloy. From a comparison with the calculated density of states curve, it was found that the Fe site of FePt ordered alloy replaced the Cu. The theoretical and experimental results in this article support the mechanism proposed in terms of the thermodynamic considerations in our previous article.

Bit Patterned Media

We fabricated exchange coupled composite (ECC) L10 ordered, (001) oriented FePt (5nm)/Fe (5 nm) bit patterned media over a large area by diblock copolymer lithography. We formed the dot arrays of the copolymer directly on the magnetic film and used them as the etching mask. The average size of the ECC FePt/Fe pillars was about 32 nm, with a center to center distance of about 35 nm and a size distribution of about 8%. The perpendicular coercivity (Hc) of the ECC FePt/Fe patterned structures was about 4.3 kOe. Both the coercivity and the saturation field of the ECC FePt/Fe patterned structure were reduced by about 50% due to the exchange coupling between FePt and Fe in the FePt/Fe patterned structure compared to the FePt patterned structure with similar dot size and distribution. The thermal stability and gain factor of the FePt/Fe ECC structure were about 260 kBT and 1.35, respectively.

Magnetic and electronic structures of FePtCu ternary ordered alloy

Etching the damage of a patterned media fabricated with an artificially assisted self-assembled mask is estimated. CoCrPt thin films were etched by ion milling into aligned dots with a diameter of 40 nm. The milling condition was optimized for reducing the etching damage. As a result, the damage to the crystal lattice and crystal orientation was estimated to be slight by transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses. Though strong perpendicular anisotropy was induced by the patterning process, magnetic measurements and Landau-Lifshitz-Gilbert simulation revealed that the magnetic anisotropy energy was almost unchanged throughout this etching process.

Fabrication of FePt type exchange coupled composite bit patterned media by block copolymer lithography

Three methods to fabricate continuous FePt films with graded magnetic anisotropy for bit patterned media (BPM) were evaluated. Continuous FePt films with surface roughness of less than 0.3 nm were achieved in continuous FePt hard magnetic films, FePt/Fe exchange coupled composite (ECC) films and FePt/Fe based graded films. Depositing an Fe-rich film on FePt at high temperature was found to form large grains and cause the film surface very rough for BPM fabrication. Depositing Fe on FePt at room temperature and then annealing it to create graded anisotropy through the layer interdiffusion process was demonstrated to fabricate FePt/Fe based graded BPM. The continuous FePt films with hard layer only, ECC structure and graded magnetic anisotropy were patterned using a di-block copolymer self-assemble hard mask method with 25 nm dot size over 2-inch substrate. The switching field distribution (SFD) broadening and degradation of FePt BPM was studied. The reduction of SFD was achieved using a postannealing process. It was confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample.