Seng Kai Wong

Antiferromagnetically Coupled Patterned Media and Control of Switching Field Distribution

Switching Field distribution (SFD) is one of the critical issues for writing in bit patterned media (BPM) for high areal densities. It is believed that the magnetostatic interaction is one of the several factors that contribute to the SFD. With the antiferromagnetically coupled (AFC) structure, the magnetostatic interaction can be tailored to understand/reduce SFD. In this study, AFC patterned media is studied with emphasis placed on the effect of the top layer coercivity, which will determine the Mr and hence the magnetostatic interaction. For this study, nanodots with a size and space of 60 and 40 nm respectively were fabricated with electron beam lithography (EBL). Remanent hysteresis curves for the nanodot arrays were obtained by counting the number of reversed dots in magnetic force microscopy (MFM) images at remanent state. The narrowest SFD at a pressure of 1 Pa for top layer was observed possibly because of good crystaline texture and reduced magnetostatic interaction.

Lateral displacement induced disorder in L1 0 -FePt nanostructures by ion-implantation

Ion implantation is a promising technique for fabricating high density bit patterned media (BPM) as it may eliminate the requirement of disk planarization. However, there has not been any notable study on the impact of implantation on BPM fabrication of FePt, particularly at nano-scale, where the lateral straggle of implanted ions may become comparable to the feature size. In this work, implantation of antimony ions in patterned and unpatterned L10-FePt thin films has been investigated. Unpatterned films implanted with high fluence of antimony exhibited reduced out-of-plane coercivity and change of magnetic anisotropy from perpendicular direction to film-plane. Interestingly, for samples implanted through patterned masks, the perpendicular anisotropy in the unimplanted region was also lost. This noteworthy observation can be attributed to the displacement of Fe and Pt atoms from the implantation sites to the unimplanted areas, thereby causing a phase disorder transformation from L10 to A1 FePt.

Magnetostatic interaction effects in switching field distribution of conventional and staggered bit-patterned media

Effect of magnetostatic interaction on the switching field distribution (SFD) of nanodots with a diameter of 30 nm was investigated in square (conventional) and hexagonal (staggered) lattice configurations. The objective of the study is to achieve different kinds of magnetostatic interaction in experimental samples and to understand their influence on SFD. It was observed that the SFD was wider in the staggered lattice. Micromagnetic simulation was carried out and a fit of experimental results to the simulation was made to understand the observed trends. In addition, magnetic layers with an antiferromagnetic coupling configuration were also studied in the two geometries. The SFD of antiferromagnetically coupled dots was further reduced, highlighting the effect of magnetostatic interaction.

Nanoimprint mold fabrication and duplication for embedded servo and discrete track recording media

A master mold for nanoimprint lithography was fabricated for discrete track recording (DTR) media using electron beam lithography and conventional etching techniques. The DTR pattern, containing 167 tracks of 120 nm pitch (60 nm land and groove widths) and embedded servo information, was automatically generated using an in-house developed program and was optimized for faster electron beam writing on an x-y stage. A daughter mold was duplicated from the master mold by nanoimprinting, using UV-curable resist and an intermediate polymer stamp technique. Scanning electron microscope images showed that the daughter mold was accurately and completely reproduced from the master mold.

Equiatomic CoPt thin films with extremely high coercivity

In this paper, magnetic and structural properties of near-equiatomic CoPt thin films, which exhibited a high coercivity in the film-normal direction—suitable for perpendicular magnetic recording media applications—are reported. The films exhibited a larger coercivity of about 6.5 kOe at 8 nm. The coercivity showed a monotonous decrease as the film thickness was increased. The transmission electron microscopy images indicated that the as fabricated CoPt film generally consists of a stack of magnetically hard hexagonal-close-packed phase, followed by stacking faults and face-centred-cubic phase. The thickness dependent magnetic properties are explained on the basis of exchange-coupled composite media. Epitaxial growth on Ru layers is a possible factor leading to the unusual observation of magnetically hard hcp-phase at high concentrations of Pt.

Sub-50-nm track pitch mold using electron beam lithography for discrete track recording media

Using electron beam lithography and ZEP 520 resist, molds with a track pitch of 50nm were fabricated on thermal silicon oxide for discrete track recording applications. In this article, the detailed process for patterning a mold with 50nm track pitch and 10nm feature size is described. Although bilayer polymethylmethacrylate of 120nm thickness is able to pattern tracks of micrometer length and pitch down to 70nm, patterned tracks with pitches below 70nm exhibited waviness and edge roughness. In contrast, fine patterns with 50nm pitch were achieved with a single 60nm layer of ZEP 520 resist. Pattern transfer using ion milling followed by reactive ion etching produced 10nm features. High resolution scanning electron microscopy was used to evaluate the size and uniformity of the tracks during each step of the process.

Microstructure investigations of hcp phase CoPt thin films with high coercivity

CoPt films have been grown in the past with a high anisotropy in L11 or L10 phase, and a high coercivity is observed only in L10 CoPt films. Recently, we have grown CoPt films which exhibited a high coercivity without exhibiting an ordered phase. In this study, high resolution transmission electron microscopy (HRTEM) investigations have been carried out to understand the strong thickness and deposition pressure dependent magnetic properties. HRTEM studies revealed the formation of an initial growth layer in a metastable hexagonal (hcp) CoPt with high anisotropy. This phase is believed to be aided by the heteroepitaxial growth on Ru as well as the formation of Ru-doped CoPt phase. As the films grew thicker, transformation from hcp phase to an energetically favourable face-centered cubic (fcc) phase was observed. Stacking faults were found predominantly at the hcp-fcc transformation region of the CoPt film. The higher coercivity of thinner CoPt film is attributed to relatively less fcc fraction, less stacking faults, and to the isolated grain structure of these films compared to the thicker films.

Anomalous Hall effect measurements on capped bit-patterned media

The role of a small exchange coupling between isolated single-domain magnetic dots through a thin continuous film—in a system called capped bit-patterned media (CBPM)—has been studied experimentally using anomalous Hall effect measurements. The exchange coupling, provided by the thin continuous layer, was effective in reducing the switching field distribution (SFD) and coercivity under optimized conditions. SFD increases and coercivity decreases for very high values of exchange coupling due to the formation of multi-domains. Besides reducing SFD, the CBPM also exhibit potential writability advantage at higher densities, indicating their potential application as bit-patterned media.

Microstructure and magnetic properties of CoCrPt–SiO2 perpendicular recording media with synthetic nucleation layers

Thermally stable, highly textured CoCrPt–SiO2 perpendicular magnetic recording media with ∼6nm size exchange decoupled grains and tight grain size distribution were prepared on synthetic nucleation layers. The media structure consisted of 14nm thick CoCrPt–SiO2 recording layer on one Ru intermediate layer. For these films, insertion of a synthetic nucleation layer directly under the recording layer was seen to reduce the grain sizes from 6.7to6.2nm and grain size distribution from 17% to 14%. Significant reduction in the intergranular exchange coupling interactions could be achieved while improving magnetic properties. The crystallographic texture of the recording layer remained strongly (00⋅2) oriented with the dispersion Δθ50 below 3.7°. Isothermal remanence magnetization and thermal stability behavior were studied. A model that describes the role of the synthetic nucleation layer in affecting the grain growth and magnetic properties of the recording layer is presented to understand these observations.

Characterization of Coupled Novel Magnetic Multilayers With Anomalous Hall Effect

We investigated the switching behavior of magnetic thin films with coupled in-plane and perpendicular anisotropy. The in-plane layer is made up of 20 A¿ of Co while the perpendicular layer consists of multiple Co/Pd bilayers. The coupling strength is moderated by a Cu intermediate layer. The method of Anomalous Hall effect was chosen because of its ability to distinguish the in-plane and normal-to-plane magnetizations in a single measurement. We discovered that without the Cu decoupling layer, the magnetization in the Co layer was induced to point out of the plane by the Co/Pd multilayer only to point back into the plane by an external in-plane field. As the coupling is reduced by increasing the Cu layer thickness, the magnetization in the Co layer behaves more independently with respect to the Co/Pd layer.