Akihiro Inomata

Longitudinal magnetic recording media with thermal stabilization layers

We introduce promising longitudinal recording media with enhanced thermal stability compared to conventional media. The media consist of one or more magnetic layers disposed between the recording layer and the underlayer with the magnetization direction of the layers being antiparallel to the adjacent layer or layers. Antiferromagnetic coupling is induced through a thin Ru layer between the Co-based magnetic layers. Comparison with conventional media with the same remanence magnetization and thickness product reveals improved thermal stability and read-write properties for the proposed media.

Spring magnet films

The properties of exchange-spring-coupled bilayer and superlattice films are highlighted for Sm-Co hard magnet (nominally Sm/sub 2/Co/sub 7/) and Fe or Co soft magnet layers. The hexagonal Sm-Co is grown via magnetron sputtering in a- and b-axis epitaxial orientations. In both cases the c-axis, in the film plane, is the easy axis of magnetization. Trends in coercivity with film thickness are established and related to the respective microstructures of the two orientations. The magnetization reversal process for the bilayers is examined by magnetometry and magneto-optical imaging, as well as by simulations that utilize a one-dimensional model to provide the spin configuration for each atomic layer. The Fe magnetization is pinned to that of the Sm-Co at the interface, and reversal proceeds via a progressive twisting of the Fe magnetization. The Fe demagnetization curves are reversible as expected for a spring magnet. Comparison of experiment and simulations indicates that the spring magnet behavior can be understood from the intrinsic properties of the hard and soft layers. Estimates are made of the ultimate gain in performance that can potentially be realized in this system.

Dual-layer patterned media “ledge” design for ultrahigh density magnetic recording

Patterned media elements comprising coupled magnetically hard and soft sections of different horizontal size, referred to as ledge elements, are characterized by several unique properties. These elements allow for remarkably reduced reversal fields, which are an order of magnitude below the Stoner-Wohlfarth limit. They also allow for precessional reversal to occur for practical field rise times (100–200ps), which are two orders of magnitude larger than those in the case of homogeneous elements (∼2ps). These attractive properties are obtained even for elements of small height (4–8nm). Patterned media implementing such ledge elements can allow for recording densities above 10Tbit∕in2.

Precessional reversal in exchange-coupled composite magnetic elements

Magnetization reversal in composite exchange-coupled dual-layer magnetic elements can occur in the regime of precessional reversal. Compared to the regime of damping reversal in composite elements, the regime of precessional reversal exhibits substantially reduced reversal fields with modified angular dependence. Precessional reversal in the composite elements can occur for write field rise times of more than an order larger than those in homogeneous (single-layer) elements. Such long rise times can be achieved in practical writing systems even for materials with an ultrahigh anisotropy. The identified phenomena have potential applications in high density hard drives and magnetic random access memory systems.

Exchange coupling strength in synthetic ferrimagnetic media

The antiferromagnetic exchange coupling strength (H/sub ex/) between two CoCrPtB magnetic layers in bilayer synthetic ferrimagnetic media (SFM) is investigated. H/sub ex/ is found to be inversely proportional to the thickness and magnetization of the initial magnetic layer (L1). H/sub ex/ also shows a decrease with increasing temperature. An exchange coupling strength of J/spl ap/0.05 erg/cm/sup 2/ was obtained from H/sub ex/. For further improvement of thermal stability at higher recording densities, the thickness or magnetic anisotropy of L1 may be increased. On the other hand, for an effective SFM, the coercivity of L1 must be lower than H/sub ex/. Therefore, a large H/sub ex/ is critical for better thermal stabilization.

Advanced synthetic ferrimagnetic media (invited)

The exchange coupling field (Hex) in two-layer synthetic ferrimagnetic media (SFM) is enhanced to extend the areal density capability of earlier structures. The thermal stability of SFM is improved by employing high magnetic anisotropy stabilizing layers without significantly affecting the required write fields. However, this necessitates larger Hex values than were earlier realized, to preserve the antiparallel layer magnetic configuration at remanence. Hex is improved by inserting thin high Co content hcp layers, called “E-layers,” between the Ru layer and the magnetic layers. Large Hex values from 1 to 4 kOe are obtained depending on the E-layer composition and thickness. As the intergranular coupling is affected by the Co-rich E-layers, a method is provided that improves Hex to values >1 kOe without degrading read-write properties.

Contribution of the magnetic anisotropy of the stabilization layer to the thermal stability of synthetic ferrimagnetic media

A study is reported on synthetic ferrimagnetic media, which is made of an initial or stabilization magnetic layer (L1) closer to the substrate and a thicker top magnetic layer (L2) separated by a Ru spacer layer. The effects of the magnetic anisotropy (KU-L1) and the thickness (tL1) of the stabilization layer on the thermal stability of synthetic ferrimagnetic media are systematically studied. The magnetic anisotropy KU-L1 is varied by changing the Pt content of CoCrPtB alloy while a constant thickness and the same composition are retained for the L2. Media coercivity, stabilization factor KUV/kBT, and thermal stability increase as a function of tL1. For a particular tL1 value, these factors are improved for higher KU-L1, though the interlayer antiferromagnetic coupling strength is decreased due to compositional changes.

Magnetic and R/W properties of CoPt-SiO/sub 2/ granular media

We fabricated Co/sub 80/Pt/sub 20/-SiO/sub 2//Cr granular media by sputtering and studied their structure and their magnetic and read/write (R/W) properties while varying the volume ratio of CoPt to SiO/sub 2/. Transmission electron microscopy (TEM) images revealed that the diameter of CoPt particles increased with an increase in volume ratio. The maximum coercivity (about 2700 Oe) was obtained at 65 vol% of CoPt. On the other hand, at over 40 vol% of CoPt media, the signal to media noise ratio (S/N/sub m/) rapidly decreased with an increase in the volume ratio. This indicates that media with finer grains exhibit low noise. In the same way, the instability of the magnetization was diminished with an increase in volume. This instability is very sensitive to the grain size of CoPt.

Magnetic properties and structure of (Co-alloy)-SiO/sub 2/ granular films

We fabricated (Co-alloy)-SiO/sub 2/ granular films by rf-sputtering with composite target and annealing in a high vacuum environment. The coercivity of the films depended on annealing temperature, the Co-alloys volume fraction in the film, and other added elements to cobalt. We found that a (Co/sub 85/Pt/sub 15/)/sub 83/-(SiO/sub 2/)/sub 17/ granular film (Co/sub 85/Pt/sub 15/: 64 vol.%) exhibited about 1600 Oe of coercivity at room temperature. The grains diameter was about 50 nm measured by AFM. X-ray diffraction patterns showed that the Co-alloy grains of most films had an fcc-phase structure. From Henkel-plot analysis, the magnetic exchange interaction between the CoPt grains was found to be weak.

Fast precessional reversal in perpendicular composite patterned media

Magnetization reversal mechanisms in composite exchange-coupled dual-layer (composite) patterned media are allowed in the regime of precessional reversal, which is characterized by substantially reduced reversal fields. An important property of precessional reversal in composite patterned media is that it can occur for recording field rise times of more than an order larger than those in patterned media comprising homogeneous elements. These longer rise times can be allowed by realistic recording systems even for materials with ultrahigh coercivity. The reversal field and rise times required for precessional reversal can be controlled by varying the soft layer parameters and coupling strength between the layers.