Byung-Ki Cheong

MgO seed layers for CoCrPt/Cr longitudinal magnetic recording media

Sputter deposited MgO thin films with the B1 crystal structure and (002) film texture are used as seed layers to improve the texture and magnetic properties of the CoCrPt/Cr magnetic thin films on glass substrates for longitudinal recording. The desired (002) textured Cr underlayers are usually obtained by sputtering Cr onto heated substrates. However, it is found that the MgO seed layers can induce the (002) film texture in the Cr underlayers without using a heated substrate and hence, the overlying CoCrPt films are formed with the (1120) texture. Vibrating‐sample magnetometry measurements show greatly improved in‐plane coercivities that are suitable for future high density recording media can be easily obtained.

The control of microstructural features of thin films for magnetic recording

Abstract Extrinsic magnetic properties play a major role in determining the performance of magnetic thin films used as magnetic recording media. These extrinsic properties are the ones which are largely determined by the microstructure of the thin magnetic films. In this paper we discuss various ways that the crystallographic texture of thin films can be controlled. We begin with a discussion of the origin of crystallographic texture of underlayers. Next, the origin of crystallographic texture of magnetic thin films by means of epitaxy with the underlayer is discussed. The control of other microstructural features such as grain size and perfection by means of underlayers and interlayers is briefly introduced. Examples from recent research are included.

Physical and magnetic microstructure of rapid thermally annealed thin film bismuth‐doped garnets

Bismuth‐doped iron garnets are promising materials for blue light magneto‐optic recording media. These materials have been deposited on glass substrates with bulk magnetic and magneto‐optic properties suitable for magneto‐optic recording. However, these films exhibit a very low carrier to noise ratio (CNR) when crystallized in conventional furnaces. The low CNR is due to the inability to write circular domains in these materials. Rapid thermal annealing (RTA) has been reported to produce films with a much finer grain size. We have confirmed by transmission electron microscopy that RTA does indeed reduce the grain size over an order of magnitude from films crystallized in a conventional furnace. Faraday microscopy has shown that films processed by RTA with smaller grain size (∼30 nm) support circular magnetic domains of much greater regularity than films crystallized in conventional furnaces. The improved regularity of static thermomagnetically written domains, in films crystallized by RTA, lends promise t...

Thermodynamic consideration of the tetragonal lattice distortion of the L10 ordered phase

Abstract In an attempt to understand the thermodynamic consequences of the tetragonal distortion accompanying the L1 0 ordering, we have conducted a theoretical investigation by treating the problem of a rigid lattice free energy model. The problem is treated as an elasticity problem and a generalized formulation is proposed in a form which can be directly utilized to study incoherent two phase equilibria. By use of the Static Concentration Wave (SCW) mean field model as a rigid lattice free energy model, the formulation is applied to the case of our interest. We show that the thermodynamic stability of the L1 0 phase may be significantly influenced by the tetragonal lattice distortion, depending on the magnitude of the associated strain energy relative to the competing positive entropic contribution in the free energy of the stress free L1 0 phase. In association with this, we suggest that the neglect of tetragonal distortion (i.e. the use of a rigid lattice free energy model) could be a source of serious errors particularly for alloys with lower L1 0 ordering transition temperatures (e.g. CuAu and InMg). A prototype phase diagram of the 〈001〉 f.c.c. ∗ special point structures, calculated with the tetragonal distortion taken into account, has indeed displayed topological features which are fundamentally different from those of a rigid lattice phase diagram and, furthermore, has reproduced the main topological features of the AuCuAu 3 Cu and the InMgIn 3 Mg sides of their respective phase diagrams.

Morphology of structural domains in a congruently ordered L10 phase FePd alloy

Atomic ordering of a disordered fcc phase into the L1[sub 0] phase is accompanied by a reduction in point group symmetry as well as in translational symmetry (Fm[bar 3]m ---> P4/mm). The reduction in point group symmetry renders three rotational modifications (or three crystallographic variants) of the L1[sub 0] phase; in terms of spatial regions, this results in three kinds of structural domains. An important point here is that each of the structural domains is distorted along its own tetragonal axis and, thus, the L1[sub 0] single phase is an aggregate of the domains of three different distortion directions. Under this circumstance, the relative volume fraction of the variants, the habit plane between structural domains and the shape of a structural domain plate, which constitute essential features of the microstructures of the L1[sub 0] phase, can not be arbitrary, but are subject to the systematics governed by minimization of the elastic strain energy. In this report, the authors discuss a few characteristics of these morphological features, as observed in thin (<100 [mu]m) specimens of L1[sub 0] single phase Fe-Pd alloys.

Transformation path dependence of the Bain strain relaxation during decomposition of a hypereutectoid CuBe alloy

A transmission electron microscopy (TEM) investigation is conducted to study the relaxation of the Bain transformation strain during decomposition of a Be-rich hypereutectoid CuBe alloy. The relaxation is found to occur mainly through two basic mechanisms; habit plane rotation and the formation of a polytwin morphology. Depending on the path of the initial stages of the decomposition, these two mechanisms interplay in different ways to give rise to two different kinds of precipitate morphologies. For quenched and aged alloys, the relaxation process involves global habit plane rotation of single-domain plates along with a reconfiguration into a macroscopic polytwin morphology. For air-cooled alloys, the relaxation process involves the formation of sawtooth type polytwin plates characterized by a segment-wise habit plane rotation. A few questions which resulted from the experiments are considered by analysis of the elastic strain energy of a polytwin plate. It is suggested that the sawtooth morphology is a metastable morphology which does not satisfy the invariant plane strain condition and which comes about through certain kinetic effects under the influence of the elastic instability of the intermediate α′ face-centered tetragonal (fct) state.

Bain strain relaxation during early stage decomposition of a hyper-eutectoid CuBe alloy

Abstract Decomposition of a hyper-eutectoid β (b.c.c.) phase CuBe alloy into the two phase [γ (B2) + α (f.c.c.)] mixture involves composition separation, b.c.c. to B2 atomic ordering and b.c.c. to f.c.c. crystal lattice rearrangement. In this investigation, the relaxation of the Bain transformation strain associated with the b.c.c. to f.c.c. lattice rearrangement is studied by TEM during the early stages of decomposition and experimental findings are interpreted within the framework of an elasticity theory of plate-like precipitates. We have found that relaxation of the Bain strain proceeds gradually with the advancement of decomposition. A decomposition sequence which is consistent with the microstructures is: b.c.c. to B2 ordering → isostructural secondary decomposition and the formation of {001} plate-like G.P. zones (b.c.t.) → b.c.c. to f.c.c. crystal lattice rearrangement and the formation of plate-like α′ (f.c.t.) precipitates with {001} habit planes → gradual relaxation to α (f.c.c.) phase. From habit plane stability analysis, we demonstrate that the intermediate α′ (f.c.t.) state is elastically unstable. Driven by this instability, the relaxation was found to occur mainly through the interplay of two basic mechanisms, that is, habit rotation and the formation of polytwin morphology. The interplay was manifested in the form of characteristic saw-tooth type polytwin plates. From a careful analysis of as-quenched states, we have found that the formation of the saw-tooth morphology initiates by plate intersection through the formation of stacking faults. We have explained this in terms of a dislocation model.