Hakjoon Lee

Mapping of magnetic anisotropy in strained ferromagnetic semiconductor GaMnAs films

The effect of strain on the magnetic anisotropy of GaMnAs films has been systematically investigated using Hall effect measurements. The magnitude of the strain, which was caused by differences in the lattice constant between the GaMnAs film and buffer layer, was controlled by adjustment of the alloy composition in the GaInAs buffer layer. The in-plane and out-of-plane components of the magnetic anisotropy were obtained from the angular dependence of the planar Hall resistance and the anomalous Hall resistance, respectively. The anisotropy constants obtained allow us to construct a three-dimensional magnetic free energy surface, which provides a clear understanding of the transition behavior of the magnetization between the in-plane and out-of-plane direction in the GaMnAs films.

Observation of a topological phase in a noncyclic case by use of a half-turn optical fiber

We describe direct observation of a geometrical phase in a noncyclic case as the rotation of the plane of polarization of a linearly polarized beam of light. The beam travels down uniformly wound half-turn single-mode optical fibers with various pitch angles.

Tunneling magnetoresistance from non-collinear alignment of magnetization in Fe/GaAlAs/GaMnAs magnetic tunnel junctions

Tunneling magnetoresistance (TMR) observed in Fe/GaAlAs/GaMnAs magnetic tunnel junctions depends strongly on relative configuration of magnetizations in the GaMnAs and Fe layers. Using a series of field orientations, we show that non-collinear alignments of magnetization in this structure occur when the magnetic field is applied away from the easy axes of the magnetic layers; and we find that the values of TMR observed for non-collinear spin configurations are very different from those of collinear configurations. The observed behavior suggests the possibility of using this effect for multivalued magnetic memory devices.

Temperature Behavior of Uniaxial Anisotropy along [100] Direction in GaMnAs Films

We have investigated a uniaxial anisotropy along the [100] direction of GaMnAs film grown on (001) GaAs. The hysteresis observed in the angular dependence of the planar Hall resistance shifted toward the [010] and [010] directions. This phenomenon was analyzed by introducing an additional uniaxial anisotropy field Hu2 along the [100] direction. The magnitude of Hu2 is much smaller than those of the cubic Hc and uniaxial Hu1 anisotropy fields for this material. The temperature behavior of Hu2 appears to be very similar to that of Hu1, suggesting the possibility that Hu1 and Hu2 have a common origin.

Observation of uniaxial anisotropy along the [100] direction in crystalline Fe film

We report an observation of uniaxial magnetic anisotropy along the [100] crystallographic direction in crystalline Fe film grown on Ge buffers deposited on a (001) GaAs substrate. As expected, planar Hall resistance (PHR) measurements reveal the presence of four in-plane magnetic easy axes, indicating the dominance of the cubic anisotropy in the film. However, systematic mapping of the PHR hysteresis loops observed during magnetization reversal at different field orientations shows that the easy axes along the and are not equivalent. Such breaking of the cubic symmetry can only be ascribed to the presence of uniaxial anisotropy along the direction of the Fe film. Analysis of the PHR data measured as a function of orientation of the applied magnetic field allowed us to quantify the magnitude of this uniaxial anisotropy field as Oe. Although this value is only 1.5% of cubic anisotropy field, its presence significantly changes the process of magnetization reversal, revealing the important role of the uniaxial anisotropy in Fe films. Breaking of the cubic symmetry in the Fe film deposited on a Ge buffer is surprising, and we discuss possible reason for this unexpected behavior.

Asymmetry in the angular dependence of the switching field of GaMnAs film

Planar Hall effect measurements were carried out on two GaMnAs ferromagnetic films with different Mn concentrations (6.2% and 8.3% Mn). The switching fields of magnetization taken from field scans of the planar Hall effect showed significantly different angular dependences in the two samples. While the angular dependence of the switching field for the sample with 8.3% Mn had a symmetric rectangular shape in the polar plot, that of the other sample with 6.2% Mn exhibited a clearly asymmetric behavior, with large steps at the 〈110〉 crystallographic directions. This switching field behavior was analyzed by considering pinning fields for crossing the 〈110〉 directions. The fitting of step features appearing at the 〈110〉 directions revealed the presence of a new uniaxial anisotropy field Hu2 along the [100] direction, in addition to the commonly observed cubic Hc anisotropy field (along the 〈100〉 directions) and uniaxial anisotropy Hu1 fields (along either the [110] or the [11¯0] direction) in the GaMnAs film.

Influence of uniaxial anisotropy on the domain pinning fields of ferromagnetic Ga1−xMnxAs films

The domain pinning fields of ferromagnetic Ga1−xMnxAs films were investigated using the planar Hall effect (PHE). Two in-plane components of GaMnAs films’ anisotropy fields (H4∥ and HU∥), which determine the direction of magnetic easy axes in the (001) plane, were obtained from the PHE’s angular dependence fitted with magnetic free energy within the scheme of the Stoner–Wohlfarth model. The domain pinning fields were obtained both with and without consideration of the deviation angle, δ, of magnetic easy axes from the ⟨100⟩ crystallographic direction of each sample. The values of domain pinning fields are clearly different between the two methods of analysis and the discrepancy increases with δ. This indicates that the correct direction of the magnetic easy axis (i.e., the influence of uniaxial anisotropy) must be considered to obtain precise values of pinning fields in GaMnAs films.

Thickness dependence of uniaxial anisotropy fields in GaMnAs films

Our investigation of thin GaMnAs films with different thicknesses revealed that the magnetic properties of this material strongly depend on film thickness. For this study, a single GaMnAs film was selectively etched, and its properties were then investigated by planar Hall effect measurements. A particularly important conclusion from the results is the emergence of a uniaxial anisotropy field along the [100] crystalline direction, which increases rapidly with increasing film thickness. We argue that such thickness dependence of the [100] uniaxial anisotropy results from the crystal structure of the film, rather than from the effects of the interface between the GaMnAs and the substrate.

Multi-Valued Planar Hall Resistance Manipulated by Current Induced Magnetic Field in Fe Films Grown on GaAs(001) Substrates

A Hall device was fabricated from single-crystal Fe film having two in-plane magnetic easy axes. Planar Hall resistance measured by sequential application of current pulses to the metal strip that was deposited on the top of a Hall bar showed a hysteresis similar to that observed by scanning an external magnetic field. It was shown that discrete Hall resistance values in the hysteresis, which correspond to specific multidomain structures in Fe film, can be created by the application of appropriate sequences of current pulses to the metal strip, and can thus be used for read/write logic applications.

Asymmetry in the planar Hall resistance of Fe films grown on vicinal GaAs substrates

We have investigated the Hall effects of the ferromagnetic Fe films grown on standard (001) and on vicinal (i.e., slightly tilted toward the [1 1- 0] direction) GaAs substrates at room temperature. ...