Taehee Yoo

Four discrete Hall resistance states in single-layer Fe film for quaternary memory devices

We report the realization of four distinct magnetic states using single layers of Fe at room temperature. When the Fe film was grown on the vicinal surface of GaAs, the fourfold-symmetric magnetization along crystallographic direction give rise to four distinct Hall resistance values due to the different combination of planar and anomalous Hall effects for the given direction. Each Hall resistance state can be written reproducibly by the sequence of field pulses and was remained constant at the written state for more than 2 h, which brings the idea of a quaternary memory device much closer to practical implementation.

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.

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.

Investigation of weak interlayer exchange coupling in GaMnAs/GaAs superlattices with insulating nonmagnetic spacers

A robust long-range antiferromagnetic coupling between ferromagnetic Ga 0.97Mn 0.03As layers has previously been realized via insertion of nonmagnetic Be-doped GaAs spacers between the magnetic lay ...

Quantitative analysis of the angle dependence of planar Hall effect observed in ferromagnetic GaMnAs film

The angle dependence of the planar Hall effect has been analyzed based on the magnetic free energy including the magnetic anisotropy and the Zeeman effects. The Zeeman effect dominated the magnetic anisotropy in high field and only a single energy minimum is shown in free energy over entire field angle, which leads to the coherent rotation of the magnetization in the form of a single domain state. When the field strength is reduced below 300Oe, multiple energy minima appear in the angle dependence of free energy due to the increase in the relative importance of magnetic anisotropy. In the low field region, reorientation of magnetization experiences abrupt transition between the free energy minima. The pinning fields obtained from the analysis showed systematic dependence on the strength of external field, which was used to rotate magnetization. We understood such pinning energy dependence in terms of the difference in the free energy density profile for the different field strengths.

Single and multidomain characteristics of GaMnAs investigated by magnetotransport measurements

We have investigated the magnetization reorientation process of GaMnAs ferromagnetic films by changing external field direction in planar Hall effect (PHE) measurement. While the angular dependences of PHE data taken with clockwise and counterclockwise under strong magnetic field (i.e., above 400Oe) are completely overlapped without hysteresis, they are significantly different under small magnetic field (i.e., below 50Oe) by exhibiting nonabrupt hysteresis. We have analyzed such angular dependence of PHE using the magnetic free energy based on Stoner-Wohlfarth model. The behavior observed under the high field was well understood in terms of coherent rotation of magnetization in the form of single domain. However, the nonabrupt hysteric behavior observed with low field cannot be explained by a single domain picture and requires involvement of multidomain structures.

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.

Room temperature weak ferromagnetism in Sn1−xMnxSe2 2D films grown by molecular beam epitaxy

We discuss growth and magnetic properties of high-quality two dimensional (2D) Sn1−xMnxSe2 films. Thin films of this 2D ternary alloy with a wide range of Mn concentrations were successfully grown by molecular beam epitaxy. Mn concentrations up to x ≈ 0.60 were achieved without destroying the crystal structure of the parent SnSe2 2D system. Most important, the specimens show clear weak ferromagnetic behavior above room temperature, which should be of interest for 2D spintronic applications.

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.