S. C. Kim

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.

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.

Vertical gradient of magnetic anisotropy in the ferromagnetic semiconductor (Ga,Mn)As film

The properties of magnetic anisotropy of GaMnAs films along the growth direction were studied by Hall effect measurements. The magnetic anisotropy fields were obtained by analyzing the angular dependence of the planar Hall resistance and the anomalous Hall resistance for in-plane and out-of plane components of the Hall signal, respectively. The magnetic anisotropy fields obtained by this process were used to construct a three-dimensional magnetic free energy diagram, which clearly shows that the out-of-plane characteristics of magnetic anisotropy become more pronounced as we approach the bottom part of the film.

Resonant, non-resonant and anomalous states of Dirac electrons in a parabolic well in the presence of magnetic fields

We report on several new basic properties of a parabolic dot in the presence of a magnetic field. The ratio between the potential strength and the Landau level (LL) energy spacing serves as the coupling constant of this problem. In the weak coupling limit the energy spectrum in each Hilbert subspace of an angular momentum consists of discrete LLs of graphene. In the intermediate coupling regime non-resonant states form a closely spaced energy spectrum. We find, counter-intuitively, that resonant quasi-bound states of both positive and negative energies exist in the spectrum. The presence of resonant quasi-bound states of negative energies is a unique property of massless Dirac fermions. As the strong coupling limit is approached resonant and non-resonant states transform into anomalous states, whose probability densities develop a narrow peak inside the well and another broad peak under the potential barrier. These properties may investigated experimentally by measuring optical transition energies that can be described by a scaling function of the coupling constant.

Confinement and deconfinement in the potential of antidot arrays of a massless Dirac electron in magnetic fields.

We have investigated the effect of inter-Landau level mixing on confinement/deconfinement in antidot potentials of states with energies less than the potential height of the antidot array. We find that, depending on the ratio between the size of the antidot R and the magnetic length [Formula: see text], probability densities display confinement or deconfinement in antidot potentials (B is the magnetic field). When R/ℓxa0<xa01 inter-Landau level mixing is strong and probability densities with energy less than the potential height are non-chiral and localized inside antidot potentials. However, in the strong magnetic field limit R/ℓxa0≫xa01, where inter-Landau level mixing is small, they are delocalized outside antidot potentials, and are chiral for Nxa0=xa00 Landau level (LL) states while non-chiral for Nxa0=xa01. In the non-trivial crossover regime R/ℓxa0∼xa01 localized and delocalized probability densities coexist. States that are delocalized outside antidots when R/ℓxa0>xa01 form a nearly degenerate band and their probability densities are independent of k, in contrast to the case of R/ℓxa0<xa01.