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.

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.

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.

Systematic planar Hall measurements have been performed on a ferromagnetic Fe film grown on a standard (001) GaAs substrate at room temperature. The angular dependence of the planar Hall effect revealed the presence of both four-fold (cubic) and two-fold (uniaxial) anisotropies in the 7 nm thick Fe film. The dominance of the four-fold symmetric anisotropy, however, provided four magnetic easy axes near the (100) direction, which results in a two step switching phenomenon in the magnetization reversal process. An interesting asymmetric hysteresis loop was observed in the planar Hall resistance (PHR) when the turning point of the field scan is set at the value in the region of the second transition. The intermediate resistance states appearing in the asymmetric PHR loop were understood in terms of mutli-domain structures formed during the second switching of magnetization. Such multi-domain structure of the Fe film showing robust time stability provided additional Hall resistance states, which can be used for multi-valued memory device applications.

A recessed-channel tunnel field-effect transistor (RCTFET) with intrinsic Si layer between gate and source/drain is proposed and its electrical characteristics are examined by technology computer-aided design simulation for lower subthreshold swing (SS) and higher on-off current ratio (I ON/I OFF) than conventional planar TFET. Although the SS and I ON/I OFF of RCTFET can be improved by optimizing the length of the intrinsic Si layer (L T), there is a trade-off in terms of turn-on voltage (V ON). To address this issue, a ferroelectric (FE) layer has been adopted to the gate stack for negative capacitance (NC) effects. Based on the study, the NC effects not only reduce V ON but also enhance the SS and I ON/I OFF characteristics. As a result, the optimized NC-RCTFET shows 3 times higher I ON and 23 mV dec−1 smaller average SS with 1 V lower V ON than the conventional RCTFET.