S. Khym

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.

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.

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.

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. ...

Field-controllable exchange bias in epitaxial Fe films grown on GaAs

We report that exchange bias observed in epitaxial Fe films grown on GaAs (001) substrates can be controlled by the direction of the cooling field. The effect is investigated by measuring the shift of field-cooled hysteresis loops toward specific field directions, as revealed by field scans of the planar Hall resistance at 3 K. The value of the unidirectional magnetic anisotropy corresponding to such field-controllable exchange is obtained from the angular dependence of the planar Hall effect.

Magnetic anisotropy of Ga1―xMnxAs films with additional nonmagnetic donor doping

We have investigated the magnetic anisotropy properties of a series of ferromagnetic Ga1−xMnxAs:Si films by transport measurements. The angular dependences of the planar Hall resistance (PHR) were analyzed in terms of the magnetic free energy to obtain the anisotropy fields. The cubic component of the magnetic anisotropy is much stronger than the uniaxial component at low temperature, although its dominance rapidly decreases with increasing temperature. The direction of the uniaxial anisotropy at 16 K lies along either the [1¯11] or the [110] direction depending on the Mn composition in the series. The uniaxial anisotropy direction, however, changed along the [110] direction regardless of the Mn concentration in the samples as the temperature increases.

Magnetic Anisotropy of GaMnAs Film and Its Application in Multi-valued Memory Devices

The magnetic anisotropy of GaMnAs ferromagnetic semiconductor film was investigated by magneto-transport measurements. Values of cubic and uniaxial anisotropies were obtained by analyzing the angular dependences of Hall effects based on magnetic free energy. The presence of strong cubic anisotropy gave four magnetic easy axes in the films plane, resulting in two-step switching behavior in field scans of the planar Hall effect. Asymmetric loops containing four resistance plateaus were observed in minor scans of the planar Hall effect owing to the formation of stable multi-domain structures during magnetization reversal. Four-valued memory function based on the four observed Hall resistance states was demonstrated by using appropriate sequences of magnetic field pulses.

Unidirectional Quantum Key Distribution System Based upon a Time-Division Mach–Zehnder Interferometer Consisting of Phase Stabilized Optical Fibers

We propose a unidirectional quantum key distribution (QKD) system based upon a time-division Mach–Zehnder interferometer (MZ) using phase stabilized optical fibers (PSOFs) for the sake of achieving the phase stability during the key distribution. A simple comparison between MZs constituent with PSOF and normal single mode fiber (SMF) is made. During the key distribution over 25 km of standard optical fiber, the phase drift of 4.6 ×10-4π/s is obtained. We discuss the demonstration of the quantum key distribution when the asymmetric MZ (AMZ) of PSOF is applied on the proposed QKD system.

Quaternary memory device fabricated from a single layer Fe film

A cross-shaped Hall device was fabricated from a single-crystal bcc Fe film having two in-plane magnetic easy axes. When a current pulse is applied to a Au strip patterned on top of the Fe Hall device, multiple domains with two orthogonal magnetization directions form in the Fe Hall device during magnetization reversal. Distributions of such domain that result in four different Hall resistance states can then be obtained by applying appropriate current pulses to the Au strip of the device. The writing of each Hall resistance state is then performed by applying a sequence of current pulses to the Au strip that generate magnetic fields required to achieve a desired domain distribution in the Fe film. Time dependent measurements show remarkably robust temporal stability for all four Hall resistance states. The demonstration of the writing process for the four distinct states in this Fe Hall device by current pulse sequences provides the possibility of realizing a practical room-temperature quaternary memory ...