Magnetic tunnel junctions (MTJs) constitute a promising building block for future nonvolatile memories and logic circuits. Despite their pivotal role, spatially resolving and chemically identifying each individual stacking layer remains challenging due to spatially localized features that complicate characterizations limiting understanding of the physics of MTJs. Here, we combine advanced electron microscopy, spectroscopy, and first-principles calculations to obtain a direct structural and chemical imaging of the atomically confined layers in a CoFeB–MgO MTJ, and clarify atom diffusion and interface structures in the MTJ following annealing. The combined techniques demonstrate that B diffuses out of CoFeB electrodes into Ta interstitial sites rather than MgO after annealing, and CoFe bonds atomically to MgO grains with an epitaxial orientation relationship by forming Fe(Co)-O bonds, yet without incorporation of CoFe in MgO. These findings afford a comprehensive perspective on structure and chemistry of MTJs, helping to develop high-performance spintronic devices by atomistic design.

While ferromagnetic nanodots are being widely studied from fundamental as well as application points of views, so far all the dots have been physically defined; once made, one cannot change their dimension or size. We show that ferromagnetic nanodots can be electrically defined. To realize this, we utilize an electric field to modulate the in-plane distribution of carriers in a ferromagnetic semiconductor (Ga,Mn)As film with a meshed gate structure having a large number of nanoscaled windows.

Conventional semiconductor devices use electric fields to control conductivity, a scalar quantity, for information processing. In magnetic materials, the direction of magnetization, a vector quantity, is of fundamental importance. In magnetic data storage, magnetization is manipulated with a current-generated magnetic field (Oersted–Ampère field), and spin current1, 2 is being studied for use in non-volatile magnetic memories3, 4. To make control of magnetization fully compatible with semiconductor devices, it is highly desirable to control magnetization using electric fields. Conventionally, this is achieved by means of magnetostriction produced by mechanically generated strain through the use of piezoelectricity5, 6, 7, 8. Multiferroics9, 10 have been widely studied in an alternative approach where ferroelectricity is combined with ferromagnetism. Magnetic-field control of electric polarization has been reported in these multiferroics using the magnetoelectric effect, but the inverse effect—direct electrical control of magnetization—has not so far been observed11. Here we show that the manipulation of magnetization can be achieved solely by electric fields in a ferromagnetic semiconductor, (Ga,Mn)As. The magnetic anisotropy, which determines the magnetization direction, depends on the charge carrier (hole) concentration in (Ga,Mn)As. By applying an electric field using a metal–insulator–semiconductor structure12, 13, 14, the hole concentration and, thereby, the magnetic anisotropy can be controlled, allowing manipulation of the magnetization direction.

The effect of vertical electric fields on the neutral exciton of GaAs “natural” quantum dots is investigated. A Stark effect with a quadratic field dependence up to 2 meV was observed and reveals a displacement of the excitonic wave function. The luminescencequenching limits the applied electric field range. No significative change on the fine structure splitting of the neutral exciton has been observed, suggesting that the lateral potential induced by the vertical electric field is too weak to modify the in-plane anisotropy of the exciton wave function.

Abstract

We report electrical manipulation of magnetization processes in a ferromagnetic semiconductor, in which low-density carriers are responsible for the ferromagnetic interaction. The coercive force H_C at which magnetization reversal occurs can be manipulated by modifying the carrier density through application of electric fields in a gated structure. Electrically assisted magnetization reversal, as well as electrical demagnetization, has been demonstrated through the effect. This electrical manipulation offers a functionality not previously accessible in magnetic materials and may become useful for reversing magnetization of nanoscale bits for ultrahigh-density information storage.

CrSb (1 ML)/GaAs (5 nm) multilayers with period up to 4 have been grown on GaAs substrates by solid-source molecular-beam epitaxy at 250°C. Reflection high-energy electron diffraction reveals zincblende characteristics throughout the growth of multilayer structures. High-resolution cross-sectional transmission electron microscopy also indicates that the crystal structure of the multilayers is zincblende and with no dislocations at the interfaces. The presence of room-temperature ferromagnetism is confirmed by magnetization measurements.

Magnetoresistance effect due to the spin-dependent scattering and the spin-polarized tunneling as well as the interlayer coupling in (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As semiconductor-based magnetic trilayer structures were studied. Both current-in-plane resistance and current-perpendicular-to-plane tunneling resistances are shown to depend on the relative magnetization directions of the two ferromagnetic (Ga,Mn)As layers. The interlayer coupling between the two (Ga,Mn)As layers is always ferromagnetic and the magnitude is weak .

We measured the electron spin relaxation time τs in n-modulation doped GaAs/AlGaAs (110) multiple quantum wells by pump probe method. The value of τs exceeds even at room temperature, which is two orders of magnitude longer than that in (001) GaAs quantum wells. The τs dependence on quantized-electron energy, pump beam power and temperature can qualitatively be explained by the reduction of the electron-hole exchange interaction due to screening.

We investigated the temperature dependence of the electroluminescence and I–V characteristics of hybrid ferromagnetic/non-magnetic semiconductor pn junction light emitting diodes, which is used for spin-injection experiments reported earlier. The observed temperature dependence is found to be reproduced by a reference sample of non-magnetic p-GaAs/(In,Ga)As/n-GaAs with an undoped (Al,Ga)As spacer layer. This suggests the existence of potential barrier at the (Ga,Mn)As/GaAs interface.

It is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits (in the form of magnetization directions established by applying external magnetic fields), the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological viewpoints, particularly in view of recent developments in magnetoelectronics and spintronics1, 2. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism has proved elusive. Here we demonstrate electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure. By applying electric fields, we are able to vary isothermally and reversibly the transition temperature of hole-induced ferromagnetism.