S. N. Holmes

Spin transport in germanium at room temperature

Spin-dependent transport is investigated in a Ni/Ge/AlGaAs junction with an electrodeposited Ni contact. Spin-polarised electrons are excited by optical spin orientation and are subsequently used to measure the spin dependent conductance at the Ni/Ge Schottky interface. We successfully demonstrate electron spin transport and electrical extraction from the Ge layer at room temperature.

Spin injection between epitaxial Co2.4Mn1.6Ga and an InGaAs quantum well

Electrical spin injection in a narrow [100] In0.2Ga0.8As quantum well in a GaAs p‐i‐n optical device is reported. The quantum well is located 300nm from an AlGaAs Schottky barrier and this system is used to compare the efficiencies and temperature dependences of spin injection from Fe and the Heusler alloy Co2.4Mn1.6Ga grown by molecular-beam epitaxy. At 5K, the injected electron spin polarizations for Fe and Co2.4Mn1.6Ga injectors are 31% and 13%, respectively. Optical detection is carried out in the oblique Hanle geometry. A dynamic nuclear polarization effect below 10K enhances the magnetic field seen by the injected spins in both devices. The Co2.4Mn1.6Ga thin films are found to have a transport spin polarization of ∼50% by point contact Andreev reflection conductivity measurements.

Magnetic and electrical properties of Co2MnGa grown on GaAs (001)

The Heusler alloys are a group of magnetic materials that will form essential components in hybrid, ferromagnet–semiconductor devices that utilize spin injection. We demonstrate that such an alloy, Co2MnGa:GaAs(001) is ferromagnetic at 300 K and has controllable magnetic properties. A weak in-plane uniaxial anisotropy is observed with the easy axis along the [0,−1,1] direction. Metallic rather than semiconducting behavior is observed over a range of wafer thicknesses. The extrapolated bulk resistivity is 20 μΩ cm at 300 K and the residual resistivity ratios range from 1.15 to 1.7 depending on the wafer thickness. An anisotropic magnetoresistance of 6% at 300 K (and 8% at 1.6 K) demonstrates the importance of spin–orbit scattering in these disordered alloys. Several issues are addressed in this letter as to whether the manifestation of the predicted spin-polarized band structure and minority spin band gap can be observed.

Enhanced current quantization in high-frequency electron pumps in a perpendicular magnetic field

We present experimental results of high-frequency quantized charge pumping through a quantum dot formed by the electric field arising from applied voltages in a GaAs/AlGaAs system in the presence of a perpendicular magnetic field B. Clear changes are observed in the quantized current plateaus as a function of applied magnetic field. We report on the robustness in the length of the quantized plateaus and improvements in the quantization as a result of the applied B field.

Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in ferromagnet/semiconductor hybrid structures. The Schottky characteristics at the interface were varied by using NiFe, Co and Fe as the ferromagnet. The Schottky characteristics were clearly observed with NiFe and Co/GaAs, while almost ohmic I-V characteristics were seen with Fe/GaAs. At negative bias a helicity-dependent photocurrent, dependent upon the magnetization configuration of the film and the Schottky barrier height, was detected upon modulating the polarization from right to left circular, For the magnetization along or perpendicular to the surface normal, the helicity-dependent photocurrent I/sup n/ or I/sup 0/, respectively, was measured. The asymmetry P=(I/sup n/-I/sup 0/)/(I/sup n/+I/sup 0/) of the helicity-dependent photocurrent decreases upon increasing the doping density of the GaAs substrates. P also decreases with photon energy h/spl nu/ as found for the polarization of photoexcited electrons in GaAs. In NiFe/GaAs samples for h/spl nu/=1.59 eV, P=16% for n/sup +/=10/sup 23/ m/sup -3/ and P=-23% for p/sup -/=10/sup 25/ m/sup -3/ doped substrates, i.e. P is comparable in magnitude to the theoretically predicted spin polarization of 50% for the optically pumped conduction band in GaAs. The results provide unambiguous evidence of spin-polarized electron transport through the ferromagnet/semiconductor interface and show that the Schottky barrier height controls the spin-polarized electron current passing from the semiconductor to the ferromagnet. The asymmetry data indicates that spin-polarized electrons are transmitted from the semiconductor to the ferromagnet with a high efficiency.

Ferromagnetic metal/semiconductor hybrid structures for magnetoelectronics

We report on the following new ferromagnetic metal/semiconductor heterostructure material systems: (1) Fe/InAs(100)-4×2, (2) Fe/InAs(graded)/GaAs(100), and (3) Fe/InAs/AlSb/GaSb/AlSb/InAs/GaAs resonant tunneling diodes. Single crystal Fe films have been stabilized in these structures using molecular beam epitaxy growth, as evidenced by low energy electron diffraction. The magnetic and electrical properties have been studied using in situ (and focused) magneto-optical Kerr effect, alternating gradient field magnetometry, and current–voltage measurements. The results show that Fe/InAs based heterostructures are very promising systems for use in future magnetoelectronic devices as they have well defined magnetic properties as well as favorable electrical properties.

Interfacial, electrical, and spin-injection properties of epitaxial Co2MnGa grown on GaAs(100)

The interfacial, electrical, and magnetic properties of the Heusler alloy Co2MnGa grown epitaxially on GaAs(100) are presented with an emphasis on the use of this metal-semiconductor combination for a device that operates on the principles of spin-injection between the two materials. Through systematic growth optimization the stoichiometry in the bulk Co2MnGa can be controlled to better than ±2%, although the interface is disordered and limits the spin-injection efficiency in a practical spintronic device irrespective of the half-metallic nature of the bulk metal. Molecular beam epitaxial growth was monitored in situ by reflection high energy electron diffraction and the bulk composition was measured ex situ with inductively coupled plasma optical emission spectroscopy. The Co2MnGa L21 cubic structure is strained below a thickness of 20 nm on GaAs(100) but relaxed in films thicker than 20 nm. Electrical measurements on the Co2MnGa reveal general characteristics of a disordered electron system with insulat...

In-plane magnetoresistance and magnetization reversal of cobalt antidot arrays

Cobalt antidot arrays defined within a Hall bar mesa have been fabricated using electron-beam lithography. The diameter of the circular antidots was fixed at 1μm with the antidot edge-to-edge spacing varying from 2to0.5μm in a square lattice and 0.4μm in a rotated square lattice. In-plane magnetoresistance measurements were carried out to investigate the magnetization reversal properties. Antidots greatly modify the domain configuration and work as domain wall pinning sites. As a result, the switching and saturation fields increase while the magnetoresistance ratio decreases with the inclusion of antidots and also with increasing antidot areal density. Micromagnetic simulations show that the magnetization reversal of antidot arrays proceeds with the formation and annihilation of domain walls, which is manifested as Barkhausen jumps in the transition regions of the magnetoresistance curves.

Hybrid Spintronic Structures With Magnetic Oxides and Heusler Alloys

Hybrid spintronic structures, integrating half-metallic magnetic oxides and Heusler alloys with their predicted high spin polarization, are important for the development of second-generation spintronics with high-efficient spin injection. We have synthesized epitaxial magnetic oxide Fe 3O 4 on GaAs(100) and the unit cell of the Fe3O4 was found to be rotated by 45deg to match the gallium arsenide GaAs. The films were found to have a bulk-like moment down to 3-4 nm and a low coercivity indicating a high-quality magnetic interface. The magnetization hysteresis loops of the ultrathin films are controlled by uniaxial magnetic anisotropy. The dynamic response of the sample shows a heavily damped precessional response to the applied field pulses. In the Heusler alloy system of Co2MnGa on GaAs, we found that the magnetic moment was reduced for thicknesses down to 10 nm, which may account for the lower than expected spin-injection efficiency from the spin-light-emitting diode structures. Using the element-specific technique of X-ray magnetic circular dichroism (XMCD), the reduced spin moments were found to originate from the Mn rather than the Co atoms, and the improvement of the interface is thus needed to increase the spin injection efficiency in this system. Further studies of the I-V characteristics of Fe3O4/GaAs(100) and Fe3O4/MgO/GaAs(100) show that the Fe3 O4 -based spintronic structures have a well-defined Schottky or tunneling barrier of moderate barrier height, which is encouraging for high-efficient spin injection.

Cobalt-Based Heusler Alloys for Spin-Injection Devices

Co2MnGa and Co2MnIn are ideal half-metallic materials for spin-injection in nanostructured semiconductor devices.Magneto-optical Kerr effect and electrical transport measurements demonstrate that although these alloys can be integrated with semiconductor devices, the metallic phase dominates over the intrinsic, minority spin phase of the alloy. Co2MnGa : GaAs(001) has a weak uniaxial magnetic anisotropy with the easy axis along the [0,−1,1] in-plane direction. Co2MnIn : GaAs(001) has no in-plane anisotropy. Metallic behavior is concomitant with ferromagnetic behavior, as the anisotropic magnetoresistance tends to zero at the same film thickness (∼100 Å) that the residual resistivity ratio extrapolates to 1. Co2MnIn has an AMR of ∼0.5%, however, an AMR as large as 6% is observed in Co2MnGa at 300 K. This demonstrates the potential for nanostructured devices, although the first 100 Å of alloy growth will determine the spin-injection efficiency.