J. Herfort

Epitaxial growth of Fe3Si/GaAs(001) hybrid structures

We have established an optimized growth temperature range, namely, 150 °C<TG<250 °C, where ferromagnetic Fe3Si/GaAs(001) hybrid structures with high crystalline and interfacial quality can be fabricated by molecular-beam epitaxy. The composition of the Fe3Si layers, which can be regarded as a Heusler alloy, was tuned within the stable Fe3Si phase. The layers show high magnetic moments with a value of 1050 emu/cm3, which is close to that of bulk Fe3Si.

Spin injection from Fe3Si into GaAs

We demonstrate room-temperature spin injection from the epitaxially grown ferromagnetic metal Fe3Si into the semiconductor GaAs. The injection efficiency is comparable to values previously obtained for the Fe∕GaAs and MnAs∕GaAs hybrid systems using the emission of similar (In,Ga)As∕GaAs light-emitting diodes for the detection of spin polarization. The temperature dependence of the detected polarization is explained by taking into account spin relaxation inside the semiconductor device.

Optical communication of spin information between light emitting diodes

For the full implementation of spintronic circuits, it is necessary to transmit spin information from one device to another. Electrons in semiconductors often suffer from high spin relaxation rates, making electrical transport of spin information highly inefficient. Here, we propose optical transport of spin information as an alternative. We demonstrate that the spin information associated with electrons injected from Co2FeSi and Fe layers into the quantum wells of spin light emitting diodes (spin-LEDs) can be transported optically in the form of circularly polarized light and deciphered electrically via the magnetic field dependence of the photocurrent in a distant detector spin-LED.

Epitaxial Heusler alloy Co2FeSi∕GaAs(001) hybrid structures

We found that Co2FeSi layers on GaAs(001) grown by molecular-beam epitaxy with high crystal and interface perfection as well as smooth surfaces can be obtained in the low-growth-temperature regime. The layers are thermally robust up to 250°C. They have long-range order and crystallize in the Heusler-type L21 structure. The easy axis of magnetization is along the [110] direction caused by a dominating uniaxial in-plane magnetic anisotropy component which has an easy axis different from that of the magnetocrystalline anisotropy component.

All-electrical spin injection and detection in the Co2FeSi/GaAs hybrid system in the local and non-local configuration

We demonstrate the electrical injection and detection of spin-polarized electrons in the Co2FeSi/GaAs hybrid system using lateral transport structures. Spin valve signatures and characteristic Hanle curves are observed both in the non-local and the local configuration. The comparatively large magnitude of the local spin valve signal and the high signal-to-noise ratio are attributed to the large spin polarization at the Fermi energy of Co2FeSi in the well-ordered L21 phase.

Ferromagnet-semiconductor nanowire coaxial heterostructures grown by molecular-beam epitaxy

GaAs–MnAs core-shell structures are grown by molecular-beam epitaxy using wurtzite GaAs nanowires on GaAs(111)B. The nanowire structures curve due to the strain at the heterointerface when the substrate is not rotated during the growth, evidencing the diffusion length in the MnAs overgrowth being less than the perimeter of the columns. The MnAs growth is thus demonstrated to take place by direct deposition on the sidewall. The MnAs envelope is m-plane-oriented with the c-axis along the nanowire axis. The magnetic easy axis hence lies in the surface plane of the substrate, which is confirmed by magnetization measurements and magnetic-force microscopy.

Thermal stability and atomic ordering of epitaxial Heusler alloy Co2FeSi films grown on GaAs(001)

The thermal stability and the atomic ordering of single-crystal Heusler alloy Co2FeSi layers grown by molecular beam epitaxy on GaAs(001) have been studied. We found that the Co2FeSi layers have a long-range atomic order and crystallize in a partly disordered L21 structure in the low growth temperature (TG) regime. The long-range atomic order of the layers is further improved with increasing TG up to 350°C. However, the increase of TG induces an interfacial reaction between the Co2FeSi layer and the GaAs substrate. The analysis of the in-plane magnetic anisotropy reveals that the interface perfection is improved up to TG=200°C and deteriorated due to an interfacial reaction above 200°C.

Thermal stability of epitaxial Fe films on GaN(0001)

Epitaxial Fe films are grown on GaN(0001) by molecular beam epitaxy at 50 °C. Several samples of one Fe/GaN structure are subjected to rapid thermal annealing from 300 to 950 °C. Using a variety of experimental techniques, we examine the impact of this annealing step upon the morphological, structural, and magnetic properties of these samples. The results demonstrate that the material system Fe/GaN is thermally stable up to a temperature of 700 °C.

Growth temperature dependent interfacial reaction of Heusler-alloy Co2FeSi/GaAs(001) hybrid structures

We have studied the growth temperature dependence of the interfacial reaction of Heusler-alloy Co2FeSi/GaAs(0 0 1) hybrid structures. The reaction proceeds dominantly by Co in-diffusion, resulting in the formation of isolated grains in the GaAs substrate starting at the growth temperature TG of 200–250 °C. The interfacial reaction is classified into two stages: (i) intermediate TG regime (250–300 °C), where a highly oriented single-crystalline phase, most likely ternary Co2GaAs, is formed in the topotaxial relationship of ; and (ii) high TG regime (≥350 °C) where binary CoAs is formed in the asymmetric topotaxial relationship of CoAs[0 0 1](2 1 0) || GaAs[1 1 0](0 0 1).

Magnetic and structural properties of ultrathin epitaxial Fe3Si films on GaAs(001)

We have studied the thickness dependence of the structural and magnetic properties of ultrathin Fe3Si films grown on GaAs(001). The onset of ferromagnetism is found to be at about 3 ML at low temperatures. Furthermore, a reorientation of the uniaxial in-plane magnetic anisotropy takes place when the thickness of the films is reduced. This is due to a competition between a volume term and an interface contribution, which have opposite sign. Whereas the origin of the volume term remains unclear, the interface contribution is attributted to an anisotropic bonding at the Fe3Si∕GaAs(001) interface, similar to other ferromagnet/semiconductor hybrid structures.