Eiji Wada

Efficient spin injection into GaAs quantum well across Fe3O4 spin filter

We demonstrate efficient spin injection into GaAs across an Fe3O4 electrode. Spin polarization of electrons injected into a GaAs quantum well becomes significantly large below 120 K, reaching a value of 33% at 10 K. The large spin polarization is likely due to spin filtering effect across the insulating ferrimagnetic Fe3O4 layer at the interface. The results indicate that spin filtering effect across Fe3O4 is a very promising means to enhance the spin injection efficiency into semiconductors.

Crossover of electron transmission mechanism and spin filtering effect at Fe∕GaAs(001) interfaces

A clear correlation between the spin filtering effect of optically pumped spin-polarized electrons at Fe∕GaAs(001) Schottky interfaces and its electron transmission mechanism is reported. Current-voltage (I‐V) characteristics of the Schottky diode demonstrate tunneling and thermionic emission transmission processes in the low and high bias voltage regions, respectively. A spin filtering current contribution, on the other hand, exhibits a significant peak at a particular bias voltage while spin selectivity shows a shoulder at the same bias voltage. The bias voltage where the features occur corresponds well to the crossover regime of the electron transmission mechanisms. The spin selectivity also shows a field dependence similar to the magnetization curve, assuring that the spin selectivity has its origin in the spin filtering effect.

Perpendicularly magnetized spin filtering Cu/Ni multilayers

Spin filtering at perpendicular magnetized Cu/Ni multilayer/GaAs(001) interfaces is demonstrated at remanence using optical spin orientation method. [Cu(9 nm)/Ni(tNi nm)]n multilayers are found to show a crossover from the in-plane to out-of-plane magnetic anisotropy at the Cu/Ni bilayer repetition n = 4 and the Ni layer thickness tNi = 3. For a perpendicularly magnetized Cu/Ni multilayer/n-GaAs(001) interface, circular polarization dependent photocurrent shows a clear hysteretic behavior under optical spin orientation conditions as a function of magnetic field out-of-plane while the bias dependence exhibits a substantial peak at a forward bias, verifying that Cu/Ni multilayers work as an efficient spin filter in the remanent state.

Electric field driven variation in magnetoresistance of Co/Cu/Fe/BaTiO3 heterostructure

We demonstrate control of the magnetoresistance of Co/Cu/Fe giant magnetoresistance structures on BaTiO3(001) by electric field. The magnetoresistance clearly increases with electric field strength in the three different ferroelectric phases of BaTiO3, while the electric field dependence is more significant in the orthorhombic and rhombohedral phases with hysteretic behaviors, associated with the different ferroelectric domain reversal processes of BaTiO3. The results clearly show that electrically induced lattice distortion at the Fe/BaTiO3 interface can provide an alternative means to manipulating the electronic structure of the Fe layer by electric field.

Electric-voltage control of magnetism in Fe/BaTiO3 heterostructured multiferroics

Electric field (E) control of the magnetic anisotropy and coercivity (HC) of a Fe film in Fe/BaTiO3 (BTO) is demonstrated at room temperature in the tetragonal phase of BTO. Polarizing microscopy and x-ray diffraction analysis of BTO (001) surface show distinctly two different regions; one with a1, a2 and c domains separated by 180° and 90° domain boundaries (DBs) (region 1) and the other with a1 domains separated by 180° DBs (region 2). The Fe film on region 1 shows complex magnetic anisotropy with the net magnetic easy axis in between [100] and [110] directions of BTO, while the magnetic anisotropy in region 2 exhibits two fold symmetry with an easy axis along [100]. In applied electric field (±10 kV/cm), the magnetic easy axis of the Fe film in region 1 is switched to the [110] direction of BTO, whereas in region 2 it stays unaffected. The HC versus E curves in region 1 show a butterfly-like behavior, while in region 2 no changes are observed. Also, the HC measured in E = ±10 kV/cm at different magneti...

Optically spin oriented electron transmission across fully epitaxial Fe3O4∕GaAs(001) interfaces

We demonstrate fully epitaxial growth of Fe3O4∕GaAs(001) heterostructures using a reactive molecular beam epitaxy (MBE) method. The Fe3O4 film was obtained by oxidizing an ultrathin Fe layer into an Fe3O4 seed layer in an O2 atmosphere at 260°C, followed by the reactive MBE growth of Fe3O4. The Fe3O4 seed layer effectively works as a template for the successive growth of Fe3O4. Also, a clear spin filtering effect of optically spin oriented electrons at the Fe3O4∕GaAs(001) interface is shown at room temperature. The results clearly demonstrate that the spin filtering effect occurs at the Fe3O4∕GaAs(001) interface and has its origin in the spin split density of states of the Fe3O4 layer.

Spin polarized electron transmission into GaAs quantum well across Fe3O4: Optical spin orientation analysis

We study electron spin transmission mechanism across an Fe3O4/GaAs quantum well interface from complementary optical approaches, i.e., optical polarization analysis of electroluminescence and spin dependent photocurrent measurement under optical spin orientation. A spin polarization over 40% is demonstrated at 10 K, as estimated from the electroluminescence due to free exciton recombination in the quantum well under spin injection conditions. The bias dependence of spin dependent photocurrent, on the other hand, shows two marked peaks due to spin filtering effect of Fe3O4, providing information on the spin-split barrier structure of the magnetic insulator Fe3O4. These combined results clearly elucidate the mechanism of the high efficiency of spin injection across Fe3O4.

Optically pumped spin-polarized carrier transport across Fewire∕GaAs interfaces

Spin-selective transport of optically pumped spin-polarized electrons across 5-nm-thick Fewires∕GaAs and 5-nm-thick Fewires∕AlOx(1.5nm)∕GaAs interfaces is studied as functions of magnetic field and temperature. We find characteristic temperature dependence of the spin selectivity at the Fe∕AlOx∕p-GaAs interface, where the selectivity shows a maximum at 200 K, which we attribute to the tunneling effect of spin-polarized electrons across the AlOx layer in the Fe∕AlOx∕p-GaAs structure.

Thermally driven magnetization switching of perpendicularly magnetized multilayers

Thermally driven magnetization switching of perpendicularly magnetized [Cu/Ni] multilayer/BaTiO3(100) heterostructures is demonstrated. While the magnetization of the multilayer shows the out-of-plane magnetic anisotropy in the tetragonal phase of BaTiO3 at room temperature, the magnetization orientation is found to switch from the out-of-plane to the in-plane in the rhombohedral phase below 180 K. The magnetization switching is attributed to the interfacial compressive strain transfer from BaTiO3 to [Cu/Ni] due to the structural phase transitions of BaTiO3. Magneto-optical polar Kerr measurement in the rhombohedral phase with and without applying an electric field supports the description that the magnetization switching arises from the compressive strain at the interface induced by the structural phase transition from the orthorhombic to rhombohedral phases.

Optically oriented electron spin transmission across ferromagnet/semiconductor interfaces

Electron spin transmission across ferromagnetic metal/semiconductor interfaces with different ferromagnetic contacts, i.e., Fe and FeGa, is studied using optical spin orientation method. The bias dependence of spin dependent photocurrent, which is the difference between the photocurrents excited with left- and right- handed circularly polarized lights, is found to show a dip-like feature at -0.058 and 0.021 V for Fe and FeGa contacts, respectively. The origin of the bias dependence of the spin dependent photocurrent is discussed on the basis of the Breit-Wigner type resonant tunneling process via interface resonant states, comparing the results for the both contacts. The results also indicate that the control of interface states is crucial to achieve efficient spin filtering effect at the ferromagnet/semiconductor interfaces.