Makoto Kohda

A Spin Esaki Diode

We demonstrate electrical electron spin injection via interband tunneling in ferromagnetic/nonmagnetic semiconductor Esaki diodes. An interband tunnel junction between ferromagnetic p+-(Ga,Mn)As and nonmagnetic n+-GaAs under reverse-bias allows spin-polarized tunneling of electrons from the valence band of (Ga, Mn)As to the conduction band of n+-GaAs. The spin polarization of tunneled electrons is probed by circular polarization of electroluminescence (EL) from an n-GaAs/InGaAs/p-GaAs light emitting structure integrated with the diode. Clear hysteresis loop with ±6.5% remanence is observed in the magnetic-field dependence of the EL polarization at 6 K, below the Curie temperature of (Ga, Mn)As.

Coercivity change in an FePt thin layer in a Hall device by voltage application

The coercivity (Hc) of a perpendicularly magnetized FePt layer was modulated by applying the voltage (Vapp) to a Hall device through MgO and Al–O insulating layers. A change in ∼40 Oe in Hc was observed by changing Vapp from −13 to 13 V. From the quantitative analysis of the voltage effect on Hc, the change in the anisotropy energy by voltage application was evaluated to be 18.6 fJ/V m, which was of the same order as the theoretical prediction. The role of the MgO layer for the voltage effect was also discussed.

All-electrical detection of the relative strength of Rashba and Dresselhaus spin-orbit interaction in quantum wires

We propose a method to determine the relative strength of Rashba and Dresselhaus spin-orbit interaction from transport measurements without the need of fitting parameters. To this end, we make use of the conductance anisotropy in narrow quantum wires with respect to the directions of an in-plane magnetic field, the quantum wire, and the crystal orientation. We support our proposal by numerical calculations of the conductance of quantum wires based on the Landauer formalism which show the applicability of the method to a wide range of parameters.

Bias voltage dependence of the electron spin injection studied in a three-terminal device based on a (Ga,Mn)As∕n+-GaAs Esaki diode

We investigated injection of spin polarized electrons in a (Ga,Mn)As∕n+-GaAs Esaki diode (ED) by using a three-terminal device integrating a (Ga,Mn)As ED and a light emitting diode (LED). Electroluminescence polarization (PEL) from the LED was measured under the Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The maximum PEL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are ballistically injected into the LED.

Spin-orbit induced electronic spin separation in semiconductor nanostructures.

The demonstration of quantized spin splitting by Stern and Gerlach is one of the most important experiments in modern physics. Their discovery was the precursor of recent developments in spin-based technologies. Although electrical spin separation of charged particles is fundamental in spintronics, in non-uniform magnetic fields it has been difficult to separate the spin states of charged particles due to the Lorentz force, as well as to the insufficient and uncontrollable field gradients. Here we demonstrate electronic spin separation in a semiconductor nanostructure. To avoid the Lorentz force, which is inevitably induced when an external magnetic field is applied, we utilized the effective non-uniform magnetic field which originates from the Rashba spin–orbit interaction in an InGaAs-based heterostructure. Using a Stern–Gerlach-inspired mechanism, together with a quantum point contact, we obtained field gradients of 108 T m−1 resulting in a highly polarized spin current.

Direct determination of spin–orbit interaction coefficients and realization of the persistent spin helix symmetry

The spin-orbit interaction plays a crucial role in diverse fields of condensed matter, including the investigation of Majorana fermions, topological insulators, quantum information and spintronics. In III-V zinc-blende semiconductor heterostructures, two types of spin-orbit interaction--Rashba and Dresselhaus--act on the electron spin as effective magnetic fields with different directions. They are characterized by coefficients α and β, respectively. When α is equal to β, the so-called persistent spin helix symmetry is realized. In this condition, invariance with respect to spin rotations is achieved even in the presence of the spin-orbit interaction, implying strongly enhanced spin lifetimes for spatially periodic spin modes. Existing methods to evaluate α/β require fitting analyses that often include ambiguity in the parameters used. Here, we experimentally demonstrate a simple and fitting parameter-free technique to determine α/β and to deduce the absolute values of α and β. The method is based on the detection of the effective magnetic field direction and the strength induced by the two spin-orbit interactions. Moreover, we observe the persistent spin helix symmetry by gate tuning.

Magnetic interaction of submicron-sized ferromagnetic rings in one-dimensional array

Magnetization characteristics of submicron-sized ferromagnetic rings in a one-dimensional array with various inter-ring distances, lx, were investigated by the magneto-optical Kerr effect and micromagnetic simulation. The onion (vortex)-to-vortex (onion) transition fields were found to be proportional to 1∕lxn with n=1.36 (0.79), instead of being a simple dipole interaction model (n=3). It was demonstrated that the transition mechanism and the inter-ring dependence are governed by the energy gain originating from the deformation of the local vortex. As a result, exchange energy as well as magnetostatic energy play important roles in the magnetization reversal of ring array.

Effect of n+-GaAs thickness and doping density on spin injection of GaMnAs/n+-GaAs Esaki tunnel junction

Abstract We investigated the influence of n + -GaAs thickness and doping density of GaMnAs/n + -GaAs Esaki tunnel junction on the efficiency of the electrical electron spin injection. We prepared seven samples of GaMnAs/n + -GaAs tunnel junctions with different n + -GaAs thickness and doping density grown on identical p-AlGaAs/p-GaAs/n-AlGaAs light emitting diode (LED) structures. Electroluminescence (EL) polarization of the surface emission was measured under the Faraday configuration with external magnetic field. All samples have the bias dependence of the EL polarization, and higher EL polarization is obtained in samples in which n + -GaAs is completely depleted at zero bias. The EL polarization is found to be sensitive to the bias condition for both the (Ga,Mn)As/n + -GaAs tunnel junction and the LED structure.

Electrical manipulation of spins in the Rashba two dimensional electron gas systems

We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the k⋅p perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate.

Platinum thickness dependence and annealing effect of the spin-Seebeck voltage in platinum/yttrium iron garnet structures

We investigate the substrate annealing effect of thermoelectric voltage induced by the spin-Seebeck effect in Pt/polycrystalline yttrium iron garnet Y3Fe5O12 (YIG) structures with different Pt thicknesses. The thermoelectric voltage is increased by decreasing the Pt thickness to 1.9 nm as well as by annealing. Annealing at 1073 K for 5 h enhances the thermoelectric voltage up to 7.4 µV/K in structures with 1.9 nm Pt thickness.