R. Jansen

Electrical creation of spin polarization in silicon at room temperature

The control and manipulation of the electron spin in semiconductors is central to spintronics, which aims to represent digital information using spin orientation rather than electron charge. Such spin-based technologies may have a profound impact on nanoelectronics, data storage, and logic and computer architectures. Recently it has become possible to induce and detect spin polarization in otherwise non-magnetic semiconductors (gallium arsenide and silicon) using all-electrical structures, but so far only at temperatures below 150 K and in n-type materials, which limits further development. Here we demonstrate room-temperature electrical injection of spin polarization into n-type and p-type silicon from a ferromagnetic tunnel contact, spin manipulation using the Hanle effect and the electrical detection of the induced spin accumulation. A spin splitting as large as 2.9 meV is created in n-type silicon, corresponding to an electron spin polarization of 4.6%. The extracted spin lifetime is greater than 140 ps for conduction electrons in heavily doped n-type silicon at 300 K and greater than 270 ps for holes in heavily doped p-type silicon at the same temperature. The spin diffusion length is greater than 230 nm for electrons and 310 nm for holes in the corresponding materials. These results open the way to the implementation of spin functionality in complementary silicon devices and electronic circuits operating at ambient temperature, and to the exploration of their prospects and the fundamental rules that govern their behaviour.

Cobalt-Al2O3-silicon tunnel contacts for electrical spin injection into silicon

The resistance of Co–Al2O3–Si tunnel contacts for electrical spin injection from a ferromagnet into silicon is investigated. The contacts form a substantial Schottky barrier, 0.7 eV, which plays a dominant role in the electronic transport. On Si with a low doping concentration ( ∼ 1015 cm−3), the contact resistance is affected by the Al2O3 tunnel barrier only in the forward bias. In the reverse bias (the spin injection condition), the Schottky barrier results in a very high contact resistance, ∼ 102 Ω m2. While the contact resistance is improved to ∼ 10−2 Ω m2 using Si with a high doping concentration ( ∼ 5×1019 cm−3), it is still about five to six orders of magnitude higher than the value needed for resistance matching to silicon

Step-induced uniaxial magnetic anisotropy of La0.67Sr0.33MnO3 thin films

The magnetic anisotropy of epitaxial La0.67 Sr0.33 MnO3 (LSMO) thin films on vicinal, TiO2 -terminated SrTiO3 substrates is investigated. Atomic force microscopy shows a regular step-terrace structure on the LSMO surface which is a replication of the surface of the substrate. The films show in-plane uniaxial magnetic anisotropy at room temperature, with the easy axis along the step direction. At low temperature the films show biaxial crystalline anisotropy with easy axes along [110], and hard axes along the [100] direction of LSMO.

Voltage tuning of thermal spin current in ferromagnetic tunnel contacts to semiconductors

Spin currents are paramount to manipulate the magnetization of ferromagnetic elements in spin-based memory, logic and microwave devices, and to induce spin polarization in non-magnetic materials. A unique approach to create spin currents employs thermal gradients and heat flow. Here we demonstrate that a thermal spin current can be tuned conveniently by a voltage. In magnetic tunnel contacts to semiconductors (silicon and germanium), it is shown that a modest voltage (~200 mV) changes the thermal spin current induced by Seebeck spin tunnelling by a factor of five, because it modifies the relevant tunnelling states and thereby the spin-dependent thermoelectric parameters. The magnitude and direction of the spin current is also modulated by combining electrical and thermal spin currents with equal or opposite sign. The results demonstrate that spin-dependent thermoelectric properties away from the Fermi energy are accessible, and open the way towards tailoring thermal spin currents and torques by voltage, rather than material design.

Anomalous Hall effect in anatase Co:TiO2 ferromagnetic semiconductor

We have investigated the effects of modification of the SrTiO3 /Co interface as well as the SrTiO3 barrier on the tunnel magnetoresistance TMR of La0.67Sr0.33MnO3 /SrTiO3 /Co junctions. Modification was realized by the introduction of one atomic layer of either TiO2 or SrO at the SrTiO3 /Co interface. Barriers with different oxygen content were also studied. In these structures we have observed positive as well as negative TMR, with a trend towards positive TMR for junctions with interfacial SrO and/or more oxygen-deficient barriers. This work offers more insight into the SrTiO3 /Co tunnel spin polarization and its sign.

Oscillatory spin-polarized tunnelling from silicon quantum wells controlled by electric field

Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Spin-polarized tunnelling, successful in ferromagnetic metal junctions, was recently used to inject and detect electron spins in organics and bulk GaAs or Si. Electric field control of spin precession was studied in III-V semiconductors relying on spin-orbit interaction, which makes this approach inefficient for Si, the mainstream semiconductor. Methods to control spin other than through precession are thus desired. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin-orbit interaction.

Spin injection and perpendicular spin transport in graphite nanostructures

Organic- and carbon-based materials are attractive for spintronics because their small spin-orbit coupling and low hyperfine interaction is expected to give rise to large spin-relaxation times. However, the corresponding spin-relaxation length is not necessarily large when transport is via weakly interacting molecular orbitals. Here we use graphite as a model system and study spin transport in the direction perpendicular to the weakly bonded graphene sheets. We achieve injection of highly 75% spin-polarized electrons into graphite nanostructures of 300–500 nm across and up to 17 nm thick, and observe transport without any measurable loss of spin information. Direct visualization of local spin transport in graphite-based spin-valve sandwiches also shows spatially uniform and near-unity transmission for electrons at 1.8 eV above the Fermi level.

Spin Accumulation in Nondegenerate and Heavily Doped p-Type Germanium

Spin accumulation induced in p-type germanium from Fe/MgO tunnel contacts is studied as a function of hole concentration p (1016–1019 cm-3). For all p, the contacts are free of rectification and Schottky barrier, guaranteeing spin injection into the Ge and preventing spin accumulation enhancement by two-step tunneling via interface states. The observed spin accumulation is smallest for nondegenerate doping (p~1016 cm-3) and increases for heavily doped Ge. This trend is opposite to what is expected from spin injection and diffusion theory. For heavily doped Ge, the observed spin accumulation is orders of magnitude larger than predicted.

Spin Accumulation and Spin Lifetime in p-Type Germanium at Room Temperature

The electrical creation and detection of spin accumulation in p-type Ge were successfully demonstrated at room temperature by spin-polarized tunneling in epitaxial Fe/MgO contacts on Ge with a hole concentration of 8×1018 cm-3. In Hanle measurements, the spin accumulation produces a spin signal of about 40 µV per mA of tunnel current. The extracted spin lifetime of holes is 13 ps, which is much longer than the momentum relaxation time. The corresponding spin-diffusion length is 80 nm, suggesting that communication of spin information in p-type Ge is possible over the typical channel length of a field-effect transistor.

Magnetic tunnel contacts to silicon with low-work-function ytterbium nanolayers

Unambiguous proof of spin transport in semiconductor spintronic devices requires a control experiment to exclude spurious signals that arise from the presence of the ferromagnetic contacts. It is shown here that insertion of a low-work-function Yb nanolayer in ferromagnetic tunnel contacts to silicon allows a selective suppression of the tunnel spin polarization for 2 nm of Yb and simultaneous control of the Schottky barrier height. The insertion of a nonmagnetic nanolayer provides a versatile method to exclude artifacts and a solution for nanoscale devices or other geometries in which the frequently employed Hanle effect cannot be applied and a control experiment did not exist.