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Research

Spin Electronics

Conventional electronics ignored the spin on the electron. New spin-based phenomena, such as spin transfer torque or the spin Hall effect open new horizons for electronic transport and magnetism, with the possibility of building new functionality in thin film devices.

Spin polarization

Spin polarization P is a key characteristic of any metallic ferromagnet to be incorporated into a spintronic device. It is defined as

Polarization equ

The method of real-time Andreev reflection where a superconducting tip is used to probe the ballistic or tunnel current passing between the ferromagnet and the tip as the tip approaches the surface is being developed to investigate rare earth metals and new magnetic materials produced in the group.

Andreev reflection. The bias-dependence of the current flowing at the superconducting tip depends on the spin polarization of the metal. The schematic compares I:V characteristics expected for a normal metal (Cu) and a half-metal (CrO2). The example shown is Tm, where P = +58%. The sign can be deduced from the asymmetry in an applied field.

Spin transport

Spin electonics depends on transport of spin-polarized electrons, or transfer of spin angular momentum in a conducting medium. The prospect of spin transport in organics is enticing, as it could add a new dimension to organic electronics. Studies of thin-film stacks with thin organic layers show quite sizable magnetoresistance effects at room temperature, which can be attributed to single or multistep tunelling across the organic layer. However, lateral spin transport which depends on diffusion of spin-polarized electrons through the organic layer is problematic. Many structures with organic semiconductor crystals or thin films exhibit no magnetoresistance. Either the spin polarization is destroyed on injection, or the carriers are spin-paired bipolarons.

An organic field-effect transistor. Room temperature (a) and low-temperature (b) characteristics. The variation with different electrodes is shown in (c).

Spin currents can be driven by unpolarized charge currents in a different direction. This is the spin Hall effect, due to spin-orbit scattering in a metal. The spin current can be used to switch a spin valve by spin-transfer torque.

Sensors

Magnetic tunnel junctions can be used as sensors when the free and pinned layers have orthogonal easy directions of magnetization in zero field. These sensors are being integrated into a microfluidic channel to detect the passage of magnetic nanowire barcodes which may be functionalized and tagged with biomolecules.

Use of a spin valve as a sensor. The yoke-type sensors used in a microfluidic channel have perpendicular pinned and free layers. The magnetic noise of several sensor types are compared, as a function of field sensitivity.

Contact Plamen Stamenov

stamenop@tcd.ie

Further Reading

Point contact Andreev reflection from Erbium: The role of external magnetic field and the sign of the spin polarization, P. Stamenov, J. Appl Phys 111 07C519 (2012)

Electron and spin transport studies of gated lateral organic devices, S. Alborghetti, J. M. D. Coey and P. Stamenov, J. Appl. Phys 112 124510 (2012); doi: 10.1063/1.4770230

Yoke-shaped MgO barrier magnetic tunnel junction sensors, J. Y. Chen. N Carroll, J F Feng and J. M. D. Coey, Appl. Phys. Letters 101 262402 (2012); doi 10.1063/1.477318