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Posted: Sep 05, 2011

Spintronics: Pumped injection

(Nanowerk News) The use of an electron's spin polarization in addition to its charge is expected to lead to faster, more efficient and more functional devices to replace the current generation of electronics. To realize these 'spintronic' devices, however, a current of electrons with uniformly polarized spin, either up or down, needs to be injected from a ferromagnet into a non-magnetic semiconductor where they can be manipulated. This has so far proved elusive due to the loss of spin information as electrons travel across the interface between these two classes of materials.
As now shown by Kazuya Ando and colleagues from Tohoku University and the Japan Atomic Energy Agency in collaboration with researchers in the UK, however, it is possible to circumvent this issue by 'spin pumping' such a spin current into the semiconductor directly ("Electrically tunable spin injector free from the impedance mismatch problem").
Spin injection without charge transfer
Spin injection without charge transfer from a ferromagnetic metal (Ni81Fe19) to a non-magnetic semiconductor (GaAs) using dynamic spin exchange interaction.
"Electrons traveling across a ferromagnet–semiconductor interface lose most of their spin information near the interface because of the considerable difference in electrical resistivity between the two layers," explains Ando. Efforts to address this problem have so far focused on growing high-quality tunneling barriers between the materials — an extremely difficult task. Ando and his co-workers instead devised a way to pump spin information from a magnetized — and therefore spin-polarized — ferromagnet to the randomly polarized spins in the semiconductor. They achieved this by making the polarized spin precess around an axis parallel with an applied magnetic field, which induces crosstalk that polarizes the spins in the semiconductor (see image).
"At the interface, the spin angular momentum of the precessing magnetization in the ferromagnet is transferred to the electrons in the semiconductor by dynamic spin exchange interaction," explains Ando, referring to a quantum mechanical effect between spatial or spin coordinates on two indistinguishable quantum objects such as electrons. As no charge is actually transported across the interface, this spin pumping process builds up a pure spin current.
"The spin current density obtained by dynamic spin injection is several orders of magnitude larger than that obtained by the conventional electrical spin injection method, and can be tuned electrically," says Ando. The method can also be used in a wide range of materials, including organic materials and high-temperature superconductors.
Source: Tokyo Institute of Technology
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