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Spin polarized injection in nanostructures and devices


A GaAs based LED has been combined with a tunnel barrier and a CoFe ferromagnetic contact. Under bias spin polarized electrons can be injected from the contact into the LED. The spin polarisation persists even at elevated temperatures. Different types of tunnel barriers have been used: (1) amorphous oxide tunnel barriers such as AlOx are very robust and can be deposited by simple techniques such as sputtering and in-situ oxidation. They give good spin injection results, up to 30% at low temperature and close to 20% at room temperature. (2) Highly doped Schottky barriers give more symmetric behaviour and are well suited for both injection AND detection. The obtained spin polarization and temperature dependence are similar to the AlOx tunnel barriers. Schottky injectors require more sophisticated growth techniques such as molecular beam epitaxy, but they also allow the growth of epitaxial ferromagnetic contacts with better control over the magnetic properties such as the anisotropy.
For the first time ferromagnetic mangamite layers have been deposited on silicon and GaAs substrates by channel spark ablation. The layers show ferromagnetism at room temperature. This result can be a first step towards room temperature spin injection into semiconductors from highly spin-polarized oxides.
For standard ferromagnetic materials like Fe or Co circular dichroism is a useful investigation tool. The same has also been tried for ferromagnetic semiconductors like GaMnAs. An additional signal can also be obtained from linear dichroism, which is typically week in ferromagnetic metals. During the project it has turned out that the linear dichroism in GaMnAs is strong and can be used for the optical investigation of the switching properties of the material.
In a stack of a ferromagnetic semiconductor, a tunnel barrier, and a non-magnetic metal, a strong tunnelling magneto-resistance can occur. The resistance is due to the change in the density of states in the ferromagnet with changing of the direction of the magnetization with respect to the lattice. In a layers stack with two ferromagnetic layers the effect can be strongly enhanced. -C. Gould, C. Ruster, T. Jungwirth, E. Girgis, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt and L.W. Molenkamp Tunnelling Anisotropic Magneto-resistance: A spin-valve like tunnel magneto-resistance using a single magnetic layer Phys. Rev. Lett. 93, 117203 (2004) -C. Ruster, C. Gould, T. Jungwirth, J. Sinova, G. M. Schott, R. Giraud, K. Brunner, G. Schmidt, and L.W. Molenkamp Very large Tunnelling Anisotropic Magneto-resistance of a (Ga,Mn)As/GaAs/(Ga,Mn)As stack Phys. Rev. Lett. In press