Präparation und Charakterisierung von Ni2MnIn-Filmen (English shop)

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Since S. Datta and B. Das proposed to develop the spin transistor, there is great interest in devices which use the orientation of the electron spin instead of the charge to store and manipulate information. The injection of spin polarized electrons into a semiconductor is challenging. A promising material for this task is the Heusler alloy Ni2MnIn.

Some Heusler alloys belong to the class of so called half-magnetic ferromagnets. In these materials the valence band of the spin-up electrons is partially occupied, i.e. these electrons take part in the charge transport. The valence band of the spin-down electrons is well beyond the Fermi-level, i.e. it shows insulating behavior. Half-metallic properties are predicted for the Ni2MnIn – indium arsenide interface.

Thin films of Ni2MnIn were grown by vapor deposition. For the preparation two methods were used, one with three evaporation sources for nickel, manganese and indium and one with two sources. In the latter case manganese and indium were alloyed and evaporated from the same crucible because of their nearly identical vapor pressures.

The Ni2MnIn films were investigated by scanning and transmission-electron-microscopy in order to determine their morphology, structure and stoichiometry. The optimum temperature of 200 °C – 300 °C for growing the ordered L21-structure of Heusler alloys has been determined. The ordered structure is essential for the magnetic properties of the Heusler alloy and therefore important for the spin polarized behavior of the conductance electrons.

The temperature dependence of the magnetization, the Curie-temperature, and the saturation magnetization have been measured by SQUID-magnetometry. Their values correspond well with the data of bulk material obtained from the literature. Deviations of the magnetic properties from bulk values are due to the morphology of the layers.

The spin polarization of the Ni2MnIn -films has been measured by point contact Andreev reflection spectroscopy. The detected degree of polarization is well above the polarization of conventional ferromagnetic electrodes.