New materials for spintronic devices we considered to the class of diluted magnetic semiconductors. Recently Ge-based magnetic semiconductors attracted special attention due to their straight-forward integration into semiconductor technology.
Here we report on Ge_100-(x+y)(Mnx,Fey) films showing ferromagnetism above RT. We show the magnetic and transport characterisation for a variety of (x,y) combinations.
The samples are grown by thermal evaporation on GaAs(001) at T=520K in a sequentional deposition scheme with a total thickness of about 100nm. In the first series the samples are annealed for one hour at elevated temperatures (T=730K).
The magnetic characterisation is done by a commercial SQUID magnetometer. The temperature dependence of the remanent magnetisation after zero field cooling shows a curve composed of two magnetic contributions with different phase transition temperatures. Thus it appears that the samples consist at least of two different magnetic phases. Transmission electron microscopy (TEM) pictures give evidence for clusters with a diameter of the order of 20 nm. Changing the final annealing conditions to 520K during 16 hours in our second series we find a reduction of the formation of clusters as evidenced by TEM. From the magnetisation measurement we can identify a high temperature phase associated with the clusters.
The van der Pauw scheme is applied to rectangular samples in order to determine the normal and anomal Hall coefficients. The temperature dependence of the transport properties gives a hint that the low temperature phase is connected to the formation of a genuine ferromagnetic semiconductor phase with a Curie-Temperature of T_c~200K.

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1. Magnetotransport properties of ultrathin GaMnAs layers