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Generating and probing spin-polarized currents in low-dimensional semiconductor systems

Final Activity Report Summary - SPIN CURRENTS (Generating and probing spin-polarised currents in low-dimensional semiconductor systems)

We performed experiments to study spin-polarised injection from ferromagnetic diluted semiconductor (Ga,Mn)As into non-magnetic GaAs. We fabricated devices with magnetic Esaki diode structures p+-(Ga,Mn)As and n+-GaAs as injection and detection contacts respectively. When reverse bias was applied to such structures spin-polarised electrons could tunnel from a valence band of p+-(Ga,Mn)As into a conduction band of n+-GaAs.

We also performed low-temperature magneto-transport experiments on those devices at a temperature of 4.2 K. The experiments were conducted in a non-local spin-valve geometry, i.e. without current flowing between injector and detector contacts. The current was flowing only between the injector contact and the ground connected to n+-GaAs and the spin-dependent voltage was to be measured between the detector and n+-GaAs. We registered a spin-valve like signal both in the detector circuit, ranging from approximately zero to almost 200 %, and in the injector circuit, at a range of about 1 to 10 %. Whereas the presence of the former might suggest that the spin-polarised injection occured in the measured structures, the latter was attributed to the novel Tunnelling anisotropic magnetoresitance effect (TAMR). This arose due to anisotropies in (Ga,Mn)As density of states with respect to the magnetic moment M, because of a strong spin-orbit coupling. Combined with a two-step magnetic reversal process it could result in a spin-valve like signal obtained from a single magnetic layer. On the one side, this would allow to fabricate spintronic devices operating with only one magnetic layer, however on the other side it could make the interpretation of spin injection experiments more difficult. It was therefore important to understand its origins and influence on the results of magneto-transport experiments.

In order to study TAMR effect in detail, we fabricated devices similar to the spin injection / detection devices which had only one magnetic contact, i.e. only one Esaki diode structure. Magnetoresistance measurements revealed TAMR-related spin-valve like signal in the order of approximately 0.5 %. We used TAMR effect to investigate different types of uniaxial anisotropy that were observed in our samples. We identified three main types of anisotropy in our devices with a uniaxial anisotropy superimposed on:
1. easy axes;
2. hard axes; and
3. both hard and easy axes of a dominant fourfold cubic anisotropy.

The type of the anisotropy, in connection with TAMR, had direct influence on the observed spin-valve like signal. The latter could change its sign by a simple rotation by 90 degrees in case the additional uniaxial anisotropy was superimposed on easy axes of a dominant anisotropy, i.e. in cases one and three. In this way it could be distinguished from the 'real' spin-injection related spin valve signal. Nevertheless, this was not the case for type two, where such a sign change was not observed. We also showed that the uniaxial anisotropy in the structure could be shaped during wafer processing.

In summary, our experiments proved the importance of TAMR effect in Esaki tunnel structures. The results showed, however, that the spin-valve like signal at the detector circuit could not be explained via only the TAMR effect. This gave a solid background for further experiments, needed to clarify if measured signal was related to spin-polarisation of injected electrons, preferably on systems with longer spin-relaxation times, e.g. lighter doped n-GaAs or two-dimensional structures.