Epitaxial Fe nanoclusters (diameter ~3.5 nm, separation ~1.9 nm) are grown at room temperature by e-beam evaporation of 2.5 ML Fe on insulating GaAs(100) substrate. The magnetoresistance (MR) of the Fe/GaAs system is measured in situ using a standard dc four terminal technique via spring loaded contact probes separated by 1.5 mm in an applied magnetic field up to 2.5 kOe . In the current set-up resistance changes of less than 0.01% can clearly be resolved. Furthermore, the MR measurements can be carried out at variable temperatures (min. 77 K).
From the temperature dependence of the resistivity and the MR measurements we are able to draw the following conclusions on the electrical carrier transport in such a nanocluster Fe/GaAs film:
Conduction of charge carriers occurs via tunneling between the Fe nanoclusters through the GaAs substrate.
Tunneling combined with a superparamagnetic phase of the Fe nanoclusters gives rise to a tunneling magnetoresistance (TMR) signal of up to 0.02%.
A 5% spin polarisation of the Fe nanocluster is estimated by fitting TMR loops.
The rather low spin polarisation and small TMR can be explained in terms of spin diffusion. The long tunneling path (minimum 1.5 mm determined by the separation of the probes) is causing a depolarisation of the current and therefore a reduced TMR. Hence, the separation of the electrical contacts has to be reduced in order to increase the TMR (spin polarisation).
 C. M. Gürtler,Y.B. Xu and J.A.C. Bland, J. Magn. Magn. Mater. 226-230, 655 (2001).
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