Observation of reversed hysteresis loops and negative coercivity in granular GaAs - Fe hybrid structures

We have studied GaAs - Fe granular magnetic-semiconductor hybrid structures by magneto-optic spectroscopy in a photon-energy range from 0.7 to 5 eV at temperatures from 7 to 300 K in magnetic fields up to 1.8 T. The structures have been grown by alternative molecular-beam deposition of GaAs and Fe. At the chosen substrate temperature of Ts=580 degreeC the Fe precipitates into clusters of nanometer to submicron size which are in part ferromagnetically ordered at 300 K. The polar Kerr spectra at 300 and 7 K show for the hybrid structures with high Fe content a broad negative peak with a Kerr rotation of -0.2 degree at a photon energy of 0.9 eV. The spectral dependence is very similar to pure Fe. Looking at the polar Kerr hysteresis loops at 300 K, a striking behavior is found. The hysteresis loops are reversed, i.e., the magneto-optic hysteresis curves decrease faster when decreasing the fields than the situation is when closing the hysteresis loop on the way back. This peculiar behavior is discussed in the context of local exchange-bias like coupling between ferromagnetic Fe clusters and an antiferromagnetic FeAs surrounding. An alternative model of two superimposed Kerr hysteresis loops is proposed as well. [copyright] 2001 American Institute of Physics