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

We have studied GaAs-Fe granular magnetic-semiconductor hybrid structures b y magneto-optic spectroscopy in a photon-energy range from 0.7 to 5 eV at t emperatures 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 T-s = 580 degreesC the Fe precipitates into clusters of nanometer to submicron size which are in part ferromagnet ically 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 r otation of -0.2 degrees at a photon energy of 0.9 eV. The spectral dependen ce is very similar to pure Fe. Looking at the polar Kerr hysteresis loops a t 300 K, a striking behavior is found. The hysteresis loops are reversed, i .e., the magneto-optic hysteresis curves decrease faster when decreasing th e 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 antiferromagnet ic FeAs surrounding. An alternative model of two superimposed Kerr hysteres is loops is proposed as well. (C) 2001 American Institute of Physics.