Epitaxial growth, magnetic properties, and lattice dynamics of Fe nanoclusters on GaAs(001)

Epitaxial bcc-Fe 001 ultrathin films have been grown at 50 °C on reconstructed GaAs 001 4 6 surfaces and investigated in situ in ultrahigh vacuum UHV by reflection high-energy electron diffraction, scanning tunneling microscopy STM , x-ray photoelectron spectroscopy XPS , and Fe conversion electron Mössbauer spectroscopy CEMS . For tFe=1 ML monolayer Fe coverage, isolated Fe nanoclusters are arranged in rows along the 110 direction. With increasing tFe the Fe clusters first connect along the −110 , but not along the 110 direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at tFe=4 ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures TB of 62±5, 80±10, and 165±5 K for tFe=1.9, 2.2, and 2.5 ML, respectively. At T TB, freezing of superparamagnetic clusters is inferred from the observed quasilinear T dependence of the mean hyperfine magnetic field Bhf . By combining the STM and CEMS results, we have determined a large magnetic anisotropy constant of 5 105 and 8 105 J /m3 at tFe=1.9–2.2 and 2.5 ML, respectively. For tFe 2.5 ML, our uncoated “free” Fe clusters exhibit intrinsic magnetic ordering below TB, contrary to literature reports on metal-coated Fe clusters on GaAs. Our present results demonstrate that the nature of the percolation transition for free Fe nanoclusters on GaAs 001 in UHV is from superparamagnetism to ferromagnetism. From the Mössbauer spectral area, a very low Debye temperature D of 196±4 K is deduced for these uncoated Fe nanoclusters in UHV, indicating a strong phonon softening in the clusters.