Single-crystal (100)Fe films 90–330 Å thick have been grown on etch-annealed (100)GaAs substrates by molecular-beam-epitaxy techniques. Ferromagnetic resonance data indicate that the two in-plane 〈110〉 directions are inequivalent and, together with magnetometry data, show that the average film magnetization decreases as the thickness decreases. The inequivalence is attributed to the nature of the interface bonding at a (100) zinc-blende surface. The decreased magnetization is attributed to the formation of Fe2As microclusters in the film due to As diffusion which is supported by Auger and electron diffraction studies. In general, the Fe films grown to date on etch-annealed (100)GaAs substrates are significantly inferior to those grown on (110)GaAs.

We have fabricated a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute ferromagnetic semiconductor matrix. The clusters form for growth temperatures of ∼300 °C with an average diameter and spacing of 100 and 150 nm, respectively. While the clusters dominate the magnetic properties, the matrix plays a subtle but interesting role in determining the transport properties. Variable range hopping at low temperatures involves both nanoclusters and MnGe sites, and is accompanied by a negative magnetoresistance attributed in part to spin-dependent scattering analogous to metallic granular systems.

The electronic and magnetic character of epitaxial Fe films on Ag(001) has been studied as a function of Fe coverage by spin-and angle-resolved photoemission. At coverages well below a monolayer, the spectra exhibit a local spin-split electronic state. Although spectra for films in the monolayer coverage range display electronic structure in close agreement with calculated monoiayer-film critical-point energies, no spin polarization is observed up to 2.5 monolayers. Thicker films approach the spin-split electronic structure and spin polarization of bulk Fe(001).PACS numbers: 75.50. Bb, 75.10.Lp, 75.70.Dp, 79.60.Cn A fundamental problem of magnetism is at what dimension long-range ferromagnetic order occurs. The question of whether two-dimensional ferromagnetism is possible was raised long ago. 1 Powerful computational methods have since been developed to investigate the electronic and magnetic structure of ultrathin films within the local-density approximation. 2 "" 4 Recent calculations describing the spin-resolved band structure and consequent magnetic character of epitaxial Fe monolayers on Ag(001) predict an enhanced magnetic moment for one 3 ' 4 or two 4 such Fe monolayers (ML). This is believed to be due to a decrease in coordination number and an increase in nearestneighbor spacing experienced by the atoms comprising the monolayer, and a lack of hybridization between the electronic states of the overlayer and substrate. These studies thus address fundamental aspects of the formation and interplay between the electronic and magnetic character of a system.Spin-polarized angle-resolved photoelectron spectroscopy is well suited for investigating such effects, as it provides an independent identification and decomposition of spin-split emission features for samples exhibiting ferromagnetic order. An additional fundamental question addressed in most studies of magnetic surface layers or thin films is a determination of the onset of long-range magnetic order as a function of temperature or film thickness. There is strong evidence to suggest that the exchange splitting alone cannot be taken as an indication of spontaneous magnetization or as a measure of long-range ferromagnetic order. 5 " 7 Since spin-polarized angle-resolved photoelectron spectroscopy measures the net polarization of the photoelectrons, it provides a direct measure of longrange magnetic order (or absence thereof) required to address the predictions of enhanced moments.In this Letter, we report the results of spin-and angle-resolved photoemission studies of Fe films epitaxially grown in situ on Ag(001). The epitaxial system of Fe on Ag is well suited for these studies, since Ag has only very weak (sp) emission between the Fermi energy E F and =*3.5-eV binding energy, the energy range over which emission from the prominent Fe 3d bands occurs. With use of a photon energy of hv = 60 eV to minimize the photoelectron escape depth, optimum surface sensitivity is achieved. Use of this photon energy also permits direct comparison with previous work on...

Conversion-electron Mossbauer measurements have been used to study hyperfine fields in Fe/Ag(lOO) single-crystal multilayer films at temperatures down to 15 K. The Fe layer thicknesses corresponded to 1.0, 2.4, and 5.5 monolayers (ML). From the relative intensity of the Mossbauer lines we show conclusively that the orientation of the magnetic moment of the 1.0-and 2.4-ML films in zero applied field is perpendicular to the film plane while the orientation of the 5.5-ML sample is in plane at room temperature and partially out of plane at low temperatures. The low-temperature hyperfine fields are enhanced compared to bulk Fe.