G. Bayreuther

Hysteresis properties of ultrathin ferromagnetic films

We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

In-plane volume and interface magnetic anisotropies in epitaxial Fe films on GaAs(0 0 1)

Epitaxial Fe films were grown on Ga-terminated GaAs(0 0 1) surfaces by molecular beam epitaxy. Samples with constant Fe thickness as well as step patterned Fe films have been studied. In-plane magnetic anisotropy energies were determined from hysteresis loops measured by alternating gradient magnetometry and magneto-optic Kerr effect. A superposition of anisotropies with fourfold and uniaxial symmetry was found in all films. From the linear variation of both contributions with the inverse Fe thickness, the volume and the interface term are determined. The fourfold anisotropy constant of the Fe/GaAs(0 0 1) interface amounts to - (1.41 ± 0.2) × 10 -2 erg/cm 2 . As a consequence, the easy and hard directions of the fourfold term are rotated by 45° below 6 ML. The uniaxial anisotropy turns out to be a pure interface term originating exclusively from the Fe/GaAs interface. The huge anisotropy constant, K Fe/GaAs U = (1.2 ± 0.2) × 10 -1 erg/cm 2 , produces an in-plane anisotropy field up to 2 kOe.

Magnetic films epitaxially grown on semiconductors

Abstract Future ‘magneto-electronics’ exploiting the spin of the electron in addition to its charge will require the injection of spin-polarized electrons from a ferromagnetic metal into a semiconductor. To this purpose an attempt has been made in the present study to avoid the formation of a non-magnetic interface phase or ‘magnetically dead layers’ which have been found in the past in Fe films epitaxially grown on GaAs(0 0 1). Fe(0 0 1) films were grown by molecular beam epitaxy and magnetron sputtering on As depleted GaAs(0 0 1) surfaces held at room temperature. Very good epitaxial growth is achieved by both deposition methods. MBE grown ultrathin films show an enhanced ground state net magnetization and the full bulk magnetic moments at the Fe/GaAs interface in contrast to previous reports. This might be an important step towards semiconductor devices using spin polarized electron transport. The in-plane magnetic anisotropy of the films generally consists of a fourfold and a uniaxial term. At 7 monolayer thickness only a strong uniaxial contribution is observed which is supposed to result from the intrinsic anisotropy of the dangling bonds at the GaAs(0 0 1) surface. This could also be a useful property for future memory or switching applications.

Increases in magnetic hyperfine field at the surface of ultra‐thin epitaxial Fe films (invited)

Previous investigations of ultra‐thin epitaxial Fe (110) films grown on Ag (111) have indicated that there is a measurable increase at 4.2 K in the magnetic hyperfine field compared with bulk value in iron atoms in the first three surface layers of the films. As these films were produced in ultra‐vacuum and then removed for study by transmission Mossbauer spectroscopy it was necessary to provide a protective overcoat to prevent oxidation of the surface. Because of this it was not possible to rule out interaction of the Fe surface with the overcoat layers as a cause of the increased surface hyperfine field. A series of new films have been made of isotopically pure 56Fe. A thin probe layer of 57Fe ranging in thickness from 4–10 A was then deposited on the film surface and a variety of different overcoat layers subsequently chosen to protect these films. Overcoating materials included Ag, Au, Cu, Ge, MnF2, and NaCl. While there were differences in the hyperfine fields and isomer shifts of the 57Fe probe laye...

Magnetism of ultrathin FeCo (001) films on GaAs(001)

Epitaxial Fe34Co66 films in a thickness range from 3 to 100 monolayers (MLs) were grown by molecular beam epitaxy on GaAs(001) at room temperature. The growth was characterized by reflection high energy electron diffraction and x-ray diffraction. The magnetic properties were investigated by alternating gradient magnetometry magneto-optic Kerr effect, and superconducting quantum interference device magnetometry. The films show a strong interface-induced uniaxial in-plane anisotropy with the easy axis along [110]. In addition, the fourfold anisotropy coefficient changes sign around 6 ML i.e., the easy axis of the fourfold anisotropy switches from 〈110〉 to 〈100〉 with decreasing thickness.

Epitaxial Fe films on GaAs(001): Does the substrate surface reconstruction affect the uniaxial magnetic anisotropy?

The in-plane magnetic anisotropy of Fe films epitaxially grown on GaAs(001), in addition to a thickness-dependent four-fold contribution has a uniaxial component originating from the Fe/GaAs interface. This has been observed in several previous investigations. The orientation of the uniaxial easy axis (e.a.), however, was found to be along the [110] direction in most studies, but also an e.a. parallel to [−110] was reported in a few cases. It has been suggested that different reconstructions of the GaAs surface prior to Fe deposition could be responsible for this discrepancy. In the present contribution, it is shown that in Fe(001) films grown by molecular-beam epitaxy on Ga-rich GaAs(001) surfaces at room temperature the uniaxial anisotropy always has its easy axis along [110] with practically the same magnitude. In particular, the surface reconstruction of the GaAs substrate — either (4×2) or (2×6) — has no effect on the resulting uniaxial magnetic anisotropy. This [together with recent results related ...

Pure nuclear Bragg reflection of a periodic 56Fe/57Fe multilayer

Grazing incidence nuclear multilayer diffraction of synchrotron radiation from a periodic stack of alternating 56Fe and 57Fe layers was observed. Resonant layer fraction, substrate size, flatness, and surface roughness limits were optimized by previous simulations. The isotopic multilayer (ML) sample of float glass/57Fe(2.25 nm)/[56Fe(2.25 nm)/57Fe(2.25 nm)]×15/Al(9.0 nm) nominal composition was prepared by molecular beam epitaxy at room temperature. Purity structure and lateral homogenity of the isotopic ML film was characterized by magnetometry, Auger electron, Rutherford backscattering, and conversion electron Mossbauer spectroscopies. The isotopic ML structure was investigated by neutron and synchrotron Mossbauer reflectometry. Surface roughness of about 1 nm of the flat substrate (curvature radius >57 m) was measured by scanning tunneling microscopy and profilometry. A pure nuclear Bragg peak appeared in synchrotron Mossbauer reflectometry at the angle expected from neutron reflectometry while no ele...

Experiments on ferromagnetic surfaces and thin films

Abstract Results of recent experiments on magnetic properties of ferromagnetic 3d metal surfaces and thin films are compared with each other and with theoretical predictions. Besides electron spin polarization data Mossbauer and SQUID magnetometer measurements of thin Fe films are discussed together with possible improvements required in future investigations.

Element-specific imaging of magnetic domains at 25 nm spatial resolution using soft x-ray microscopy

The combination of magnetic circular dichroism as a magnetic contrast mechanism and a transmission x-ray microscope allows imaging of magnetic structures with lateral resolutions down to 25 nm. Results on magneto-optical Tb25(Fe75Co25)75 layers system with thermomagnetically written bits of various sizes were obtained at the x-ray microscope XM-1 at the Advanced Light Source in Berkeley, CA. The results prove the thermal stability of the bits in the recording process. Furthermore the capability of soft x-ray microscopy with respect to the achievable lateral resolution, element specificity and sensitivity to thin magnetic layers is demonstrated. The potential of imaging in applied magnetic fields for both out-of-plane and in-plane magnetized thin magnetic films is outlined.

Onset of ferromagnetism in Fe epitaxially grown on GaAs(001) (4×2) and (2×6)

Ultrathin Fe films were epitaxially grown at room temperature on GaAs(001) with either predominant (4×2) or (2×6) surface reconstruction. At nominal Fe coverages of tFe⩾2.8 monolayers (ML), a ferromagnetic state is observed below a certain critical temperature, TC. Surprisingly, the magnetic phase transition at TC appears even sharper than for Fe films on metallic single-crystal substrates, which were believed to be an excellent representation of two-dimensional (2D) ferromagnets. This may be due to the extremely short lateral length scale of film inhomogeneities. The critical exponent β=0.26 is close to the value expected for 2D XY systems of finite size. For tFe=3.6 ML, TC is close to room temperature. TC decreases steeply with decreasing Fe coverage, with an average slope of 270 K/ML. From a power law extrapolation, TC seems to vanish at tFe=2.5 ML. The onset of ferromagnetism at tFe=2.5 ML is interpreted as a percolation phenomenon during the coalescence process of Fe islands.