U. Gradmann

Magnetic surface anisotropies

Abstract A review is given on experimental methods to determine magnetic surface anisotropies, and on experimental results for well-defined single-crystal surfaces, both for out-of-plane and in-plane anisotropies. Results are compared with Neels phenomenological theory.

Magnetic anisotropies in Fe(110) films on W(110)

Magnetic anisotropies in epitaxial Fe(110) films on W(110) were analyzed using torsion oscillation magnetometry (TOM) in situ in UHV. Films with clean surfaces and films coated by Cu, Ag, and Au were analyzed. Out-of-plane anisotropies were determined by standard TOM methods, in-plane anisotropies from hard-axis magnetization loops. Since all anisotropies showed a clear linear dependence on reciprocal film-thickness, a straightforward separation of volume and surface type anisotropies could be performed. Volume anisotropies can be explained as a superposition of standard fourth-order magnetocrystalline and strain anisotropies, caused by residual strain of the Fe coincidence lattice on W. Surface anisotropies, both in-plane and out-of-plane, are related by Néels model to the bulk magnetoelastic constants, to a surprisingly good approximation.

Kinetic facetting in homoepitaxy of Fe(110) on Fe(110)

Abstract The homoepitaxial growth of Fe(110) on atomically smooth Fe(110) surfaces was analyzed using high angular resolution, spot profile low energy electron diffraction (SPA-LEED). Spot profiles were observed near normal incidence for films consisting of up to 10 atomic layers, grown at temperatures between 200 and 500 K. The profiles show a clear separation of a central spike of instrumental width from a background consisting of two broad shoulders, arranged symmetrically with respect to the central spike along the [110] direction. The distance between the shoulders varies linearly with the scattering vector, being zero for in-phase and maximum for out-pf-phase conditions. The surface structure therefore consists of a quasi-periodic sequence of up and down staircases along [110], with ridges along [001]. It forms by a facetting process governed by free diffusion on atomic planes in combination with an energy barrier which prevents hopping between the planes (layer restricted diffusion, LRD). The one-dimensional facet structure results from the lowered symmetry of the surface. Apparently, an anisotropic sticking probability of atoms on atomic steps, being high at steps with edges along [110] directions and low at steps with edges along [001], results in nucleation by stripes elongated along [001]. The distance of these stripe nuclei defines the period of the ridge structure which therefore is roughly independent of film thickness. Apparently, the growth virtually consists in a step flow of atoms towards the ridges, with subsequent nucleation of new planes on top of them, and a roughness which increases with the square root of the thickness. The period increases with increasing preparation temperatures. Annealing of the facets at 600 K restores the initial flat surface, thus confirming the kinetic nature of the facetting process.

Perpendicular magnetization and dipolar antiferromagnetism in double layer nanostripe arrays of Fe(110) on W(110)

Fe(110) nanostripe arrays, consisting of alternating monolayer and double layer stripes, have been grown by step flow on vicinal W(110) substrates. The magnetic easy axis switches from in-plane in the monolayer to perpendicular in the double layer stripes. The data strongly suggest that magnetostatic interactions induce antiferromagnetic order in the double layer nanostripe array. It can be switched into a ferromagnetic arrangement by low external fields.

Magnetic anisotropies of Fe(110) interfaces

Abstract Magnetic surface and interface anisotropies were determined for interfaces of Fe(110) with UHV, W, Cr, Cu, Ag and Au. This determination of single interface anisotropies is based on torsion oscillation magnetometry in situ in UHV of Fe(110)-films on Cr(110), in extension of previous work on Fe(110)-films on W(110). The out-of-plane anisotropy of the W(110)/Fe(110) interface is easy plane with an extraordinary strength of 1.92 mJ/m 2 , whereas all other interfaces show perpendicular out-of-plane anisotropy. The in-plane surface anisotropy supports [001] for Fe / Cr and Fe / Au interfaces, it supports [1 1 0] for W / Fe, Fe / UHV, Fe / Cu and Fe / Ag interfaces. Higher order terms of angular dependence beyond Neels quadratic approximation are needed for a full description of magnetic surface anisotropies. A convenient notation is proposed.

Ferromagnetism in the thermodynamically stable monolayer Fe(110) on W(110), coated by Ag

Ferromagnetic order in the thermodynamically stable, pseudomorphic monolayer Fe(110) on W(110), coated by Ag, was studied in situ in UHV using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). Films near the monolayer coverage, prepared at 475 K, consist of nearly independent monolayer and double-layer patches. The properties of monolayer patches are nearly independent of the mean film thickness resulting in excellent conditions to determine the true monolayer properties. The Curie temperature is reduced toTc, mono= 282 K = 0.27 Tc,bulk, the ground state hyperfine field is reduced toBhf(0)=11.9 T = 0.35Bhf,bulk(0) and the magnetic moment per atom is enhanced toμ(0) = 2.53 μB=1.14μ(0)bulk. Remanent magnetization is detected forT ≦260 K=0.92Tc, mono, square loop magnetization forT ≦230 K=0.82Tc, mono. Unusual properties of the phase transition are detected by the combination of both experimental techniques.

Magnetometric analysis of the interaction of Ni(111) with oxygen

The interaction of oxygen with Ni(111) films on Re(0001), consisting of 18–50 atomic layers, was analyzed using LEED, AES and torsion oscillation magnetometry in situ in UHV (U-TOM) at room temperature. The sequence of rapid chemisorption up to θ = 13, followed by slow oxidation to 3 layers NiO, by lateral growth of islands, known from bulk crystals, has been confirmed. The initial sticking coefficient is determined to be So = 0.52 ± 0.1; it persists up to θ = 0.2. In the chemisorption state θ = 13 a minimum Smin ≤ 0.005 is observed. For the initial chemisorption (θ ≤ 0.2) the reduction of magnetic moment per O atom, ΔμOx, in comparison with the metallic Ni moment μNi, is given by ΔμOx = -4.5μNi. It is reduced to ΔμOx = -3.8μNi for θ = 13. The total reduction Δm of the magnetic moment saturates at ∼ 100 L with Δm = -2.7mML (moment mML of bulk monolayer Ni), corresponding to 3 monolayers of antiferromagnetic NiO. The surface anisotropy field is reduced from 9.9 ± 1 T for the free surface to 0 ± 1 T, both for the chemisorbed and the oxidized surface.

Magnetization of free Fe(110) surfaces from thin film magnetometry

Abstract Using torsion oscillation magnetometry of uncovered Fe(110) films on W(110) in situ in UHV, we have determined a surface excess moment in free Fe(110) surfaces of 0.39(16) monolayer equivalents. The results are compared with previous determinations using SPLEED, and with theoretical values.

Magnetic interface anisotropies of Co/Cu(111) and Co/Au(111) interfaces from ultrathin Co films on Cu(111)

Abstract Magnetic interface anisotropies of Co/Cu(111) and Co/Au(111) were determined by magnetometry of Cu/Co/Cu(111) and Cu/Co/Au(111) sandwiches. Being determined using Co films which were coherent with the Cu-substrate, the resulting anisotropy constants K s CoCu = -0.18 mJ/m 2 and K s CoAu = -0.37 mJ/m 2 are free from magnetoelastic contributions.

Oscillating indirect coupling of perpendicular magnetized Co(111)-monolayers through Cu(111)

Abstract Oscillating indirect coupling has been observed between perpendicularly magnetized Co(111)-monolayers on Cu(111). through a Cu(111)-interlayer. Contrarily, a weak indication of oscillation only was observed between in-plane magnetized Co(111)-films containing 5 monolayers each. Possible mechanisms for the strongly differing coupling behaviour observed in the Cu/Co(111) system are discussed.