Properties Of Ultrathin Epitaxial Films Research Articles

Second-order magnetoelastic effects: From the Dirac equation to the magnetic properties of ultrathin epitaxial films for magnetic thin-film applications

Abstract It has always been the endeavour of Professor Kronmüller to combine various methods in order to attain a comprehensive understanding of a physical phenomenon on all relevant scales. In this spirit, we combine the phenomenological nonlinear theory of magnetoelasticity with the relativistic density functional electron theory to describe the magnetoelastic behaviour of epitaxial films. We explain the two already performed cantilever bending beam experiments on the magnetoelasticity and underpin the assumption that second-order magnetoelastic effects are very important for epitaxial Fe films. A complete set of six cantilever experiments is introduced which allows to calculate the intrinsic first- and second-order magnetoelastic constants for cubic materials, and these constants are calculated ab initio for Fe, face-centred cubic Co, Ni, Ni3Fe and CsCl-type CoFe.

Liquid-Gated Ambipolar Transport in Ultrathin Films of a Topological Insulator Bi2Te3

Using ionic-liquid (IL) gating in electric-double-layer transistors (EDLTs), we investigate field-effect electrical transport properties of ultrathin epitaxial films of a topological insulator (TI), Bi(2)Te(3). Because of their extreme thinness, the Bi(2)Te(3) films show a band gap opening and resulting semiconducting transport properties. Near room temperature, an obvious ambipolar transistor operation with an ON-OFF ratio close to 10(3) was observed in the transfer characteristics of liquid-gated EDLTs and further confirmed by a sign change of the Hall coefficients. Modulation of the electronic states and a phase transition from a semiconducting conduction (dR(xx)/dT < 0) to a metallic transport (dR(xx)/dT > 0) were observed in the temperature-dependent resistance of the ultrathin Bi(2)Te(3) channel, demonstrating that the liquid gating is an effective way to modulate the electronic states of TIs.

Second-order magnetoelastic effects: From the Dirac equation to the magnetic properties of ultrathin epitaxial films for magnetic thin-film applications

Abstract It has always been the endeavour of Professor Kronmuller to combine various methods in order to attain a comprehensive understanding of a physical phenomenon on all relevant scales. In this spirit, we combine the phenomenological nonlinear theory of magnetoelasticity with the relativistic density functional electron theory to describe the magnetoelastic behaviour of epitaxial films. We explain the two already performed cantilever bending beam experiments on the magnetoelasticity and underpin the assumption that second-order magnetoelastic effects are very important for epitaxial Fe films. A complete set of six cantilever experiments is introduced which allows to calculate the intrinsic first- and second-order magnetoelastic constants for cubic materials, and these constants are calculated ab initio for Fe, face-centred cubic Co, Ni, Ni3Fe and CsCl-type CoFe.

Magnetic properties of ultrathin epitaxial films of iron

Ultrathin Fe films epitaxially grown on Cu(100), Au(100), Pd(100) and Ru(0001) substrates are used to study monolayer magnetism, surface magnetic anisotropy and two-dimensional (2D) critical behavior, by means of in-situ surface magneto-optical Kerr-effect measurements. Epitaxial structures and growth modes are characterized by auxiliary low-energy electron diffraction and Auger electron studies. The magnetic properties of the films depend on the growth temperature. For 100 K growth, all films exhibit a universal behavior of perpendicular spin orientations in the monolayer regime, while for 300 K growth, Fe/Pd(100) and Fe/Au(100) have in-plane easy-axis of magnetization for all thickness. Fe/Ru(0001) lacks a ferromagnetic response for thickness less than two monolayers (ML). Fe/Pd(100) has remarkable thermal stability in the monolayer and submonolayer range. Reversible magnetization data are obtained for 0.6 to 3.0 ML thick films, which show second-order phase transitions at thickness-dependent Curie temperatures ( T c ). An effective magnetization exponent β eff of 0.127 ± 0.004 is obtained for a 1.2 ML Fe/Pd (100) film, in agreement with the theoretical value of β c = 0.125 for an ideal 2D Ising model. The T c trend with film thickness for Fe/Pd(100) is also in agreement with Monte Carlo calculations for Ising films.

Spin-polarised neutron reflection studies of epitaxial films

The application of polarised neutron reflection (PNR) to investigations of the static magnetic properties of ultrathin epitaxial films is discussed. PNR yields the absolute value of the magnetic moment per atom of sandwiched monolayer thickness films with high accuracy, while in contrast with SQUID magnetometry for example, no magnetic signal requiring correction arises due to the substrate. PNR also provides valuable information relating to the interface quality. The roughness amplitude and correlation length can be estimated from the specular reflectivity and the angular width of the diffusely reflected beam respectively. We discusse the observed dependence of the magnetisation upon the transition metal layer thickness in the 1–10 ML range for Ag/Fe/Ag(001), Ag/Co/Ag(001), Cu/Co/Cu(001) and Ag/Cr/Ag(001) sandwich structures as an illustration of PNR as a magnetometric technique. The sensitivity of the wavevector-dependent reflectivity to the layer dependent magnetisation is illustrated by our recent investigations of sputtered Fe/Cr/Fe layers in the antiparallel coupling regime and of a ‘thick’ bcc Co/GaAs(001) film.

MAGNETO-OPTICAL STUDIES OF NOVEL EPITAXIAL FILMS

The magneto-optic Kerr-effect technique is used to explore the magnetic properties of ultra-thin epitaxial films of iron and chromium grown on (100) faces of gold and copper. The studies extend into the monolayer range, and are performed in the ultra-high-vacuum environment in which the films are grown. Auxiliary low-energy electron diffraction and Auger electron spectroscopy studies are used to characterize the epitaxy and growth-mode behavior. Highlights include: (a) an observed lowering of the Curie temperature, compared to that of the bulk, for monolayer-range iron grown epitaxially on the lattice-matched (100) face of gold; (b) correlations between magnetism and morphology; (c) a search for the predicted ferromagnetism of chromium monolayers; and (d) the observation of a new, metastable surface magnetic state for the high-temperature, face-centered-cubic phase of iron, which is stabilized via epitaxy on the (100) face of copper. The opportunities, challenges and limitations posed by the present magneto-optical approach in assessing the properties of these atomically-engineered materials will be evaluated.