Ch. Morawe

Structure and thermal stability of sputtered metal/oxide multilayers: The case of Co/Al2O3

Co/Al2O3 multilayers have been grown on sapphire (1120) substrates by rf sputtering techniques. In situ resistance measurements reveal percolation thresholds of Co between 1.5 and 2.5 nm. Continuous metallic layers appear at thicknesses beyond 3.0 nm. Co layers grow in a polycrystalline manner, whereas Al2O3 layers are amorphous, as inferred from x‐ray and transmission electron microscope studies. By fitting reflectivity data, interface roughnesses of only 0.25 nm were obtained. Annealing experiments up to 800u2009°C show a gradual internal oxidation at the Co/Al2O3 interface, while the overall structure and the multilayer period remain stable with constant interface roughnesses and reflectivity values. At 900u2009°C the layered structure is partly destroyed and Co forms crystallites thicker than one former Co layer.

Structural and magnetic studies of FexCo1−x(001) alloy films on MgO(001) substrates

Thin epitaxial FexCo1−x alloy films sputtered on MgO(001) substrates were studied by x‐ray scattering and by the magneto‐optical Kerr effect in order to investigate the structural phase stability induced by the substrate and to relate the magnetic properties to the alloy film structure. Alloy films with a thickness of about 20 nm and concentrations from x=1 to x=0.3 exhibit a bcc (001) growth with the in‐plane [100] axis parallel to the MgO[110] axis. For pure Co (x=0) a fcc (001) structure with the Co[100] axis parallel to the MgO[100] axis was obtained. For x=0.15 no out‐of‐plane or in‐plane Bragg peak could be observed, possibly due to the bcc‐fcc structural phase transition. Kerr rotation angles in saturation as a function of x reveal a shape similar to the Slater–Pauling curve. Magnetic hysteresis measurements reveal a fourfold anisotropy with easy axes parallel to the FexCo1−x[100] crystal axis for 1≥x≥0.8, and parallel to the FexCo1−x[110] crystal axis for 0.7≥x≥0.3. In pure Co the easy axis lies p...

Optimization of sputtered Co films

Abstract Co films in the thickness range of 120–230 nm have been sputtered on quartz glass and monocrystalline sapphire (11 2 0) substrates. A preferential orientation of the hcp [0001]- and the fcc [111]-axis, respectively, has been observed at low sputtering rates (0.05 nm/s) and after optimization of the substrate temperature Ts. For 200°C ≤ Ts ≤ 300°C the structural coherence length normal to the film-plane reaches its maximum. The measured lattice spacings d are between the bulk values d(0002) and d(111) of the hcp- and the fcc-phase, respectively, and reach a minimum in the same range of temperatures. REM investigations supplement the X-ray measurements. Furthermore, thinner Co films (10–25 nm) were sputtered at 250 and 300°C. X-ray patterns show superior film quality at 300°C, slightly dependent on Ar pressure.

METAL/AL2O3 : A NEW CLASS OF X-RAY MIRRORS

We have examined sputtered metal/Al2O3 multilayers (metal: Co, Pt, W) with respect to their suitability as x‐ray mirrors in high temperature environments. The rf‐sputtering technique leads to layered structures with interface roughnesses of only 0.2 nm as confirmed by x‐ray scattering and transmission electron microscopy. In situ resistance measurements characterize the percolation process and indicate the lower thickness limits for the chosen materials which lead to minimum modulation periods of ≊2 nm. The samples were characterized after stepwise annealing up to 1000u2009°C. Reflectivity values of nearly 100% at the first satellite‐reflection were found in the case of Pt and W (for λ=0.15418 nm). The most stable W/Al2O3 multilayer reflects 70% of the primary beam even after annealing at 900u2009°C.

FMR studies of magnetic properties of Co and Fe thin films on Al2O3 and MgO substrates

The effect of substrates on the magnetic properties has been studied for Co and Fe films both on Al2O3 (1120) and MgO (001) substrates by using ferromagnetic resonance techniques. For Fe(001)/MgO(001) samples the thickness dependence of the magnetocrystalline constant and of the effective magnetization values have been determined from the in‐plane angular variation of the resonance field H0. Different reasons for the thickness dependencies of these parameters are discussed. For Co(111)/Al2O3(1120) the angular variation of H0 exhibits an uniaxial anisotropy, for which several causes are discussed. For Co(1120)/MgO(100) a four‐fold in‐plane anisotropy was observed which is due to the twinned structure of these samples.

Spin polarized neutron reflectivity study of a Co/Cu superlattice

We present spin polarized neutron reflectivity data on a Co/Cu(111) superlattice and show how not only the magnitude but also the orientation of the average magnetic moment of each layer can be extracted by analyzing the polarization of the reflected beam. This method allows more detailed conclusions about the exchange coupling of magnetic layers across nonmagnetic interlayers and the magnetic in‐plane anisotropy in such systems. We present a theoretical fit to the spin‐flip and non‐spin‐flip data which leads to quantitative conclusions about the spin structure. These spin polarized neutron reflectivity results coincide well with the macroscopic magnetic properties which were measured using the magneto‐optic Kerr effect revealing a newly discovered uniaxial anisotropy in this system.

Structural and magnetic properties of Co/face-centered-cubic Mn(001) multilayers

The structural and magnetic properties of Mn in Co/Mn (001) multilayers were investigated by x‐ray scattering, by magneto‐optical Kerr effect measurements, and with a Faraday balance. The multilayers were deposited by rf‐sputtering on MgO(001) substrates. Small angle x‐ray reflectivity scans and high angle Bragg scattering confirmed good layer quality and crystal coherence properties. In‐ and out‐of‐plane x‐ray scattering revealed a coherent growth of face‐centered‐cubic (fcc) (or fct) Mn on fcc Co(001). The stability limit of the metastable fct Mn structure is about 20 A or 10 atomic layers. The magnetic exchange coupling appears to be partly antiferromagnetic with a first maximum located between 20 and 30 A. Saturation magnetization measurements indicate no ferromagnetic moment for the fct Mn layers. Furthermore, the saturation moment per unit volume is reduced as compared to the bulk Co value, suggesting the presence of ‘‘dead layers’’ at the Co/Mn interfaces.

Thermal stability and superlattice formation of sputtered platinum/alumina multilayers

By high‐frequency sputtering techniques we prepared high‐quality platinum/alumina artificial multilayers (Pt/Al2O3) with interface roughnesses below 0.2 nm and x‐ray reflectivities close to 100% at the first satellite reflection using a wavelength of 0.1542 nm. The multilayer structure remains stable up to at least 700u2009°C without loss in reflectivity. Further annealing causes a structural phase transition with the formation of a coherent superlattice structure, which is finally destroyed at 1000u2009°C.

59Co NMR investigations of sputtered Co/Cu (100) and (111) multilayers

Abstract In this paper we present the results of a 59Co NMR investigation of quasi-single crystal Co/Cu (100) and (111) multilayers grown by sputtering. The NMR spectra show that these films have a high degree of crystallinity with the distribution of effective magnetic fields (hyperfine field) similar to that found in the first MBE-grown materials.

Structure and magnetic properties of Fe films on Al2O3(112¯0) and MgO(001)

Abstract Structural and magnetic properties of Fe thin films (4–60 nm thick) on Al 2 O 3 (112¯0) and MgO(001) have been studied by X-ray scattering and Kerr effect measurements. Fe on Al 2 O 3 exhibits a strong in-plane uniaxial anisotropy, possibly due to the oxidation of Fe films on Al 2 O 3 substrates. In contrast, no interfacial anisotropy has been observed for Fe on MgO, indicating a very weak interaction of Fe with MgO.