K. Bröhl

Epitaxial growth of Co films and Co/Cu superlattices on sapphire substrates with and without buffer layers

Co(0001)/(111) films and Co/Cu(111) multilayers were grown by MBE on sapphire (1120) substrates. Co grows in a good epitaxial manner directly on the sapphire substrate as well as on a Cu(111)/Nb(110) buffer layer system. Co/Cu(111) superlattices were grown on the same buffer system. Extensive RHEED and X-ray characterizations demonstrate the high film quality.

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.

UNIAXIAL MAGNETIC ANISOTROPY OF CO FILMS ON SAPPHIRE

Abstract We have studied the magnetic properties of thin Co(0001) films on sapphire (11 2 0) substrates for thicknesses ranging from 10 to 40 nm. In this range the Co films, grown by molecular beam epitaxy techniques, exhibit a well defined orientational relationship to the sapphire substrate and a high structural coherence parallel to the film normal. Longitudinal magneto-optic Kerr effect measurements are presented which show that in addition to the usual in-plane anisotropy the magnetization strongly depends on the azimuthal angle between the magnetic field and an in-plane crystallographic direction. Instead of the expected six-fold crystal anisotropy we find a uniaxial anisotropy. This azimuthal anisotropy is solely due to an interface interaction and may be caused by internal oxidation of those Co atoms which are close to the sapphire substrate.

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.

Magnetic in-plane anisotropy of MBE grown Co/Cu(111) superlattices

Abstract The magnetic in-plane anisotropy of molecular-beam epitaxy (MBE) grown Co/Cu(111) superlattices is investaged by ferromagnetic resonance (FMR). It is shown that the sixfold anisotropy contribution expected for fcc(111) and hcp(0001) oriented films is small, but nevertheless can be detected. This is mainly due to the resonance condition being governed by the derivatives of the free energy density, so that contributions from higher order terms enter with higher weighting. Evidence is found for a correlation between the in-plane anisotropy parameters and the structural quality as well as the magnetic coupling behaviour. It is proposed to use the sixfold in-plane anisotropy as a very sensitive test for the structural in-plane coherence of thin films and superlattices containing Co grown on a (111)-oriented plane.

NMR studies of 59Co in Cu/Co/Cu(111) trilayer systems grown by MBE

Abstract NMR using the 59 Co nucleus has been measured for a series of Cu/Co/Cu(111) trilayers grown by MBE with Co thicknesses ranging from 25 to 178 A. The Co layer retains the Cu fcc structure up to a thickness of ∼ 60 A at which point the hcp structure is favoured. This structural transition is not identical for each sample suggesting that the point at which the transition occurs depends on the exact growth conditions including the thickness of the Cu underlayer. Our results suggest that for very thin (15 A) Cu underlayers the transition to hcp occurs at thicknesses less than 60 A.

FMR study of MBE-grown Co films on Al2O3 and MgO substrates

Abstract FMR studies of epitaxial Co(0001) films on Al2O3(11 2 0) and Co(11 2 0) on MgO(001) substrates have been performed at the X-band and in the temperature range 80–400 K. For the samples on sapphire substrates the resonance field behavior as a function of the dc magnetic field orientation in the plane of the film exhibits a uniaxial anisotropy. In contrast, for the Co films on MgO substrates a fourfold anisotropy in the film plane has been observed. Both in-plane anisotropies are unexpected with regard to the Co film structure. The FMR analysis shows that the fourfold anisotropy for Co/MgO occurs due to a twinned film structure consisting of two Co domains with their c-axes in the film plane and perpendicular to each other.

Anisotropy studies of molecular‐beam‐epitaxy‐grown Co(111) thin films by ferromagnetic resonance

The magnetic anisotropy of Co/Cu(111) thin films has been investigated using the ferromagnetic resonance (FMR) technique. The films were prepared by molecular‐beam epitaxy in ultrahigh vacuum on sapphire substrates with niobium as a buffer layer. In situ RHEED investigations, ex situ x‐ray low‐angle reflectivity measurements, and high‐angle Bragg scans confirm the structural quality of the films. Angular dependent FMR measurements are performed in the plane of the films and out‐of‐plane. The angular dependence of the line positions in‐plane shows a competition between the sixfold anisotropy, which is expected for bulk Co, and a twofold anisotropy contribution. It is demonstrated that by FMR one can detect even small values of the higher‐order anisotropy terms. In the analysis, we put emphasis on the relationship between magnetic and structural properties.

Anisotropy studies of AFM coupled MBE grown Co/Cu(001) superlattices

We have studied the anisotropy behavior of antiferromagnetically (AFM) coupled fcc Co/Cu(001) sandwiches and superlattices. Magneto‐optical Kerr effect measurements on samples with Cu film thicknesses in the range of the second antiferromagnetic maximum reveal characteristic shapes of the hysteresis loops in close agreement with theoretical predictions. For the AFM coupled samples we infer antiparallel spin orientations in remanence perpendicular to the originally applied magnetic field direction. Changing the magnetic layer thickness and maintaining a constant thickness of the spacer completely alters the shape of the hysteresis loops. The loops of weakly coupled AFM layers show characteristic steps, indicative for a rather unusual nonsymmetric spin state. This behavior can be explained by the competing effects of anisotropy with the exchange coupling.

Evidence for an anisotropy-induced non-collinear spin state in exchange-coupled Co/Cu(001)

The magnetization behaviour of a Co/Cu/Co(OOl) sandwich has been studied by magneto-optical Kerr effect measurements. The sample was grown by molecular beam epitaxy onto a sapphire (11.2) substrate with a Cu/Cr/Nb(OOl) buffer system. The copper layer had the form of a wedge with the thickness range chosen to be around the second region of antiferromagnetic exchange coupling. The hysteresis loops in the regime of weak antiferromagnetic coupling show characteristic steps, which can be explained by an anisotropy-induced non-collinear spin state. Indication for a similar behaviour is also found in the regime of strong antiferromagnetic coupling. This behaviour is explained by taking into account the competition between anisotropy, interlayer exchange coupling and external field energy. The nature of this metastable non-collinear magnetization state is in marked contrast to the biquadratic (90”) exchange coupling which was discovered in Fe/Cr(OOl).