P. Schuster

Influence of the growth conditions on the magnetic properties of fcc cobalt films: from monolayers to superlattices

Abstract The growth and magnetic properties of films of fcc cobalt on Cu(100) substrates has been characterized by a multitechnique approach. The films are ferromagnetically ordered in-plane at temperatures below Tc. The Curie temperature of the films displays a linear dependence with the coverage reaching bulk-like behaviour at coverages of 5–6 monolayers. Spin-polarized photoemission shows that the band structure is already close to that of the bulk at 5 ML. Crystalline Co/Cu sandwiches and superlattices have been grown on Cu(100) substrates. The magnetic ordering of Co slabs across Cu layers of varying thicknesses, as explored by SMOKE, changes from ferromagnetic to antiferromagnetic and back to ferromagnetic. The antiferromagnetic ordering has been confirmed by polarized neutron diffraction. As a function of the external magnetic field, the magnetic ordering changes to ferromagnetic with a complex intermediate behaviour.

Experimental evidence of an oscillatory magnetic coupling in Co / Cu / Co epitaxial layers

Abstract Crystalline Co / Cu / Co sandwiches have been grown epitaxially on Cu(100) substrates. In-situ surface magneto-optic Kerr effect data indicate that the magnetic ordering of Co slabs across Cu layers of varying thicknesses oscillates from ferromagnetic to antiferromagnetic several times as function of the Cu thickness. The number and period of the oscillations and the ferromagnetic character of the coupling at low Cu thickness, are indications that an RKKY-like coupling is acting.

Spin- and angle-resolved photoemission from single crystals and epitaxial films using circularly polarized synchrotron radiation

Abstract We describe some novel results on the experimental spin-resolved electronic structure of a layered system, consisting of ultrathin films of fcc-Cobalt epitaxially grown onto a Cu(100) single crystal surface. Films more than 2 mono-atomic layers (ML) thick are found to be ferromagnetic at room temperature with an in-plane remanent magnetization. The electronic structure of 5 ML fcc-Co films turns out to be already bulk-like. On the other hand, the excitation with circularly polarized light leads to strong spin-polarization effects from the clean Cu(100) surface. This proves a significant influence of the spin-orbit interaction even in copper, resulting in strongly hybridized electronic energy bands.

Epitaxy and magnetic properties of fcc cobalt films on Cu(100)

The epitaxial growth of fcc cobalt films on top of a Cu(100) substrate has been investigated by a variety of methods including Auger Electron Spectroscopy (AES), Medium Energy Electron Diffraction (MEED) and Thermal Energy Atom Scattering (TEAS). In a simultaneous AES/MEED experiment the growth mode was determined to be of the Frank-v.d. Merwe type. The magnetic properties of these films have been studied in situ by means of the Surface Magneto Optical Kerr Effect (SMOKE). We observed a strong thickness dependence of the Curie temperature leading to T c ≈ 300 K for a cobalt coverage of 2 atomic layers. The orientation of the magnetic moments, which was always found to be within the film plane, shows a distinctive in-plane magnetocrystalline anisotropy. In fcc cobalt films with a thickness of more than 3 monolayers we identified the 〈110〉 directions as the easy axes of magnetization.

Crystallography of epitaxial face centered tetragonal Co/Cu(100) by low energy electron diffraction

A crystallographic LEED I/V characterization of thin Co/Cu(100) films is presented. Results from dynamical LEED calculations are compared with experiments via Pendrys reliability factor. The first Co layer grows with a small, but noticeable, amount of Co atoms in the second layer. For thicker films up to at least 10 ML, Co is found to grow on Cu layer by layer in a tetragonally distorted structure. The film adopts the lateral Cu spacing with an in-plane lattice expansion and a contraction of the vertical interplanar distances. By subsequent Cu on Co deposition, lateral and vertical Cu spacings are kept, reproducing the initial suhstrate structure.

Spin-Polarized Photoemission from f.c.c.-Cobalt above the Curie Temperature: Evidence of Short-Range Magnetic Order

We report on the first spin- and momentum-resolved photoemission studies on epitaxial f.c.c.-cobalt films above the Curie temperature. In order to obtain a thorough characterization of the electronic states, the photoelectrons have been excited by circularly polarized VUV synchrotron radiation and a vectorial spin analysis has been employed to determine size and spatial orientation of the photoelectron spin polarization. This novel experimental approach allows a direct comparison of the spin-resolved photoemission spectra from the ferromagnetic cobalt overlayer and the paramagnetic copper substrate. The results indicate a failure of the Stoner model in f.c.c.-Co and suggest the existence of a short-range ferromagnetic order above TC.

Spin polarised angle resolved uv photoelectron spectroscopy study of Cu3Au(001) using circularly polarised synchrotron radiation

We report the first spin-resolved photoemission study of the electronic structure of Cu 3 Au(001) using circularly polarised synchrotron radiation in the energy range 9-30 eV. In particular, this unique technique was applied to study the spin-orbit splitting of electronic bands in Cu 3 Au, the interpretation of which is still a controversy. We have identified the symmetry characters of the initial states in the Au region (i.e. between 4–7 eV binding energy). Near the X-point, we find the splitting between Au-like bands of Δ 5 7 and Δ 5 6 symmetry to be about 0.8 eV. This value is in agreement with recent calculations [J W Davenport, R E Watson and M Weinert, Phys Rev, B37 , 9985 (1988)]. The experimental value of spin-orbit splitting of 0.2 eV in the Cu region is more than in pure Cu, indicating at least some hybridisation with the Au states.

Influence of Growth and Structure on the Magnetism of Epitaxial Cobalt Films on Cu(001)

Artificially layered materials and ultrathin films of ferromagnetic substances currently receive considerable interest in materials science. These systems often show novel magnetic properties which differ dramatically from those of the corresponding bulk ferromagnets, e. g. , unexpected magnetic anisotropics or surprisingly low Curie temperatures. Research in this field is stimulated by both scientific curiosity and potential applications in magnetic storage and sensor technology. A successful development of new storage devices and technologies requires a fundamental knowledge of the underlying physical mechanisms, in order to be able to deliberately modify the magnetic properties of a given material or system.

Spin‐resolved electronic band structure of fct‐cobalt (100) (abstract)

The spin‐polarized electronic structure of the fcc high‐temperature modification of cobalt has not been investigated yet, because of the experimental difficulties of photoemission at high temperature. We stabilized fcc Co by molecular‐beam epitaxy on a Cu(100) substrate with large, atomically flat terasses (0.5–1 μm wide) as revealed by STM. The structure of the layers was studied by LEED and MEED, showing a tetragonal distortion of the fcc lattice perpendicular to the (100) surface plane. The dispersion of the exchange split bands perpendicular to the surface was determined for a 5‐monolayer‐thick sample (tetragonal distortion on average 4%–5%) by spin‐ and momentum‐resolved photoemission. The results are compared to two relativistic spin‐polarized band‐structure calculations for fcc cobalt. Somewhat surprisingly, even a 5‐ML‐thick sample shows three‐dimensional dispersion in good agreement with the calculations, as far as the average exchange splitting (1.2±0.2 eV), and the symmetry character of the ban...

Correlation of Crystalline and Electronic Structure in Epitaxial FCC-Cobalt Monolayers on Cu(100)

Initiated by the need for improved magnetic recording media, recently the ferromagnetism of low-dimensional systems has received wide interest. A layered system may be realized by epitaxially growing films of a ferromagnetic material on top of a suitable non-magnetic substrate. The magnetism of these systems, however, is strongly determined by the electronic structure of the films which in turn depends on their crystalline structure. The knowledge of this correlation is of major importance in the understanding of the magnetic properties of ultrathin films (being a necessary basis for successful technological application). The task to characterize the crystalline and electronic structure may be accomplished by a combination of in situ LEED and angle-resolved photoemission (ARUPS) experiments. Ambiguities in the classification of majority and minority spin states in a ferromagnet (being immanent to the conventional spin-integrated ARUPS method) can only be avoided by including a spin-polarization analysis of the photoelectrons. The present contribution outlines exemplarily the interplay of crystalline and electronic structure in the model system fcc-Cobalt on Cu(100) by means of the approach sketched above.