Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at ¹¹¹In(¹¹¹Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed -angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.     

Crystallographic structure of ultrathin Fe films on Cu(100)

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.     

Photoinduced interface charging in multiphoton photoemission from ultrathin Ag films on Si(100)

The fluence dependence of two-photon photoemission and time resolved two-color pump–probe photoemission spectroscopy of a 25 ML thick Ag-film grown on n-doped Si(100) reveal a photoinduced work function reduction that is attributed to a reduction of the surface dipole. Time-resolved two-color pump–probe spectroscopy shows that this reduction persists for at least several microseconds. This and the pump-induced modification of the 4.65 eV two-photon photoemission spectrum indicate that the excitation of long-lived trap states at the Ag-Si interface affects the charge distribution in the Ag film and consequently is responsible for the reduction of the surface dipole. PACS 79.60.Bm; 79.60.-i; 82.50.Nd, 73.50.-h     

Kinetic limitations in electronic growth of Ag films on Fe(100)

Quantum well (QW) resonances are identified in Ag films on an Fe(100) surface and are used in low energy electron microscopy to monitor film morphology during annealing and growth. We find that Ag films thermally decompose to thicknesses that are stabilized by QW states at the Gamma point. Novel growth morphologies are also observed that highlight the competition between kinetic limitations and the QW state energetics that promote electronic growth. These combined observations help to explain the unusual bifurcation mode of thermal decomposition that was reported previously for this system.     

Theoretical calculation of magnetic properties of ultrathin Fe films on Cu(100)

The temperature dependence of the magnetization in fcc Fe on Cu(100) is calculated using a self-consistent local mean-field theory. The model reproduces an experimental magnetization oscillation as a function of film thickness and supports a picture where the top two layers are ferromagnetically coupled, and the remaining layers are antiferromagnetically coupled. The origin of the puzzling linear temperature dependence in oscillation amplitude is understood as a "surface phenomena" of the antiferromagnetic layer at the Fe/Cu interface. Proximity effects between a thin antiferromagnet with a low Neel temperature and a neighboring ferromagnet with a higher Curie temperature are discussed.     

Noncollinear magnetic hyperfine fields in the Ag spacers of Fe/Ag multilayers

Nearly perpendicular magnetic hyperfine fields have been observed for the first time in the Ag "spacers" of Fe/Ag multilayers using low temperature nuclear orientation of (110)Ag(m) at 6 mK. At the same time, vibrating sample magnetometry measurements at temperatures down to 4 K have shown the magnetic anisotropy of the Fe to be in plane. The direction of the Ag hyperfine field is thus noncollinear (nearly orthogonal) to the Fe anisotropy. These results are compared with full potential linearized augmented plane wave calculations using the wien97 code.     

Analysis of the electronic structure of ultrathin NiO/Ag(100) films

Thin films of nickel oxide on a silver substrate have been extensively studied both experimentally and theoretically. In this paper we present band structure calculations of one, two, three and five layer NiO/Ag(100) systems using a GGA+U density functional method and study the approach of the system towards the bulk situation. We find that the interfacial layer is metallised and that even for a five-layer system, the substrate still affects the properties of the outermost and central layers, suggesting that these layers have not yet reached convergence towards bulk properties. This may affect some of the more sensitive properties of the system.     

Direct evidence for block-by-block growth in high-temperature superconductor ultrathin films

Charge neutrality and stoichiometry impose severe restrictions on the mechanisms of epitaxial growth of complex oxides. The fundamental question arises of what is the minimum growth unit when sample thickness is reduced beyond the size of the unit cell. We have investigated the growth mechanism of YBa2Cu3O7 cuprate superconductor, using a consistent approach based on the growth of noninteger numbers of YBa2Cu3O7 layers in YBa(2)Cu(3)O(7)/PrBa(2)Cu3O7 superlattices. Ex situ chemical and structural analysis evidence a 2D block-by-block mechanism in which the minimum growth units are complete unit cell blocks, growing coherently over large lateral distances.     

Magnetism of ultrathin Fe(110) films

Application of conversion electron Mössbauer spectroscopy (CEMS) to structural and magnetic analysis of ultrathin films and their interfaces is reviewed. Fe(110) films were prepared on W(110) under UHV conditions and analyzed in situ. CEMS provides detailed information on the mode of growth and film structure and on magnetic hyperfine fields, B _hf. Local structure of B _hf across the film is discussed in relation to modifications of magnetic order caused by the finite (including monolayer) film thickness and by the electronic structure of the interface.