Local spectroscopy of two-dimensional states in Na films on Cu(111)

Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we found two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands, confinement of the QWS was observed.     

Three-dimensional spin structure on a two-dimensional lattice: Mn/Cu(111)

Based on first-principles vector spin-density total-energy calculations of the magnetic and electronic structure of Cr and Mn transition-metal monolayers on the triangular lattice of a (111) oriented Cu surface, we propose for Mn a three-dimensional noncollinear spin structure on a two-dimensional triangular lattice as magnetic ground state. This new spin structure is a multiple spin-density wave of three row-wise antiferromagnetic spin states and comes about due to magnetic interactions beyond the nearest neighbors and due to higher order spin interactions (i.e., four spin). The magnetic ground state of Cr is a coplanar noncollinear periodic 120 degrees Néel structure.     

Electrical conduction in Cu/Mn multilayer films

Metallic superlattices of copper and manganese have been synthesized on glass and mica substrates by a sequential evaporation technique. The electrical resistivity and the temperature coefficient of resistance (TCR) of layered Cu/Mn has been studied for various thicknesses (d) in the range 2–6 nm by varying the number of double layers (n) from 5–35. The transition from a negative to positive TCR has been observed ford >5 nm. The thickness dependence of room temperature resistivity ( _RT) and TCR shows oscillatory behaviour.     

Ion beam triangulation of ultrathin Mn and CoMn films grown on Cu(001)

Total target currents for grazing scattering of keV protons from a crystal target are used to investigate the structure of surfaces and ultrathin films. This current shows pronounced maxima whenever the azimuthal incidence angle coincides with close-packed rows of atoms in the surface and subsurface layers. The real-space method is applied to study monolayer and bilayer films of Mn and of CoMn epitaxially grown on a Cu(001) surface.     

Effective mass enhancement and spin-glass behaviour in CeCu4Mn(y)Al(1-y) compounds

We report on the magnetic ac/dc susceptibility and specific heat measurements for the CeCu(4)Mn(y)Al(1-y) series of compounds with 0 ≤ y ≤ 1. All compounds investigated crystallize in the hexagonal CaCu(5)-type structure with the space group P6/mmm. The results reveal that the frustration of the interactions dominates and leads to a spin-glass (SG) behaviour with a linear change of the freezing temperature T(f) as a function of y. The SG state has been confirmed by the frequency dependence of the ac magnetic susceptibility, the relaxation of the remanent magnetization and the split of the field-cooled-zero-field-cooled dc magnetic susceptibility. The electronic specific heat coefficient γ is enhanced for all y and increases with the Al content. Below a threshold at y ≈ 0.3, γ is large even if determined for temperatures above T(f); moreover the paramagnetic Curie-Weiss temperature θ(p) changes sign to negative. These observations indicate the possible presence of the heavy fermion (HF) state, at least below y ≈ 0.3, and the possible coexistence of the SG and HF states down to y = 0, i.e. for the CeCu(4)Al compound. A tentative magnetic phase diagram for CeCu(4)Mn(y)Al(1-y) has been constructed.     

Quantum spin-glass transition in the two-dimensional electron gas

We discuss the possibility of spin-glass order in the vicinity of the unexpected metallic state of the two-dimensional electron gas in zero applied magnetic field. An average ferromagnetic moment may also be present, and the spin-glass order then resides in the plane orthogonal to the ferromagnetic moment. We argue that a quantum transition involving the destruction of the spinglass order in an applied in-plane magnetic field offers a natural explanation of some features of recent magnetoconductance measurements. We present a quantum field theory for such a transition and compute its mean field properties.     

Effect of hydrostatic pressure on the static susceptibility of the spin-glass systems Ag:Mn,Au:Mn,Cu:Mn and Au:Fe

Previous studies of the magnetic susceptibility of a Ag-3 at.% Mn spin-glass under high pressure are extended to include the systems Au-3at.% Mn, Cu-3at.% Mn and Au-3at.% Fe. In all four alloys the spin-glass freezing temperatureT ₀ increases with pressure, yielding the volume dependences lnT ₀/ lnV=–3.6±0.4, –4.1±0.6, –2.1±0.4 and –1.64±0.25, respectively. These results give information about the nature of the interaction mechanisms responsible for the spin-glass freezing phenomena. The pressure dependence of the Curie temperature of ferromagnetic Au-20at.% Fe was also studied.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Ferromagnetism in noncompensated (Mn,Ga)-codoped ZnO films

Mn/Ga noncompensated codoped ZnO films were prepared on c-cut sapphire substrates via pulsed laser deposition. The structural, magnetic, transport, and optical properties of the films were then investigated. Addition of the Ga donor increases the electron concentration and enhances the magnetization in these films because of the net negative charge of the special noncompensated codoping, which can adjust the carrier concentration as well as the magnetic moment. Moreover, the Fermi level moves into the conduction band because of the increase in electron concentration, which results in an increase in the optical band gap value, from 3.28 eV for the undoped ZnO film to 3.61 eV for the (Mn,Ga)-codoped ZnO film.     

Role of manganese in ferromagnetic (Al,Mn)N films

We have studied the (Al,Mn)N films grown by plasma-enhanced molecular beam epitaxy via Raman spectroscopy to verify how Mn atoms were incorporated into AlN lattice. In Raman spectra of (Al,Mn)N, the frequency of E₂ (high) mode blueshifts. Together with XRD results, the strain calculations support the existence of the substitutional dopant. Also, an additional peak, attributed to local vibrational mode of Mn, coincides with the calculated phonon frequency on the basis of a simple model. The Raman results reveal that Mn atoms replace Al atoms at the substitutional sites in the lattice. The (Al,Mn)N films were found to exhibit insulating characteristics and ferromagnetic ordering above room temperature. These results suggest that the ferromagnetism in the (Al,Mn)N films is due to the intrinsic nature originating from the substitutional Mn within (Al,Mn)N.