D. Spišák

Does density-functional theory predict a spin-density-wave ground state for Cr?

We present ab initio spin-density-functional investigations of static long-period spin-density waves (SDWs) in Cr. Calculations performed in the generalized-gradient approximation (GGA) and using both projector-augmented-wave and muffin-tin orbital techniques show that the overestimation of the strength of the antiferromagnetism suppresses the formation of an SDW. In the local density approximation we find that a static SDW is also higher in energy than a commensurate antiferromagnetic structure, but with an energy difference that tends towards zero as the lattice constant shrinks to its low-temperature limit. A possible scenario for the origin of the observed SDW state is developed.

Magnetism of monoatomic wires on vicinal surfaces

Abstract There are three key steps in the development of one-dimensional nanostructures: fabrication, characterization and application. In our contribution we concentrate on the second step, the characterization of the structure and magnetic behavior of monoatomic wires grown on Cu and Ag surfaces with a high density of steps from the theoretical point of view. At first, the geometry of clean Cu(11n) and Cu(10n) vicinal surfaces is optimized by means of ab initio density-functional calculations, and the formation energies of steps along the 〈1 1 0〉 and 〈1 0 0〉 directions are compared with experimental estimates. In the following, the monoatomic Fe wires are placed at various positions onto Cu substrate. The preferential positions of wires are predicted to be the inner corner sites of the steps. From the total energy differences between ferromagnetic and antiferromagnetic configurations the effective intrawire and interwire magnetic coupling constants are estimated. In all investigated cases the resulting magnetic order is ferromagnetic. Finally, the possibility of magnetism in monoatomic wires made of fifth and sixth row elements, which are nonmagnetic in a solid, is explored and discussed. The obtained results are in complete agreement with the known trends that a reduced dimensionality promotes magnetism.

Electronic and magnetic structure of Mn-Ni alloys in two and three dimensions

The electronic and magnetic structure of face-centred tetragonal Mn-Ni compounds and of structurally related c(2 × 2) Mn-Ni alloy films on Ni(001) substrates has been investigated using ab initio local-spin-density calculations including generalized gradient corrections. For the intermetallic compound a layered antiferromagnetic high-spin ground state with Mn moments of ±3.2 µB (LSDA) and ±3.4 µB (GGCs) and non-magnetic Ni atoms is predicted, in good agreement with the estimates from magnetic neutron scattering. Calculations of the magnetic anisotropy energy show that the moments are aligned in the Mn planes, parallel to the edges of the unit cell. An alloy with unit cell dimensions of the Mn-Ni planes strained to match the lattice parameter of the Ni(001) substrate has a very similar magnetic structure, albeit with slightly reduced moments. A monolayer c(2 × 2) Mn-Ni alloy shows high-spin ferromagnetic order (µMn = 3.9 µB). Films with two and more monolayers show antiferromagnetic interlayer coupling, with a quite pronounced enhancement of the surface moments over those in the deeper layers. The predicted antiferromagnetic ordering of the films emphasizes the similarity of the atomic and magnetic structures of the two-dimensional films with that of the three-dimensional compounds and contradicts recent claims as to a ferromagnetic order of the films not only in the monolayer limit. Possible explanations of this discrepancy are discussed.

Ab initio local-spin-density study of oscillatory exchange coupling in Fe/Au multilayers

Fully self-consistent ab initio local-spin-density calculations of the interlayer exchange coupling (IEC) in Fe/Au multilayers have been performed. The thickness of the ferromagnetic body-centred cubic Fe layers has been kept fixed at five monolayers (ML), the thickness of the face-centred cubic Au spacer layers has been varied between 2 and 30 ML. Agreement of the calculated and observed oscillation periods of the interlayer exchange coupling has been obtained. To verify the Rudermann-Kittel-Kasuya-Yoshida and quantum-well (QW) pictures for the IEC, detailed investigations of the Fermi surface of Au and of the modulation of the Bloch states in the spacer layer by means of QW oscillations have been performed.

Ab initio local-spin-density study of oscillatory exchange coupling and spin density waves in Fe/Cr multilayers

The interlayer exchange coupling and the magnetic structure of bcc (001) Fe/Cr multilayers have been studied using a first-principles self-consistent tight-binding linear-muffin-tin-orbital method in the atomic sphere approximation. A generalized gradient approximation to the exchange correlation potential was used. Antiferromagnetic coupling between neighboring Fe layers is found to be energetically favorable for an even number of monolayers in the Cr spacer, while ferromagnetic coupling is preferred for an odd number of monolayers. Also it is found that for certain Cr thicknesses incommensurate spin density waves with different wavelengths coexist.

Shear instability in fcc Fe and thin Fe-Cu[001] films

Using total energy local-spin-density calculations, it is shown that fcc /spl gamma/-Fe is unstable with respect to a monoclinic shear deformation producing a nearly bcc structure. The instability persists in ultrathin iron films on Cu[001] substrate for film thicknesses above a monolayer limit. A strong correlation between the shear instability and the magnetic states is reported.

Ab initio study of hexagonal Fe/Ru multilayers

Calculations of structural and electronic properties of hexagonal Fe/Ru multilayers, as well as of pure Fe and Ru metals, were performed using the spin-polarized density functional technique. Apart from the conventional hexagonal AB stacking of close-packed planes, an AB′ stacking with atoms in a B′ plane shifted over the triangle edges formed by atoms in an A plane was examined in the Fe part of the multilayers. It was concluded that the most stable solution is a ferromagnetic configuration with an AB′ stacking within Fe layers with the axial ratio cFe/aRu=1.47 and with an AB stacking in Ru layers with cRu/aRu=1.60. Interfacial interdiffusion effects and antiferromagnetic order were also investigated in connection with the reported disappearance of a magnetism at the Fe/Ru interfaces. The equilibrium lattice parameters of hexagonal close-packed Fe metal, aFe=2.44 A, cFe/aFe=1.60, were obtained for a nonmagnetic state. At an only slightly larger lattice constant an antiferromagnetic state appears. For bul...

Magnetism in amorphous Fe(Co)-Y alloys

Abstract We present calculations of the structural, electronic and magnetic properties of amorphous FeY and CoY alloys. We show that over a wide range of compositions both a-FeY and a-CoY are ferrimagnetic, but with some significant differences: a-CoY retains a simple collinear magnetic structure, but the disorder-induced broadening of the d-band leads to enhanced magnetism. In Fe-rich a-FeY alloys competing exchange interactions lead to complex spin structures.

Structural and magnetic properties of manganese overlayer on Fe(100)

This paper investigates the magnetic structure of Mn overlayer on a ferromagnetic body-centered cubic Fe(100). The calculations reported here were performed within the framework of density functional theory. The system is modeled by a slab comprising six Fe layers, one Mn layer and a sufficiently thick vacuum region separating the adjacent slabs. The surface c(2times2) periodicity was used in order to regard both FM and in-plane AFM magnetic order. The four topmost layers were allowed to relax in the vertical direction. A single Mn monolayer on Fe(100) assumes a ferrimagnetic structure with the magnetic moments 3.32 and -3.59 muB.