R. Lorenz

Noncollinear magnetic structures in amorphous iron and iron-based alloys

Abstract We present a novel technique for calculating noncollinear spin structures in itinerant magnets. Our approach is based on a tight-binding Hubbard Hamiltonian constructed by a canonical transformation of a self-consistent spin-polarized linear muffin-tin orbital Hamiltonian to a tight-binding form, and on a real-space recursion technique for the self-consistent calculation of the local exchange splitting and local moments. Applications to amorphous Fe at different densities and to Fe-Zr alloys of different compositions are presented. Our results show that with increasing density amorphous Fe shows a continuous transition from a ferromagnetic to an asperomagnetic and finally to a speromagnetic (spin-glass-like) state. This spin-glass state exists over a considerable range of densities before the system becomes paramagnetic. Similar transitions are observed in amorphous Fe-Zr alloys with increasing Fe content.

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.

Noncollinear magnetism in amorphous Fe-Y alloys

Abstract Iron-rich amorphous alloys with nonmagnetic transition metals (TM) or rare-earth (RE) metals show a rich variety of complex spin-structures that have been described as spin-glass-like, spero- or asperomagnetic. Amorphous Fe-Y alloys have attracted particular attention because it has been suggested that, in contrast to other amorphous Fe-TM and Fe-RE alloys showing re-entrant spin-glass behaviour, in Fe-Y alloys the spin-glass transition could occur directly from the paramagnetic phase. It has also been suggested that, unlike most other alloys where the spin-glass-like phase is restricted to high Fe concentrations, amorphous Fe-Y alloys are noncollinear at all compositions. We have recently presented a technique for a selfconsistent calculation of noncollinear spin-structures in crystalline and amorphous alloys based on a tight-binding-Hubbard Hamiltonian generated via a canonical transformation of the local-spin-density (LSD) Hamiltonian. This approach has been used to analyze the magnetic properties of amorphous Fe-Y alloys. We show that, unlike, for example, amorphous Fe-Zr alloys where the net magnetization is well defined in the spin-glass and in the ferromagnetic phases and independent of the initialization of the magnetic structure, for Fe-Y energetically nearly degenerate low- and high-moment phases can be induced by different initializations. This could explain the observed strong dependence of the magnetic state on the preparation of the amorphous phase and its strong variation under applied external fields or pressures.

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.

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=2.44u200aA, cFe/aFe=1.60, were obtained for a nonmagnetic state. At an only slightly larger lattice constant an antiferromagnetic state appears. For bul...

Magnetic order in YFe9Mo3

Abstract High-field 57 Fe Mossbauer and DC-magnetization measurements are reported for YFe 9 Mo 3 . Mo occupies preferentially the 8i-sites and to a small extent 8f. From the hyperfine field arrangement a noncollinear magnetic order of Fe is obtained in agreement with the results of band-structure calculations, explaining the observed low saturation in high external fields.

Results on thin film magnetic anisotropy by real-space TB-LMTO

Abstract In this work, we report results in calculation of the magnetic anisotropy of thin films by means of the real-space tight-binding linearized muffin-tin orbitals method (RS-TB-LMTO).A self-consistent spin-polarized version including spin-orbit coupling and with arbitrary spin-quantization axis is used. Within this framework, and as an example, the results of the magnetic anisotropy properties of Ni films on a Cu(001) substrate are reported.

Theoretical Study of the Magnetic Anisotropy of Ni Films on Cu(001)

The anisotropy properties of Ni films on Cu(001) are quite unusual compared to other systems: The magnetization direction of Ni is in-plane for a coverage smaller than a critical thickness of 7 monolayers and out-of-plane for a coverage larger than 7 monolayers. As a first step in the study of this unusual behaviour, we report results of ab-initio calculations of the magnetic order of Ni films on a Cu(001) substrate. The magnetic moments are computed by means of the real-space Tight-Binding LMTO method allowing non-collinear magnetic moments and including spin-orbit coupling to account for magnetic anisotropy effects. As the number of Ni layers is increased, we discuss the stability of the system with a magnetization in-plane or out-of-plane.