H. Dreyssé

Antiferromagnetic interlayer coupling in Fe/V and Fe/Cr

Antiferromagnetic (AF) coupling between Fe and V atoms is observed experimentally at the Fe/V and Fe/Cr interfaces. In the self‐consistent tight‐binding derivation of the unrestricted approximation of the Hubbard Hamiltonian, we have investigated Fe3Vn (n=1–5) superlattices and Fe3Vn (n=1–5) films. In all cases one finds an AF coupling between Fe and V together with a decreasing oscillation of the induced magnetic moment on the V atoms when the distance from Fe increases. The distributions of local magnetic moments on Fe and Cr layers in Fe3Crn (n=3–5) superlattices have been determined self‐consistently. Both AF and ferromagnetic (F) coupling between Fe layers separated by Cr have been studied. In all cases one finds strong AF couplings between Fe and Cr nearest neighbors with an important increase of the Cr magnetic moment at the interface as compared to the Cr bulk value.

The Pd polarization at the Pd/Fe interface

Abstract The electronic and magnetic properties of the ground state ( T =0 K) at the Pd/Fe(001) interface were investigated through the mean-field parameterized tight-binding method. Structurally, the Pd overlayers occupy the fcc sites of the substrate, with a few percent expansion of the Pd-Pd distances as compared to the bulk fcc Pd. We report results concerning deposition of n Pd layers on semi-infinite Fe. Ferromagnetic polarization of the Pd n layers are obtained up to n =3. For n =4, we have obtained ferromagnetic polarization of the nearest neighbour Pd overlayer but negative polarization for the three other planes. For n =5, all the Pd atoms are polarized ferromagnetically unless the surface which has a quasi-zero magnetic moment. In the case of sandwiches Fe m /Pd n /Fe m or superlattices (Fe m Pd n ) p we investigate both ferromagnetic and antiferromagnetic couplings between Fe films through the Pd spacer. For completeness we perform also total energy calculations in order to study the exchange coupling displayed by Celinski and Heinrich [J. Magn. Magn. Mater. 99 (1991) L25]. Similar trends are found: ferromagnetic coupling is shown to be stable until n =5 for Fe 5 /Pd n /Fe 5 sandwiches; antiferromagnetic coupling is however stable for n =6. We discuss the importance of the interface roughness on this oscillation.

Ab initio study of the CuPd one-dimensional long period superstructure phase diagram

Abstract The f.c.c. superstructure phase diagram for Cu Pd is calculated by means of isotropic effective interactions derived from an ab initio KKR-CPA electronic structure calculation. We prove the existence of a region in which one-dimensional long period superstructures are stable. This region is composed of several single phase fields in which the modulation period is constant and in close agreement with results from High Resolution Electron Microscopy. The change from wavy antiphase boundaries at low Pd concentration to sharp antiphase boundaries at high Pd concentration is related to a significant change of the interactions with Pd concentration. The role of the Fermi surface and the applicability of the ANNNI model are clarified.

Calculated magnetic properties of low-dimensional systems: the AuCu- and AuCu3-type ferromagnets

Abstract Using the state-of-the-art relativistic full-potential linear muffin–tin orbital method the magnetic properties of some of the most well-known ferromagnetic alloys, FePt and CoPt that crystallize in the L 1 0 structure and the MnPt 3 , CoPt 3 , Fe 3 Pt and Ni 3 Pt that crystallize in the L 1 2 structure, are studied. The growth axis is chosen so that these alloys consist of alternating layers of pure platinum or transition-metal atoms. Both the local-spin-density approximation and the generalized gradient approximation to the exchange correlation potential are used in the computation. The calculated spin and orbital magnetic moments of the transition-metal atoms agree nicely with previous theoretical and experimental results. The shape of the Pt-projected density of states reflects the difference in the crystalline structure. The application of the sum rules to the theoretical X-ray magnetic circular dichroism spectra to compute both the spin and orbital magnetic moments produced results that are in good agreement with these obtained directly from band structure.

C(2 × 2) ferrimagnetic order versus P(1 × 1) ferromagnetic order in V, Cr, Mn monolayers on Fe(001)

Abstract The possibility of a ferrimagnetic-in-plane configuration with C(2 × 2) symmetry is analyzed for V, Cr and Mn monolayers deposited on Fe(001). The spin-polarized electronic distribution is calculated by solving self-consistently a tight-binding Hamiltonian in a mean field approximation. The relative stability of this solution with respect to the usual P(1 × 1) ferromagnetic-in-plane solution is determined by means of the difference of total energy. A nearly zero surface magnetization is obtained in the C(2 × 2) arrangement. The results of this study are discussed in relation with recent experimental observations.

Strong or weak ferromagnetism in transition metals

Abstract The various new magnetic phases obtained by epitaxial growth have re-given a large interest to the study of magnetic moment versus the lattice parameter. Considering systems with equivalent sites in the Stoner model, we propose simple criteria to obtain the energetically stablest magnetic moment as a function of the lattice parameter. Strong and weak ferromagnetism are clearly predicted. Also we point out that the well-known Stoner criterion is not enough to describe the onset of magnetism, first order transition from paramagnetic to ferromagnetic state are often present. Finally we show that, for a given “d” band occupation, the possible values for bulk fcc or bcc magnetic moment are surprisingly, rather limited.

Magnetic-Order Transition in Thin Fe Overlayers on Cr: Role of the Interfacial Roughness

The distribution of magnetic domains in a thin Fe overlayer on Cr is calculated as a function of the coverage thickness in the presence of roughness at the interface. The spin-polarized electronic structure is determined by solving self-consistently a d-band model Hamiltonian in the mean-field approximation. Arising from a magnetic multidomain arrangement, a zero total magnetic moment is obtained when starting the Fe deposition. A transition towards a single Fe domain at a critical coverage thickness leads to a net magnetization in qualitative agreement with various experimental observations. This macroscopic magnetic behaviour is traced back to environment-dependent microscopic properties such as the local magnetic moments and magnetic ordering.

Onset of magnetism in transition-metal monolayers: V and Rh on Ag(001)

Abstract We discuss the onset of magnetic structures in very thin V and Rh overlayer slabs on an Ag(001) substrate in terms of the exchange integral J . We use a self-consistent real-space tight-binding method in the unrestricted Hartree-Fock approximation to the Hubbard Hamiltonian. In both cases, the results display onset of magnetism essentially in the monolayer range. For thicker coverage of Ag(001), the magnetism is quickly killed. This thickness dependence of the magnetism may explain discrepancies between the experimental results performed with various techniques in different groups, in the case of V on Ag(001). Our simple model for Rh on Ag(001) is not able to reproduce the experimental situation plagued by interdiffusion.

Magnetism of Vicinal Surfaces of Vanadium

Hubbard tight-binding Hamiltonian is used to derive the local magnetic moments of low-index and vicinal V surfaces. The polarization of the low-index surface atoms can be qualitatively explained through the coordination number rule but inwards relaxation tends to suppress it. For vicinal surfaces, both the single-cell and double-cell magnetic periodicities are considered. The magnetic moments at the edge of the step are much lower than on their neighbouring atoms in the plane of the step and to atoms of the less coordinated crystallographic faces (100) and (111).

Magnetic properties of semi-infinite systems of Cr(Fe) on a Fe(Cr) (001) surface

We have investigated the magnetic properties of semi-infinite systems of n (n = 1–5) overlayers of Cr(Fe) on the Fe(Cr) (001) surface by means of self-consistent tight-binding calculations in the framework of the Hubbard Hamiltonian within the unrestricted Hartree-Fock approximation including s, p and d electrons. Particularly, we have obtained a decrease of the magnetic moment of Cr at the interface of CrnFe(001) and FenCr(001), when the number of layers n increases. The same effect, although less pronounced, appears for iron at the interface of FenCr(001), whereas the contrary happens at the interface of CrnFe(001). The general results can be understood in terms of the competition between a surface effect and the antiparallel alignment of spins of Fe and Cr atoms.