Ph. Kurz

Magnetism and electronic structure of hcp Gd and the Gd(0001) surface

We investigate hcp Gd and the Gd(0001) surface on the basis of density functional theory. The localized 4f states of Gd, which represent a challenge for first-principles theory, are treated in four different models, employing consistently the full-potential linearized augmented plane-wave method. Our results support previous findings that within the local density approximation (LDA) or generalized gradient approximation (GGA) the itinerancy of the 4f states is overestimated. In particular, the large density of states at the Fermi energy due to the minority 4f electrons is unphysical, and our results show that this is the origin of the incorrect prediction of the antiferromagnetic ground state for hcp Gd by many LDA and GGA calculations. We show that different models of removing these states from the region close to the Fermi energy, for example the treatment of the 4f electrons as localized core electrons or by using the LDA + U formalism, lead to the prediction of the correct ferromagnetic ground state for the bulk and a ferromagnetically coupled (0001) surface layer. With these models ground-state properties such as the magnetic moment and structural parameters can be determined in good agreement with experiment. The energetic positions of the surface states of the Gd(0001) surface are compared with experimental data.

First-principles theory of ultrathin magnetic films

We report on a set of systematic first-principles electronic structure investigations of the magnetic spin moments, the magnetic spin configurations, and the magnetic coupling of ultrathin magnetic films on (001)- and (111)-oriented noble-metal substrates and on the Fe(001) substrate. Magnetism is found for 3d-, 4d-, and 5d-transition-metal monolayers on noble-metal substrates. For V, Cr, and Mn on (001) substrates a c(2 × 2) antiferromagnetic superstructure has the lowest energy, and Fe, Co, Ni are ferromagnetic. On (111) substrates, for Cr the energy minimum is found for a 120° non-collinear magnetic configuration in a ( × )R30° unit cell, and for Mn a row-wise antiferromagnetic structure is found. On Fe(001), V and Cr monolayers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni monolayers favour the ferromagnetic coupling to Fe(001). The magnetic structure of Mn on Fe(001) is a difficult case: at least two competing magnetic states are found within an energy of 7 meV. The Cr/Fe(001) system is discussed in more detail as the surface-alloy formation is investigated, and this system is used as a test case to compare theoretical and experimental scanning tunnelling spectroscopy (STS) results. The possibility of resolving magnetic structures by STS is explored. The results are based on the local spin-density approximation and the generalized gradient approximation to the density functional theory. The calculations are carried out with the full-potential linearized augmented-plane-wave method in film geometry.

Noncollinear magnetism of Cr and Mn monolayers on Cu(111)

Cr and Mn monolayers on a triangular lattice are prototypical examples of frustrated spin systems in two dimensions. Collinear and noncollinear magnetic structures of these monolayers on Cu(111) substrate are investigated on the basis of first-principles total-energy calculations using the full-potential linearized augmented plane-wave method extended by the vector spin-density description for the interstitial and vacuum region. The search for the magnetic minimum-energy configurations included unit cells with one, two, and three atoms. For Cr the minimal energy was found for a 120° spin configuration in a (∛×∛)R30° unit cell, which is in agreement with the classical nearest-neighbor Heisenberg model with antiferromagnetic exchange interaction. The same behavior is expected for Mn, but a surprising result was found: the minimal energy was found for a collinear row-wise antiferromagnetic structure.

Itinerant Magnets on a Triangular Cu(111) Lattice

We investigate complex spin structures of frustrated two-dimensional Cr, Mn, and Fe monolayer magnets on a triangular lattice provided by the Cu(111) substrate. First we establish a zero-temperature phase diagram of possible spin structures on the basis of the classical Heisenberg model up to the third-nearest neighbor exchange interaction. Second we carried out first-principles total energy calculations on the basis of the vector-spin density formulation of the density functional theory using the full potential linearized augmented plane wave (FLAPW) method in film geometry for a set of complex non-collinear spin structures. We found, the ground state of Fe is ferromagnetic, Cr exhibits a coplanar, two-dimensional non-collinear 120 Néel state and Mn a three-dimensional non-collinear ground state, the 3Q-state. Incommensurate spin-spiral states are expected for a FeMn alloy on Cu(111). We employ the constrained local moment method to estimate the exchange parameters of the model Hamiltonians. We show that for Mn higher-order spin interactions are the origin of the 3Q-state for Mn. The combination of ab initio calculations and model Hamiltonians provides a powerful tool to investigate the magnetic structures of complex magnetic systems.