Abstract:
By using a fully relativistic embedded cluster Green's function technique we investigated the magnetic anisotropy properties of four different compact Cr trimers (equilateral triangles) and Cr mono-layers deposited on Au(111) surface in both fcc and hcp stackings. For all trimers the magnetic ground state was found a frustrated 120$^\circ$ N\'eel configuration. Applying global spin rotations to the magnetic ground state, the predictions of an appropriate second order spin Hamiltonian were reproduced with high accuracy by the first principles calculations. For the Cr trimers with adjacent Au atoms in similar geometry we obtained similar values for the in-plane and out-of-plane anisotropy parameters, however, the Dzyaloshinskii-Moriya (DM) interactions appeared to differ remarkably. For two kinds of trimers we found an unconventional magnetic ground state showing 90$^\circ$ in-the-plane rotation with respect to the high symmetry directions. Due to higher symmetry, the in-plane anisotropy term was missing for the mono-layers and distinctly different DM interactions were obtained for the different stackings. The chiral degeneracy of the N\'eel configurations was lifted by less then 2 meV for the trimers, while this value raised up to about 15 meV per 3 Cr atoms for the hcp packed mono-layer.

Abstract:
To calculate the magnetic ground state of nanoparticles we present a self-consistent first principles method in terms of a fully relativistic embedded cluster multiple scattering Green's function technique. Based on the derivatives of the band energy, a Newton-Raphson algorithm is used to find the ground state configuration. The method is applied to a cobalt nanocontact that turned out to show a cycloidal domain wall configuration between oppositely magnetized leads. We found that a wall of cycloidal spin-structure is about 30 meV lower in energy than the one of helical spin-structure. A detailed analysis revealed that the uniaxial on-site anisotropy of the central atom is mainly responsible to this energy difference. This high uniaxial anisotropy energy is accompanied by a huge enhancement and anisotropy of the orbital magnetic moment of the central atom. By varying the magnetic orientation at the central atom, we identified the term related to exchange couplings (Weiss-field term), various on-site anisotropy terms, and also those due to higher order spin-interactions.

Abstract:
We propose an atom-superposition-based method for simulating spin-polarized scanning tunneling microscopy (SP-STM) from first principles. Our approach provides bias dependent STM images in high spatial resolution, with the capability of using either constant current or constant height modes of STM. In addition, topographic and magnetic contributions can clearly be distinguished, which are directly comparable to results of SP-STM experiments in the differential magnetic mode. Advantages of the proposed method are that it is computationally cheap, it is easy to parallelize, and it can employ the results of any ab initio electronic structure code. Its capabilities are illustrated for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer on Ag(111) in a noncollinear magnetic $120^{\circ}$ N\'eel state. We show evidence that the magnetic contrast is sensitive to the tip electronic structure, and this contrast can be reversed depending on the bias voltage.

Abstract:
Based on a simple model for spin-polarized scanning tunneling spectroscopy (SP-STS) we study how tip magnetization and electronic structure affects the differential conductance (dI/dV) tunneling spectrum of an Fe(001) surface. We take into account energy dependence of the vacuum decay of electron states, and tip electronic structure either using an ideal model or based on ab initio electronic structure calculation. In the STS approach, topographic and magnetic contributions to dI/dV can clearly be distinguished and analyzed separately. Our results suggest that the sensitivity of STS on a magnetic sample can be tuned and even enhanced by choosing the appropriate magnetic tip and bias setpoint, and the effect is governed by the effective spin-polarization.

Abstract:
We propose a computationally efficient atom-superposition-based method for simulating spin-polarized scanning tunneling spectroscopy (SP-STS) on complex magnetic surfaces based on the sample and tip electronic structures obtained from first principles. We go beyond the commonly used local density of states (LDOS) approximation for the differential conductance, dI/dV. The capabilities of our approach are illustrated for a Cr monolayer on a Ag(111) surface in a noncollinear magnetic state. We find evidence that the simulated tunneling spectra and magnetic asymmetries are sensitive to the tip electronic structure, and we analyze the contributing terms. Related to SP-STS experiments, we show a way to simulate two-dimensional differential conductance maps and qualitatively correct effective spin polarization maps on a constant current contour above a magnetic surface.

Abstract:
By calculating the spectral density of states in the ferromagnetic (FM) ground state and in the high temperature paramagnetic (PM) phase we provide the first concise study of finite temperature effects on the electronic structure of the bulk and the surface of gadolinium metal. The variation of calculated spectral properties of the Fermi surface and the density of states in the bulk and at the surface are in good agreement with recent photoemission experiments performed in both ferromagnetic and paramagnetic phases. In the paramagnetic state we find vanishing spin splitting of the conduction band, but finite local spin moments both in bulk and at the surface. We clearly demonstrate that the formation of these local spin moments in the conduction band is due to the asymmetry of the density of states in the two spin channels, suggesting a complex, non-Stoner behavior. We, therefore, suggest that the vanishing or nearly vanishing spin splitting of spectral features cannot be used as an indicator for Stoner-like magnetism.

Abstract:
We present detailed investigations of the magnetic properties of an Fe monolayer on W and Ta (110) surfaces based on the ab initio screened Korringa-Kohn-Rostoker method. By calculating tensorial exchange coupling coefficients, the ground states of the systems are determined using atomistic spin dynamics simulations. Different types of ground states are found in the systems as a function of relaxation of the Fe layer. In case of W(110) substrate this is reflected in a reorientation of the easy axis from in-plane to out-of-plane. For Ta(110) a switching appears from the ferromagnetic state to a cycloidal spin spiral state, then to another spin spiral state with a larger wave vector and, for large relaxations, a rotation of the normal vector of the spin spiral is found. Classical Monte Carlo simulations indicate temperature-induced transitions between the different magnetic phases observed in the Fe/Ta(110) system. These phase transitions are analyzed both quantitatively and qualitatively by finite-temperature spin wave theory.

Abstract:
We determined the magnetic B-T phase diagram of PdFe bilayer on Ir(111) surface by performing Monte Carlo and spin dynamics simulations based on an effective classical spin model. The parameters of the spin model were determined by ab initio methods. At low temperatures we found three types of ordered phases, while at higher temperatures, below the completely disordered paramagnetic phase, a large region of the phase diagram is associated with a fluctuation-disordered phase. Within the applied model, this state is characterized by the presence of skyrmions with finite lifetime. According to the simulations, this lifetime follows the Arrhenius law as a function of temperature.

Abstract:
We introduce an orbital dependent electron tunneling model and implement it within the atom superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS). Applying our method, we analyze the convergence and the orbital contributions to the tunneling current and the corrugation of constant current STM images above the W(110) surface. In accordance with a previous study [Heinze et al., Phys. Rev. B 58, 16432 (1998)], we find atomic contrast reversal depending on the bias voltage. Additionally, we analyze this effect depending on the tip-sample distance using different tip models, and find two qualitatively different behaviors based on the tip orbital composition. As an explanation, we highlight the role of the real space shape of the orbitals involved in the tunneling. STM images calculated by our model agree well with Tersoff-Hamann and Bardeen results. The computational efficiency of our model is remarkable as the k-point samplings of the surface and tip Brillouin zones do not affect the computation time, in contrast to the Bardeen method.

Abstract:
The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co$_{2}$FeSi, Co$_{2}$FeAl, Co$_{2}$MnSi and Co$_{2}$MnAl are investigated by means of {\em ab initio} calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane wave code Quantum Espresso and magnetic exchange interactions are determined using the KKR method. From the magnetic exchange interactions the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of the trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in between are obtained from the fully relativistic KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly polarized currents within the systems. This is attributed to the recovery of half-metallicity with increasing thickness. The absence of electronic states of the spin down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductivity. This strongly influences the thermoelectric properties of such systems and results in polarized thermoelectric currents.