Relativisztikus hatások elméleti vizsgálata bulk anyagok és nanoszerkezetek mágneses tulajdonságaiban = Study of relativistic effects on the magnetic properties of bulk materials and nanostructures

Általánosítottuk az infinitezimális elforgatások módszerét relatívisztikus esetre, meghatároztuk réz hordozóra növesztett Fe és Co mono rétegek magnon spektrumát [9,20] és megvizsgáltuk az MnAu2 rendszer helikális mágneses szerkezetét[26]. Megvizsgáltuk réz hordozóra növesztett vas vékony réteg Curie hőmérséklet változását a fedőréteg vastagságának függvényében [4]. Tanulmányoztuk a rács relaxáció hatását réz szubsztrátron növesztett Co réteg mágneses anizotrópiájára [3]. Megvizsgáltuk Cu3Au felületén kialakított fcc vas vékonyréteg reorientációs fázisátalakulását [22,23]. Foglalkoztunk a mágneses vékonyrétegek közötti kicserélődési kölcsönhatás és a GMR kapcsolatával [2,10]. Kifejlesztettünk egy, a véges atomcsoportok mágneses transzportjának számítására alkalmas programot [2,8,18,20]. Tanulmányoztuk fcc szerkezetű permalloy ötvözetek anizotróp mágneses ellenállását [12] és doménfalainak a tulajdonságait [11]. Megmutattuk, hogy Pt és Cu felületre helyezett Fe és Co, nanodrótoknak megnövekedett a spin és pálya momentumuk valamint a mágneses anizotrópiájuk a tömbi értékekhez képest [5,6,14]. Különböző geometriájú arany nanokontaktusok vezetőképességét határoztuk meg [20]. Adiabatikus spin-dinamika módszert alkalmaztunk fémes felületekre helyezett mágneses ''drótok'' alapállapotának tanulmányozására [15,13]. Egy 52 vas atomot tartalmazó kvantum "karám" esetén ab-initio alapuló számításokkal mutattuk ki a felületen kialakuló interferincia jelenséget [24]. | The magnon spectra of Co and Fe monolayers on Cu (001) surface has been determined using the relativistic extension of the method of infinitesimal rotation [9,20]. The formation of magnetic helix in MnAu2 has been investigated [26]. The dependence of the Curie temperature on the width of the cover layer has been studied for Fe thin films on Cu (001) surface [4]. The effect of the lattice relaxation on the magnetic anisotropy of epitaxial Co thin films on Cu (001) has been investigated [3] as well as the reorientation phase transition of fcc Fe on Cu3Au ordered alloy [22,23]. A code in the framework of the SKKR method has been developed for determining the transport properties of embedded clusters and spin valves[2,8]. We explored a connection between the GMR and the exchange coupling [2,10]. The anisotropic magneto-resistance [12] and the formation of domain walls in permalloys have been studied [11]. Our calculations have revealed enhanced spin and orbital moments and magnetic anisotropy energy for Fe and Co nanowires on Pt and Cu surfaces [5,6,14]. We have studied the effect of magnetic impurities on the transport properties of nano contacts [18,20]. Ab-initio spin dynamics has been applied in order to find the ground state magnetic configuration of nano wires [13,15]. The interference of the valence electrons of the substrate has been pointed out from first principle for a quantum corral consisting of 52 Fe atoms on Cu (111) surface [24]

Magnetic phase diagram of an Fe monolayer on W(110) and Ta(110) surfaces based on ab initio calculations

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.Comment: 18 pages, 11 figure

Orbital dependent electron tunneling within the atom superposition approach: Theory and application to W(110)

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.Comment: 28 pages manuscript, 7 figures, 1 tabl

Complex magnetic phase diagram and skyrmion lifetime in an ultrathin film from atomistic simulations

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.Comment: 11 pages, 8 figure

Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

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.Comment: 28 pages manuscript, 1 table, 5 figure

Theoretical study of the role of the tip in enhancing the sensitivity of differential conductance tunneling spectroscopy on magnetic surfaces

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.Comment: 22 pages manuscript, 4 figures; http://link.aps.org/doi/10.1103/PhysRevB.83.21441