TY - JOUR AU - Csire, Gábor AU - Deák, András AU - Nyári, Bendegúz Tamás AU - Ebert, H AU - Annett, JF AU - Ujfalussy, Balázs TI - Relativistic spin-polarized KKR theory for superconducting heterostructures: Oscillating order parameter in the Au layer of Nb/Au/Fe trilayers JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 97 PY - 2018 IS - 2 PG - 9 SN - 2469-9950 DO - 10.1103/PhysRevB.97.024514 UR - https://m2.mtmt.hu/api/publication/3331048 ID - 3331048 LA - English DB - MTMT ER - TY - JOUR AU - Kim, Howon AU - Palacio-Morales, Alexandra AU - Posske, Thore AU - Rózsa, Levente AU - Palotás, Krisztián AU - Szunyogh, László AU - Thorwart, Michael AU - Wiesendanger, Roland TI - Toward tailoring Majorana bound states in artificially constructed magnetic atom chains on elemental superconductors JF - SCIENCE ADVANCES J2 - SCI ADV VL - 4 PY - 2018 IS - 5 PG - 7 SN - 2375-2548 DO - 10.1126/sciadv.aar5251 UR - https://m2.mtmt.hu/api/publication/3371112 ID - 3371112 AB - Realizing Majorana bound states (MBS) in condensed matter systems is a key challenge on the way toward topological quantum computing. As a promising platform, one-dimensional magnetic chains on conventional superconductors were theoretically predicted to host MBS at the chain ends. We demonstrate a novel approach to design of model-type atomic-scale systems for studying MBS using single-atom manipulation techniques. Our artificially constructed atomic Fe chains on a Re surface exhibit spin spiral states and a remarkable enhancement of the local density of states at zero energy being strongly localized at the chain ends. Moreover, the zero-energy modes at the chain ends are shown to emerge and become stabilized with increasing chain length. Tight-binding model calculations based on parameters obtained from ab initio calculations corroborate that the system resides in the topological phase. Our work opens new pathways to design MBS in atomic-scale hybrid structures as a basis for fault-tolerant topological quantum computing. LA - English DB - MTMT ER - TY - JOUR AU - Simon, Eszter AU - Rózsa, Levente AU - Palotás, Krisztián AU - Szunyogh, László TI - Magnetism of a Co monolayer on Pt(111) capped by overlayers of 5d elements: A spin-model study JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 97 PY - 2018 IS - 13 PG - 11 SN - 2469-9950 DO - 10.1103/PhysRevB.97.134405 UR - https://m2.mtmt.hu/api/publication/3363139 ID - 3363139 AB - Using first-principles calculations, we study the magnetic properties of a Co monolayer on a Pt(111) surface with a capping monolayer of selected 5d elements (Re, Os, Ir, Pt, and Au). First we determine the tensorial exchange interactions and magnetic anisotropies characterizing the Co monolayer for all considered systems. We find a close relationship between the magnetic moment of the Co atoms and the nearest-neighbor isotropic exchange interaction, which is attributed to the electronic hybridization between the Co and the capping layers, in the spirit of the Stoner picture of ferromagnetism. The Dzyaloshinskii-Moriya interaction is decreased for all overlayers compared to the uncapped Co/Pt(111) system, while even the sign of the Dzyaloshinskii-Moriya interaction changes in the case of the Ir overlayer. We conclude that the variation of the Dzyaloshinskii-Moriya interaction is well correlated with the change of the magnetic anisotropy energy and of the orbital moment anisotropy. The unique influence of the Ir overlayer on the Dzyaloshinskii-Moriya interaction is traced by scaling the strength of the spin-orbit coupling of the Ir atoms in Ir/Co/Pt(111) and by changing the Ir concentration in the Au1-xIrx/Co/Pt(111) system. Our spin dynamics simulations indicate that the magnetic ground state of Re/Co/Pt(111) thin film is a spin spiral with a tilted normal vector, while the other systems are ferromagnetic. LA - English DB - MTMT ER - TY - JOUR AU - Lászlóffy, András AU - Udvardi, László AU - Szunyogh, László TI - Atomistic simulation of finite-temperature magnetism of nanoparticles: Application to cobalt clusters on Au(111) JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 95 PY - 2017 IS - 18 PG - 10 SN - 2469-9950 DO - 10.1103/PhysRevB.95.184406 UR - https://m2.mtmt.hu/api/publication/3254535 ID - 3254535 AB - We developed a technique to determine suitable spin models for small embedded clusters of arbitrary geometry by combining the spin-cluster expansion with the relativistic disordered local moment scheme. We present results for uncovered and covered hexagonal Co clusters on Au(111) surface, and use classical Monte Carlo simulations to study the temperature dependent properties of the systems. To test the new method we compare the calculated spin-model parameters of the uncovered clusters with those of a Co monolayer deposited on Au(111). In general, the isotropic and Dzyaloshinsky-Moriya interactions are larger between atoms at the perimeter than at the center of the clusters. For Co clusters covered by Au, both the contribution to the magnetic anisotropy and the easy axis direction of the perimeter atoms differ from those of the inner atoms due to reduced symmetry. We investigate the spin reversals of the covered clusters with perpendicular magnetic anisotropy and based on the variance of the magnetization component parallel to the easy direction we suggest a technique to determine the blocking temperature of superparamagnetic particles. We also determine the Neel relaxation time from the Monte Carlo simulations and find that it satisfies the Neel-Arrhenius law with an energy barrier close to the magnetic anisotropy energy of the clusters. LA - English DB - MTMT ER - TY - JOUR AU - Vida, György József AU - Simon, Eszter AU - Rózsa, Levente AU - Palotás, Krisztián AU - Szunyogh, László TI - Domain-wall profiles in Co/Irn /Pt(111) ultrathin films: Influence of the Dzyaloshinskii-Moriya interaction JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 94 PY - 2016 IS - 21 PG - 9 SN - 2469-9950 DO - 10.1103/PhysRevB.94.214422 UR - https://m2.mtmt.hu/api/publication/3163501 ID - 3163501 LA - English DB - MTMT ER - TY - JOUR AU - Csire, Gábor AU - Ujfalussy, Balázs AU - Cserti, József AU - Gyorffy, B TI - Multiple scattering theory for superconducting heterostructures JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 91 PY - 2015 IS - 16 PG - 11 SN - 2469-9950 DO - 10.1103/PhysRevB.91.165142 UR - https://m2.mtmt.hu/api/publication/2897341 ID - 2897341 LA - English DB - MTMT ER - TY - JOUR AU - Rózsa, Levente AU - Udvardi, László AU - Szunyogh, László TI - Langevin spin dynamics based on ab initio calculations: numerical schemes and applications JF - JOURNAL OF PHYSICS-CONDENSED MATTER J2 - J PHYS CONDENS MAT VL - 26 PY - 2014 IS - 21 PG - 13 SN - 0953-8984 DO - 10.1088/0953-8984/26/21/216003 UR - https://m2.mtmt.hu/api/publication/2698941 ID - 2698941 AB - A method is proposed to study the finite-temperature behaviour of small magnetic clusters based on solving the stochastic Landau–Lifshitz–Gilbert equations, where the effective magnetic field is calculated directly during the solution of the dynamical equations from first principles instead of relying on an effective spin Hamiltonian. Different numerical solvers are discussed in the case of a one-dimensional Heisenberg chain with nearest-neighbour interactions. We performed detailed investigations for a monatomic chain of ten Co atoms on top of a Au(0 0 1) surface. We found a spiral-like ground state of the spins due to Dzyaloshinsky–Moriya interactions, while the finite-temperature magnetic behaviour of the system was well described by a nearest-neighbour Heisenberg model including easy-axis anisotropy. LA - English DB - MTMT ER - TY - JOUR AU - Balogh, László AU - Palotás, Krisztián AU - Udvardi, László AU - Szunyogh, László AU - Nowak, U TI - Theoretical study of magnetic domain walls through a cobalt nanocontact JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 86 PY - 2012 IS - 2 PG - 10 SN - 2469-9950 DO - 10.1103/PhysRevB.86.024406 UR - https://m2.mtmt.hu/api/publication/2194545 ID - 2194545 AB - 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. LA - English DB - MTMT ER - TY - JOUR AU - Antal, A AU - Lazarovits, Bence AU - Udvardi, László AU - Szunyogh, László AU - Ujfalussy, Balázs AU - Weinberger, P TI - First-principles calculations of spin interactions and the magnetic ground states of Cr trimers on Au(111) JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 77 PY - 2008 IS - 17 PG - 8 SN - 2469-9950 DO - 10.1103/PhysRevB.77.174429 UR - https://m2.mtmt.hu/api/publication/117126 ID - 117126 LA - English DB - MTMT ER - TY - JOUR AU - Ujfalussy, Balázs AU - Lazarovits, B AU - Szunyogh, László AU - Stocks, GM AU - Weinberger, P TI - Ab initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 70 PY - 2004 IS - 10 PG - 4 SN - 2469-9950 DO - 10.1103/PhysRevB.70.100404 UR - https://m2.mtmt.hu/api/publication/144189 ID - 144189 LA - English DB - MTMT ER - TY - JOUR AU - Udvardi, László AU - Szunyogh, László AU - Palotás, Krisztián AU - Weinberger, P TI - First-principles relativistic study of spin waves in thin magnetic films JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 68 PY - 2003 IS - 10 PG - 11 SN - 2469-9950 DO - 10.1103/PhysRevB.68.104436 UR - https://m2.mtmt.hu/api/publication/2633165 ID - 2633165 AB - In order to study spin-wave excitations of itinerant ferromagnets a relativistic first-principles method based on the adiabatic approach is presented. The derivatives of the free energy up to second order with respect of the polar and azimuthal angles are derived within the framework of the magnetic force theorem and the fully relativistic Korringa-Kohn-Rostoker method. Exchange and spin-orbit coupling are thus incorporated on equal footing in the Hamiltonian. Furthermore, a detailed comparison to classical spin Hamiltonians is given and it is shown that the magnetocrystalline anisotropy energy contains contributions from both the on-site anisotropy and the off-site exchange coupling terms. The method is applied to an Fe monolayer on Cu(001) and Au(001) surfaces and for a Co monolayer on Cu(001). The calculations provide with the gap at zero wave number due to the spin-orbit coupling and uniaxial anisotropy energies in good agreement with the results of the band energy difference method. It is pointed out that the terms in the spin-wave Hamiltonian related to the mixed partial derivatives of the free energy, absent within a nonrelativistic description, introduce an asymmetry in the magnon spectrum with respect to two in-plane easy axes. Moreover, in the case of an in-plane magnetized system the long-wavelength magnons are elliptically polarized due to the difference of the second-order uniaxial and fourth-order in-plane magnetic anisotropy. LA - English DB - MTMT ER - TY - JOUR AU - Lazarovits, B AU - Szunyogh, László AU - Weinberger, P TI - Fully relativistic calculation of magnetic properties of Fe, Co, and Ni adclusters on Ag(100) JF - PHYSICAL REVIEW B J2 - PHYS REV B VL - 65 PY - 2002 IS - 10 PG - 8 SN - 2469-9950 DO - 10.1103/PhysRevB.65.104441 UR - https://m2.mtmt.hu/api/publication/2633220 ID - 2633220 AB - We present first-principles calculations of the magnetic moments and magnetic anisotropy energies of small Fe, Co, and Ni clusters on top of a Ag(100) surface as well as the exchange-coupling energy between two single adatoms of Fe or Co on Ag(100). The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method. The magnetic anisotropy and the exchange-coupling energies are calculated by means of the force theorem. In the case of adatoms and dimers of iron and cobalt we obtain enhanced spin moments and, especially, unusually large orbital moments, while for nickel our calculations predict a complete absence of magnetism. For larger clusters, the magnitudes of the local moments of the atoms in the center of the cluster are very close to those calculated for the corresponding monolayers. Similar to the orbital moments, the contributions of the individual atoms to the magnetic anisotropy energy strongly depend on the position, hence, on the local environment of a particular atom within a given cluster. We find strong ferromagnetic coupling between two neighboring Fe or Co atoms and a rapid, oscillatory decay of the exchange-coupling energy with increasing distance between these two adatoms. LA - English DB - MTMT ER - TY - JOUR AU - Ujfalussy, Balázs AU - Stocks, GM AU - Wang, X AU - Nicholson, DMC AU - Shelton, WA AU - Wang, Y AU - Gyorffy, BL TI - Constrained density functional theory for first principles spin-dynamics JF - JOURNAL OF APPLIED PHYSICS J2 - J APPL PHYS VL - 85 PY - 1999 IS - 8 II A SP - 4824 EP - 4826 PG - 3 SN - 0021-8979 DO - 10.1063/1.370494 UR - https://m2.mtmt.hu/api/publication/142937 ID - 142937 LA - English DB - MTMT ER - TY - JOUR AU - Stocks, GM AU - Ujfalussy, Balázs AU - Wang, XD AU - Nicholson, DMC AU - Shelton, WA AU - Wang, Y AU - Canning, A AU - Gyorffy, BL TI - Towards a constrained local moment model for first principles spin dynamics JF - PHILOSOPHICAL MAGAZINE B-PHYSICS OF CONDENSED MATTER ELECTRONIC, OPTICAL AND MAGNETIC PROPERTIES J2 - PHILOS MAG B - PHYS CONDENS MATTER ELECTRON OPT MAGN PROP VL - 78 PY - 1998 IS - 5-6 SP - 665 EP - 673 PG - 9 SN - 0141-8637 DO - 10.1080/13642819808206775 UR - https://m2.mtmt.hu/api/publication/2508681 ID - 2508681 LA - English DB - MTMT ER -