Controlling the preferred direction of the magnetic moments is essential for the design of spintr... more Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and heterostructures. As the film thickness or the temperature is increased, the easy anisotropy axis is typically reoriented from an out-of-plane direction preferred by surface and interface energy contributions to an in-plane alignment favored by the volume anisotropy terms. We study the temperature-driven spin reorientation transition in two atomic layers of Fe on W(110) using well-tempered metadynamics simulations based on a spin model parametrized by ab initio calculations and find that the transition only takes place in the presence of the Dzyaloshinsky-Moriya interaction (DMI). This demonstrates that the chiral DMI does not only differentiate between noncollinear spin structures of different rotational senses, but it also influences the magnetic orientation of collinear magnetic configurations.
We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature... more We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T , in metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced 'local moment' magnetic fluctuations are described within the relativistic generalisation of the 'disordered local moment' (R-DLM) theory from which the T dependence of the magnetisation, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry,L1 0 -ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L1 0ordered FePt and which agrees with experiment. This account, however, differs qualitatively from that extracted from a single ion anisotropy model. We also study the magnetically soft cubic magnet, the F e 50 P t 50 solid solution, and find that its small magnetic anisotropy constant K 1 rapidly diminishes from 8 µeV to zero. K evolves from being proportional to m 7 at low T to m 4 near the Curie temperature.
We employ metadynamics simulations to calculate the free energy landscape of thin ferromagnetic f... more We employ metadynamics simulations to calculate the free energy landscape of thin ferromagnetic films and perform a systematic study of the temperature dependence of magnetic anisotropy and of the spin-reorientation transitions. By using a simple spin model we recover the well-known power-law behavior of the magnetic anisotropy energy against magnetization and present a rather detailed analysis of the spin-reorientation transitions in ultrathin films. Based on tensorial exchange interactions and anisotropy parameters derived from first-principles calculations we perform simulations for Fe double layers deposited on Au(001) and W(110). In case of Fe2W(110) our simulations display an out-of-plane to in-plane spin-reorientation transition in agreement with experiments.
We present a computational method to determine the exchange constants in isotropic spin models. T... more We present a computational method to determine the exchange constants in isotropic spin models. The method uses the Hamiltonian and overlap matrices computed from density functional schemes that are based on nonorthogonal basis sets. We demonstrate that the new method as implemeted in the SIESTA code reproduces the Heisenberg interactions of simple metallic bulk ferromagnets as obtained from former well-established computational approaches. Then we address sp magnetism in graphene nanostructures. For fluorinated graphene we obtain exchange interactions in fairly good agreement with previous calculations using maximally localized Wannier functions and we confirm the theoretical prediction of a 120 • Néel state. Associated with the magnetic edge-states of a zigzag graphene nanoribbon we find rapidly decaying exchange interactions, however, with an unconventional distance dependence of exp(-r/δ). We show that the stiffness constant derived from the exchange interactions is consistent with previous estimate based on total energy differences of twisted spin configurations. We highlight that our method is an efficient tool for the analysis of novel hybrid nano-structures where metallic and organic components are integrated to form exotic magnetic patterns.
Magnetic skyrmions are localized nanometer-sized spin configurations with particle-like propertie... more Magnetic skyrmions are localized nanometer-sized spin configurations with particle-like properties, which are envisioned to be used as bits in next-generation information technology. An essential step toward future skyrmion-based applications is to engineer key magnetic parameters for developing and stabilizing individual magnetic skyrmions. Here we demonstrate the tuning of the non-collinear magnetic state of an Fe double layer on an Ir(111) substrate by loading the sample with atomic hydrogen. By using spin-polarized scanning tunneling microscopy, we discover that the hydrogenated system supports the formation of skyrmions in external magnetic fields, while the pristine Fe double layer does not. Based on ab initio calculations, we attribute this effect to the tuning of the Heisenberg exchange and the Dzyaloshinsky–Moriya interactions due to hydrogenation. In addition to interface engineering, hydrogenation of thin magnetic films offers a unique pathway to design and optimize the s...
We observe metastable localized spin configurations with topological charges ranging from Q = -3 ... more We observe metastable localized spin configurations with topological charges ranging from Q = -3 to Q = 2 in a (Pt0.95Ir0.05)/Fe bilayer on Pd(111) surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The Dzyaloshinsky-Moriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with Q = -1, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.
Kutatásainkat elsősorban a mágneses vékonyrétegeknek és nanoszerkezeteknek a nagysűrűségű adattár... more Kutatásainkat elsősorban a mágneses vékonyrétegeknek és nanoszerkezeteknek a nagysűrűségű adattárolásban való alkalmazhatósága valamint az alkalmazások alapjait jelentő fizikai jelenségek megértése motiválta. A relativisztikus hatások, közöttük is elsősorban a spin-pálya kölcsönhatás, számos jelenségben meghatározóak, mint pl. a mágneses anizotrópia, különböző reorientációs fázis átalakulások vagy a domén-falak képződése, amelyek fontosak pl. a merőleges mágneses adattároló anyagok (PMRM, perpendicular magnetic recording media) tervezésében is. A jelen pályázat szerves folytatása az OTKA 2002-ben lezárult T030240 nyilvántartási számú projektjének, amelyenek Dr. Szunyogh László volt a témavezetője. Az e projekt keretein belül alkalmazott eljárásokat használtuk fel kuatásaink során illetve fejlesztettük tovább. A szerződésben a következő főbb kutatási program pontokat adtuk meg: 1. Mágneses vékonyrétegek -Co/Cu(100), Fe/Cu(100) -magnon spektrumának és mágneses anizotrópiájának meghatározása különböző rétegvastagság esetén. Ugyanezen rendszerek Curie hômérsékletének vizsgálata a rétegszám függvényében a spinhullám közelítés segítségével. A témakörben 7 publikáció született: [4,3,9,20,22,23,26] 2. Réteges szerkezetű ferromágneses anyagokban -CoPt, CoPt3 -mágneses doménfalak vastagságának first principles meghatározása. Megvizsgáljuk a doménfal szerkezetének eltéréseit a fenomenologikus modell által jósolt szerkezettől. Transzport tulajdonságok meghatározása a doménfalon kersztül. A témakörben 7 publikáció született: [2,
We determined the magnetic B -T phase diagram of PdFe bilayer on Ir(111) surface by performing Mo... more 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.
Effective Cluster Interactions at Disordered Surfaces
MRS Proceedings, 1991
ABSTRACTEffective cluster interactions (ECI) at surfaces of substitutionally disordered alloys ar... more ABSTRACTEffective cluster interactions (ECI) at surfaces of substitutionally disordered alloys are determined using the generalization of the TB-LMTO-CPA method to inhomogeneous systems. Illustrative results are reported for the (Ag,Pd) alloy system.
The 13th international winterschool on electronic properties of novel materials- science and technology of molecular nanostructures, 1999
The energy of the triplet and singlet states of the N@C60−1 have been calculated at ROHF and MCSC... more The energy of the triplet and singlet states of the N@C60−1 have been calculated at ROHF and MCSCF level. The ground state of the anion appeared to be triplet and the nitrogen atom has been found to sit at the center of the molecule. A possible reason why the endohedral molecule remains neutral in the AC60 phase is discussed at the end of the paper.
The UV-Vis spectra of Diels-Alder adducts of Cso show characteristic differences with respect to ... more The UV-Vis spectra of Diels-Alder adducts of Cso show characteristic differences with respect to the spectra of isolated C6o. New bands appear between 400 and 500 nm, as well as at around 700 nm. In order to explain the origin of these bands we performed quantum chemical calculations for model molecules: C6oH2~ (n=1,2,3). The upper bands correspond to singlet-singlet transitions while the lowest energy band can originate only from a triplet level and spin dependent interactions should be responsible for this excitation. Symmetry arguments indicate that such transitions remain forbidden for pure Cs0.
To calculate the magnetic ground state of nanoparticles we present a self-consistent first-princi... more 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.
By using the generalized perturbation method within the screened Korringa-Kohn-Rostoker formalism... more By using the generalized perturbation method within the screened Korringa-Kohn-Rostoker formalism we calculate magnetic exchange interactions in the helimagnetic bulk MnAu 2 compound upon alloying Mn with Fe and Cr. The obtained interactions reproduce well the results of neutron diffraction experiments concerning the periodicity of the helix in MnAu 2 and its weakening with Fe substitution thus supporting the conventional interpretation of a metamagnet-ferromagnet transitions due to external fields. In the case of Mn 1-x Cr x Au 2 alloys our results reveal that the experimentally observed magnetization process follows a completely different scenario, predicted theoretically a few decades ago for nearly orthogonal helimagnetics.
Chiral magnetic ordering due to Dzyaloshinsky-Moriya interaction on two-dimensional lattices is s... more Chiral magnetic ordering due to Dzyaloshinsky-Moriya interaction on two-dimensional lattices is studied theoretically. Several competing Dzyaloshinsky-Moriya vectors are introduced on the basis of symmetry arguments. The role of the exchange interaction, magnetic anisotropy, and dipolar coupling for the ordering in chiral nanomagnets is investigated. It is demonstrated that the periodicity of the modulated structure, which is determined by all interactions involved, is lattice dependent; the direction of spiral propagation and orientation of magnetization is determined by the competition between different Dzyaloshinsky-Moriya vectors and anisotropy; the anisotropy can induce a domain formation or destroy the chiral ordering depending on its orientation. We show that the Dzyaloshinsky-Moriya coupling is responsible for the chiral magnetic ordering in Fe/ W͑110͒.
We present a multiscale study of the formation of magnetic patterns in Mn monolayers deposited on... more We present a multiscale study of the formation of magnetic patterns in Mn monolayers deposited on Wð0 0 1Þ and Wð1 1 0Þ surfaces. We derive the parameters of an extended classical Heisenberg Hamiltonian from relativistic spin-polarized first principle calculations. By using these parameters we perform Monte Carlo simulations to explore the low-temperature spin structures. We find that the antisymmetric Dzyaloshinskii-Moriya interactions induce spin-spirals propagating along an in-plane axis with a wavelength of about 2.2 and 7.2 nm, respectively.
The spatial and energy-dependent fluctuations of the local density of states (LDOS) are calculate... more The spatial and energy-dependent fluctuations of the local density of states (LDOS) are calculated by means of a transfer matrix formalism for a model point contact (PC). It is shown that decreasing the size of the orifice the LDOS fluctuations become more and more pronounced in the region near to the orifice. As the Kondo temperature of a magnetic impurity is determined by the LDOS at the Fermi energy a broadening of the Kondo resonance is predicted for small metallic PC's in agreement with the results of recent measurements.
A lattice-gas model is suggested for describing the ordering phenomena in alkali-metal fullerides... more A lattice-gas model is suggested for describing the ordering phenomena in alkali-metal fullerides of facecentered-cubic structure assuming that the electric charge of alkali ions residing in either octahedral or tetrahedral sites is completely screened by the first-neighbor C60 molecules. This approximation allows us to derive an effective ion-ion interaction. The van der Waals interaction between the ion and C60 molecule is characterized by introducing an additional site energy at the tetrahedral sites. This model is investigated by using a three-sublattice mean-field approximation and a simple cluster-variation method. The analysis shows a large variety of phase diagrams when changing the site energy parameter.
In order to study the finite temperature behavior of magnetic nanoparticles a novel Monte Carlo m... more In order to study the finite temperature behavior of magnetic nanoparticles a novel Monte Carlo method has been developed. The energy of a new trial configuration during the simulation is calculated directly from the expansion of the band energy avoiding a set up of an a priori Heisenberg-type model. The electronic structure of the cluster is determined by means of the embedded-cluster Green's function technique based on the Korringa-Kohn-Rostoker method within the local spin-density approximation of the density functional theory. As a benchmark the ground state of anti-ferromagnetic clusters and the temperature dependence of the magnetization of a flat square cluster of 16 Co atoms on a Cu(001) surface have been studied.
Controlling the preferred direction of the magnetic moments is essential for the design of spintr... more Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and heterostructures. As the film thickness or the temperature is increased, the easy anisotropy axis is typically reoriented from an out-of-plane direction preferred by surface and interface energy contributions to an in-plane alignment favored by the volume anisotropy terms. We study the temperature-driven spin reorientation transition in two atomic layers of Fe on W(110) using well-tempered metadynamics simulations based on a spin model parametrized by ab initio calculations and find that the transition only takes place in the presence of the Dzyaloshinsky-Moriya interaction (DMI). This demonstrates that the chiral DMI does not only differentiate between noncollinear spin structures of different rotational senses, but it also influences the magnetic orientation of collinear magnetic configurations.
We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature... more We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T , in metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced 'local moment' magnetic fluctuations are described within the relativistic generalisation of the 'disordered local moment' (R-DLM) theory from which the T dependence of the magnetisation, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry,L1 0 -ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L1 0ordered FePt and which agrees with experiment. This account, however, differs qualitatively from that extracted from a single ion anisotropy model. We also study the magnetically soft cubic magnet, the F e 50 P t 50 solid solution, and find that its small magnetic anisotropy constant K 1 rapidly diminishes from 8 µeV to zero. K evolves from being proportional to m 7 at low T to m 4 near the Curie temperature.
We employ metadynamics simulations to calculate the free energy landscape of thin ferromagnetic f... more We employ metadynamics simulations to calculate the free energy landscape of thin ferromagnetic films and perform a systematic study of the temperature dependence of magnetic anisotropy and of the spin-reorientation transitions. By using a simple spin model we recover the well-known power-law behavior of the magnetic anisotropy energy against magnetization and present a rather detailed analysis of the spin-reorientation transitions in ultrathin films. Based on tensorial exchange interactions and anisotropy parameters derived from first-principles calculations we perform simulations for Fe double layers deposited on Au(001) and W(110). In case of Fe2W(110) our simulations display an out-of-plane to in-plane spin-reorientation transition in agreement with experiments.
We present a computational method to determine the exchange constants in isotropic spin models. T... more We present a computational method to determine the exchange constants in isotropic spin models. The method uses the Hamiltonian and overlap matrices computed from density functional schemes that are based on nonorthogonal basis sets. We demonstrate that the new method as implemeted in the SIESTA code reproduces the Heisenberg interactions of simple metallic bulk ferromagnets as obtained from former well-established computational approaches. Then we address sp magnetism in graphene nanostructures. For fluorinated graphene we obtain exchange interactions in fairly good agreement with previous calculations using maximally localized Wannier functions and we confirm the theoretical prediction of a 120 • Néel state. Associated with the magnetic edge-states of a zigzag graphene nanoribbon we find rapidly decaying exchange interactions, however, with an unconventional distance dependence of exp(-r/δ). We show that the stiffness constant derived from the exchange interactions is consistent with previous estimate based on total energy differences of twisted spin configurations. We highlight that our method is an efficient tool for the analysis of novel hybrid nano-structures where metallic and organic components are integrated to form exotic magnetic patterns.
Magnetic skyrmions are localized nanometer-sized spin configurations with particle-like propertie... more Magnetic skyrmions are localized nanometer-sized spin configurations with particle-like properties, which are envisioned to be used as bits in next-generation information technology. An essential step toward future skyrmion-based applications is to engineer key magnetic parameters for developing and stabilizing individual magnetic skyrmions. Here we demonstrate the tuning of the non-collinear magnetic state of an Fe double layer on an Ir(111) substrate by loading the sample with atomic hydrogen. By using spin-polarized scanning tunneling microscopy, we discover that the hydrogenated system supports the formation of skyrmions in external magnetic fields, while the pristine Fe double layer does not. Based on ab initio calculations, we attribute this effect to the tuning of the Heisenberg exchange and the Dzyaloshinsky–Moriya interactions due to hydrogenation. In addition to interface engineering, hydrogenation of thin magnetic films offers a unique pathway to design and optimize the s...
We observe metastable localized spin configurations with topological charges ranging from Q = -3 ... more We observe metastable localized spin configurations with topological charges ranging from Q = -3 to Q = 2 in a (Pt0.95Ir0.05)/Fe bilayer on Pd(111) surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The Dzyaloshinsky-Moriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with Q = -1, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.
Kutatásainkat elsősorban a mágneses vékonyrétegeknek és nanoszerkezeteknek a nagysűrűségű adattár... more Kutatásainkat elsősorban a mágneses vékonyrétegeknek és nanoszerkezeteknek a nagysűrűségű adattárolásban való alkalmazhatósága valamint az alkalmazások alapjait jelentő fizikai jelenségek megértése motiválta. A relativisztikus hatások, közöttük is elsősorban a spin-pálya kölcsönhatás, számos jelenségben meghatározóak, mint pl. a mágneses anizotrópia, különböző reorientációs fázis átalakulások vagy a domén-falak képződése, amelyek fontosak pl. a merőleges mágneses adattároló anyagok (PMRM, perpendicular magnetic recording media) tervezésében is. A jelen pályázat szerves folytatása az OTKA 2002-ben lezárult T030240 nyilvántartási számú projektjének, amelyenek Dr. Szunyogh László volt a témavezetője. Az e projekt keretein belül alkalmazott eljárásokat használtuk fel kuatásaink során illetve fejlesztettük tovább. A szerződésben a következő főbb kutatási program pontokat adtuk meg: 1. Mágneses vékonyrétegek -Co/Cu(100), Fe/Cu(100) -magnon spektrumának és mágneses anizotrópiájának meghatározása különböző rétegvastagság esetén. Ugyanezen rendszerek Curie hômérsékletének vizsgálata a rétegszám függvényében a spinhullám közelítés segítségével. A témakörben 7 publikáció született: [4,3,9,20,22,23,26] 2. Réteges szerkezetű ferromágneses anyagokban -CoPt, CoPt3 -mágneses doménfalak vastagságának first principles meghatározása. Megvizsgáljuk a doménfal szerkezetének eltéréseit a fenomenologikus modell által jósolt szerkezettől. Transzport tulajdonságok meghatározása a doménfalon kersztül. A témakörben 7 publikáció született: [2,
We determined the magnetic B -T phase diagram of PdFe bilayer on Ir(111) surface by performing Mo... more 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.
Effective Cluster Interactions at Disordered Surfaces
MRS Proceedings, 1991
ABSTRACTEffective cluster interactions (ECI) at surfaces of substitutionally disordered alloys ar... more ABSTRACTEffective cluster interactions (ECI) at surfaces of substitutionally disordered alloys are determined using the generalization of the TB-LMTO-CPA method to inhomogeneous systems. Illustrative results are reported for the (Ag,Pd) alloy system.
The 13th international winterschool on electronic properties of novel materials- science and technology of molecular nanostructures, 1999
The energy of the triplet and singlet states of the N@C60−1 have been calculated at ROHF and MCSC... more The energy of the triplet and singlet states of the N@C60−1 have been calculated at ROHF and MCSCF level. The ground state of the anion appeared to be triplet and the nitrogen atom has been found to sit at the center of the molecule. A possible reason why the endohedral molecule remains neutral in the AC60 phase is discussed at the end of the paper.
The UV-Vis spectra of Diels-Alder adducts of Cso show characteristic differences with respect to ... more The UV-Vis spectra of Diels-Alder adducts of Cso show characteristic differences with respect to the spectra of isolated C6o. New bands appear between 400 and 500 nm, as well as at around 700 nm. In order to explain the origin of these bands we performed quantum chemical calculations for model molecules: C6oH2~ (n=1,2,3). The upper bands correspond to singlet-singlet transitions while the lowest energy band can originate only from a triplet level and spin dependent interactions should be responsible for this excitation. Symmetry arguments indicate that such transitions remain forbidden for pure Cs0.
To calculate the magnetic ground state of nanoparticles we present a self-consistent first-princi... more 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.
By using the generalized perturbation method within the screened Korringa-Kohn-Rostoker formalism... more By using the generalized perturbation method within the screened Korringa-Kohn-Rostoker formalism we calculate magnetic exchange interactions in the helimagnetic bulk MnAu 2 compound upon alloying Mn with Fe and Cr. The obtained interactions reproduce well the results of neutron diffraction experiments concerning the periodicity of the helix in MnAu 2 and its weakening with Fe substitution thus supporting the conventional interpretation of a metamagnet-ferromagnet transitions due to external fields. In the case of Mn 1-x Cr x Au 2 alloys our results reveal that the experimentally observed magnetization process follows a completely different scenario, predicted theoretically a few decades ago for nearly orthogonal helimagnetics.
Chiral magnetic ordering due to Dzyaloshinsky-Moriya interaction on two-dimensional lattices is s... more Chiral magnetic ordering due to Dzyaloshinsky-Moriya interaction on two-dimensional lattices is studied theoretically. Several competing Dzyaloshinsky-Moriya vectors are introduced on the basis of symmetry arguments. The role of the exchange interaction, magnetic anisotropy, and dipolar coupling for the ordering in chiral nanomagnets is investigated. It is demonstrated that the periodicity of the modulated structure, which is determined by all interactions involved, is lattice dependent; the direction of spiral propagation and orientation of magnetization is determined by the competition between different Dzyaloshinsky-Moriya vectors and anisotropy; the anisotropy can induce a domain formation or destroy the chiral ordering depending on its orientation. We show that the Dzyaloshinsky-Moriya coupling is responsible for the chiral magnetic ordering in Fe/ W͑110͒.
We present a multiscale study of the formation of magnetic patterns in Mn monolayers deposited on... more We present a multiscale study of the formation of magnetic patterns in Mn monolayers deposited on Wð0 0 1Þ and Wð1 1 0Þ surfaces. We derive the parameters of an extended classical Heisenberg Hamiltonian from relativistic spin-polarized first principle calculations. By using these parameters we perform Monte Carlo simulations to explore the low-temperature spin structures. We find that the antisymmetric Dzyaloshinskii-Moriya interactions induce spin-spirals propagating along an in-plane axis with a wavelength of about 2.2 and 7.2 nm, respectively.
The spatial and energy-dependent fluctuations of the local density of states (LDOS) are calculate... more The spatial and energy-dependent fluctuations of the local density of states (LDOS) are calculated by means of a transfer matrix formalism for a model point contact (PC). It is shown that decreasing the size of the orifice the LDOS fluctuations become more and more pronounced in the region near to the orifice. As the Kondo temperature of a magnetic impurity is determined by the LDOS at the Fermi energy a broadening of the Kondo resonance is predicted for small metallic PC's in agreement with the results of recent measurements.
A lattice-gas model is suggested for describing the ordering phenomena in alkali-metal fullerides... more A lattice-gas model is suggested for describing the ordering phenomena in alkali-metal fullerides of facecentered-cubic structure assuming that the electric charge of alkali ions residing in either octahedral or tetrahedral sites is completely screened by the first-neighbor C60 molecules. This approximation allows us to derive an effective ion-ion interaction. The van der Waals interaction between the ion and C60 molecule is characterized by introducing an additional site energy at the tetrahedral sites. This model is investigated by using a three-sublattice mean-field approximation and a simple cluster-variation method. The analysis shows a large variety of phase diagrams when changing the site energy parameter.
In order to study the finite temperature behavior of magnetic nanoparticles a novel Monte Carlo m... more In order to study the finite temperature behavior of magnetic nanoparticles a novel Monte Carlo method has been developed. The energy of a new trial configuration during the simulation is calculated directly from the expansion of the band energy avoiding a set up of an a priori Heisenberg-type model. The electronic structure of the cluster is determined by means of the embedded-cluster Green's function technique based on the Korringa-Kohn-Rostoker method within the local spin-density approximation of the density functional theory. As a benchmark the ground state of anti-ferromagnetic clusters and the temperature dependence of the magnetization of a flat square cluster of 16 Co atoms on a Cu(001) surface have been studied.
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Papers by László Udvardi