Ulrich Nowak - Academia.edu (original) (raw)

Papers by Ulrich Nowak

AIP Advances, 2020

Antidot lattices are potential candidates to act as bit patterned media for data storage as they ... more Antidot lattices are potential candidates to act as bit patterned media for data storage as they are able to trap nanoscale magnetic domains between two adjacent holes. Here, we demonstrate the combination of micromagnetic modeling and x-ray microscopy. Detailed simulation of these systems can only be achieved by micromagnetic modeling that takes thermal effects into account. For this purpose, a Landau–Lifshitz–Bloch approach is used here. The calculated melting of magnetic domains within the antidot lattice is reproduced experimentally by x-ray microscopy. Furthermore, we compare conventional scanning transmission x-ray microscopy with resolution enhanced ptychography. Hence, we achieve a resolution of 13 nm. The results demonstrate that ptychographic imaging can also recover magnetic contrast in the presence of a strong topological variation and is generally applicable toward magnetic samples requiring ultimate resolution.

Information transport and processing by pure magnonic spin currents in insulators is a promising ... more Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current driven spintronic devices. The absence of Joule heating as well as the reduced spin wave damping in insulating ferromagnets has been suggested to enable the implementation of efficient logic devices. After the proof of concept for a logic majority gate based on the superposition of spin waves has been successfully demonstrated, further components are required to perform complex logic operations. A key component is a switch that corresponds to a conventional magnetoresistive spin valve. Here, we report on magnetization orientation dependent spin signal detection in collinear magnetic multilayers with spin transport by magnonic spin currents. We find in Y3Fe5O12|CoO|Co tri-layers that the detected spin signal depends on the relative alignment of Y3Fe5O12 and Co. This demonstrates a spin valve behavior with an effect amplitude of 120 % in our syste...

Antiferromagnetic materials are in the focus of current research in magnetism because of their po... more Antiferromagnetic materials are in the focus of current research in magnetism because of their potential for applications in spintronics. As for ferromagnets, their magnetic stability in nanostructures will be limited by thermal excitations. Here, we investigate the superparamagnetic limit of antiferromagnetic nanoparticles theoretically, focusing on a comparison to the known properties of ferromagnetic particles. We find a drastically reduced thermal stability because of the exchange enhancement of the attempt frequencies and the effective damping during the antiferromagnetic switching process. We show that the order parameter in antiferromagnetic particles may strongly oscillate during the reversal at low damping values.

Physical Review B

The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and fe... more The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and ferrimagnetic systems. The linear response to an oscillating external magnetic field is calculated within the framework of the inertial Landau-Lifshitz-Gilbert equation using analytical theory and computer simulations. Precession and nutation resonance peaks are identified, and it is demonstrated that the precession frequencies are reduced by the spin inertia, while the lifetime of the excitations is enhanced. The interplay between precession and nutation is found to be the most prominent in antiferromagnets, where the timescale of the exchange-driven sublattice dynamics is comparable to inertial relaxation times. Consequently, antiferromagnetic resonance techniques should be better suited for the search for intrinsic inertial spin dynamics on ultrafast timescales than ferromagnetic resonance.

Physical Review B

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferr... more We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states.

Physical Review B

We present a study on spin-superfluid transport based on an atomistic, classical spin model. Easy... more We present a study on spin-superfluid transport based on an atomistic, classical spin model. Easy-plane ferroas well as antiferromagnets are considered, which allows for a direct comparison of these two material classes based on the same model assumptions. We find a spin-superfluid transport which is robust against variations of the boundary conditions, thermal fluctuations, and dissipation modeled via Gilbert damping. Though the spin accumulation is smaller for antiferromagnets the range of the spin-superfluid transport turns out to be identical for ferro-and antiferromagnets. Finally, we calculate and explore the role of the driving frequency and especially the critical frequency, where phase slips occur and the spin accumulation breaks down.

Applied Physics Letters

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnet... more Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown-the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal.

Physical Review B

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of t... more Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of ≈100 mJ/cm 2. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

Nature communications, Mar 14, 2018

Information transport and processing by pure magnonic spin currents in insulators is a promising ... more Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current-driven spintronic devices. The absence of Joule heating and reduced spin wave damping in insulating ferromagnets have been suggested for implementing efficient logic devices. After the successful demonstration of a majority gate based on the superposition of spin waves, further components are required to perform complex logic operations. Here, we report on magnetization orientation-dependent spin current detection signals in collinear magnetic multilayers inspired by the functionality of a conventional spin valve. In YFeO|CoO|Co, we find that the detection amplitude of spin currents emitted by ferromagnetic resonance spin pumping depends on the relative alignment of the YFeOand Co magnetization. This yields a spin valve-like behavior with an amplitude change of 120% in our systems. We demonstrate the reliability of the effect and identify its or...

Advanced materials (Deerfield Beach, Fla.), Jan 20, 2017

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unkn... more Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom-up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps-"structure control" and "chemical transformation"-are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dep...

Physical Review B

We use a multiscale approach linking ab initio calculations for the parametrization of an atomist... more We use a multiscale approach linking ab initio calculations for the parametrization of an atomistic spin model with spin dynamics simulations based on the stochastic Landau-Lifshitz-Gilbert equation to investigate the thermal magnetic properties of the ferrimagnetic rare-earth transition-metal intermetallic DyCo 5. Our theoretical findings are compared to elemental resolved measurements on DyCo 5 thin films using the x-ray magnetic circular dichroism technique. With our model, we are able to accurately compute the complex temperature dependence of the magnetization. The simulations yield a Curie temperature of T C = 1030 K and a compensation point of T comp = 164 K, which is in a good agreement with our experimental result of T comp = 120 K. The spin reorientation transition is a consequence of competing elemental magnetocrystalline anisotropies in connection with different degrees of thermal demagnetization in the Dy and Co sublattices. Experimentally, we find this spin reorientation in a region from T SR1,2 = 320 to 360 K, whereas in our simulations the Co anisotropy appears to be underestimated, shifting the spin reorientation to higher temperatures.

Physical Review B

Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ... more Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition-metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurrence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.

Physical Review B

Magnetic nanoparticles are important building blocks for future technologies ranging from nano-me... more Magnetic nanoparticles are important building blocks for future technologies ranging from nano-medicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly-understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size-and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature only superparamagnetic behavior is observed in our

Physical Review Letters

Domain wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is s... more Domain wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is studied theoretically. It is shown by numerical calculations based on a classical spin model that the wall moves towards the hotter regions, as in ferromagnets. However, for larger driving forces the so called Walker breakdown which usually speeds down the wall is missing. This is due to the fact that the wall is not tilted during its motion. For the same reason antiferromagnetic walls have no inertia and, hence, no acceleration phase leading to higher effective mobility.

Journal of Magnetism and Magnetic Materials

This "Critical Focused Issue" presents a brief review of experiments and models which describe th... more This "Critical Focused Issue" presents a brief review of experiments and models which describe the origin of exchange bias in epitaxial or textured ferromagnetic/antiferromagnetic bilayers. Evidence is presented which clearly indicates that inner, uncompensated, pinned moments in the bulk of the antiferromagnet (AFM) play a very important role in setting the magnitude of the exchange bias. A critical evaluation of the extensive literature in the field indicates that it is useful to think of this bulk, pinned uncompensated moments as a new type of a ferromagnet which has a low total moment, an ordering temperature given by the AFM Néel temperature, with parallel aligned moments randomly distributed on the regular AFM lattice.

Physical Review B

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.

Physical Review Letters, 2016

We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111... more We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111) surface with a Pt 1−x Ir x alloy overlayer from ab initio calculations. While the ground state of the system is ferromagnetic for x ¼ 0.00, it becomes a spin spiral state as Ir is intermixed into the overlayer. Although the Dzyaloshinsky-Moriya interaction is present in the system, we will demonstrate that the frustrated isotropic exchange interactions play a prominent role in creating the spin spiral state, and these frustrated couplings lead to an attractive interaction between Skyrmions at short distances. Using spin dynamics simulations, we show that under these conditions the individual Skyrmions form clusters, and that these clusters remain stable at finite temperature.

AIP Advances, 2020

Antidot lattices are potential candidates to act as bit patterned media for data storage as they ... more Antidot lattices are potential candidates to act as bit patterned media for data storage as they are able to trap nanoscale magnetic domains between two adjacent holes. Here, we demonstrate the combination of micromagnetic modeling and x-ray microscopy. Detailed simulation of these systems can only be achieved by micromagnetic modeling that takes thermal effects into account. For this purpose, a Landau–Lifshitz–Bloch approach is used here. The calculated melting of magnetic domains within the antidot lattice is reproduced experimentally by x-ray microscopy. Furthermore, we compare conventional scanning transmission x-ray microscopy with resolution enhanced ptychography. Hence, we achieve a resolution of 13 nm. The results demonstrate that ptychographic imaging can also recover magnetic contrast in the presence of a strong topological variation and is generally applicable toward magnetic samples requiring ultimate resolution.

Information transport and processing by pure magnonic spin currents in insulators is a promising ... more Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current driven spintronic devices. The absence of Joule heating as well as the reduced spin wave damping in insulating ferromagnets has been suggested to enable the implementation of efficient logic devices. After the proof of concept for a logic majority gate based on the superposition of spin waves has been successfully demonstrated, further components are required to perform complex logic operations. A key component is a switch that corresponds to a conventional magnetoresistive spin valve. Here, we report on magnetization orientation dependent spin signal detection in collinear magnetic multilayers with spin transport by magnonic spin currents. We find in Y3Fe5O12|CoO|Co tri-layers that the detected spin signal depends on the relative alignment of Y3Fe5O12 and Co. This demonstrates a spin valve behavior with an effect amplitude of 120 % in our syste...

Antiferromagnetic materials are in the focus of current research in magnetism because of their po... more Antiferromagnetic materials are in the focus of current research in magnetism because of their potential for applications in spintronics. As for ferromagnets, their magnetic stability in nanostructures will be limited by thermal excitations. Here, we investigate the superparamagnetic limit of antiferromagnetic nanoparticles theoretically, focusing on a comparison to the known properties of ferromagnetic particles. We find a drastically reduced thermal stability because of the exchange enhancement of the attempt frequencies and the effective damping during the antiferromagnetic switching process. We show that the order parameter in antiferromagnetic particles may strongly oscillate during the reversal at low damping values.

Physical Review B

The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and fe... more The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and ferrimagnetic systems. The linear response to an oscillating external magnetic field is calculated within the framework of the inertial Landau-Lifshitz-Gilbert equation using analytical theory and computer simulations. Precession and nutation resonance peaks are identified, and it is demonstrated that the precession frequencies are reduced by the spin inertia, while the lifetime of the excitations is enhanced. The interplay between precession and nutation is found to be the most prominent in antiferromagnets, where the timescale of the exchange-driven sublattice dynamics is comparable to inertial relaxation times. Consequently, antiferromagnetic resonance techniques should be better suited for the search for intrinsic inertial spin dynamics on ultrafast timescales than ferromagnetic resonance.

Physical Review B

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferr... more We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states.

Physical Review B

We present a study on spin-superfluid transport based on an atomistic, classical spin model. Easy... more We present a study on spin-superfluid transport based on an atomistic, classical spin model. Easy-plane ferroas well as antiferromagnets are considered, which allows for a direct comparison of these two material classes based on the same model assumptions. We find a spin-superfluid transport which is robust against variations of the boundary conditions, thermal fluctuations, and dissipation modeled via Gilbert damping. Though the spin accumulation is smaller for antiferromagnets the range of the spin-superfluid transport turns out to be identical for ferro-and antiferromagnets. Finally, we calculate and explore the role of the driving frequency and especially the critical frequency, where phase slips occur and the spin accumulation breaks down.

Applied Physics Letters

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnet... more Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown-the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal.

Physical Review B

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of t... more Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of ≈100 mJ/cm 2. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

Nature communications, Mar 14, 2018

Information transport and processing by pure magnonic spin currents in insulators is a promising ... more Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current-driven spintronic devices. The absence of Joule heating and reduced spin wave damping in insulating ferromagnets have been suggested for implementing efficient logic devices. After the successful demonstration of a majority gate based on the superposition of spin waves, further components are required to perform complex logic operations. Here, we report on magnetization orientation-dependent spin current detection signals in collinear magnetic multilayers inspired by the functionality of a conventional spin valve. In YFeO|CoO|Co, we find that the detection amplitude of spin currents emitted by ferromagnetic resonance spin pumping depends on the relative alignment of the YFeOand Co magnetization. This yields a spin valve-like behavior with an amplitude change of 120% in our systems. We demonstrate the reliability of the effect and identify its or...

Advanced materials (Deerfield Beach, Fla.), Jan 20, 2017

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unkn... more Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom-up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps-"structure control" and "chemical transformation"-are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dep...

Physical Review B

We use a multiscale approach linking ab initio calculations for the parametrization of an atomist... more We use a multiscale approach linking ab initio calculations for the parametrization of an atomistic spin model with spin dynamics simulations based on the stochastic Landau-Lifshitz-Gilbert equation to investigate the thermal magnetic properties of the ferrimagnetic rare-earth transition-metal intermetallic DyCo 5. Our theoretical findings are compared to elemental resolved measurements on DyCo 5 thin films using the x-ray magnetic circular dichroism technique. With our model, we are able to accurately compute the complex temperature dependence of the magnetization. The simulations yield a Curie temperature of T C = 1030 K and a compensation point of T comp = 164 K, which is in a good agreement with our experimental result of T comp = 120 K. The spin reorientation transition is a consequence of competing elemental magnetocrystalline anisotropies in connection with different degrees of thermal demagnetization in the Dy and Co sublattices. Experimentally, we find this spin reorientation in a region from T SR1,2 = 320 to 360 K, whereas in our simulations the Co anisotropy appears to be underestimated, shifting the spin reorientation to higher temperatures.

Physical Review B

Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ... more Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition-metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurrence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.

Physical Review B

Magnetic nanoparticles are important building blocks for future technologies ranging from nano-me... more Magnetic nanoparticles are important building blocks for future technologies ranging from nano-medicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly-understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size-and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature only superparamagnetic behavior is observed in our

Physical Review Letters

Domain wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is s... more Domain wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is studied theoretically. It is shown by numerical calculations based on a classical spin model that the wall moves towards the hotter regions, as in ferromagnets. However, for larger driving forces the so called Walker breakdown which usually speeds down the wall is missing. This is due to the fact that the wall is not tilted during its motion. For the same reason antiferromagnetic walls have no inertia and, hence, no acceleration phase leading to higher effective mobility.

Journal of Magnetism and Magnetic Materials

This "Critical Focused Issue" presents a brief review of experiments and models which describe th... more This "Critical Focused Issue" presents a brief review of experiments and models which describe the origin of exchange bias in epitaxial or textured ferromagnetic/antiferromagnetic bilayers. Evidence is presented which clearly indicates that inner, uncompensated, pinned moments in the bulk of the antiferromagnet (AFM) play a very important role in setting the magnitude of the exchange bias. A critical evaluation of the extensive literature in the field indicates that it is useful to think of this bulk, pinned uncompensated moments as a new type of a ferromagnet which has a low total moment, an ordering temperature given by the AFM Néel temperature, with parallel aligned moments randomly distributed on the regular AFM lattice.

Physical Review B

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.

Physical Review Letters, 2016

We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111... more We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111) surface with a Pt 1−x Ir x alloy overlayer from ab initio calculations. While the ground state of the system is ferromagnetic for x ¼ 0.00, it becomes a spin spiral state as Ir is intermixed into the overlayer. Although the Dzyaloshinsky-Moriya interaction is present in the system, we will demonstrate that the frustrated isotropic exchange interactions play a prominent role in creating the spin spiral state, and these frustrated couplings lead to an attractive interaction between Skyrmions at short distances. Using spin dynamics simulations, we show that under these conditions the individual Skyrmions form clusters, and that these clusters remain stable at finite temperature.