| 51. |
- Li, Guijiang, et al.
(author)
-
Kinetic arrest induced antiferromagnetic order in hexagonal FeMnP0.75Si0.25 alloy
- 2014
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:26, s. 262405-
-
Journal article (peer-reviewed)abstract
- The magnetic state of the FeMnP0.75Si0.25 alloy was investigated by first principles calculations. The coexistence of ferromagnetic and antiferromagnetic phases in FeMnP0.75Si0.25 with the same hexagonal crystal structure was revealed. It was found that kinetic arrest during the transition from the high temperature disordered paramagnetic phase to the low temperature ordered ferromagnetic phase results in the intermediate metastable and partially disordered antiferromagnetic phase. We propose that the ratio of the ferromagnetic and antiferromagnetic phases in the FeMnP0.75Si0.25 sample can be tuned by adjusting the kinetic process of atomic diffusion. The investigations suggest that careful control of the kinetic diffusion process provides another tuning parameter to design candidate magnetocaloric materials.
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| 52. |
- Locht, Inka L. M., 1986-, et al.
(author)
-
Magnetic asymmetry around the 3p absorption edge in Fe and Ni
-
Other publication (other academic/artistic)abstract
- This work is a joint theoretical and experimental study of the relation between the magneto- optical response of a material in the sub-picosecond timescale and its instantaneous magnetisation. We perform pump-probe experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry. We measure the magnetic asymmetry of elemental Fe and Ni before and after the laser pulse. The observed differences between the magnetic asymmetry curves for various photon energies suggest that the relation between asymmetry and sample magnetization is more complex than a simple proportionality. Further insight is obtained by means of theoretical simulations based on density-functional theory. Our calculations show that non-linear effects in the asymmetry are most prominent at energies corresponding to the absorption edge and that the proportionality is recovered outside of this region. In conclusion, our experimental and theoretical results emphasize the need of including the complex relation between asymmetry and magnetization in the interpretation of ultrafast magnetization experiments in terms of microscopic properties.
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| 53. |
- Locht, Inka L. M., et al.
(author)
-
Standard model of the rare earths analyzed from the Hubbard I approximation
- 2016
-
In: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 94:8
-
Journal article (peer-reviewed)abstract
- In this work we examine critically the electronic structure of the rare-earth elements by use of the so-called Hubbard I approximation. From the theoretical side all measured features of both occupied and unoccupied states are reproduced, without significant deviations between observations and theory. We also examine cohesive properties like the equilibrium volume and bulk modulus, where we find, in general, a good agreement between theory andmeasurements. In addition, we have reproduced the spin and orbital moments of these elements as they are reflected from measurements of the saturation moment. We have also employed the Hubbard I approximation to extract the interatomic exchange parameters of an effective spin Hamiltonian for the heavy rare earths. We show that the Hubbard I approximation gives results which are consistent with calculations where 4f electrons are treated as core states for Gd. The latter approach was also used to address the series of the heavy/late rare earths. Via Monte Carlo simulations we obtained ordering temperatures which reproduce measurements within about 20%. We have further illustrated the accuracy of these exchange parameters by comparing measured and calculated magnetic configurations for the heavy rare earths and the magnon dispersion for Gd. The Hubbard I approximation is compared to other theories of the electronic structure, and we argue that it is superior. We discuss the relevance of our results in general and how this makes it possible to treat the electronic structure of materials containing rare-earth elements, such as permanent magnets, magnetostrictive compounds, photovoltaics, optical fibers, topological insulators, and molecular magnets.
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| 54. |
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| 55. |
- Lojewski, Tobias, et al.
(author)
-
The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
- 2023
-
In: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 11:8, s. 655-661
-
Journal article (peer-reviewed)abstract
- The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be captured by electron repopulation respectively correlation-induced modifications of the electronic structure, requiring to take the local Coulomb interaction into account.
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| 56. |
- Lüder, Johann, et al.
(author)
-
Theory of L-edge spectroscopy of strongly correlated systems
- 2017
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:24
-
Journal article (peer-reviewed)abstract
- X-ray absorption spectroscopy measured at the L edge of transition metals (TMs) is a powerful element selective tool providing direct information about the correlation effects in the 3d states. The theoretical modeling of the 2p -> 3d excitation processes remains to be challenging for contemporary ab initio electronic structure techniques, due to strong core-hole and multiplet effects influencing the spectra. In this work, we present a realization of the method combining the density-functional theory with multiplet ligand field theory, proposed in Haverkort et al. [Phys. Rev. B 85, 165113 (2012)]. In this approach, a single-impurity Anderson model (SIAM) is constructed, with almost all parameters obtained from first principles, and then solved to obtain the spectra. In our implementation, we adopt the language of the dynamical mean-field theory and utilize the local density of states and the hybridization function, projected onto TM 3d states, in order to construct the SIAM. The developed computational scheme is applied to calculate the L-edge spectra for several TM monoxides. A very good agreement between the theory and experiment is found for all studied systems. The effect of core-hole relaxation, hybridization discretization, possible extensions of the method as well as its limitations are discussed.
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| 57. |
- Maciel, Renan P., et al.
(author)
-
Resistive switching in graphene : A theoretical case study on the alumina-graphene interface
- 2023
-
In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 5:4
-
Journal article (peer-reviewed)abstract
- Neuromorphic computing mimics the brain's architecture to create energy-efficient devices. Reconfigurable synapses are crucial for neuromorphic computing, which can be achieved through memory-resistive (memristive) switching. Graphene-based memristors have shown nonvolatile multibit resistive switching with desirable endurance. Through first-principles calculations, we study the structural and electronic properties of graphene in contact with an ultra-thin alumina overlayer and demonstrate how one can use charge doping to exert direct control over its interfacial covalency, reversibly switching between states of conductivity and resistivity in the graphene layer. We further show that this proposed mechanism can be stabilized through the p-type doping of graphene, e.g., by naturally occurring defects, the passivation of dangling bonds or defect engineering.
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| 58. |
- Maldonado, Pablo, et al.
(author)
-
Microscopic theory of ultrafast out-of-equilibrium magnon-phonon dynamics in insulators
- 2019
-
In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:1
-
Journal article (peer-reviewed)abstract
- The interaction between lattice and spins is at the heart of an extremely intriguing ultrafast dynamics in magnetic materials. In this paper, we formulate a general nonequilibrium theory that disentangles the complex interplay between phonons and magnons in a THz laser-excited antiferromagnetic insulator. The theory provides a quantitative description of the transient energy flow between the spin and lattice subsystems, subject to magnon-phonon and phonon-phonon scatterings, giving rise to finite lifetimes of the quasiparticles and to the equilibration time of the system. We predict a kind of scattering process where two magnons of opposite polarizations decay into a phonon, previously omitted in the literature. The theory is combined with first-principles calculations and then applied to simulate realistic dynamics of NiO. The main relaxation channels and hot spots in the reciprocal space, giving the strongest contribution to the energy transfer between phonons and magnons are identified. The diverse interaction strengths lead to distinct coupled dynamics of the lattice and spin systems and subsequently to different equilibration timescales.
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| 59. |
- Malik, Rameez Saeed, 1987-, et al.
(author)
-
Ultrafast dynamics in Fe65Co35 alloys: Effect of Re doping
-
Other publication (pop. science, debate, etc.)abstract
- Soft magnetic FeCo alloys are of great interest due to their potential spintronics applications. The magnetic damping parameter plays a vital role in the performance of these spintronics devices. The Gilbert damping parameter increase in these alloys with doping of 5d elements. Here, we have investigated the effect of Re doping on the element-specific magnetization dynamics of Ru/Fe65Co35/Ru thin films using the time-resolved magneto-optical Kerr effect. When varying the concentration of Re from 0 to 12.6 %, no change of the demagnetization time constant is observed. However, a gradual change of the remagnetization time is observed with the increase of Re concentration. This remagnetization dynamics can be related to the Gilbert damping parameter of these films. An interesting time-resolved dynamics at the Ru-edge is observed. A significant increase (40%) of the asymmetry signal is observed for the undoped sample and drops down with the Re doping. This effect is possibly a super diffusive spin current going from the magnetic layer to the non magnetic capping layer.
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| 60. |
- Malik, Rameez Saeed, 1987-, et al.
(author)
-
Ultrafast magnetization dynamics in half-metallic Co2FeAl Heusler alloy
-
Other publication (pop. science, debate, etc.)abstract
- We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl,probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to resultsfrom electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we findthat the demagnetization time (τM) in films of Co2FeAl is almost independent of varying structuralorder, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower processof magnetization recovery, specified byτR, is found to occur on picosecond time scales, and isdemonstrated to correlate strongly with the Gilbert damping parameter (α). Our results showthat Co2FeAl is unique, in that it is the first material that clearly demonstrates the importance ofthe damping parameter in the remagnetization process. Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which mighthave general applicabilit
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| 61. |
- Malik, Rameez Saeed, 1987-, et al.
(author)
-
Ultrafast magnetization dynamics in magnetically frustrated NiFe2O4 ferrimagnet
-
Other publication (pop. science, debate, etc.)abstract
- NiFe2O4 shows complex dynamics between its elements after an ultrashort laser pulse excitation. The reflectivity shows oscillations, indicating either a reordering of the charge distribution several picoseconds after absorption of the laser pulse or a coherent phonon mode with a frequency of 0.5 THz. Also, the magnetic asymmetry exhibits oscillations that cannot be attributed to transverse magnetic excitations (magnons) but correspond to longitudinal excitations. The demagnetization shows different behavior depending on the excitation photon energy, for energies close to the band gap (1.55 eV) oscillations in the asymmetry are prominent, while for higher energies (3.1 eV) the demagnetization becomes dominant and the oscillations vanish. A fast demagnetization is observed when using 3.1 eV compared to 1.55 eV, while the electronic response remains similar, which indicates that the fast demagnetization dynamics is driven by holes created deeper in the valence band while the dynamics of electrons excited to d-states in the conduction band provides a smaller contribution to the demagnetization.The oscillations in the asymmetry can be a consequence of the competing exchange interactions that are antiferromagnetic between all three sublattices, but where a ferromagnetic alignment between octahedral Ni and octahedral Fe is the ground state.
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| 62. |
- Malik, Rameez Saeed, 1987-, et al.
(author)
-
Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl
- 2021
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
-
Journal article (peer-reviewed)abstract
- We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (tau(M)) in films of Co2FeAl is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by tau(R), is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter (alpha). Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which might have general applicability.
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| 63. |
- Mazurenko, V. V., et al.
(author)
-
A DMI Guide to Magnets Micro-World
- 2021
-
In: Journal of Experimental and Theoretical Physics. - : PLEIADES PUBLISHING INC. - 1063-7761 .- 1090-6509. ; 132:4, s. 506-516
-
Journal article (peer-reviewed)abstract
- Dzyaloshinskii-Moriya interaction, DMI in short, represents an antisymmetric type of magnetic interactions that favor orthogonal orientation of spins and competes with Heisenberg exchange. Being introduced to explain weak ferromagnetism in antiferromagnets without an inversion center between magnetic atoms such an anisotropic interaction can be used to analyze other non-trivial magnetic structures of technological importance including spin spirals and skyrmions. Despite the fact that the corresponding DMI contribution to the magnetic energy of the system has a very compact form of the vector product of spins, the determination of DMI from first-principles electronic structure is a very challenging methodological and technical problem whose solution opens a door into the fascinating microscopic world of complex magnetic materials. In this paper we review a few such methods developed by us for calculating DMI and their applications to study the properties of real materials.
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| 64. |
- Mazurenko, V. V., et al.
(author)
-
First-principles modeling of magnetic excitations in Mn-12
- 2014
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:21, s. 214422-
-
Journal article (peer-reviewed)abstract
- We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet Mn-12. First, the intramolecular magnetic properties have been studied by means of first-principles density functional based methods, with local correlation effects being taken into account within the local density approximation plus U (LDA + U) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing one to work with as large a Hilbert space dimension as 108 without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.
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| 65. |
- Ntallis, Nikolaos, et al.
(author)
-
Connection between magnetic interactions and the spin-wave gap of the insulating phase of NaOsO3
- 2021
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:13
-
Journal article (peer-reviewed)abstract
- The scenario of a metal-insulator transition driven by the onset of antiferromagnetic order in NaOsO3 calls for a trustworthy derivation of the underlying effective spin Hamiltonian. To determine the latter we rely on ab initio electronic-structure calculations, linear spin-wave theory, and comparison to experimental data of the corresponding magnon spectrum. We arrive this way to Heisenberg couplings that are less than or similar to 45 to less than or similar to 63% smaller than values presently proposed in the literature and Dzyaloshinskii-Moriya interactions in the region of 15% of the Heisenberg exchange J. These couplings together with the symmetric anisotropic exchange interaction and single-ion magnetocrystalline anisotropy successfully reproduce the magnon dispersion obtained by resonant inelastic x-ray scattering measurements. In particular, the spin-wave gap fully agrees with the measured one. We find that the spin-wave gap is defined from a subtle interplay between the single-ion anisotropy, the Dzyaloshinskii-Moriya exchange, and the symmetric anisotropic exchange interactions. The results reported here underpin the local-moment description of NaOsO3, when it comes to analyzing the magnetic excitation spectra. Interestingly, this comes about from a microscopic theory that describes the electron system as Bloch states, adjusted to a mean-field solution to Hubbard-like interactions.
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| 66. |
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| 67. |
- Pal, Somnath, et al.
(author)
-
Peculiar magnetic states in the double perovskite Nd2NiMnO6
- 2019
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:4
-
Journal article (peer-reviewed)abstract
- We present magnetic measurements on Nd2NiMnO6 which exhibits a well-known insulating paramagnetic state to an insulating ferromagnetic state transition when cooled below 200 K. Beyond this basic fact, there is a great deal of diversity in the reported magnetic properties and interpretation of specific anomalies observed in the magnetic data of this compound below the Curie temperature. We address specifically two anomalies discussed in the past, namely, a spin-glass like behavior observed in some samples near 100 K and a downturn in the magnetization with a lowering of the temperature below approximately 50 K. We show for the first time that the application of an increasing magnetic field can systematically change the low-temperature behavior to make the down-turn in the magnetization into an upturn. With the help of first principle calculations and extensive simulations along with our experimental observations, we provide a microscopic understanding of all magnetic properties observed in this interesting system to point out that the glassiness around 100 K is absent in well-ordered samples and that the low-temperature magnetic anomaly below 50 K is a consequence of a ferromagnetic coupling of the Nd spin moments with the spin of the Ni-Mn ordered sublattice without giving rise to any ordering of the Nd sublattice that remains paramagnetic, contrary to earlier claims. We explain this counter-intuitive interpretation of a ferromagnetic coupling of Nd spins with Ni-Mn spin giving rise to a decrease in the total magnetic moment by noting the less than half-filled 4f occupation of Nd that ensures orbital and spin moments of Nd to be opposite to each other due to the spin-orbit coupling. Since the ground state total magnetic moment of Nd has a contribution from the orbital moment, that is larger than the spin moment, the total moment of Nd is indeed pointing in a direction opposite to the direction of spin moments of the Ni-Mn sublattice as a consequence of the ferromagnetic exchange coupling between Nd and Ni-Mn spins.
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| 68. |
- Panda, Swarup K., et al.
(author)
-
Electronic structure and exchange interactions of insulating double perovskite La2CuRuO6
- 2016
-
In: PHYSICAL REVIEW B. - 2469-9950. ; 94:6
-
Journal article (peer-reviewed)abstract
- We have performed first-principles calculations of the electronic and magnetic properties of insulating double perovskite compound La2CuRuO6 (LCRO) which has recently been reported to exhibit intriguing magnetic properties. We derived a tight-binding Hamiltonian for LCRO based on the Nth-order muffin-tin orbital (NMTO) downfolding technique. The computed on-site energies and hopping integrals are used to estimate the dominant exchange interactions employing an extended Kugel-Khomskii model. This way the dominant exchange paths were identified and a low-energy spin model was proposed. The Green function method based on the magnetic force theorem has also been used to extract the exchange interactions to provide a more accurate estimation and to justify the model calculations. Our results show that the nearest neighbor (NN) Cu-Ru magnetic interactions are very much direction dependent and a strong antiferromagnetic next nearest neighbor Ru-Ru interaction along the crystallographic b axis is responsible for the magnetic frustration observed experimentally in this system. We argue that due to the broken symmetry, NN Cu-Ru interaction becomes stronger along one direction than the other, which essentially reduces the amount of frustration and helps the system to achieve an antiferromagnetic ground state at low temperature. A detailed microscopic explanation of the exchange mechanism is discussed. We also find that spin-orbit coupling effect is significant and causes a canting of the Ru spin with respect to the Cu moments.
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| 69. |
- Panda, Swarup K., et al.
(author)
-
High photon energy spectroscopy of NiO : Experiment and theory
- 2016
-
In: PHYSICAL REVIEW B. - 2469-9950. ; 93:23
-
Journal article (peer-reviewed)abstract
- We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.
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| 70. |
- Paul, Souvik, et al.
(author)
-
Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory
- 2018
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:12
-
Journal article (peer-reviewed)abstract
- Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of BiFeO3, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the LDA+U approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe 3d states, whereas the LDA+U method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe 3d orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe 3d states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.
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| 71. |
- Pincini, D., et al.
(author)
-
Role of the orbital moment in a series of isostructural weak ferromagnets
- 2018
-
In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:10
-
Journal article (peer-reviewed)abstract
- The orbital contribution to the magnetic moment of the transition-metal ion in the isostructural weak ferromagnets ACO(3) (A = Mn,Co,Ni) and FeBO3 was investigated by a combination of first-principles calculations, nonresonant x-ray magnetic scattering, and x-ray magnetic circular dichroism. A nontrivial evolution of the orbital moment as a function of the 3d orbitals filling is revealed, with a particularly large value found in the Co member of the family. Here, the coupling between magnetic and lattice degrees of freedom produced by the spin-orbit interaction results in a large single-ion anisotropy and a peculiar magnetic-moment-induced electron cloud distortion, evidenced by the appearance of a subtle scattering amplitude at space-group-forbidden reflections and significant magnetostrictive effects. Our results, which complement a previous investigation on the sign of the Dzyaloshinskii-Moriya interaction across the series, highlight the importance of spin-orbit coupling in the physics of weak ferromagnets and prove the ability of modern first-principles calculations to predict the properties of materials where the Dzyaloshinskii-Moriya interaction is a fundamental ingredient of the magnetic Hamiltonian.
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| 72. |
- Sadhukhan, Banasree, et al.
(author)
-
Spin-lattice couplings in two-dimensional CrI3 from first-principles computations
- 2022
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:10
-
Journal article (peer-reviewed)abstract
- Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to atomic displacement and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e., how the magnetic interactions depend on atomic displacement, of the prototypical two-dimensional ferromagnet CrI3. We extract an effective measure of the spin-lattice coupling in CrI3, which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to antiferromagnetic (AFM) for atomic displacements larger than 0.16 (0.18) angstrom for Cr (I) atoms. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr-3d orbitals. The competition between the AFM t(2g)-t(2g) and FM t(2g)-e(g) contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign changes when the I-Cr-I bond angle approaches 90 degrees. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis, can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.
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| 73. |
- Sarkar, S., et al.
(author)
-
Magnetism between magnetic adatoms on monolayer NbSe2
- 2022
-
In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 9:4, s. 045012-
-
Journal article (peer-reviewed)abstract
- In this work, we report on an ab-initio computational study of the electronic and magnetic properties of transition metal adatoms on a monolayer of NbSe2. We demonstrate that Cr, Mn, Fe and Co prefer all to sit above the Nb atom, where the d states experience a substantial hybridization. The inter-atomic exchange coupling is shown to have an oscillatory nature accompanied by an exponential decay, in accordance with what theory predicts for a damped Ruderman-Kittel-Kasuya-Yosida interaction. Our results indicate that the qualitative features of the magnetic coupling for the four investigated adatoms can be connected to the fine details of their Fermi surface. In particular, the oscillations of the exchange in Fe and Co are found to be related to a single nesting vector, connecting large electrons and hole pockets. Most interestingly, this behavior is found to be unaffected by changes induced on the height of the impurity, which makes the magnetism robust to external perturbations. Considering that NbSe2 is a superconductor down to a single layer, our research might open the path for further research into the interplay between magnetic and superconducting characteristics, which could lead to novel superconductivity engineering.
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| 74. |
- Shaw, Justin M., et al.
(author)
-
Magnetic damping in sputter-deposited Co2MnGe Heusler compounds with A2, B2, and L2(1) orders : Experiment and theory
- 2018
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:9
-
Journal article (peer-reviewed)abstract
- We show that very low values of the magnetic damping parameter can be achieved in sputter deposited polycrystalline films of Co2MnGe annealed at relatively low temperatures ranging from 240 degrees C to 400 degrees C. Damping values as low as 0.0014 are obtained with an intrinsic value of 0.0010 after spin-pumping contributions are considered. Of importance to most applications is the low value of inhomogeneous linewidth that yields measured linewidths of 1.8 and 5.1 mT at 10 and 40 GHz, respectively. The damping parameter monotonically decreases as the B2 order of the films increases. This trend is reproduced and explained by ab initio calculations of the electronic structure and damping parameter. Here, the damping parameter is calculated as the structure evolves from A2 to B2 to L2(1) orders. The largest decrease in the damping parameter occurs during the A2 to B2 transition as the half-metallic phase becomes established.
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| 75. |
- Shaw, Justin M., et al.
(author)
-
Quantifying Spin-Mixed States in Ferromagnets
- 2021
-
In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 127:20
-
Journal article (peer-reviewed)abstract
- We quantify the presence of spin-mixed states in ferromagnetic 3D transition metals by precise measurement of the orbital moment. While central to phenomena such as Elliot-Yafet scattering, quantification of the spin-mixing parameter has hitherto been confined to theoretical calculations. We demonstrate that this information is also available by experimental means. Comparison of ferromagnetic resonance spectroscopy with x-ray magnetic circular dichroism results show that Kittel's original derivation of the spectroscopic g factor requires modification, to include spin mixing of valence band states. Our results are supported by ab initio relativistic electronic structure theory.
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| 76. |
- Suturin, Sergey M., et al.
(author)
-
Short nanometer range optically induced magnetic fluctuations accompanying ultrafast demagnetization of nanoscale ferromagnetic domains
- 2023
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:17
-
Journal article (peer-reviewed)abstract
- We have studied the nature of optically induced short-range magnetic fluctuations occurring at 10-nm length scale during ultrafast demagnetization in ferromagnetic Co/Pt multilayers. The time resolved probing of magnetization dynamics was performed with femtosecond soft x-ray pulses at the European x-ray free-electron laser. A transient high-q magnetic scattering accompanying and directly correlated to the destruction of the maze domain network has been observed at picosecond time scale in the wave vector region of 0.2–0.8nm−1. This high-q scattering has a purely magnetic nature and is ascribed to the optically induced short-range magnetic fluctuations developing in the disturbed but not fully destroyed magnetic domain network. Finally, we have simulated the optically induced response of the domain system using a two-temperature atomistic spin model and have concluded that the reason behind the high-q scattering is the laser-driven heating inducing thermal fluctuations of the domain magnetic structure.
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| 77. |
- Swarbrick, Janine C., et al.
(author)
-
Ligand Identification in Titanium Complexes Using X-ray Valence-to-Core Emission Spectroscopy
- 2010
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 49:18, s. 8323-8332
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Journal article (peer-reviewed)abstract
- The identification of ligands in metalloorganic complexes is crucial for understanding many important biological and chemical systems. Nonresonant K beta valence-to-core X-ray emission spectroscopy (XES) has been demonstrated as a ligand identification technique which is complementary to other spectroscopies, such as X-ray absorption. In this study we show the K beta valence-to-core XES alongside the Ti K-edge X-ray absorption near edge structure spectra for a series of chemically relevant low-symmetry Ti organometallic complexes. The spectra are modeled using density functional theory calculations. XES spectra are analyzed in terms of the molecular orbitals probed, in order to understand the effects of bond length, bond nature, orbital hybridization, and molecular symmetry on the observed spectral features.
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| 78. |
- Szilva, Attila, et al.
(author)
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Quantitative theory of magnetic interactions in solids
- 2023
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In: Reviews of Modern Physics. - : American Physical Society. - 0034-6861 .- 1539-0756. ; 95:3
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Research review (peer-reviewed)abstract
- This review addresses the method of explicit calculations of interatomic exchange interactions of magnetic materials. This involves exchange mechanisms normally referred to as a Heisenberg exchange, a Dzyaloshinskii-Moriya interaction, and an anisotropic symmetric exchange. The connection between microscopic theories of the electronic structure, such as density functional theory and dynamical mean-field theory, and interatomic exchange is examined. The different aspects of extracting information for an effective spin Hamiltonian that involves thousands of atoms, from electronic structure calculations considering significantly fewer atoms (1-50), is highlighted. Examples of exchange interactions of a large group of materials is presented, which involves heavy elements of the 3d period, alloys between transition metals, Heusler compounds, multilayer systems as well as overlayers and adatoms on a substrate, transition metal oxides, 4f elements, magnetic materials in two dimensions, and molecular magnets. Where possible, a comparison to experimental data is made that becomes focused on the magnon dispersion. The influence of relativity is reviewed in a few cases, as is the importance of dynamical correlations. Development to theories that handle out-of-equilibrium conditions is also described here. The review ends with a description of extensions of the theories behind explicit calculations of interatomic exchange to nonmagnetic situations, such as those that describe chemical (charge) order and superconductivity.
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| 79. |
- Szilva, Attila, et al.
(author)
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Theory of noncollinear interactions beyond Heisenberg exchange : Applications to bcc Fe
- 2017
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:14
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Journal article (peer-reviewed)abstract
- We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an Eg-T2g symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the T2g orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.
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| 80. |
- Söderlind, P., et al.
(author)
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Prediction of the new efficient permanent magnet SmCoNiFe3
- 2017
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 96:10
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Journal article (peer-reviewed)abstract
- We propose a new efficient permanent magnet, SmCoNiFe3, which is a development of the well-known SmCo5 prototype. More modern neodymium magnets of the Nd-Fe-B type have an advantage over SmCo5 because of their greater maximum energy products due to their iron-rich stoichiometry. Our new magnet, however, removes most of this disadvantage of SmCo5 while preserving its superior high-temperature efficiency over the neodymium magnets. We show by means of first-principles electronic-structure calculations that SmCoNiFe3 has very favorable magnetic properties and could therefore potentially replace SmCo5 or Nd-Fe-B types in various applications.
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| 81. |
- Tengdin, Phoebe, et al.
(author)
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Direct light–induced spin transfer between different elements in a spintronic Heusler material via femtosecond laser excitation
- 2020
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In: Science Advances. - : American Association for the Advancement of Science. - 2375-2548. ; 6:3
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Journal article (peer-reviewed)abstract
- Heusler compounds are exciting materials for future spintronics applications because they display a wide range of tunable electronic and magnetic interactions. Here, we use a femtosecond laser to directly transfer spin polarization from one element to another in a half-metallic Heusler material, Co2MnGe. This spin transfer initiates as soon as light is incident on the material, demonstrating spatial transfer of angular momentum between neighboring atomic sites on time scales < 10 fs. Using ultrafast high harmonic pulses to simultaneously and independently probe the magnetic state of two elements during laser excitation, we find that the magnetization of Co is enhanced, while that of Mn rapidly quenches. Density functional theory calculations show that the optical excitation directly transfers spin from one magnetic sublattice to another through preferred spin-polarized excitation pathways. This direct manipulation of spins via light provides a path toward spintronic devices that can operate on few-femtosecond or faster time scales.
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| 82. |
- Thersleff, Thomas, 1980-, et al.
(author)
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Towards sub-nanometer real-space observation of spin and orbital magnetism at the Fe/MgO interface
- 2017
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In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 7
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Journal article (peer-reviewed)abstract
- While the performance of magnetic tunnel junctions based on metal/oxide interfaces is determined by hybridization, charge transfer, and magnetic properties at the interface, there are currently only limited experimental techniques with sufficient spatial resolution to directly observe these effects simultaneously in real-space. In this letter, we demonstrate an experimental method based on Electron Magnetic Circular Dichroism (EMCD) that will allow researchers to simultaneously map magnetic transitions and valency in real-space over interfacial cross-sections with sub-nanometer spatial resolution. We apply this method to an Fe/MgO bilayer system, observing a significant enhancement in the orbital to spin moment ratio that is strongly localized to the interfacial region. Through the use of first-principles calculations, multivariate statistical analysis, and Electron Energy-Loss Spectroscopy (EELS), we explore the extent to which this enhancement can be attributed to emergent magnetism due to structural confinement at the interface. We conclude that this method has the potential to directly visualize spin and orbital moments at buried interfaces in magnetic systems with unprecedented spatial resolution.
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| 83. |
- Thonig, Danny, 1986-, et al.
(author)
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Nonlocal Gilbert damping tensor within the torque-torque correlation model
- 2018
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In: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:1
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Journal article (peer-reviewed)abstract
- An essential property of magnetic devices is the relaxation rate in magnetic switching, which depends strongly on the damping in the magnetization dynamics. It was recently measured that damping depends on the magnetic texture and, consequently, is a nonlocal quantity. The damping enters the Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping parameter a, which does not, in a straightforward formulation, account for nonlocality. Efforts were spent recently to obtain Gilbert damping from first principles for magnons of wave vector q. However, to the best of our knowledge, there is no report about real-space nonlocal Gilbert damping aij. Here, a torque-torque correlation model based on a tight-binding approach is applied to the bulk elemental itinerant magnets and it predicts significant off-site Gilbert damping contributions, which could be also negative. Supported by atomistic magnetization dynamics simulations, we reveal the importance of the nonlocal Gilbert damping in atomistic magnetization dynamics. This study gives a deeper understanding of the dynamics of the magnetic moments and dissipation processes in real magnetic materials. Ways of manipulating nonlocal damping are explored, either by temperature, materials doping, or strain.
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| 84. |
- Torchio, R., et al.
(author)
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Structure and magnetism of cobalt at high pressure and low temperature
- 2016
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In: PHYSICAL REVIEW B. - 2469-9950. ; 94:2
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Journal article (peer-reviewed)abstract
- The magnetic and structural properties of cobalt were investigated under high pressure (160 GPa) and low temperature (50 K), by synchrotron K-edge x-ray magnetic circular dichroism and x-ray diffraction. A quasihydrostatic equation of state was measured up to 160 GPa. We found that uniaxial stress plays a role in the hexagonal close packed-face centered cubic (hcp-fcc) structural transition pressure. Also, our data provide the first experimental evidence that changes of the c/a ratio pressure derivative are related to the magnetic behavior. The complete extinction of ferromagnetism is observed above 130 GPa in a mixed hcp-fcc phase with no recovery upon cooling to 50 K, indicating that cobalt at 150 GPa is very likely nonmagnetic, i.e., characterized by zero local spin polarization. Density functional theory calculations point out that the K-edge x-ray magnetic circular dichroism (XMCD) signal is related to the 4p orbital moment rather than to the total spin moment and allow us to get a deeper insight into the K-edge XMCD measurements interpretation. The combination of novel theoretical results and experimental outputs provides a detailed scenario of the structural and magnetic properties of cobalt at these extreme conditions answering some previously unsolved issues.
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| 85. |
- Turenne, Diego, et al.
(author)
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Nonequilibrium sub–10 nm spin-wave soliton formation in FePt nanoparticles
- 2022
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:13
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Journal article (peer-reviewed)abstract
- Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.
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| 86. |
- Keshavarz, Samara, et al.
(author)
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- 2018
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In: Physical Review Materials. - 2475-9953. ; 2:4
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Journal article (peer-reviewed)abstract
- We present a comprehensive study of the magnetic properties of Sr3-xYx(Fe1.25Ni0.75)O-7(-delta )(0 <= x <= 0.75). Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Ned temperature (T-N) with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above T-N.
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