Magnons in a Ferromagnetic Monolayer

Prokop, J.; Tang, Wing Man; Zhang, Y; Tudosă, Ioan; Peixoto, T. R. F.; Zakeri, Kh.; Kirschner, J.

doi:10.1103/physrevlett.102.177206

Cited by 87 publications

(91 citation statements)

References 48 publications

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“…Spin-polarized high-resolution electron energy loss spectroscopy (SPEELS) was used to study the magnetic excitations. Due to its monolayer sensitivity, possibility of measuring magnetic excitations over the whole surface Brillouin zone and the capability of investigating spin-dependent scattering, SPEELS is a powerful technique for probing terahertz magnons in ultrathin ferromagnetic films [22][23][24][25][26][27][28][29][30] . A schematic representation of the scattering geometry in our SPEELS experiments is shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

Long-living terahertz magnons in ultrathin metallic ferromagnets

et al. 2015

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The main idea behind magnonics is to use the elementary magnetic excitations (magnons) for information transfer and processing. One of the main challenges, hindering the application of ultrafast terahertz magnons in magnonics, has been the short lifetime of these excitations in metallic ferromagnets. Here, we demonstrate that the engineering of the electronic structure of a ferromagnetic metal, by reducing its dimensionality and changing its chemical composition, opens a possibility to strongly suppress the relaxation channels of terahertz magnons and thereby enhance the magnons' lifetime. For the first time, we report on the long-lived terahertz magnons excited in ultrathin metallic alloy films. On the basis of the first-principles calculations, we explain the microscopic nature of the long lifetime being a consequence of the peculiar electronic hybridizations of the species. We further demonstrate a way of tailoring magnon energies (frequencies) by varying the chemical composition of the film.

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Section: Resultsmentioning

confidence: 99%

Long-living terahertz magnons in ultrathin metallic ferromagnets

et al. 2015

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“…Indeed, spin waves up to the Brillouin-zone boundary are observed in Fe and Co, which have substantially larger exchange splittings than Ni. [89][90][91][92] On the theoretical side, the first attempt to use realistic energy bands for Ni in the calculations of a generalized susceptibility was undertaken by Lowde and Windsor. 84 The authors calculated the magnetic susceptibility within the RPA for rigidly spin-split bands, but the agreement with the available experimental data was not good.…”

Section: B Collective Spin-wave Excitationsmentioning

confidence: 99%

Wannier-function approach to spin excitations in solids

et al. 2010

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We present a computational scheme to study spin excitations in magnetic materials from first principles. The central quantity is the transverse spin susceptibility, from which the complete excitation spectrum, including single-particle spin-flip Stoner excitations and collective spin-wave modes, can be obtained. The susceptibility is derived from many-body perturbation theory and includes dynamic correlation through a summation over ladder diagrams that describe the coupling of electrons and holes with opposite spins. In contrast to earlier studies, we do not use a model potential with adjustable parameters for the electron-hole interaction but employ the random-phase approximation. To reduce the numerical cost for the calculation of the four-point scattering matrix we perform a projection onto maximally localized Wannier functions, which allows us to truncate the matrix efficiently by exploiting the short spatial range of electronic correlation in the partially filled d or f orbitals. Our implementation is based on the full-potential linearized augmented-plane-wave method. Starting from a ground-state calculation within the local-spin-density approximation ͑LSDA͒, we first analyze the matrix elements of the screened Coulomb potential in the Wannier basis for the 3d transition-metal series. In particular, we discuss the differences between a constrained nonmagnetic and a proper spin-polarized treatment for the ferromagnets Fe, Co, and Ni. The spectrum of single-particle and collective spin excitations in fcc Ni is then studied in detail. The calculated spin-wave dispersion is in good overall agreement with experimental data and contains both an acoustic and an optical branch for intermediate wave vectors along the ͓1 0 0͔ direction. In addition, we find evidence for a similar double-peak structure in the spectral function along the ͓1 1 1͔ direction. To investigate the influence of static correlation we finally consider LSDA+ U as an alternative starting point and show that, together with an improved description of the Fermi surface, it yields a more accurate quantitative value for the spin-wave stiffness constant, which is overestimated in the LSDA.

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“…3. As can be inferred from this figure, ∆E(q) exhibits local extrema at about q = ±0.44Å The magnon spectrum of FeW(110) along the Y direction has been measured very recently by using spin-polarized electron energy loss spectroscopy (SPEELS), [32] a highly suitable technique to probe high wave-vector magnetic excitations of ultrathin films. Surprisingly, the measured magnon energies are about half of the theoretical values reported here and smaller by even a factor of four than the calculated values in Ref.…”

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“…3. Thus, the low energy of the measured magnon spectrum [32] should most probably be attributed to effects not included in the first principles calculations, such as spin-charge coupling [34] or phonon-magnon interaction. [35] Considering the size of the SW asymmetry obtained from our calculations, e.g., about 20 % at q = ±0.44 A −1 with respect to the average energy, (E + (q) + E − (q))/2, we strongly suggest that it should be accessible to experiments.…”

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Chiral Asymmetry of the Spin-Wave Spectra in Ultrathin Magnetic Films

2009

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We raise the possibility that the chiral degeneracy of the magnons in ultrathin films can be lifted due to the presence of Dzyaloshinskii-Moriya interactions. By using simple symmetry arguments, we discuss under which conditions such a chiral asymmetry occurs. We then perform relativistic first principles calculations for an Fe monolayer on W(110) and explicitly reveal the asymmetry of the spin-wave spectrum in case of wave-vectors parallel to the (001) direction. Furthermore, we quantitatively interpret our results in terms of a simplified spin-model by using calculated DzyaloshinskiiMoriya vectors. Our theoretical prediction should inspire experiments to explore the asymmetry of spin-waves, with a particular emphasis on the possibility to measure the Dzyaloshinskii-Moriya interactions in ultrathin films. It is by now well-established that relativistic effects play a fundamental role in the magnetism of nanostructures, in particular, for thin films and finite deposited nanoparticles. Over the past two decades, a vast number of experimental and theoretical studies has been published to explore related phenomena such as magnetic anisotropies, spin-reorientation phase transitions, and non-collinear magnetic orderings. [1,2,3,4,5] The antisymmetric exchange interaction between two magnetic atoms,, where M i and M j denote the spin-moments of the atoms labeled by i and j, has been proposed 50 years ago by Dzyaloshinskii [6] and Moriya [7]. The D ij is called the Dzyaloshinskii-Moriya vector being identical to zero if the sites i and j experience inversion symmetry. It has been put forward just about ten years ago that an enhanced Dzyaloshinskii-Moriya interaction (DMI) at surfaces or interfaces can give rise to novel phenomena in nanomagnetism such as to noncollinear interlayer coupling, [8,9] to unidirectional competing magnetic anisotropies, [10] or to stabilization of non-collinear (chiral) magnetic orderings. [11,12] A breakthrough on this field happened when the resolution of spin-polarized scanning tunneling microscopy enabled to detect magnetic pattern formation on the atomic scale in monolayer-thin films. Such periodic modulations have been observed for Mn monolayers deposited on W(110) and W(001) and, could successfully be interpreted in terms of a combination of relativistic first principles calculations and a simple micromagnetic model as the consequence of large DM interactions. [13,14]. Using the same theoretical basis it was even possible to explain the homochirality of the domain walls in two monolayers of Fe on W(110), [15] in agreement with previous experimental observation. [16] In this Letter, we investigate a consequence of the DM interactions on the spin-wave spectra in ultrathin films, not yet explored in the literature. We argue that the chiral degeneracy of the spin-wave (SW) spectrum can be lifted due to the Dzyaloshinskii-Moriya interactions and discuss under which conditions such a chiral asymmetry occurs. Based on relativistic first principles calculations, we explicitly evidence the asy...

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Magnons in a Ferromagnetic Monolayer

Cited by 87 publications

References 48 publications

Long-living terahertz magnons in ultrathin metallic ferromagnets

Long-living terahertz magnons in ultrathin metallic ferromagnets

Wannier-function approach to spin excitations in solids

Chiral Asymmetry of the Spin-Wave Spectra in Ultrathin Magnetic Films

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