Emission of coherent spin waves from a magnetic layer excited by a uniform microwave magnetic field

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.We have developed an analytical theory of the Schlömann spin wave generation from a ferromagnetic layer sandwiched between two semi-infinite media of another ferromagnetic material and pumped by a uniform microwave magnetic field. Our calculations show that, under identical conditions, such a non-uniformity can boost more than twice the emitted spin wave amplitude relative to that emitted from an isolated magnetic interface. The theory provides further support in favour of the dominant role played in the process by the local difference of the microwave magnetic susceptibilities of the adjacent magnetic materials.Engineering and Physical Sciences Research CouncilEuropean Union’s Horizon 2020 researchMarie Skłodowska-Curi

An effect of the curvature induced anisotropy on the spectrum of spin waves in a curved magnetic nanowire

This is the final version of the article. Available from the American Institute of Physics via the DOI in this record.Within the framework of the solid state theory, an expression for the spectrum of spin waves propagating in a thin magnetic nanowire curled into a helix (spiral) is obtained. Its modification under the effect of a periodic modulation of the helical pitch is analyzed. In particular, it is shown that the periodic modulation of the helix pitch leads to the appearance of band gaps in the spectrum of spin waves. The influence of the modulation depth of the helical pitch on a size of the first gap is considered. © 2013 American Institute of Physics.This work was supported in part by the project NoWaPhen (FP7 GA 247556)

Magnetic interfaces as sources of coherent spin waves

This is the final version of the article. Available from APS via the DOI in this recordWe have developed a simple but general analytical theory that elucidates the mechanism of spin-wave generation from interfaces between ferromagnetic media pumped by a uniform microwave magnetic field. Our calculations show that, provided there is a finite coupling between the two media, the amplitude of the emitted spin waves depends linearly on the difference between their magnetic susceptibilities. The theory is successfully applied to interpret qualitatively three recent experimental studies in which such a spin-wave emission was observed. Furthermore, we describe how our approach can be extended to several more complicated spin-wave excitation schemes employing electric, elastic, and optical stimuli.The research leading to these results has received funding from the Engineering and Physical Sciences Research Council of the United Kingdom (Project No. EP/L019876/1) and from the European Union’s Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement No. 644348 (MagIC)

Scattering of exchange spin waves from a helimagnetic layer sandwiched between two semi-infinite ferromagnetic media

This is the final version. Available from the publisher via the DOI in this record.We have calculated the scattering (reflection and transmission) coefficients of linear exchange spin waves normally incident upon a helimagnetic layer sandwiched between two semi-infinite ferromagnetic media. Our calculations show that, despite the helimagnetic order induced in the layer by the Dzyaloshinskii-Moriya interaction (DMI), the scattering is reciprocal and insensitive to the presence of the helimagnetic order in the layer. This comes as a result of the disappearance of the DMI from the boundary conditions in the considered geometry under the small-amplitude approximation and from the specific form of the nonreciprocity of the spin-wave dispersion relation in the helimagnetic material. We show that the helimagnetic layer’s interfaces act as a system of two semicrossed polarizers for the circularly polarized spin waves incident from the ferromagnetic media. This results from the ellipticity of the magnetic precession induced by the easy-plane anisotropy in the helimagnetic layer. Our calculations also reveal the importance of evanescent solutions to correctly describe the spin-wave scattering in samples with elliptical precession. Our findings will aid development of magnonic devices containing helimagnetic constituents.European Union’s Horizon 202

Magnetization boundary conditions at a ferromagnetic interface of finite thickness.

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.We develop a systematic approach to derive boundary conditions at an interface between two ferromagnetic materials in the continuous medium approximation. The approach treats the interface as a two-sublattice material, although the final equations connect magnetizations outside of the interface and therefore do not explicitly depend on its structure. Instead, the boundary conditions are defined in terms of some average properties of the interface, which may also have a finite thickness. In addition to the interface anisotropy and symmetric exchange coupling, this approach allows us to take into account coupling resulting from inversion symmetry breaking in the vicinity of the interface, such as the Dzyaloshinskii-Moriya antisymmetric exchange interaction. In the case of negligible interface anisotropy and Dzyaloshinskii-Moriya exchange parameters, the derived boundary conditions represent a generalization of those proposed earlier by Barnaś and Mills and are therefore named 'generalized Barnaś-Mills boundary conditions'. We demonstrate how one could use the boundary conditions to extract parameters of the interface via fitting of appropriate experimental data. The developed theory could be applied to modeling of both linear and non-linear spin waves, including exchange, dipole-exchange, magnetostatic, and retarded modes, as well as to calculations of non-uniform equilibrium micromagnetic configurations near the interface, with a direct impact on the research in magnonics and micromagnetism.The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007–2013) under Grant Agreement no. 247556 (NoWaPhen)

Magnonic band spectrum of spin waves in an elliptical helix

This is the final version of the article. Available from the Royal Society via the DOI in this record.Data accessibility: All data related to this research are contained in the manuscript.We show that the spin-wave spectrum in an elliptical helix has a band character. The size of the first band gap calculated using the perturbation theory is shown to scale as square root of the eccentricity. Curved magnonic waveguides of the kind considered here could be used as structural elements of future three-dimensional magnonic architectures.This research has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 247556 (NoWaPhen), the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant Agreement no. 644348 (MagIC)

Graded magnonic index and spin wave fano resonances in magnetic structures: Excite, direct, capture

This is the author accepted manuscript. The final version is available from Pan Stanford via the DOI in this record Starting from the general topic and fundamentals of magnonics, we discuss and provide demonstrations of exciting new physics and technological opportunities associated with the graded magnonic index and spin wave Fano resonances, highlighting them as the next big thing in magnonics research.Engineering and Physical Sciences Research Council (EPSRC)European Union Horizon 202

Phenomenological description of the nonlocal magnetization relaxation in magnonics, spintronics, and domain-wall dynamics

This is the final version of the article. Available from the American Physical Society via the DOI in this record.A phenomenological equation called the Landau-Lifshitz-Baryakhtar (LLBar) [Zh. Eksp. Teor. Fiz 87, 1501 (1984) [Sov. Phys. JETP 60, 863 (1984)]] equation, which could be viewed as the combination of the Landau-Lifshitz (LL) equation and an extra "exchange-damping" term, was derived by Baryakhtar using Onsager's relations. We interpret the origin of this exchange damping as nonlocal damping by linking it to the spin current pumping. The LLBar equation is investigated numerically and analytically for the spin-wave decay and domain-wall motion. Our results show that the lifetime and propagation length of short-wavelength magnons in the presence of nonlocal damping could be much smaller than those given by the LL equation. Furthermore, we find that both the domain-wall mobility and the Walker breakdown field are strongly influenced by the nonlocal damping.We acknowledge the financial support from EPSRC's DTC Grant No. EP/G03690X/1. W.W. thanks the China Scholarship Council for financial assistance. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 247556 (NoWaPhen) and from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement No. 644348 (MagIC)

Formation of the band spectrum of spin waves in 1D magnonic crystals with different types of interfacial boundary conditions

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.We report a theoretical study of the spin-wave band spectrum of magnonic crystals formed by stacking thin-film magnetic layers, with general assumptions about the properties of the interfaces between the layers. The use of the Barnaś–Mills magnetization boundary conditions has enabled us to systematically trace the origin of the magnonic band gaps in the spin-wave spectrum of such systems. We find that the band gaps are a ubiquitous attribute of a weakened interlayer coupling and a finite interface anisotropy (pinning). The band gaps in such systems represent a legacy of the discrete spin-wave spectrum of the individual magnetic layers periodically stacked to form the magnonic crystal rather than resulting from Bragg scattering. At the same time, magnonic crystals with band gaps due to the Bragg scattering can be described by natural boundary conditions (i.e. those maintaining continuity of the magnetization direction across the whole sample). We generalize our conclusions to systems beyond thin-film magnonic crystals and propose magnonic crystals based on the ideas of graded-index magnonics and those formed by Fano resonances as a possible way to circumvent the discovered issues.This research has received funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement No. 644348 (MagIC) and from Ukrainian State Fund for Fundamental Research under project No. F71/73-2016 ('Multifunctional Photonic Structures')