Topological spin dynamics in cubic FeGe near room temperature

Abstract

Understanding spin-wave dynamics in chiral magnets is a key step for the development of highspeed, spin-wave based spintronic devices that take advantage of chiral and topological spin textures for their operation. Here we present an experimental and theoretical study of spin-wave dynamics in a cubic B20 FeGe single crystal. Using the combination of waveguide microwave absorption spectroscopy (MAS), micromagnetic simulations, and analytical theory, we identify the resonance dynamics in all magnetic phases (field polarized, conical, helical, and skyrmion phases). Because the resonance frequencies of specific chiral spin textures are unique, quantitative agreement between our theoretical predictions and experimental findings for all resonance frequencies and spin wave modes enables us to unambiguously identify chiral magnetic phases and to demonstrate that MAS is a powerful tool to efficiently extract a magnetic phase diagram. These results provide a new tool to accelerate the integration of chiral magnetic materials into spintronic devices.Identifying and understanding of spin-wave excitations in chiral and topological magnetic materials is a fundamental step towards their integration into spintronic devices. Moreover, some of these materials host magnetic skyrmions, in which spins form a topologically non-trivial excitation with an integer winding number. 1,2 Magnetic skyrmions can be driven to move by spin torques with an ultralow current threshold because there is weak or no pinning, 3,4 which makes them appealing for low-power memory, 5 magnetic logic, 6 and auto-oscillator 7 devices. In support of these potential applications, magnetic skyrmions have been investigated using several methods including Lorentz transmission electron microscopy (LTEM), [8][9][10] inelastic neutron scattering, 11,12 magnetic susceptibility, 13,14 and recently by microwave absorption spectroscopy. 15,16 Microwave absorption spectroscopy (MAS), in particular, is a powerful tool for identifying and studying dynamical magnetic excitations in materials. In materials with several complex magnetic phases, such as chiral magnetic materials with a volume Dzyaloshinskii-Moriya interaction, it may be possible to use MAS to quickly establish a magnetic phase diagram because the spin texture in each magnetic phase has a unique resonance response. This specificity gives MAS an advantage as compared to electrical measurements such as the topological Hall effect, [17][18][19][20][21][22] which is more difficult to interpret because the signals are not unique to skyrmion phases or helical phases. [19][20][21][22][23][24] In this letter, we present a study of spin-wave dynamics in a single crystal of B20 FeGe using MAS. Specifically, we measure the gigahertz frequency dynamic susceptibility of FeGe using a field-referenced lock-in technique, which significantly increases our sensitivity to magnetic phase boundaries as compared to conventional MAS. 15 We observe the resonance response in all the magnetic phases (field polarized, conical, helical and...

Crossover of skyrmion and helical modulations in noncentrosymmetric ferromagnets

“…During the last years, the precursor states of noncentrosymmetric ferromagnets have been investigated experimentally and theoretically [14][15][16][17][18][19][20][21][22][23]. Particularly, the recent observations in bulk cubic helimagnets MnSi [18] and FeGe [16][17][18] are consistent with the crossover effects of skyrmionic states theoretically described in [9][10][11] and earlier reported in a cubic helimagnet FeGe [10,11].…”

Dislocation-driven relaxation processes at the conical to helical phase transition in FeGe

“…H A indicates the transition to the well-established skyrmion phase in FeGe and H x marks the transition into the paramagnetic state determined by a fluctuation-disorder regime [15]. The measured phase diagram is in good agreement with literature [16][17][18][19][20][21], representing the basis for our magnetic relaxation investigations.…”

Skyrmion Lattice Topological Hall Effect near Room Temperature

“…Recently, two bulk crystalline materials have been shown by Lorentz Transmission Electron Microscopy (LTEM) and Small Angle Neutron Scattering (SANS) to have a skyrmions lattice near or at room temperature: Co x -Zn y -Mn z alloys 38 , 39 , and stoichiometric cubic FeGe 10 , 37 , 40 – 42 . In FeGe, magnetisation 43 , specific heat measurements 44 , and microwave absorption spectroscopy 45 also identified several magnetic phases compatible with skyrmions. Transport studies in FeGe thin films 26 – 30 reported an extraordinarily large Hall signature (up to 1000 n .cm in 36 nm thin films) that occurs in an “extended” phase spanning from cryogenic temperatures to the Néel temperature.…”