Session X40: Magnetic Excitations

Live

We study gauge fields produced by gradients of the Dzyaloshinskii-Moriya interaction and propose a model of AFM topological insulator of magnons. In the long wavelength limit, the Landau levels induced by the inhomogeneous Dzyaloshinskii-Moriya interaction exhibit relativistic physics described by the Klein-Gordon equation. We further study Landau levels in our model of AFM topological insulator and observe unconventional Hofstadter's butterfly. We calculate the spin Nernst response due to formation of magnonic Landau levels and compare it to similar topological responses in skyrmion and vortex-antivortex crystal phases of AFM insulators. Our studies show that AFM insulators exhibit rich physics associated with topological magnon excitations.   

Live

Magnetism-dependent phonons in ferromagnetic materials have been found even in simple magnetic materials such as bcc Fe. Although this phenomenon was recognized, its influences on equilibrium magnetic states have been overlooked. In this presentation, we introduce the impact of the magnetism-dependent phonon on the magnetic phase transition and the electronic origin of the dependence. The magnetism-dependent phonons stabilize magnetically disordered states, and thereby the Curie temperature is also influenced [1]. Additionally, our first-principles phonon calculations considering magnetically disordered states reveal a clear relationship between the degree of dependence and crystal symmetry [2]. We explain the origin of the relationship by introducing a concept, exchange ligand field.

[1] T. Tanaka and Y. Gohda, arXiv:2006.02604.
[2] T. Tanaka and Y. Gohda, J. Phys. Soc. Jpn. 89, 093705 (2020).   

Live

The use of spin waves (SWs) as data carriers in spintronic and magnonic logic devices offers operation at low power consumption, free of Joule heating. Nevertheless, the controlled emission and propagation of SWs in magnetic materials remains a significant challenge. Here, we propose that skyrmion-antiskyrmion bilayers form topological charge dipoles and act as efficient sub-100 nm SW emitters when excited by in-plane ac magnetic fields [1]. The propagating SWs have a preferred radiation direction, with clear dipole signatures in their radiation pattern, suggesting that the bilayer forms a SW antenna. The characteristics of the emitted SWs are linked to the topology of the source, allowing for full control of the SW features, including their amplitude, preferred direction of propagation, and wavelength. I will also discuss the response of skyrmions under an oscillating magnetic field gradient. In a recent work, we treated the unavoidable impact of the driving field on the magnon bath and demonstrated that, new time-dependent dissipation terms arise for the skyrmion, which result in a new type of unidirectional propagation [2]. This work opens up new possibilities to control skyrmion dissipation under certain non-equilibrium protocols.

[1] Spin Wave Radiation by a Topological Charge Dipole, S. A. Diaz, T. Hirosawa, D. Loss, and C. Psaroudaki, Nano Lett. 20, 9, 6556 (2020).
[2] Skyrmions Driven by Intrinsic Magnons, C. Psaroudaki and D. Loss, Phys. Rev. Lett. 120, 237203 (2018).   

Live

Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order. We compute the dynamical spin structure factor, extracting the spin-wave spectrum in the SkX, in the vicinity of the paramagnet to SkX transition [1]. Inside the SkX, we find six spin-wave modes, which are supplemented by another mode originating from the ferromagnetic background. Above the critical temperature Ts for the skyrmion crystallization, a diffusive regime, reminiscent of the liquid-to-crystal transition, reveals that topological spin texture of skyrmionic character starts to develop above Ts as the precursor of the SkX [2]. We discuss the opportunities for the detection of the spin waves of the SkX using inelastic-neutron-scattering experiments in manganite-iridate heterostructures [3].

References:
1. N. Mohanta, E. Dagotto and S. Okamoto, Phys. Rev. B 100, 064429 (2019)
2. N. Mohanta, A. D. Christianson, S. Okamoto, and E. Dagotto, arXiv:2005.11399 (2020)
3. N. Mohanta, S. Okamoto, and E. Dagotto, Phys. Rev. B 102, 064430 (2020)   

Live

The role played by the magnetic frustration arising from competing exchange interactions on the magnetic properties of honeycomb systems continues to be the subject of intense research. Competition between first, second and third neighbor exchange interactions results in rich magnetic phase diagrams that include Néel, zigzag, stripy, and spiral orders. Anisotropic exchange interactions that arise from spin-orbit coupling can lead to various exotic quantum-disordered states. Here we discuss the results of neutron scattering measurements performed on the honeycomb systems Na_2+xM₂TeO₆ (M=Co and Ni). The diffusive Na atoms provide an effective control of the interlayer magnetic couplings, and therefore a route for tuning the lattice dimensionality. The changes in the static magnetic order and spin-dynamics for samples with different Na content are discussed. We also show that spin-wave excitation spectra obtained from stoichiometric powder samples cannot be well described using a simple isotropic Heisenberg spin Hamiltonian. An alternative model with anisotropic exchange coupling is evaluated and discussed.   

Live

We study coupling of electromagnetic waves to magnetization waves at THz frequencies [1]. The Purcell effect and magnon-polaritons are intensively studied in ferromagnets at microwave frequencies [2]. Antiferromagnets are interesting due to their high-frequency magnetization dynamics, however, there are only a few examples of weak [3] or indirect [4] magnon-photon coupling in that materials at THz frequencies. We report strong magnon-phonon coupling in the high-temperature antiferromagnet hematite α-Fe₂O₃. A cube of hematite was placed inside a 3-dimensional cavity of a mode at about 0.24 THz. We measured transmission though the cavity in 0.2-0.3 THz band at above room temperature. Frequency of the antiferromagnetic resonance of hematite rises with temperature and interacts with the cavity mode, showing a very clear avoided crossing of about 6 GHz splitting and the cooperativity factor of about 50.

[1] D. L. Mills et al, Rep. Prog. Phys. 37, 817 (1974).
[2] Y. S. Gui et al, Phys. Rev. Lett. 95, 056807 (2005).
[3] M. Bialek et al, Phys. Rev. B 101, 024405 (2020).
[4] X. Li et al, Science 361, 794 (2018).   

Live

The interface between photons and magnons has introduced the opportunity to perform quantum information processing in macroscopic systems. So far, strong coupling and Rabi oscillations have been exploited to control the coupling between photons and magnons in microwave cavities (1–3). Here, we present a new scheme to achieve spectral control of magnon-photon coupling via Fano interference. The Fano resonance is controlled via cavity detuning, which acts as the background channel in the Fano picture at low coupling. An incoherent term in the Fano picture, normally attributed to decoherence in open quantum systems (4), actually arises here due to the coupling of a discrete matter resonance with a resonant cavity mode. Magnon-photon hybridization at higher coupling strengths effects the breakdown of the Fano picture. These results establish spectral Fano control in cavity electromagnonics, which may lead to advances in room temperature quantum information processing.
References:
1. X. Zhang, C.-L. Zou, J. Liang, and X. H. Tang, Phys. Rev. Lett.113, 156401 (2014).
2. X. Zhang, et al., Nat. Communications. 6, 8914 (2015).
3. L. Bai et al., Phys. Rev. Lett. 114, 227201 (2015).
4. A. Bärnthaler et al., Phys. Rev. Lett. 105, 056801 (2010).   

Live

The inertial mass of a skyrmion arising from spin-phonon interaction is computed exactly within a toy model of the magnetoelastic coupling in a ferromagnetic film. The mass scales as the square of the strength of the magnetoelastic coupling, as the square of the film thickness, and as the first power of the lateral size of the skyrmion. For nanometer skyrmions it is in the ballpark of a few electron masses but may be significantly greater in materials with large magnetostriction. These findings are expected to stand for any complex structure of spin-phonon interaction in real materials. They must be taken into account when addressing the speed of information processing based upon skyrmions.

D. Capic, E.M. Chudnovsky, and D.A. Garanin, Skyrmion mass from spin phonon interaction, Phys. Rev. B. 102, 060404(R) (2020).   

Live

Spin waves in magnetic insulators can transport information without the dissipation caused by moving charges, and could therefore play a major role in future information processing technology. Using electron spins in diamond, we introduce a phase-sensitive detection scheme to image spin waves via their magnetic field, which we use to quantify the spin-wave precession amplitude in YIG and image spin-wave interference and caustics. The observed patterns are well reproduced by a model based on the chiral spin-wave excitation and coupling to the sensor spins. Detecting spin-waves via their magnetic field allows imaging through optically opaque materials, paving the way for studies of multilayers and top-gated systems, and to study the interaction of spin waves and electric currents. The sensitivity of our technique, which can reach a nanometric spatial resolution when implemented in a scanning geometry, allows to detect spin waves in ultrathin magnetic films, down to the monolayer limit.   

Live

Domain walls (DW) - topological defects separating oppositely oriented magnetic regions - are known to host confined spin wave modes [Phys. Rev. 124, 452 (1961)]. These modes are typically excited by power-hungry magnetic fields, which undermines the advantages of low-dissipation propagation and nanoscale confinement offered by spin waves in DWs. In this work, we show that layered van der Waals antiferromagnets with recently demonstrated voltage control of magnetic order [Nat. Nanotechnol. 13, 549 (2018)] provide an excellent platform to realize a voltage-based DW spin wave excitation scheme. The linear dispersion offered by these channels in the long-wavelength limit enables dispersionless transfer of spin information. Moreover, utilizing the interplay between charge and magnetization dynamics, we demonstrate that the group velocity of excited spin waves in the long-wavelength limit can be tuned via DC voltage. Our results provide a tunable, local, dispersionless, ultra-low dissipation means to excite and transport spin waves.   

Live

We study generation and dynamics of skyrmions and antiskyrmions using current-induced torques at edges of magnetic regions with DMI (Dzyaloshinskii-Moriya interaction). The generation of skyrmions and antiskyrmions can be interpreted as taking place in the presence of a Doppler-like shift associated with current which occurs for magnons localized at the edge. These localized modes are analyzed by using the Bogoliubov-de-Gennes Hamiltonian written for magnons. We confirm our theoretical predictions using micromagnetics simulations where we observe that a current pulse closes the magnon band gap, leading to instabilities in the magnetic texture at the edge. We observe that the closure of the magnon band gap effectively causes the system to form skyrmions or antiskyrmions, depending on the type of DMI present.   

Live

Spin pumping (SP) is a well-known way generating DC spin current by the irradiation of electromagnetic (EM) waves. Its usual setup is a junction of a ferromagnetic insulator and a paramagnetic heavy metal. Magnons are resonantly created by applying a GHz wave to the ferromagnet, and then spin current is injected to the metal via the angular momentum transfer from magnons to conducting electrons at the interface. On the other hand, in multiferroics driven by EM waves, electron spins can be coupled to not only the magnetic field but also the electric one via the magnetoelectric coupling. Electro-active magnons are called electromagnons, and their resonant frequency is usually in the range of 0.1-1.0 THz. In the present work, we theoretically study the SP in a multiferroicmagnet, focusing on its spiral ordered phase such as perovskite manganites RMnO₃. Based on the non-equilibrium Green’s function method, we derive the formula of DC spin currents pumped by AC electric or magnetic fields, and show that the spin current is created via THz waves in addition to GHz pumping. Comparing our results to the usual pumping in ferromagnets, we show that currently available THz laser is enough to realize the SP in multiferroics.   

On Demand

Due to the reduced dimensions of magnetic nanowires (NWs), the possibility to control spin wave (SW) confinement and to couple electromagnetic waves to the magnetization textures with non-trivial topologies (e.g. vortex state), makes these structures good candidates for SW based information processing technologies. Here, we present experimental results of dynamic stimulation of Fe28Co63Cu10 NWs with 120 nm diameter and 25 micron length. Microwave permeability with DC and microwave magnetic fields perpendicular to the NW axis shows enhanced losses in the low frequency range for magnetic field below 2 kOe. In simulations, we observed the formation of a vortex state on the NW ends for a field close to the one with experimental losses. Investigations on how the modes depend on the distance from the NW end and on the product of vortex polarity and chirality were carried out. We distinguish two types of SW modes: lower frequency modes localized close to the NW ends and higher frequency delocalized modes, which are described as plane waves with a finite pinning at the NW ends. Simulation results are in agreement with the analytical model based on the generalized Thiele equation for the vortex core string, which accounts for the exchange and non-local magnetostatic interactions.