Session X39: Magnons, Spin Waves, and Magnetic Dynamics

Topological magnons on a kagome lattice

A system of spins on a kagome lattice interacting via the nearest-neighbor XXZ interaction (anisotropy ratio △) and Dzyaloshinskii-Moriya interaction (strength D) is found to have a very rich phase diagram. In the classical limit, five distinct phases with different ground-state spin configurations are found by varying the two interaction parameters. In some of these phases, spin-wave theory using the Holstein-Primakoff transformations is used to find the bulk energy bands of the magnons. In the ferromagnetic phase, where all the spins point along the +z direction, the bulk bands are separated from each other by finite energy gaps. Finding the Chern numbers here, one finds that there are four topologically distinct phases sharing the same ground state spin-configuration. Hence an infinite strip of the system hosts robust edge states which are directly related to the Chern number of the bands. The other phases, however, are found to have gapless magnon bands.   

Spin waves in doped graphene with in-plane magnetic fields

Plasmonics in graphene and related 2D materials has attracted much interest, with many prospects for novel applications. However, the dynamics of collective spin excitations and spin waves in 2D materials has been much less explored. Here, we study the spin-wave excitations of itinerant electrons in doped graphene in the presence of in-plane magnetic fields. We calculate the spin-wave dispersions using time-dependent density-functional methods within a standard tight-binding approach. Dynamical exchange-correlation effects are treated using the Singwi-Tosi-Land-Sjolander (STLS) approach, generalized for systems with noncollinear spins.   

Relation between unidirectional magnetoresistance and magnon generation by spin current

Ferromagnet/spin Hall effect (SHE) material bialyers are central to modern active nanomagnetic devices. The unidirectional magnetoresistance (UMR) observed in such structures – the asymmetry of resistance with respect to the current direction – has been explained in terms of spin-dependent electronic transport [1], or alternatively the effects of SHE-induced magnon generation [2].
We report on electrical measurements of UMR in a Permalloy/Pt bilayer, performed in conjunction with magneto-optical Brillouin light spectroscopy of spin current-driven magnon population. We show that the current dependence of UMR closely follows the dipolar magnon density, and that both dependences exhibit the same scaling over a large temperature range of 80-400 K. These findings demonstrate a close relationship between spin current-driven magnon generation and UMR, and indicate that the latter is likely dominated by the dipolar magnons. Our results may facilitate the development of new magnetoelectronic approaches for the characterization of SHE-induced magnetization dynamics.

[1] C. O. Avci et al., Nat. Phys. 11, 570 (2015).
[2] S. Langenfeld et al., APL 108, 192402 (2016); T. Li et al., Appl. Phys. Expr. 10, 073001 (2017).   

How to generate whispering gallery magnons

One of the most fascinating topics in current quantum physics are hybridized systems, in which resonators of different quantum systems are strongly coupled. Prominent examples are circular resonators with high quality factors that allow the coupling of optical whispering gallery modes to microwave cavities or magnon resonances. However, the coupling to magnons with finite wave vectors has not yet been achieved due to the lack of efficient excitation schemes.
Here, we present the generation of whispering gallery magnons with unprecedented high azimuthal wave vectors via nonlinear 3-magnon scattering in a µm-sized magnetic disk exhibiting a vortex state. These modes show a strong localization at the perimeter of the disk and practically zero amplitude in an extended area around the vortex core. They originate from the splitting of the fundamental radial magnon modes, which can be resonantly excited in a vortex state by an out-of-plane microwave field. We will shed light on the basics of this non-linear scattering mechanism from experimental and theoretical point of view. Using Brillouin light scattering (BLS) microscopy, we investigated the frequency and power dependence of this nonlinear mechanism. The spatially resolved mode profiles give evidence for the localization at the boundaries of the disk and allow for a direct determination of the modes’ wavenumbers. Furthermore, time resolved BLS in combination with pulsed microwave excitation revealed the temporal evolution of the 3-magnon splitting and its dependence on the applied microwave power.   

Spin and Charge Pumping and Spin Wave Emission by Magnetic Domain Wall Annihilation: Quantum-Classical Micromagnetics Approach

Recent experiments on magnetic domain wall collisions [1] showed their importance in generating short-lived spin wave bursts, low-power signals which can be used in future spintronics devices. Using a new, time-dependent method to propagate the non-equilibrium density matrix (the TD-NEGF method) combined with the classical Landau-Lifshitz-Gilbert (LLG) equation [2], we analyze spin and charge currents produced by an annihilation of two colliding domain walls in a magnetic material coupled to two electron reservoirs. The advantage of this approach is in its self-consistent coupling between quantum electron spin and classical magnetic moments which gives rise to a dynamical Gilbert damping and allows for calculation of the exact charge current pumped out of the system, in contrast to an approximate solution given by the perturbative spin motive force formula combined with the LLG equation [3]. The pumped currents provide enough information to indirectly determine details of the annihilation event.

[1] S. Woo, T. Delaney, and G. S. D. Beach, Nature Physics 13, 448 (2017).
[2] M. D. Petrović, B. S. Popescu, U. Bajpai, P. Plechac, and B. K. Nikolić, arXiv:1802.05682 (2018).
[3] U. Bajpai, and B. K. Nikolić, arXiv:1810.11016 (2018).   

Magnon Damping in Yttrium Iron Garnet Films at Millikelvin Temperatures

Magnon-based quantum devices require low damping materials to maintain coherence. One of the most promising candidates is yttrium iron garnet (YIG). A ferrimagnetic insulator, YIG has the narrowest known linewidth of any practical material at room temperature. Bulk YIG has been shown to have low damping at millikelvin (mK) temperatures. However, there is some debate about whether this is the case in YIG films. Much of the uncertainty centres around the fact that the low-temperature magnetic behaviour of the substrate upon which high-quality samples are grown -- gadolinium gallium garnet (GGG) -- is not fully understood.

In this work, we study magnon damping in YIG films with and without GGG substrates by measuring the resonance linewidth at low temperatures. We observe that GGG contributes to significantly increased damping at low temperatures. Further measurements up to 9K reveal that temperature-peak processes are active above 1K and the presence of additional damping at mK temperatures which we attribute to the two-level fluctuators. Upon saturating the fluctuators, linewidths of 1.4 MHz are achievable in substrate-free YIG films at 20 mK, lower than the measured linewidth in the same films at room temperature.   

Non-linear parallel pump FMR studies in magnetic insulators

Non-linear magnetic response of magnetic insulator materials is often used in the realization of radio-frequency (RF) devices. Although in use, there exists physical aspects that still need investigation for such devices. Here, we use high performance parallel computing based CUDA simulations and a theoretical model for magnon scattering to explore the direct and parametric generation of magnons, excited by microwaves in parallel pumped configurations. The sample is 10-100s of um and the response is largely dominated by the demagnetization fields in the sample. The DC applied magnetic field is in-plane and the RF field is linearly polarized and
parallel to the DC field. We first determine the dispersion relation which plays a vital role in the understanding of nonlinear magnons. We then study the effect of ferrimagnet thickness, damping constant, RF frequency and power as well as DC applied magnetic field,
on properties like frequency response, operational bandwidth, and power absorption of the material. It is found that the degree of non-linearity can be controlled using parameters like RF power. Analytically, Holstein and Primakoff based analysis is used to arrive at the threshold RF amplitude and dispersion relations.   

Magnon transport in quasi-two-dimensional van der Waals antiferromagnets

The recent emergence of 2D van der Waals magnets down to atomic layer thickness provides an exciting platform for exploring quantum magnetism and spintronics applications. However, the potential of 2D van der Waals magnets for magnonics, has not been explored yet. In this talk, we will present the experimental observation of long distance magnon transport over several micrometers in quasi-2D van der Waals antiferromagnet MnPS₃. Experimentally, we exfoliated mechanically MnPS₃ flakes down to a few nanometers, and then fabricated the devices of non-local geometry where MnPS₃ is the transport channel of magnon. Then magnon is injected via thermal method and detected based on inverse spin Hall effect of platinum. As the quasi-2D MnPS₃ thickness decreases, a shorter magnon diffusion length is observed, which could be attributed to the surface-impurity-induced magnon scattering.   

Spin-wave Confinement and Coupling in Organic-Based Magnetic Nanostructures

We present the synthesis of a new class of organic-based magnetic nanostructures consisting of nanowires of V[TCNE]_x that assemble along the ridges of a grooved substrate. These nanowires exhibit uniaxial magnetic anisotropy in direct contrast to the isotropic in-plane response of typical thin-films. When different magnon modes excited in these structures are brought into resonance by varying the orientation of an in-plane magnetic field, we observe anticrossing behavior, indicating strong coherent coupling between excitations. Furthermore, micromagnetic simulations using real nanowire profiles extracted from cross-sectional scanning electron microscopy faithfully reproduce the experimentally measured spectra without any free parameters, including spin-wave and other higher-order modes. These results introduce a new degree of freedom for organic-based magnetism and spintronics, and together with recent demonstration of encapsulation technologies and demonstrated functional microwave devices that exhibit high quality factors across a frequency range, suggest future promising applications in microwave electronics and quantum magnonics.   

Revisiting the Einstein-de Haas Effect

The Einstein-de Haas (EdH) effect manifests the intrinsic connection between magnetic moment and mechanical angular momentum. In the original experiment [1], angular momentum changes induced in an iron cylinder by field-driven changes in the net magnetic moment were detected via resonant motion of a torsion pendulum. The miniaturization of torque sensors [2], combined with ultrasensitive optical detection, enables routine measurements of EdH effects. Millimeter to nanometer-scale sensors affixed to yttrium iron garnet structures were fabricated to explore the frequency-dependence of the EdH effect. Described are experimental methods for distinguishing EdH and conventional magnetic torques simultaneously.

[1] A. Einstein and W. J. de Haas, Proc.—KNAW 18, 696 (1915).
[2] J.E. Losby, V.T.K. Sauer, and M.R. Freeman, J. Phys. D: Appl. Phys. 51, 483001 (2018).   

Observation of Ultrastrong Magnon-Magnon Coupling in YFeO3 in High Magnetic Fields

There is currently much interest in exploring resonant light-matter interaction in the ultrastrong-coupling (USC) regime where light and matter mix to an extreme degree and a variety of new phenomena emerge [1]. Recently, we have extended the concept of USC to a magnetic context in the form of matter-matter interaction [2]. Specifically, the exchange interaction of paramagnetic Er³⁺ spins with an Fe³⁺ magnon field in ErFeO₃ exhibited a large vacuum Rabi splitting. Here, we report another type of matter-matter USC between two magnon modes in YFeO₃. In rare-earth orthoferrite materials, there are ferromagnetic (FM) and antiferromagnetic (AFM) modes of magnons in the terahertz (THz) frequency range. By performing single-shot THz time-domain spectroscopy with a table-top pulsed magnet [3], we have investigated the magnetic field dependent magnon frequency and discovered anticrossing behavior for the FM and AFM magnon modes in high magnetic fields. The magnitude of the vacuum Rabi splitting suggests USC strength between the magnon modes when applying high magnetic fields at certain finite angles with respect to the crystallographic axes of bulk YFeO₃.
[1] P. Forn-Díaz et al., arXiv:1804.09275.
[2] X. Li et al., Science 361, 794 (2018).
[3] G. T. Noe et al., Opt. Exp. 24, 30328 (2016).   

Magnon-photon coupling in orthoferrites

We study the magnon-photon coupling at THz frequencies [1]. The Purcell effect and magnon-polaritons were shown in ferromagnetic materials [2, 3]. It is interesting to observe these interactions in antiferromagnetic materials due to their high-frequency magnetic resonances. We report on an magnon-phonon coupling in high-temperature antiferromagnets, bismuth ferrite BiFeO₃ (BFO) and dysprosium ferrite DyFeO₃ (DFO). We investigated polycrystaline samples of different thicknesses, thus having different sets of Fabry-Perrot-type cavity modes. We measured transmission spectra in 200-350 GHz frequency band at 300-600 K temperature range using frequency extenders to a vector network analyzer. Magnetic-resonance frequencies soften with rising temperature and cross with cavity modes. In BFO samples, we found that the width of the resonant line broadens at these crossings, consistent with the Purcell effect. In DFO samples, our data show avoided crossings of resonances with cavity modes, which is an indication of magnon-polaritons.

[1] D. L. Mills et al,, Rep. Prog. Phys. 37, 817 (1974).
[2] H. Huebl et al., Phys. Rev. Lett. 111, 127003 (2013).
[3] L. Bai et al., Phys. Rev. Lett. 114, 227201 (2015).   

Spin Waves Across 3-Dimensional, Close-Packed Nanoparticles

While there is much practical and theoretical interest in characterizing magnons within 3-dimensional self-assembled nanoscale systems, few experimental techniques are appropriate. Here inelastic neutron scattering, although intensity limited, is utilized to measure inter-nanoparticle spin waves, or magnons, which arise from coupling between 8.4 nm manganese ferrite nanoparticles that are self-assembled into a close-packed lattice, yet physically separated by oleic acid surfactant. The observed magnons are dispersive, respond to an applied magnetic field, and display the expected temperature-dependent Bose population factor. Moreover, the dispersion yields a non-negative energy gap only when the effective Q is reduced by the inter-particle spacing, confirming that it is a collective excitation between the nanoparticles, rather than originating within individual nanoparticles. The experimental results are well explained by a limited parameter model which treats the 3-dimensional ordered, magnetic nanoparticles as dipolar-coupled superspins.