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Einstein-de Haas (EdH) effect is the fundamental phenomenon that the microscopic spin angular momentum is converted to the angular momentum of mechanical rotation. A mechanical torque due to the EdH effect, the magnitude of which is proportional to the time derivative of the magnetization, is difficult to be detected, which keeps its practical application away. However, recent development of microfabrication technology has enabled to sensitively detect the tiny EdH torque using micro-fabricated mechanical resonators [1,2]. These findings show that the EdH effect can be utilized for the new principle of the mechanical torque generation in sub-micron scale device. Moreover, it was demonstrated that the ultrafast demagnetization due to the laser heating excited the transverse strain wave in a Fe film due to the EdH effect, where the timescale and conversion efficiency of the angular momentum transfer were quantitatively estimated [3].&lt;br/&gt;Rayleigh-type surface acoustic waves (RSAWs), the displacement of which oscillates elliptically, have been widely used to excite the spin waves in a ferromagnetic thin film via the magnetoelastic interaction [4]. Recently, as the phenomena based on the angular momentum conversion from the lattice rotation to the magnetization or electron spin, the gyromagnetic spin-wave excitation [5] and the spin current generation in a nonmagnetic metal with a weak spin-orbit coupling [6] were demonstrated.&lt;br/&gt;In this study, we demonstrate both acceleration and deceleration of RSAW velocity in the ferromagnetic (FM) film based on the EdH effect. When the spin-wave is gyromagnetically excited by RSAW propagation through the FM film, the EdH torque is generated in the FM lattice as the back action of the spin-wave resonance (SWR) (Fig. 1(a)). As shown in Fig. 1(a), the direction of EdH torque is opposite to the time derivative of the magnetization. We theoretically expect that the transverse velocity of RSAW can be modulated by the EdH torque, resulting in the phase change of RSAW propagation. In the experiment, we successfully observed the phase change of RSAW propagation, whose magnitude and variation were consistent with our analytical prediction.&lt;br/&gt;Figure 1(b) shows the schematical experimental setup for measuring the phase change of RSAW. A pair of inter-digital transduces (IDTs) consisting of Ti(3)/Au(30) is fabricated on a piezoelectric LiNbO&lt;sub&gt;3&lt;/sub&gt; substrate by the electron beam evaporation. The numbers in the parentheses indicate the layer thickness in nanometers. The width and space of the electrode of IDT are 550 nm. The Ni&lt;sub&gt;81&lt;/sub&gt;Fe&lt;sub&gt;19&lt;/sub&gt; (NiFe) film with a lateral size of 400×400 μm&lt;sup&gt;2&lt;/sup&gt; is sputtered between a pair of IDTs. By applying a microwave with the fundamental frequency of RSAW excitation into one of IDTs, RSAWs are excited via the piezoelectric effect. The RSAWs propagating through the NiFe film are electrically detected by the other of IDTs using a vector network analyzer (VNA) as the inverse process of the RSAW excitation. Figures 2(a) and 2(b) show the reduced RSAW energy dissipation Δ&lt;i&gt;P&lt;/i&gt;&lt;sub&gt;norm&lt;/sub&gt; and phase difference Δ&lt;i&gt;θ&lt;/i&gt;, respectively, as a function of external magnetic field swept in the direction parallel to the RSAW wave vector at the RSAW frequency of 1.75 GHz. Steep peaks appear at 4.6 and -4.0 mT in Fig. 2(a), showing that the spin-wave is excited because the values of the resonant field and resonant frequency are matched with the spin wave dispersion of a 20-nm-thick NiFe film. A bipolar change of Δ&lt;i&gt;θ&lt;/i&gt; is observed in the vicinity of the resonance field of spin-waves. From the angle between the applied external field and RSAW wave vector, it can be seen that the spin-wave is excited via gyromagnetic effect, not the magnetoelastic coupling [5]. The increase and decrease in Δ&lt;i&gt;θ&lt;/i&gt; can be explained by our analytical model consisting of the elastic motion equation with the EdH torque term. The maximum value of phase change Δ&lt;i&gt;θ&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; is -0.035 rad at 5.0 mT. This value is as large as that in case using the magnetoelastic coupling in a single Ni film [7]. In addition, the order of Δ&lt;i&gt;θ&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; is comparative to the value analytically obtained by assuming a loss-less conversion of the angular momentum. The consistency in Δ&lt;i&gt;θ&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; suggests that the EdH effect is robust in the angular translation from the magnetization to the lattice in the gigahertz regime.

**References:**

[1] K. Harii, et. al., Nat. Commun. 10, 2616 (2019).&lt;br/&gt;[2] K. Mori, et. al., Phys. Rev. B 102, 054415 (2020).&lt;br/&gt;[3] C. Dornes, et. al., Nature 565, 209 (2019).&lt;br/&gt;[4] M. Weiler, et. al., Phys. Rev. Lett. 106, 117601 (2011).&lt;br/&gt;[5] Y. Kurimune, et. al., Phys. Rev. Lett. 124, 217205 (2020).&lt;br/&gt;[6] D. Kobayashi, et. al., Phys. Rev. Lett. 119, 077202 (2017).&lt;br/&gt;[7] R. Sasaki, et. al., Phys. Rev. B 95, 020407 (R) (2017).&lt;br/&gt;[8] S. Tateno, et.al., submitted.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/95a80b47499a8efb662564ebc84f2325.jpg)
Fig. 1 (a) Schematic of the stress based on EdH effect &lt;i&gt;σ&lt;/i&gt;&lt;sup&gt;EdH&lt;/sup&gt;&lt;sub&gt;zx&lt;/sub&gt;  and the EdH torque &lt;i&gt;τ&lt;/i&gt;&lt;sup&gt;EdH&lt;/sup&gt; in the Ni-Fe lattice. The direction of the EdH torque is opposite to the time derivative of the magnetization. (b) Experimental setup for measuring the phase changeΔθ of RSAW using a VNA. The external field is applied along the RSAW propagation direction.

![](https://assets.underline.io/uploads/markdown_image/1/image/6d3a013484d0f414781f2f3ebf4436f3.jpg)
Fig. 2 The field dependence of (a) Δ&lt;i&gt;P&lt;/i&gt;&lt;sub&gt;norm&lt;/sub&gt; and (b) Δ&lt;i&gt;θ&lt;/i&gt; at the fundamental frequency of RSAW excitation, respectively. The red solid line in (b) shows the result of the best fit using our analytical model.

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 Velocity Modulation of Surface Acoustic Waves via Einstein-de Haas Effect

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Einstein-de Haas (EdH) effect is the fundamental phenomenon that the microscopic spin angular momentum is converted to the angular momentum of mechanical rotation. A mechanical torque due to the EdH effect, the magnitude of which is proportional to the time derivative of the magnetization, is difficult to be detected, which keeps its practical application away. However, recent development of microfabrication technology has enabled to sensitively detect the tiny EdH torque using micro-fabricated mechanical resonators [1,2]. These findings show that the EdH effect can be utilized for the new principle of the mechanical torque generation in sub-micron scale device. Moreover, it was demonstrated that the ultrafast demagnetization due to the laser heating excited the transverse strain wave in a Fe film due to the EdH effect, where the timescale and conversion efficiency of the angular momentum transfer were quantitatively estimated [3]. Rayleigh-type surface acoustic waves (RSAWs), the displacement of which oscillates elliptically, have been widely used to excite the spin waves in a ferromagnetic thin film via the magnetoelastic interaction [4]. Recently, as the phenomena based on the angular momentum conversion from the lattice rotation to the magnetization or electron spin, the gyromagnetic spin-wave excitation [5] and the spin current generation in a nonmagnetic metal with a weak spin-orbit coupling [6] were demonstrated. In this study, we demonstrate both acceleration and deceleration of RSAW velocity in the ferromagnetic (FM) film based on the EdH effect. When the spin-wave is gyromagnetically excited by RSAW propagation through the FM film, the EdH torque is generated in the FM lattice as the back action of the spin-wave resonance (SWR) (Fig. 1(a)). As shown in Fig. 1(a), the direction of EdH torque is opposite to the time derivative of the magnetization. We theoretically expect that the transverse velocity of RSAW can be modulated by the EdH torque, resulting in the phase change of RSAW propagation. In the experiment, we successfully observed the phase change of RSAW propagation, whose magnitude and variation were consistent with our analytical prediction. Figure 1(b) shows the schematical experimental setup for measuring the phase change of RSAW. A pair of inter-digital transduces (IDTs) consisting of Ti(3)/Au(30) is fabricated on a piezoelectric LiNbO3 substrate by the electron beam evaporation. The numbers in the parentheses indicate the layer thickness in nanometers. The width and space of the electrode of IDT are 550 nm. The Ni81Fe19 (NiFe) film with a lateral size of 400×400 μm2 is sputtered between a pair of IDTs. By applying a microwave with the fundamental frequency of RSAW excitation into one of IDTs, RSAWs are excited via the piezoelectric effect. The RSAWs propagating through the NiFe film are electrically detected by the other of IDTs using a vector network analyzer (VNA) as the inverse process of the RSAW excitation. Figures 2(a) and 2(b) show the reduced RSAW energy dissipation ΔPnorm and phase difference Δθ, respectively, as a function of external magnetic field swept in the direction parallel to the RSAW wave vector at the RSAW frequency of 1.75 GHz. Steep peaks appear at 4.6 and -4.0 mT in Fig. 2(a), showing that the spin-wave is excited because the values of the resonant field and resonant frequency are matched with the spin wave dispersion of a 20-nm-thick NiFe film. A bipolar change of Δθ is observed in the vicinity of the resonance field of spin-waves. From the angle between the applied external field and RSAW wave vector, it can be seen that the spin-wave is excited via gyromagnetic effect, not the magnetoelastic coupling [5]. The increase and decrease in Δθ can be explained by our analytical model consisting of the elastic motion equation with the EdH torque term. The maximum value of phase change Δθmax is -0.035 rad at 5.0 mT. This value is as large as that in case using the magnetoelastic coupling in a single Ni film [7]. In addition, the order of Δθmax is comparative to the value analytically obtained by assuming a loss-less conversion of the angular momentum. The consistency in Δθmax suggests that the EdH effect is robust in the angular translation from the magnetization to the lattice in the gigahertz regime.

**References:**

[1] K. Harii, et. al., Nat. Commun. 10, 2616 (2019). [2] K. Mori, et. al., Phys. Rev. B 102, 054415 (2020). [3] C. Dornes, et. al., Nature 565, 209 (2019). [4] M. Weiler, et. al., Phys. Rev. Lett. 106, 117601 (2011). [5] Y. Kurimune, et. al., Phys. Rev. Lett. 124, 217205 (2020). [6] D. Kobayashi, et. al., Phys. Rev. Lett. 119, 077202 (2017). [7] R. Sasaki, et. al., Phys. Rev. B 95, 020407 (R) (2017). [8] S. Tateno, et.al., submitted.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/95a80b47499a8efb662564ebc84f2325.jpg)
Fig. 1 (a) Schematic of the stress based on EdH effect σEdHzx and the EdH torque τEdH in the Ni-Fe lattice. The direction of the EdH torque is opposite to the time derivative of the magnetization. (b) Experimental setup for measuring the phase changeΔθ of RSAW using a VNA. The external field is applied along the RSAW propagation direction.

![](https://assets.underline.io/uploads/markdown_image/1/image/6d3a013484d0f414781f2f3ebf4436f3.jpg)
Fig. 2 The field dependence of (a) ΔPnorm and (b) Δθ at the fundamental frequency of RSAW excitation, respectively. The red solid line in (b) shows the result of the best fit using our analytical model.

technical paper

Bose-Einstein condensate is a collective phenomenon which takes place when many bosons – particles having integer value of spin – experience transition to the same single quantum state. The famous example is formation of Cooper pairs of electrons, responsible for the superconducting properties of some materials at extremely low temperatures. Magnons – quasi-particles corresponding to spin waves propagating in magnetic media – are bosons as well. Hence, if we describe oscillation of magnetization as a set of magnons, the phenomenon like Bose-Einstein condensation should also exist in this case. Indeed, the Bose-Einstein condensation of magnons at room temperature has been observed under parametric pumping in thick ferrite films [1], and, recently, by rapid cooling in ultra- thin films [2]. The stability of Bose-Einstein condensate of magnons (mBEC) is determined by the nonlinear interaction between magnons. This includes both interaction between magnons in the same spectral minimum (self-interaction), and the interaction between magnons belonging to two different spectral minima (cross-interaction). mBEC can be stable if the coefficient of nonlinear interaction is positive, i.e. if the interaction is repulsive [3]. It is well known that the self-interaction between magnons in relatively thick films is attractive [3]. However, mBEC has been experimentally proven to be stable [1, 2, 4]. It was shown in [5] that coefficient of nonlinear interaction in mBEC is negative, but rather small, so relatively strong repulsive interaction between magnons in different spectral minima may provide stability to mBEC. Recently, it has been also shown [6] that it is possible to separate mBEC in two parts, each containing magnons from only one spectral minimum, which means that mBEC in each of the minima is stable, and this stability is independent of the strong cross-interaction of magnons from different minima. Thus, the theory developed in [5] cannot be used to explain stability of separated mBEC. Also, the theory [5] assumes a uniform profile of a magnon mode across the film thickness, and infinite in-plane sizes of the magnetic film sample. Both these simplifications are, generally, incorrect. Due to the long-range nature of the dipolar interaction, finite-size effects may have a significant impact on the nonlinear interactions of magnons [4]. Here, we present a simple, but rather accurate model of the nonlinear interactions in mBEC that takes into account the finite-size effects. Using Landau-Lifshits equation and vector Hamiltonian approach [7], we derived approximate analytical expressions for the linear dispersion characteristics and nonlinear interaction coefficients of magnons near the minima of the magnon spectrum (i.e. in the mBEC region). We showed that magnons undergoing mBEC can be described as a gas of weakly-interacting bosonic particles in an infinite potential well formed by a ferromagnetic film. The rest energy of the magnons in mBEC is determined by the dipolar interaction inside a film sample. This leads to a strong anisotropy of the effective mass of magnons in the magnetic film. In the direction along the applied in-plane magnetic field, the effective mass is m∥ = hMs/(32πγA) (where Ms is the saturation magnetization, h is the Planck constant, γ is the gyromagnetic ratio, and A is the exchange stiffness). For yttrium-iron garnet (YIG), this “longitudinal” mass is m∥ ≈ 1.5×10-30 kg ≈ 1.6 me (me is the electron mass). In the directions perpendicular to the applied field magnons are described by the “perpendicular” mass m⊥ = (18λ/d)m∥, which depends on the ratio of the exchange length λ = (2A/μ0Ms2)1/2 to the thickness of the film d. For typical experimental conditions m⊥ « m∥; for example, m⊥ ≈ 0.06 m∥ in a YIG film of the thickness d = 5 μm. The huge anisotropy of the effective mass leads to the anisotropy of the dynamical mBEC properties. In particular, the speed of the Bogolyubov waves [8] depends strongly on the direction of the wave propagation. Since the perpendicular mass m⊥ depends on the thickness of the film, it is possible to achieve the inverse mass relation m⊥ &gt; m∥ in ultra-thin YIG films with d &lt; 300 nm. Under the assumption of totally pinned spins, which is an accurate approximation in the mBEC region, we derived approximate analytical expressions for the nonlinear frequency shift due to the self- and cross- interactions in mBEC: Δf = (Fself + 2Fcross)n; Fself = F0 (2Nd – 3πλ/d√2); Fcross = F0. Here F0 = γ2μ0h/4π2 is the normalisation constant, n is the magnon density, Nd is the component of the demagnetization tensor N = diag(Nx, Ny, Nz) corresponding to the direction along the applied in-plane magnetic field. These expressions show that the nonlinear cross-interaction stays large and positive independently of the confinement of the sample in the applied field direction, while the self-interaction coefficient changes its sign when Nd become greater than the critical value Ncr: Ncr = (3π/2√2)(λ/d). The result explains possibility to separate mBEC into two stable components corresponding to the different minima of the mBEC spectrum [4, 6]. The developed model allows one to make simple and accurate predictions about the stability of the mBEC depending on the shape of the sample used, and to describe dynamics of fluctuations and waves of magnon density in the mBEC. Also, it makes possible to directly apply theoretical methods developed to describe condensates of atoms to study the mBEC dynamics, and, therefore, it might be very useful in further investigations of coherent magnonic phenomena.

**References:**

[1] S. O. Demokritov, V. E. Demidov, O. Dzyapko et al., Nature, vol. 443, pp. 430-433, 2006. [2] M. Schneider, T. Brächer, D. Breitbach et al., Nat. Nanotechnol., vol. 15, pp. 457–461, 2020. [3] V. S. L’vov, Wave turbulence under parametric excitation, Springer Series in Nonlinear Dynamics (Springer, Berlin, 1994). [4] I. V. Borisenko, B. Divinskiy, V. E. Demidov et al., Nat. Commun., vol. 11, no. 1691, 2020. [5] O. Dzyapko, I. Lisenkov, P. Nowik-Boltyk et al., Phys. Rev. B, vol. 96, no. 064438, 2017. [6] I. V. Borisenko, V. E. Demidov, V. L. Pokrovsky et al., Sci. Rep., vol. 10, no. 14881, 2020. [7] V. Tyberkevych, A. Slavin, P. Artemchuk, et al., arXiv:2011.13562 (2020). [8] D. A. Bozhko, A. J. E. Kreil, H. Yu. Musiienko-Shmarova et al., Nat. Commun., vol 10, no. 2460, 2019.

**Images:**

 A Model for Description of Linear Properties and Stability of Bose-Einstein Condensate of Magnons at Room Temperature

The magnetism in curved geometries is attracting attention because of several theoretical predictions with possible potential applications, such as a curvature induced effective Dzyaloshinskii-Moriya like interaction [1]. Within this setting, analogously to a geometric curvature, a curvature induced by a gauge field [2] is a fruitful aspect to consider in the interaction of the magnetization with an applied static electric field. In a quantum description of the motion of a magnetic moment, an electric field does not modify the energy but it rather modifies the linear momentum, therefore introducing an additional phase for the corresponding Schrödinger wave function, an effect which is known as the Aharonov-Casher’s effect [3]. The classical analog is the spin wave of a ferromagnet. It has been shown that a spin wave, which is carrying a magnetic moment, will acquire a phase when the system is subjected to an electric field [4]. This effect, even if rather small, can be experimentally observed [5]. It is therefore of interest to extend this idea to a general micromagnetic description of the problem. The aim of this research is to pose the problem of the micromagnetics under the effect of an electric field by using the so-called gauge invariance. Briefly the spin-waves, which are small oscillations, deviate from the uniform magnetization with respect to a local reference frame and around the ez direction. They interact with an applied electric field E Fig.(1) modifying the spin wave dispersion relation. Therefore, according to the so-called local exchange invariance [6], we propose a phase shift model, in the exchange spin waves regime, in order to possibly describe and interpret the data shown in [5]. More in details the local gauge symmetry is approximately a local invariance property (because of its thermodynamic origin), of the lagrangian density of the magnetization m(x, t) of a ferromagnet. It is known that it is invariant with respect to the SO(3) global symmetry [7]. In our case this symmetry reduces to a unitary representation of the SO(2) group, (i.e., U(1)). Due to the reason that this symmetry has to be local (i.e., to be position dependent) the invariance of the lagrangian density is broken unless introducing a gauge field able to solve the technical problem of the parallel transport of the magnetization vector. Therefore we show that a particular orientation of a reference frame has no physical relevance. In fact, gauge invariance should occur whenever one can define non-collinear, local reference frames. In our model, the orientation of the local non-coordinate reference frames cannot be uniquely defined because of the presence of an electric field. Summarizing in our invariant gauge formulation the reference frames’ orientation is unspecified with respect to a local phase which defines an invariance property of the exchange interaction terms in the lagrangian density of the magnetization m(x, t). The Lagrangian density of a ferromagnet, interacting with an electric field, is transformed into a new Lagrangian density upon a formal redefinition of the standard partial derivative. Moreover the transformation group, which expresses this invariance, imposes definite restrictions on the dispersion relation that at the first order in a Tylor expansion was previously found to be ω=ω(ky)-γLc-2∂ω/∂kyEx [4], with γL the gyromagnetic ratio and c the speed of light. Therefore our findings point to a more advanced theoretical interpretation with respect to previous discoveries [4] in order to drive experiments in manipulating spin waves and developing electrically tunable magnonic devices.

**References:**

[1] R. Streubel et al., J. Phys. D: Appl. Phys. 49, 363001 (2016). [2] S.G. Tan, et al., Physics Reports, Volume 882, Pages 1-36, (2020). [3] Y. Aharonov and A. Casher, Phys. Rev. Lett. 53, 319, (1984). [4] V. Basso and P. Ansalone, EPL, Vol. 130, Number 1, Article 17008, (2020). [5] X. Zhang, T.Liu, M.E. Flatté and H.X. Tang Phys. Rev. Lett. 113, (2014). [6] I.E. Dzyaloshinskii and G.E.Volovik, J.Physique, 39, 693, (1978). [7] W. F. Brown, Magnetoelastic interactions, pp. 26 and 79, (Vol. 9), Berlin: Springer, (1966).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/62bbb322a9e11ec137f86fd168238d14.jpg)
Fig. 1 The system under study is a two dimensional infinite ferromagnetic layer with primary magnetization along the direction ez , an external electric field is applied perpendicularly to both the magnetization and k spin-wave vector.

 Effect of the Local Exchange Invariance on the Magnetization Dynamics in a Ferromagnet

wo recent reports of the Münster University experimental team led by S.O. Demokritov [Borisenko et al., Nat. Comm. 2020, Borisenko et al., Sci. Rep. 10, 14881 (2020)] displayed several important facts contradicting to the existing theories of the Bose-Einstein condensate of magnons (BECM) [Tupitsyn et al. 2008, Rezende 2009, F. Li et al. 2013]. We present simplified theoretical arguments to determine what are properties of the uniform stationary BECM established under the action of permanent parametric pumping that follows from the experimental facts. We point out that the complete equilibrium was not established in the magnon system because the inter-condensates relaxation time is much longer than the lifetime of condensate magnons. Since the magnons repulse each other, the uniform stationary state of the condensate does not collapse. If the inter-condensates relaxation processes are negligible, then the stationary state of the condensate must be symmetric since the processes of parametric pumping and those responsible for finite magnons lifetime are identical for the two condensates. The weak inter- condensates relaxation processes lead to a weak spontaneous violation of the use of reflection symmetry. This prediction agrees with the inter-condensate interference structure of the condensate discovered by Novik-Boltyk et al. [Novik- Boltyk 2012]

**References:**

 Borisenko, I., Divinskiy, B., Demidov, V. et al. Direct evidence of spatial stability of Bose-Einstein condensate of magnons. Nat Commun 11, 1691 (2020). Borisenko, I.V., Demidov, V.E., Pokrovsky, V.L. et al. Spatial separation of degenerate components of magnon Bose–Einstein condensate by using a local acceleration potential. Sci Rep 10, 14881 (2020) Tupitsyn, I. S., Stamp, P. C. E. & Burin, A. L. Stability of Bose–Einstein condensates of hot magnons in yttrium iron garnet films. Phys. Rev. Lett. 100, 257202 (2008). Rezende, S. M. Theory of coherence in Bose–Einstein condensation phenomena in a microwave-driven interacting magnon gas. Phys. Rev. B 79, 174411 (2009) Li, F., Saslow, W. M. & Pokrovsky, V. L. Phase diagram for magnon condensate in yttrium iron garnet film. Sci. Rep. 3, 1372 (2013) Nowik-Boltyk, P., Dzyapko, O., Demidov, V. E., Berloff, N. G. & Demokritov, S. O. Spatially non-uniform ground state and quantized vortices in a two-component Bose–Einstein condensate of magnons. Sci. Rep. 2, 482 (2012)

**Images:**

 Stationary state of Bose-Einstein condensate of magnons: theory and experiment

Due to low transport dissipation property, magnon was considered as a candidate of next-generation carrier of spintronic industry. In 2016, the van Wees group[1] found an interesting difference between electrically and thermally excited magnons on the nonlocal transport in a very short range. Unlike electrically excited magnons, thermally excited magnons showed a sign-reversal in less than about 300 nm distance between heat injector and magnons detector. Some people developed a bulk SSE theory to explain this phenomenon. According to this theory, a heat flow Jq in YIG will excite a thermal magnon flow Jm,q along with it, A diffusive magnon flow is induced to balance the thermal magnon flow, until the system reaches a steady-state. The sign-reversal derived from a competition of these two magnon flow. While in our point of view, this theory may not reach the nature of transmission properties of thermally excited magnons. To clarify the physical essence and gain complete information in the reciprocal space of this process, we adapted mesoscopic Boltzmann method to study magnon transports[2]. In the method, like phonon, we introduced a special scattering process named “Normal Scattering”, which prevents magnons’ momenta during scattering process. When normal scattering is dominant, magnons turned to a uniform velocity. This uniform velocity makes magnons flux acting as a viscous fluid. Based on Boltzmann method, we developed a new diffusion method of magnons viscous fluid to simplify the calculation and found its nonlinear property. Two-dimensional figures of magnon flux showed the viscosity of magnon fluid created two eddies near the heat source. And the numerical result could fit Experimental data precisely. Our results show magnon flow may be controlled like a viscous fluid by applied magnetic field, which has promising applications in nanoscale magnon-based spintronic devices.

**References:**

1, J. Shan, L. Cornelissen, N. Vlietstra, J. Youssef, T. Kuschel, R. Duine, and B. van Wees, Phys. Rev. B 94, 174437 (2016) 2, T. Liu, W. Wang & J. Zhang, Phys. Rev. B, 99, 214407 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6442af17d25c4eca7d4827e3e6b415bc.jpg)

 Novel Transport Properties of Viscous-Fluid type Magnon in Magnetic Thin Film

Studying spin system dynamics concerns different rapidly growing fields of technology such as developing new types of information transfer systems [1], quantum computing [2, 3], new approaches for magnetic recording and reading [4]. The most common way to excite spin oscillations is by using the inverse Faraday effect (IFE). It is based on a Raman-like coherent optical scattering process and does not require absorption of light. This gives us two main advantages of this effect: it is instantaneous and non-thermal [5]. IFE results in magnetization induced by high-power laser radiation according to the following formula: Min=χ/16π [E×E*], (1) where χ is magneto-optical susceptibility, E is the electric field of the incident light, and E* is its complex conjugate. The cross product in formula (1) is zero for linearly polarized incident light and is maximum for circularly polarized light. The excitation of high order spin waves requires the nonuniform distribution of the electromagnetic field in the medium. It is commonly achieved by using the nanostructured materials in which the resonant phenomena can occur breaking the uniformity of the field. For example, in work [6] the all-dielectric 1D grating was used to generate spin waves due to the excitation of propagating waveguide modes. Another way to excite spin waves is the utilization of anisotropic materials in which the polarization conversion of the incident light occurs. Such a transformation leads to the spatial dependency of magnetization induced via IFE. This nonuniformity of the magnetic torque allows one to excite the spin waves of a high order. Let us consider the case of biaxial crystals the influence on the incident light polarization of which is described by the permittivity tensor εij. The diagonal components of this tensor are assumed not to be equal ε11≠ε22≠ε33 and connected with cartesian coordinates x, y, and z respectively. Let the light fall under the normal incidence to the plane of the material and propagate along the z-axis. Placing the material under study in an external magnetic field along the z-axis leads to the appearance of additional off-diagonal components which are ε12=-i×g and ε21=i×g. Polarization of the light propagating in such a medium transforms due to the anisotropy and Faraday effect. This process is described theoretically in [7]. To be more specific, consider the YFeO3 crystal with an orthorhombic cell. Its crystallographic axes a, b, and c are oriented along x, y, and z axes respectively. The main refractive indices of such material are n1=2.365, n2=2.4, n3=2.337 at the wavelength of 633 nm (ε'' is assumed to be zero). The transformation of the incident light with circular polarization when it passes through anisotropic medium results in oscillations of the magnetization induced via IFE (blue line on Fig. 1). It worth noting that oscillations of magnetization occur for linearly polarized light. The maximum value of cross-product in formula (1) can be achieved for the polarization of incident light at an angle of 45° to the a crystal axis (green line on Fig. 1). In this case, the material behaves as a quarter-wave plate fully transforming the initial linear polarization to circular. One can observe the oscillations even for light polarized linearly along the crystal axis but their maximum value is two times smaller compared to the previous scenarios (Fig. 2). In this situation, the polarization transformation due to birefringence must disappear but the Faraday effect slightly rotates the polarization off the axis allowing the anisotropy to take its place. The oscillations of IFE due to the birefringent properties of the material under study have been theoretically demonstrated. Such a behavior of the induced magnetization is important for generating spin waves of a high order allowing one to avoid additional nanostructuring of the material. This work was supported by the Ministry of Science and Higher Education of the Russian Federation (mega-grant no. 075-15-2019-1934).

**References:**

[1] S. Neusser and D. Grundler, Advanced Materials, Vol. 21 p. 2927 (2009) [2] K. Ganzhorn, S. Klingler, T. Wimmer, et al, Applied Physics Letters, Vol. 109, p. 022405 (2016) [3] D. Lachance-Quirion, Y. Tabuchi, A. Gloppe, et al, Applied Physics Express, Vol. 12, p. 070101 (2019) [4] D.O. Ignatyeva, C.S. Davies, D.A. Sylgachev, et al, Nature Communications, Vol. 10, p. 1 (2019) [5] A.V. Kimel, A. Kirilyuk, P.A. Usachev, et al, Nature, Vol. 435, p. 655 (2005) [6] A.I. Chernov, M.A. Kozhaev, D.O. Ignatyeva, et al, Nano Letters, Vol. 20, p. 5259 (2020) [7] A.K. Zvezdin, V.A. Kotov, Modern magnetooptics and magnetooptical materials, CRC Press (1997)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9fd7730e609d00e4f4b1fe8becda2c13.jpg)
Fig. 1. Cross product of electric field and its complex conjugate depending on the propagation length for circularly (blue line) and linearly polarized at an angle of 45° to the crystal axis (green line) incident light.

![](https://assets.underline.io/uploads/markdown_image/1/image/a63c9245721032602764f78c072ba99b.jpg)
Fig. 2. Cross product of electric field and its complex conjugate depending on the propagation length for incident light polarized along the a crystal axis.

 Oscillating Behavior of Inverse Faraday Effect in YFeO3

Cavity magnonics built on strongly coupled cavity photons and magnons has gradually demonstrated its unique advantages in fundamental and applied research in the past few years. It has several advantages, including flexible turnability, excellent controllability, and high compatibility. Furthermore, a versatile platform based on coherently coupled microwave photons, magnons, phonons, superconducting qubits, and optical photons has begun to take shape. The hybrid system is expected to be applied in quantum information processing, transfer, and storage. Before 2018, all experiments were carried out in the case of coherent coupling between cavity photons and magnons. In 2018, an intriguing dissipative coupling between cavity photon and magnon was observed for the first time, which originated from the cooperative dissipation of the cavity mode and magnon mode. Unlike the coherent coupling case, where the spectrum is shown as an anti-crossing between cavity mode and magnon mode, the spectrum is shown as a level attraction in the case of dissipative coupling. The revelation of dissipative coupling greatly enhances the richness and functionality of the cavity magnonics and stimulates some fundamental research. We will introduce this trend with two recent works from our group. After revealing the dissipative coupling mechanism, we first realized nonreciprocity and unidirectional invisibility in cavity magnonics by utilizing the interference effect between the coherent and dissipative couplings. The isolation ratio can be relatively large with optimized small insertion loss. With the dissipative coupling mechanism and magnon mode linewidth engineering technique, we construct an anti-parity-time (APT) symmetric cavity magnonics system. Besides the dissipative coupling, the APT symmetric system also needs (i) the cavity mode and magnon mode have the opposite frequency, which can be effectively realized with a detuned cavity magnonics system in the rotating frame, (ii) the same damping or gain rates of the cavity mode and magnon mode, which can be engineered by adjusting the damping rate of the magnon mode to match that of the cavity mode. Further, we observed the exceptional points in this APT symmetric cavity magnonics system, which are the transition points between the PT-symmetry-preserved and PT-symmetry-broken phases. Between two EPs, the maximal coherent superposition of photon and magnon states is robustly sustained by the preserved APT symmetry. Moreover, we observed another kind of singularity, arising from the dissipative coupling of two antiresonances, is an unconventional bound state in the continuum (BIC). At the settings of BICs, the coupled system exhibits infinite discontinuities in the group delay. We find that both singularities coexist at the equator of the eigenvector mapped Bloch sphere, revealing a unique hybrid state that simultaneously exhibits the maximal coherent superposition and slow light capability. We expect to explore more interesting physics and applications based on the dissipative coupling in cavity magnonics.

**References:**

1. H. Huebl, et al, Phys. Rev. Lett. 111, 127003 (2013). 2. Y. Tabuchi, wt al, Phys. Rev. Lett. 113, 083603 (2014). 3. X. Zhang, et al, Phys. Rev.Lett. 113, 156401 (2014). 4. M. Goryachev, et al, Phys. Rev. Applied 2, 054002 (2014). 5. L. Bai, et al, Phys. Rev. Lett. 114, 227201 (2015). 6. Y.-P. Wang, Phys. Rev. Lett. 123, 127202 (2019). 7. Y. Yang, Phys. Rev. Lett. 125, 147202 (2020).

**Images:**

 Nonreciprocity and Unconventional Singularities in Cavity Magnonics INVITED

The field of magnonics considers the operations with data carried by spin waves and their quanta, magnons [1]. Traditionally, magnonics has been a room-temperature research discipline. Nowadays, the new direction of cryogenic magnonics attracts increasing attention. The decrease in temperature, in particular, results in the reduction of thermal population n of magnons below 1 opening access to single-magnon quantum physics [2]. Moreover, cryogenic temperatures allow for combining magnonics with superconductivity [3-5]. Ferromagnetism and superconductivity are fundamental cooperative phenomena in condensed matter physics. Entailing opposite spin orders, they share an essential conceptual similarity: Disturbances in magnetic ordering in magnetic materials can propagate in the form of spin waves (magnons) while magnetic fields penetrate the majority of technologically relevant (type II) superconductors as a lattice of magnetic flux quanta (fluxons, Abrikosov vortices). Despite a rich choice of wave and quantum phenomena predicted, magnon-fluxon coupling has lacked experimental scrutiny so far. In this talk, a selection of our recent results [5] on the interaction of spin waves with the flux lattice in ferromagnet/superconductor Py/Nb bilayers will be presented - see Fig. 1. In this system we found the magnon frequency spectrum to exhibit a Bloch-like band structure (as shown in Fig. 1(b)) which can be tuned by the biasing magnetic field. Furthermore, the frequency spectra of spin waves scattered on the flux lattice moving under the action of a transport current in the superconductor exhibit Doppler shifts. The resonance absorption of spin waves is explained by their inelastic scattering on the vortex lattice constituting a reconfigurable magnonic crystal [6]. The observed Doppler shifts arise due to the modified dispersion relation for spin waves scattered on the moving vortex lattice constituting a moving Bragg grating. In addition, manipulation of spin waves and tailoring of their transmission spectra in Py will be exemplified by using adjacent Nb layers with nanofabricated pinning potential landscapes. A special attention will be devoted to direct-write hybrid superconductor-based systems with fast relaxation of non-equilibrium effects and supporting ultra-fast vortex motion at &gt;10km/s velocities [7]. Research leading to these results was done in collaboration with R. Sachser, T. Brächer, T. Böttcher, V. Kruglyak, R. Vovk, V. Shklovskij, M. Huth, B. Hillebrands and A. Chumak.

**References:**

[1] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, et al., Nat. Phys. 11 (2015) 453 [2] D. Lachance-Quirion, S. P. Wolski, Y. Tabuchi,et al., Science 367 (2020) 425 [3] I. A. Golovchanskiy, N. N. Abramov, V. S. Stolyarov, et al., Adv. Func. Mater. 28 (2018) 1802375 [4] A. A. Bespalov, A. S. Mel’nikov, and A.I. Buzdin, Phys. Rev. B 89 (2014) 054516 [5] O. V. Dobrovolskiy, R. Sachser, T. Brächer, et al., Nat. Phys. 15 (2019) 477 [6] A. V. Chumak, A. A. Serga, and B. Hillebrands, J. Phys. D: Appl. Phys. 50 (2017) 244001 [7] O. V. Dobrovolskiy, D. Yu. Vodolazov, F. Porrati, et al., Nat. Commun. 11 (2020) 3291

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/02511907edbb80ebd00a971a77b36b08.jpg)
(a) Sketch of the experimental system. (b) Fluxon-induced reconfigurable magnonic crystal: Bandgaps in the spin-wave transmission spectra in Py are observed when the Nb layer contains an array of Abrikosov vortices.

 Magnon Fluxonics INVITED

Acoustic wave has been widely integrated into modern electronics to enable various functions, including sensing, communication, signal filtering, etc. Conventionally, acoustic waves propagate equally forward and backward. Here, we employ a fundamentally new mechanism, magneto-rotation coupling[1], to break the symmetry of the propagation, and consequently induces an unprecedented rectification ratio to 100% at the optimized condition, in accordance with the theoretical prediction. As a unique feature of surface acoustic waves, individual lattice point undergoes a rotation motion, which changes its sign according to the wave propagation direction (see the blue and red oriented circles in figure 1(a)). More than 40 years ago, Maekawa and Tachiki developed a theoretical framework [1], proposing the coupling between the lattice rotation and the magnetization via magnetic anisotropy, namely magneto- rotation coupling. Thus, to break the symmetry of the propagation, here, we employ magneto-rotation coupling to capture the chirality of the acoustic waves in a perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO (see figure 1(b)), achieving a giant nonreciprocal acoustic wave attenuation (see figure 1(c)) with a rectification ratio up to 77% [2].

**References:**

[1] S. Maekawa and M. Tachiki, Surface acoustic attenuation due to surface spin wave in ferro and antiferromagnets, AIP Conf. Proc. 29, 542 (1976). [2] M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D. Grundler, S. Maekawa, and Y. Otani, Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling, Vol. 6, no. 32, eabb1724 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/404c74cc9b48109c9508f92526b2d977.jpg)
Fig. 1. (a) Schematic of the nonreciprocity mechanism. (b) Illustration of the experimental setup. (c) Attenuation ofacoustic waves, P±k, near a spin-wave resonance condition for surface acoustic wavenumbers +k and -k.

 Magneto-rotation coupling inducing nonreciprocal surface acoustic waves

Very recently magnetic stripe domains, characterized by alternating up and down out-of-plane orientation of the magnetization, have received great interest due to the possibility to use stripe patterns to manipulate spin-wave (SW) propagation as in artificial magnonic crystals [1]. In this work, we demonstrate the control of the SW propagation by using reconfigurable regular stripe-pattern domain structure in the hybrid system. The investigated system consists of 64-nm-thick NdCo layer and 10-nm-thick NiFe layer, coupled through an Al layer of different thicknesses. Magnetic force microscopy (MFM) measurements show that, due to the perpendicular magnetic anisotropy of the NdCo film, the system develops stripe domains aligned with the last in-plane saturation direction [2]. The domain pattern is found to have a period of about 140 nm, which is almost independent on the thickness of the Al layer. The magnetization reversal of the trilayer system was investigated by vibrating sample magnetometer, showing that the hysteresis loop is characterized by a two-step process, due to the different coercivity of the NiFe and NdCo films. Detailed analysis of the hysteresis loops along with micromagnetic simulations indicates that the stray magnetic field coming from the NdCo layer induces a regular domain structure also in the NiFe layer, which is tuned by the thickness of Al spacer. In addition, upon reversing the applied magnetic field, an antiparallel state, characterized by an antiparallel alignment of the magnetization component parallel to the domain axis in the NdCo and NiFe stripes, is formed. Then, Brillouin light scattering spectroscopy has been used to measure the spectra of the SWs propagating in the direction perpendicular to stripe domains for the parallel and the antiparallel state. For both configurations, the dispersion relation shows a strongly nonreciprocal mode (Figs. 1-2). However, in the parallel state SWs propagating with positive and negative wavevector are both characterized by a positive dispersion, while in the reversed state SWs propagating with negative wavevector show a negative dispersion. The above experimental results have been satisfactorily reproduced by numerical simulations. The latter show that the detected SW mode is mainly localized in the NiFe layer and its frequency nonreciprocity can be ascribed to the static magnetization configuration as well as to the interaction with the NdCo induced by the SWs via the dynamic stray field.

**References:**

[1] C. Banerjee et al., Phys. Rev. B 96, 024421 (2017) [2] D. Markó et al., Appl. Phys. Lett. 115, 082401 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1f952ef7a0b2bb8cf6cafa5252632c82.jpg)
Fig. 1. Dispersion relation in the NdCo(64 nm)/Al(5)/NiFe(10) in the parallel state. Empty and full dots mark the results of Brillouin light scattering measurements and simulations, respectively. Size and color intensity of the numerical results indicate the predicted intensity in the measurements.

![](https://assets.underline.io/uploads/markdown_image/1/image/dbd287cdf366b5549a79bdda3afc82b8.jpg)
Fig. 2. Dispersion relation in the NdCo(64 nm)/Al(5)/NiFe(10) in the antiparallel state.

 Reconfigurable spin-wave propagation in magnetic stripe domains in hybrid system

In recent years, the coupling between the elastic and magnetic domain in magnetostrictive materials has gained strong interest [1]. The magnetoelastic coupling in these materials results in elastic response originating from magnetic excitations and vice versa. At microwave frequencies, this mutual interaction leads to coupling between elastic and magnetic waves, forming magnetoelastic waves. Whereas plane magnetoelastic waves are already studied in bulk materials some decades ago [2-4], the magnetoelastic waves in thin magnetostrictive films gained attention only recently [5,6]. By contrast, many recent spin-wave-based information processing applications employ nanoscale waveguides for information transfer and computation [7]. At these nanoscales, confinement of both the elastic and magnetic waves leads to multiple modes each of which with a specific mode profile and consequently also to confined magnetoelastic waves. The shape of the magnetoelastic field depends on the displacement profile and the shape of the magnetoelastic body force depends on the magnetization profile. As a consequence, it is expected that every mode experiences different magnetoelastic coupling strength. This presents a rather complex multiphysics problem where analytical methods are hard or even impossible, and thus numerical approaches are preferred. Therefore, in this work, we present the development of a novel mumax3 extension that allows the numerical calculation of magnetoelastic waves in scaled waveguides [8]. Furthermore, we present results of magnetoelastic waves in a nanoscale waveguide with static magnetization along the wave propagation direction. The studied system consists of a thin CoFeB waveguide with width of 200 nm and thickness of 20 nm. A weak external field of 5 mT is applied in the longitudinal direction of the waveguide to initiate the static magnetization along this direction. The resonant modes in the structure are numerically found by applying a 5 ps excitation pulses and performing a two-dimensional fast Fourier transform. By comparing the numerical result with the analytical uncoupled dispersion relations of confined elastic and magnetic waves, all modes were identified. The uncoupled resonant waves of this structure consist of confined magnetic spin waves, antisymmetric and symmetric lamb waves and out-of-plane shear elastic waves. Fig. 1a shows the numerically obtained dispersion relations and Fig. 1b shows the overlay with the analytically calculated dispersion relations for uncoupled elastic and magnetic waves. The magnetoelastic coupling leads to the formation of anti-crossings near the reciprocal point where the uncoupled elastic and magnetic dispersion relations would intersect with each other. These anti-crossings identify regions of strong interaction between the elastic and magnetic domain and correspond to pure magnetoelastic waves. The magnetoelastic waves are shown to have different behavior as compared to the pure elastic or magnetic waves. The magnetoelastic wave group velocity is rather unaffected in the quasi-magnetic or quasi-elastic regime whereas it strongly differs close to the anti-crossing. Furthermore, the magnetization and displacement value are shown to take both high values close to the anti-crossing. However, when approaching the quasi-elastic regime there is a very strong decay of the magnetization component of the magnetoelastic wave and vice versa for the displacement component and the quasi-magnetic regime. Lastly, careful analysis of the dispersion relations also shows reciprocal points with weak or even without anti-crossing. The frequency gap formed by the anti-crossing is a good measure of the magnetoelastic coupling strength and thus provides much information. The variation in amplitude of this frequency gap is attributed to the wave confinement and their mode profiles. Every elastic and magnetic mode has its own mode profile and thus also its own magnetoelastic body force and field. Considering these mode dependent body forces and fields together with the mode’s overlap integral allows to identify the strength of the coupling between the different elastic and magnetic waves. The strongest coupling is shown to exist between the odd magnetic modes and symmetric lamb modes and even magnetic modes and anti-symmetric lamb modes. This work has been partially funded by the European Union’s Horizon 2020 research and innovation programme within the FET-OPEN project CHIRON under grant agreement No. 801055. FV acknowledge the financial support from Fonds voor Wetenschappelijk Onderzoek (FWO) under the grant No. 1S05719N.

**References:**

[1] A. D. Kent and D. C. Worledge, Nature Nanotechnol. Vol. 10, p. 187 (2015) [2] C. Kittel, Phys. Rev. Vol. 110, p. 836 (1958). [3] E. Schlomann and R. I. Joseph, J. Appl. Phys. Vol. 35, p. 2382 (1964). [4] P. A. Fedders, Phys. Rev. B. Vol. 9, p. 3835 (1974). [5] Y. Nozaki and S. Yanagisawa, Electr. Eng. Jpn. Vol. 204, p. 3 (2018). [6] S. Bhuktare, A. Bose, H. Singh, and A. A. Tulapurkar, Sci. Rep. Vol. 7, p. 840 (2017). [7] A. Khitun and K. L. Wang, J. Appl. Phys. Vol. 110, p. 34306 (2011). [8] F. Vanderveken, et al., https://github.com/Fredericvdv/Magnetoelasticity_MuMax3.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c4a42e298a703a137c4d16ff97d65c5e.jpg)
Figure 1: Dispersion relations of magnetoelastic waves in a 200 nm wide and 20 nm thick CoFeB waveguide. (a) Numerically calculated dispersion relations. The inset shows a magnification of a region near a magnetoelastic gap. (b) Analytically calculated dispersion relations of uncoupled elastic and spin waves superimposed to the numerically obtained results.

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