France



One current goal of spintronics is the development of a sustainable information technology based on the transport of pure spin currents. One promising way to do this is to use spin waves (SWs), the elementary excitations of magnetically ordered materials: by collective precession of magnetic moments, they can transport angular momentum. SW information processing also offers interesting alternatives to conventional microelectronics [1]. For this, yttrium iron garnet (YIG), an insulating ferrimagnet, is the ideal material because it has the lowest magnetic damping. Furthermore, thanks to the spin orbit torque (SOT), it is possible to control the relaxation time of SWs in YIG by an electric current injected into an adjacent platinum layer [2]. However, such metal/insulator hybrid devices presently have certain limitations, as a sudden drop in the amplitude of the main mode beyond a certain excitation threshold, which originates from nonlinear coupling with other SW modes [2].&lt;br/&gt;&lt;br/&gt;Different strategies can be considered to overcome these issues. In nanopatterned samples the available SW modes become quantized, which severely limits the available nonlinear processes. As a result, nonlinear saturation thresholds are postponed, which allows to reach much higher amplitudes of single mode dynamics [3]. The perpendicular magnetic anisotropy (PMA) can also be used to control the sign of the nonlinear frequency shift. Recently, the growth of ultra-thin films of Bismuth doped YIG (BiYIG) with tunable PMA has been achieved while preserving a high dynamic quality [4]. It was shown that the emission characteristics of SWs emitted by SOT in thin BiYIG films where PMA compensates the in-plane shape anisotropy are greatly improved [5].&lt;br/&gt;&lt;br/&gt;Here, we study the magnetization dynamics in individual nanodisks patterned from such a 30 nm thick BiYIG film, with diameters ranging from 1 µm down to 200 nm. The static magnetic field is applied perpendicularly to the sample plane. To excite the ferromagnetic resonance (FMR) of the BiYIG nanodisks, we use the spatially uniform in-plane microwave field produced by an antenna integrated on top. To detect it, we employ a magnetic resonance force microscope [6] in which a cobalt nanosphere grown at the tip of a soft cantilever [7] is dipolarly coupled to the longitudinal component of the magnetization of the nanodisk positioned underneath (Fig. 1).&lt;br/&gt;&lt;br/&gt;By studying the SW spectra of these BiYIG nanodisks in the linear regime, we find that their effective anisotropy field is weakly dependent of the diameter and negative, about -10 mT, which means that the PMA slightly overcompensates the shape anisotropy. The damping parameter is found to increase as the diameter is decreased and lies in the high 10&lt;sup&gt;-4&lt;/sup&gt; – low 10&lt;sup&gt;-3&lt;/sup&gt; range for all disks.&lt;br/&gt;&lt;br/&gt;We then perform measurement of the main resonance line, corresponding to the quasi-uniform mode, as a function of the microwave power injected into the antenna, which is varied by up to four orders of magnitude (excitation field ranging from 0.03 mT to 3 mT at 5 GHz). For all the disks, we observe a distortion of the resonance line at intermediate excitation power, corresponding to the onset of foldover. As expected, due to the negative effective anisotropy field, the resonance line shifts towards higher magnetic field as the precession angle increases, which is opposite to the behavior reported on undoped YIG nanodisks [3]. For the largest BiYIG disks, the dynamics witnessed at higher power is quite more complex and interesting, as shown in Fig. 2 for the 700 nm diameter nanodisk. We observe a rapid saturation of the peak amplitude, corresponding to average precession angles of the order of 10° to 20°, accompanied with a broadening of the line towards both lower and higher magnetic field. To understand this behavior, we perform extensive micromagnetic simulations. Using the magnetic parameters extracted in the linear regime and the nominal geometry of the nanodisks, these simulations reproduce the main characteristics of the measured FMR lines in the strongly nonlinear regime. They reveal that a dynamic instability is responsible of the observed behavior: as the amplitude of the quasi-uniform mode increases with the driving field, its precession profile gets more and more localized at the center of the disk, which eventually leads to the formation of a dynamic soliton similar to a wave bullet [8]. Moreover, this soliton is not stable over time, resulting in a rich temporal dynamics, which, depending on the exact driving conditions, can be quasi-periodic or intermittent. To experimentally access this complex temporal dynamics, we use the method introduced in [3], which consists in applying a second, much weaker excitation field of varying frequency, in addition to the main driving field at high power. The obtained frequency modulation spectra indeed reflect the existence of rich low-frequency temporal variations in the dynamics of the magnetization, as suggested by the simulations.

**References:**

[1] A. Chumak et al. Nat. Phys. 11, 453 (2015)&lt;br/&gt;[2] A. Hamadeh et al. Phys. Rev. Lett. 113, 197203 (2014); M. Evelt et al. Appl. Phys. Lett. 108, 172406 (2016)&lt;br/&gt;[3] Y. Li et al. Phys. Rev. X 9, 041036 (2019)&lt;br/&gt;[4] L. Soumah et al. Nat. Commun. 9, 3355 (2018)&lt;br/&gt;[5] M. Evelt et al. Phys. Rev. Appl. 10, 041002 (2018)&lt;br/&gt;[6] O. Klein et al. Phys. Rev. B 78, 144410 (2008)&lt;br/&gt;[7] S. Sangiao et al. Beilstein J. Nanotechnol. 8, 2106 (2017)&lt;br/&gt;[8] A. Slavin and V. Tiberkevich, Phys. Rev. Lett. 95, 237201 (2005)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4929da18838a5e687ca4c091422c4fa2.jpg)
Fig.1 - Schematics of the experiment. The microwave field supplied by the antenna drives the magnetization of a BiYIG nanodisk into FMR, opening a precession angle around the perpendicularly applied field, which is detected by the cantilever of a magnetic resonance force microscope.

![](https://assets.underline.io/uploads/markdown_image/1/image/4ea71d26a653b41e10c95b4dc047a304.jpg)
Fig.2 - Main resonance line of the 700 nm diameter nanodisk recorded at 5 GHz as a function of the driving power, increased by step of 1 dB from -8 dBm to 10 dBm. The spectra have been vertically offset for clarity.

INTERMAG 2021

 Strongly nonlinear ferromagnetic resonance of Bi-doped YIG nanodisks

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One current goal of spintronics is the development of a sustainable information technology based on the transport of pure spin currents. One promising way to do this is to use spin waves (SWs), the elementary excitations of magnetically ordered materials: by collective precession of magnetic moments, they can transport angular momentum. SW information processing also offers interesting alternatives to conventional microelectronics [1]. For this, yttrium iron garnet (YIG), an insulating ferrimagnet, is the ideal material because it has the lowest magnetic damping. Furthermore, thanks to the spin orbit torque (SOT), it is possible to control the relaxation time of SWs in YIG by an electric current injected into an adjacent platinum layer [2]. However, such metal/insulator hybrid devices presently have certain limitations, as a sudden drop in the amplitude of the main mode beyond a certain excitation threshold, which originates from nonlinear coupling with other SW modes [2]. Different strategies can be considered to overcome these issues. In nanopatterned samples the available SW modes become quantized, which severely limits the available nonlinear processes. As a result, nonlinear saturation thresholds are postponed, which allows to reach much higher amplitudes of single mode dynamics [3]. The perpendicular magnetic anisotropy (PMA) can also be used to control the sign of the nonlinear frequency shift. Recently, the growth of ultra-thin films of Bismuth doped YIG (BiYIG) with tunable PMA has been achieved while preserving a high dynamic quality [4]. It was shown that the emission characteristics of SWs emitted by SOT in thin BiYIG films where PMA compensates the in-plane shape anisotropy are greatly improved [5]. Here, we study the magnetization dynamics in individual nanodisks patterned from such a 30 nm thick BiYIG film, with diameters ranging from 1 µm down to 200 nm. The static magnetic field is applied perpendicularly to the sample plane. To excite the ferromagnetic resonance (FMR) of the BiYIG nanodisks, we use the spatially uniform in-plane microwave field produced by an antenna integrated on top. To detect it, we employ a magnetic resonance force microscope [6] in which a cobalt nanosphere grown at the tip of a soft cantilever [7] is dipolarly coupled to the longitudinal component of the magnetization of the nanodisk positioned underneath (Fig. 1). By studying the SW spectra of these BiYIG nanodisks in the linear regime, we find that their effective anisotropy field is weakly dependent of the diameter and negative, about -10 mT, which means that the PMA slightly overcompensates the shape anisotropy. The damping parameter is found to increase as the diameter is decreased and lies in the high 10-4 – low 10-3 range for all disks. We then perform measurement of the main resonance line, corresponding to the quasi-uniform mode, as a function of the microwave power injected into the antenna, which is varied by up to four orders of magnitude (excitation field ranging from 0.03 mT to 3 mT at 5 GHz). For all the disks, we observe a distortion of the resonance line at intermediate excitation power, corresponding to the onset of foldover. As expected, due to the negative effective anisotropy field, the resonance line shifts towards higher magnetic field as the precession angle increases, which is opposite to the behavior reported on undoped YIG nanodisks [3]. For the largest BiYIG disks, the dynamics witnessed at higher power is quite more complex and interesting, as shown in Fig. 2 for the 700 nm diameter nanodisk. We observe a rapid saturation of the peak amplitude, corresponding to average precession angles of the order of 10° to 20°, accompanied with a broadening of the line towards both lower and higher magnetic field. To understand this behavior, we perform extensive micromagnetic simulations. Using the magnetic parameters extracted in the linear regime and the nominal geometry of the nanodisks, these simulations reproduce the main characteristics of the measured FMR lines in the strongly nonlinear regime. They reveal that a dynamic instability is responsible of the observed behavior: as the amplitude of the quasi-uniform mode increases with the driving field, its precession profile gets more and more localized at the center of the disk, which eventually leads to the formation of a dynamic soliton similar to a wave bullet [8]. Moreover, this soliton is not stable over time, resulting in a rich temporal dynamics, which, depending on the exact driving conditions, can be quasi-periodic or intermittent. To experimentally access this complex temporal dynamics, we use the method introduced in [3], which consists in applying a second, much weaker excitation field of varying frequency, in addition to the main driving field at high power. The obtained frequency modulation spectra indeed reflect the existence of rich low-frequency temporal variations in the dynamics of the magnetization, as suggested by the simulations.

**References:**

[1] A. Chumak et al. Nat. Phys. 11, 453 (2015) [2] A. Hamadeh et al. Phys. Rev. Lett. 113, 197203 (2014); M. Evelt et al. Appl. Phys. Lett. 108, 172406 (2016) [3] Y. Li et al. Phys. Rev. X 9, 041036 (2019) [4] L. Soumah et al. Nat. Commun. 9, 3355 (2018) [5] M. Evelt et al. Phys. Rev. Appl. 10, 041002 (2018) [6] O. Klein et al. Phys. Rev. B 78, 144410 (2008) [7] S. Sangiao et al. Beilstein J. Nanotechnol. 8, 2106 (2017) [8] A. Slavin and V. Tiberkevich, Phys. Rev. Lett. 95, 237201 (2005)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4929da18838a5e687ca4c091422c4fa2.jpg)
Fig.1 - Schematics of the experiment. The microwave field supplied by the antenna drives the magnetization of a BiYIG nanodisk into FMR, opening a precession angle around the perpendicularly applied field, which is detected by the cantilever of a magnetic resonance force microscope.

![](https://assets.underline.io/uploads/markdown_image/1/image/4ea71d26a653b41e10c95b4dc047a304.jpg)
Fig.2 - Main resonance line of the 700 nm diameter nanodisk recorded at 5 GHz as a function of the driving power, increased by step of 1 dB from -8 dBm to 10 dBm. The spectra have been vertically offset for clarity.

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Analyzing the magnetic structure factor of a field demagnetized artificial square ice, qualitative deviations from what would predict the square ice model are observed. Combining micromagnetic and Monte Carlo simulations, we demonstrate that these deviations signal the presence of interactions between nanomagnets that extend beyond nearest neighbors. Including further neighbor, dipolar-like couplings in the square ice model, we find that the first seven or eight coupling strengths are needed to reproduce semi-quantitatively the main features of the magnetic structure factor measured experimentally. An alternative, more realistic numerical scenario is also proposed in which the ice condition is slightly detuned. In that case as well, the features evidenced in the experimental magnetic structure factor are only well-described when further neighbor couplings are taken into account. Our results show that long range dipolar interactions are not totally washed out in a field demagnetized artificial square ice, and cannot be neglected as they impact the magnetic correlations within the ice manifold.

**References:**



**Images:**





 Signatures of long range dipolar interactions in artificial square ice



Field demagnetization protocols may be employed to bring athermal, artificial spin systems into an arrested, low-energy magnetic configuration. Such protocols, applied to systems in which thermal fluctuations are absent, are intrinsically deterministic: if the field history and the sample properties were known, the final configuration would be completely determined. Here, we study the response of a given athermal, artificial square ice submitted to successive, albeit similar, field demagnetization protocols. We show that each resulting arrested state differs from the previous one. Analyzing these magnetic states, we demonstrate that our artificial ice magnet behaves as a stochastic system. We propose a mechanism by which stochasticity can be induced.

**References:**



**Images:**





 Are field demagnetized, athermal, artificial square ice magnets stochastic systems?

The dynamics of spin waves propagating in thin magnetic films has been studied intensively for several decades, and is relatively well-understood [1,2]. However, investigations have been mainly focused on the cases of either perpendicular or in-plane magnetization, where forward volume, backward volume, and surface magnetostatic (or dipolar) long-wavelength wave modes have been identified [1]. A more general dipole-exchange theory of the spin wave spectrum in magnetic films applicable also to the short-wavelength spin waves has been later developed by the research group of B.A. Kalinikos [2]. The conventional theory led to the understanding that a spin wave in a magnetic film with spin-sink-free surfaces is a non-uniform travelling wave having a stationary distribution of the variable magnetization along the film thickness, and can transfer energy and angular momentum only along the in-plane direction of the spin wave propagation. It turned out, that the situation becomes substantially more complicated in the case when the magnetic film is magnetized obliquely (see Fig.1), and short-wavelength dipole-exchange spin waves are considered. The results of recent experiments [3], performed using Brillouin light-scattering spectroscopy, in combination with the numerical results, based on the theory of dipole-exchange spin-wave spectra [2], have shown two effects: (i) thickness profiles of the propagating spin waves (both in in-plane and obliquely magnetized films) are strongly dependent on the in-plane wavenumber k due to the strong dipolar hybridization between the modes having different thickness profiles, and demonstrate “dipolar pinning” [4] with the increase on the in-plane wavenumber (see Fig.2, top panel); (ii) transverse (thickness) profiles of the spin waves propagating in the film plane in an obliquely magnetized film are non-stationary , i.e. are changing with time in the process of the wave propagation (see Fig.2, bottom panel). This last phenomenon might mean that there could be a transverse (along the film thickness) transfer of spin angular momentum without corresponding transverse transfer of energy. In the end, this issue of transverse spin transport is related to the boundary conditions at the surfaces of the magnetic film. In our simple system Fig. 1, we consider a magnetic film with spin-sink-free surfaces. However, if there is a spin-sink formed by a layer of a heavy metal, such as Pt, present at the film surface, it may be possible to experimentally detect the transfer of spin angular momentum along the film thickness in an obliquely magnetized magnetic film supporting the unidirectional spin wave propagation in the film plane. Further experimental studies are necessary to clarify the issue of possible transverse spin angular momentum transfer in obliquely magnetized magnetic films.

**References:**

[1] R. Damon and J. Eshbach, J. Phys. Chem. Solids, vol. 19, p.308 (1960). [2] B. Kalinikos and A. Slavin, J. Phys. C, vol. 19, p.7013 (1986). [3] D. Bozhko, H. Musiienko-Shmarova, V. Tiberkevich, et al., Phys. Rev. Research, vol. 2, p.023324 (2020). [4] K.Yu. Guslienko, S.O. Demokritov, B. Hillebrands, and A.N. Slavin, Phys. Rev. B , vol.66, 132402 (2002).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d2c295c636a8de1bab19ac4b6cee516b.jpg)
Fig. 1. Geometry of an obliquely magnetized (by an internal magnetic field Hi directed at an angle θ to the surface) spin-sink-free magnetic film of the thickness L carrying a propagating spin wave having a wavenumber k. An example of a transverse profile of the dynamic magnetization in a propagating spin wave is shown along the film thickness.

![](https://assets.underline.io/uploads/markdown_image/1/image/81be77679a816fef838e8f4e5180f41f.jpg)
Fig. 2. Numerically calculated transverse dynamic magnetization profiles of spin wave modes propagating in a 5.6 μm thick yttrium-iron garnet (YIG) film. Top panel: profiles in an in-plane magnetized ( θ =0) film , corresponding to three different values of the in-plane wavenumber, and demonstrating the effect of “dipolar pinning”. Bottom panel: profiles of the wave having in-plane wavenumber k =1 rad μm-1 in an obliquely magnetized ( θ =π/4 ) film at three different time moments, demonstrating that the transverse spin wave profile is non-stationary. 
T - is the period of the spin wave frequency.

 Non-Stationary Thickness Profiles of Spin Wave Modes Propagating in Obliquely Magnetized Magnetic Films

The concept of spin current, introduced analogously to the charge current, has gained a great significance in spintronic and magnonic systems. Two distinct types of spin transport mechanisms can be identified. The first one is associated with diffusion of spin-polarized conduction electrons from one part of the system to another, followed by their spin relaxation mainly due to the exchange interaction with the localized electrons. The second mechanism is mediated by the transfer of spin between localized magnetic moments due to their mutual interaction. The latter can be also described as spin transport by propagating spin waves. Interplay between these two phenomena enables generation of propagating spin waves by spin currents carried by conduction electrons. Reciprocally, spin waves can produce spin currents carried by spin-polarized electrons, which is known as spin pumping. In my talk I will discuss several phenomena involving interplay between spin waves and electron spin currents, including the excitation of propagating spin waves by spin polarized electric current [1] and the spin-Hall effect (SHE) [2]. In the first case a simplified model considering only the exchange interaction provides an adequate description of the observed phenomena, based on the conservation of angular momentum in the spin subsystem. However, the physical origin of SHE is the relativistic spin-orbit coupling, which mixes the angular momentum of spin subsystem with that of the lattice. Surprisingly, the same is valid for the magnetic dipole interaction. I will illustrate the importance of mixing between spin and orbital degrees of freedom with the examples of spin-wave pumping experiments [3] and the dynamics of Bose-Einstein condensate of magnons [4].

**References:**

 [1] S. Urazhdin et al. Nature Nanotech. 9 509 (2014) [2] B. Divinskiy et al. Adv. Mater. 1802837 (2018) [3] H. Kurebayashi et al., Nature Mat. 10, 660 (2011). [4] I.V. Borisenko et al., Nature Comm. 11, 1691443 (2020).

**Images:**

 Spin waves and spin currents

Magnons in an easy-axis antiferromagnet with uniaxial anisotropy have two circularly polarized modes, which, very similar to the spin states of electrons, carry opposite angular momentum. Spin transport mediated by these antiferromagnetic magnons has attracted much attention recently. In this talk, we will discuss explicitly the role of different microscopic mechanisms behind. Specifically, we will show that the exchange interaction can lead to strong mode splitting and confluence processes, which change the magnon number but conserve the total angular momentum. Such processes can drive the two oppositely polarized magnon species to opposite chemical potentials, resulting in a pure magnon spin current, i.e., a counterflow of the two species without net magnon current [1]. The dipole-dipole interaction, on the other hand, is demonstrated to be able to introduce an effective magnon spin-orbit coupling, which then gives rise to electron-like spin-orbit phenomena, such as an intrinsic magnon spin Hall effect and D’yakonov-Perel’-type magnon spin relaxation [2]. Moreover, we show that the dipole-dipole interaction is able to generate Damon-Eshbach-type magnon surface modes [3]. In contrast to the nonreciprocity of the conventional Damon-Eshbach mode in ferromagnets, the surface modes in antiferromagnets experience a spin-momentum locking, just like the surface modes in an electronic topological insulator. Our work suggests that the antiferromagnetic insulators could be a suitable platform for exploring eletron-like spin dynamics in insulators and designing novel spintronic devices.

**References:**

[1] Ka Shen, Phys. Rev. B 100, 094423 (2019). [2] Ka Shen, Phys. Rev. Lett. 124, 077201 (2020). [3] Jie Liu, Lin Wang, and Ka Shen, Phys. Rev. Research 2, 023282 (2020)

**Images:**

 Pure spin current and spin Hall effect in antiferromagnetic insulators

Recent progresses in quantum information technologies have demonstrated the benefits of leveraging coherence effects such as collective spin precession in ferromagnets in order to process information more efficiently. This coherent transfer between different waveforms is an important ingredient of the quantum information processing as it allows transport of quantum states without loss of frequency or phase. An efficient hybridization process requires to reach a strong coupling regime, where the interaction rate between two collective states becomes larger than their relaxation rates. While increasing the overlap integral between two spatio-temporal patterns is primarily a design issue, diminishing the internal friction inside an object is a much more complex task, which requires know-how in material research and growth technology. In spintronics, this has revived interest for insulating materials and in particular garnets, which are the magnetic materials benefiting from the lowest magnetic damping. The sound wave attenuation coefficient in garnets is also exceptional, i.e. up to an order of magnitude lower than that in single crystalline quartz. In addition to the low damping of magnetic and sound waves, a strong coupling can be established between spin-waves (magnons) and lattice vibrations (phonons) through the magnetic anisotropy and strain dependence of the magnetocrystalline energy in magnetic garnets. The magnetoelasticity leads to new hybrid quasiparticles (“magnon polarons”) when spin wave and acoustic wave dispersions cross [1]. This coupling has been exploited in the past to produce microwave acoustic transducers [2]. The adiabatic conversion between magnons and phonons in magnetic field gradients proves their strong coupling in yttrium iron garnet (YIG) [3]. Here we first demonstrate that the spin waves can be strongly coupled to coherent transverse sound waves that have very long characteristic decay length and propagate ballistically over millimetric distances. The experiment was performed at room temperature with a magnetic field applied perpendicular to the film. Our sample consists of two 200 nm thick YIG layers deposited on both sides of a 0.5 mm thick gadolinium gallium garnet (GGG) substrate. The circularly polarized standing sound waves couple to the magnetization oscillations in both layers. An interference pattern is observed and it is explained as the strong coupling of the magnetization dynamics of the two YIG layers either in phase or out of phase by the standing transverse sound waves [4]. This long range coherent transport of spin by phononic angular momentum can add new functionalities to insulator spintronic circuits and devices. Furthermore, such mediation of coherent spins could induce a novel dynamic exchange coupling in a spin-valve system when the magnetic resonances of two layers are brought within the strength of the magnetoelastic coupling. To detect such coupling, a Pt strip electrode was deposited on one YIG layer to monitor its dynamics through the inverse spin Hall effect (ISHE). This improves the measurement sensitivity of the phonon mediated spin transfer. In our sample, the two YIG layers have initially different spontaneous magnetizations. We tune the difference between two magnetic resonances either i) by varying the polar angle of an external magnetic field or ii) by applying a temperature gradient along the stack direction. The existence of the novel dynamic coupling is evidenced by an emergence of the “bright” and “dark” collective states in both measurements. Our observation of such collective modes resembles the level attraction/repulsion behavior between two hybridized modes [5]. This feature is similar to the coupling control via microwave phase [6]. We show that the observed behavior is in agreement with theoretical predictions. Finally we discuss the detection of heat carried by coherent phonons. We measure thermal voltages across Pt in YIG|GGG|Pt system, where no ISHE voltage should be present. The signal appears only at the acoustic phonon resonances indicating that the heat can be efficiently carried by circularly polarized coherent phonons excited by magnons. When the magnetic field direction is tilted away from the film normal direction, the signal vanishes, suggesting that the heat carried by acoustic phonons becomes less coherent. We perform several control experiments, which shows the universality of the signal and possible enhancement schemes.

**References:**

[1] T. Kikkawa, K. Shen, B. Flebus et al., Phys. Rev. Lett. 117, 207203 (2016) [2] M. Pomerantz, Phys. Rev. Lett. 7, 312 (1961) [3] J. Holanda, D. S. Maior, A. Azevedo et al., Nat. Phys. 14, 500 (2018) [4] K. An, A.N. Litvinenko, R. Kohno et al., Phys. Rev. B 101, 060407(R) (2020) [5] M. Harder, Y. Yang, B. M. Yao et al., Phys. Rev. Lett. 121, 137203 (2018) [6] I. Boventer, C. Dörflinger, T. Wolz et al., Phys. Rev. Research 2, 013154 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6b745abdcceba5bf7d855dd387b11ad1.jpg)
Theoretical calculation of (a,b) the dark and (c,d) bright states formation. The insets show the magnetic precession states of both layers when (a) even or (c) odd phonon mode mediates the coupling between two magnetic layers. The dashed lines show the calculated eigenvalues. The linecuts in (b) and (d) are along the zero detuning case.

 Long range coupling of magnetic bilayers by coherent phonons

The most important parameters of complementary CMOS electronics, such as high speed information processing, small size of transistors, rather small energy consumption and low thermal radiation have fundamental restrictions. Magnonics - is one of the alternative conteptual solutions to overcome these restrictions [1-3]. Basically, the main idea of this is to combine magnonics with conventional CMOS technologies [4,5]. Interaction of magnons (spin waves) in micron- or nanosized ferromagnetic films with transport of electrons in semiconductors adjacent to these films permits to develop novel component base of the microwave or THz nanolectronics with unique characteristics. The most suitable materials for combination in spintonics (magnonics) and electronics beyond CMOS are yttrium-iron-garnet (YIG) and Si. YIG possess very low damping for spin waves propagation. Si has very high mobility of current carriers and wide band gap. Novel technologies in YIG - semiconductor materials in which Si and GaAs are used as substrates for YIG films show possibilities for their integration in beyond CMOS electronics [6]. In this work we study spin waves propagation in a heterostructure consisting of corrugated YIG film grown on gallium-gadolinium-garnet (GGG) substrate attached to Si wafer. By using electric and magnetic control of wave propagation we are able to study linear and nonlinear effects accompanying spin waves propagation. Fig. 1 shows the layout of the investigated structure. The heterostructure was made on the base of YIG film with a thickness of =10 μm and saturation magnetization G grown on GGG substrate. On the surface of the YIG film, the periodic structure was created with the period L =200 μm in the form of grooves with the depth of 1 μm and the width of 100 μm (1D magnonic crystal (MC). To form an MC/SC (semiconductor) type structure on the MC surface, the n-type doped silicon semiconductor wafer (with electrons carrier density N = 3 011 cm-3 and conductivity = 10 10-5 Ohm-1 m-1) was placed. The thickness of SC layer = 500 μm, the length 3 mm, and the width is equal the width of MC =4 mm. Copper contacts with chromium adhesive layer were deposited on the SC surface at the distance 2.5 mm relative to each other. When a control voltage U is applied to the contacts, a directional drift of charges in the SC (EC) takes place. For the excitation and receiving of the magnetostatic spin wave, 30 μm-wide microstrip transducers were used. The transducers were placed on the MC surface at a distance of 9.5 mm relative to each other. An external constant magnetic field was applied parallel to the microstrip transducers, and the magnetostatic surface waves (MSSW) was excited in the MC. In Fig. 2 the amplitude-frequency characteristic (AFC) curves for surface spin waves in MC/SC structure are shown at the input power P0 = 23 dBm and various voltages applied to SC. The strongly pronounced minima in AFC curves respect to frequency band gaps of magnonic crystals (Bragg frequencies): at the absence of the voltage (at U=0 V the central frequency of band gap (BG) - fnl=4.11 GHz), at the applied voltage U=20 V the central frequency of band gap - =4.08 GHz. At the voltage increase the BG central frequency moves to lower values. In Fig. 2 the band gap central frequency is marked fl=4.12 GHz for linear case (P0 = 11 dBm) at U=0 V. It can be seen that for fl&gt;fnl, thus at the increase of the input power the band gap in nonlinearly shifted. We also considered the features of spin wave pulses propagation through the MC/SC structure depending on the electric voltage variation. The nonlinear shift of the band gap permits to form conditions for band gap solitons. We developed the adequate nonlinear theory for such wave propagation. The theoretical results are obtained using the coupled waves method for forward wave and wave reflected from the spatial non-uniformities of periodic structure. The microwave method for measuring amplitude versus frequency data of propagating spin waves was applied. The parameters of these solitons further can be governed by the double electric asnd magnetic control. In particular, the exciting number of solitons, their threshold appearance and propagation velocity can be controlled by the electric current applied to the semiconductor. We have further discovered the effect of the nonlinear switching at which the YIG film with periodic structure can transmit the wave signal through it at the frequencies of the band gap due to high power signal wave. In this case the heterostructure demonstrates the bistability properties and hysteresis appearance at the wave power increase. The obtained results permit to consider heterostructures based on magnonic crystals and semiconductor layers as the possible candidates for integration of magnonics component base into semiconductor electronics architecture. The work was supported by the grant from the Government of the Russian Federation for state support of academic research conducted under the guidance of leading scientists (project no. 075- 15-2019-1874), by the Russian Foundation for Basic Research (grant 19-29-03049) and by the State Task No. 0030-2019-0013.

**References:**

1. V. Chumak, V. I. Vasyuchka, A. A. Serga et al., Nature Physics, VOL. 11, p. 453 (2015). 2. S. A. Nikitov, D. V. Kalyabin, I. V. Lisenkov et al., Phys. Usp. VOL.58, p. 1002 (2015). 3. S. A. Nikitov, A.R. Safin, D. V. Kalyabin, et al., Phys. Usp. VOL.63, p. 945 (2020). 4. C. Felser and G. Fecher, eds., Spintronics: From materials to devices (Springer Netherlands, 2013). 5. A. Sidorenko, ed., Functional Nanostructures and Metamaterials (Springer International Publ., New York, 2018). 6. A.V. Sadovnikov, E.N. Beginin et al., Phys. Rev. B, VOL. 99, p.054424 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fa2045de12b024c91f130704e71963f4.jpg)
Fig. 1. Experimental layout of the MC/SC structure.

![](https://assets.underline.io/uploads/markdown_image/1/image/e0f4289025acdb4865a95ac8c1b0d536.jpg)
Fig. 2. The amplitude-frequency characteristics for MSSW in the MC/SC structure and application of voltage U= 0 kV/cm (green curve), U= 10 V (orange), U= 20 V (red).

 Spin Waves in YIG-Semiconductor Heterostructures

The advent of materials with complex, topologically-stabilized magnetic textures, such as skyrmions, stemming from careful tuning of the Dzyaloshinskii–Moriya interaction, direct light-induced manipulation of spins, or geometric frustration [1], has opened up exciting avenues of research. Understanding the structure and dynamics of, and achieving control over, spin on the scale of the magnetic exchange interaction is both a question of fundamental physics and at the forefront of developing quantum technologies like spintronics and low-energy data transport and memory [2]. Because of the small size of these intricate nanoscale magnetic textures, few to hundreds of nm, and their low contrast, it is quite challenging to image them in 3D directly without prior knowledge of the sample or extensive simulation. They dynamics of such systems can also prove challenging to capture, as some magnetic processes occur on the timescale of twenty femtoseconds or less [3]. To fully understand the static and dynamic electronic, magnetic, and structural properties of complex samples we combine complementary nanoscale imaging and spectroscopy techniques: resonant x-ray magnetic circular dichroism (MCD) provides element-specific magnetic sensitivity; ptychography yields diffraction-limited images with phase contrast, ideal for low-contrast materials; and time-resolved extreme ultraviolet (EUV) spectroscopies can capture atto-to-nanosecond dynamics. By integrating soft x-ray MCD imaging with scalar ptycho-tomography [4, 5] we demonstrate a technique capable of directly imaging the 3D magnetic field within a Ni metalattice material, with no need for prior knowledge or simulation, that is applicable to a wide range of magnetic nanostructured samples.

**References:**

[1] J. E. Han and V. H. Crespi, Phys. Rev. Lett., Vol. 89, p. 197203 (2002) [2] P. Fischer, IEEE Trans. Magn., Vol. 51, p. 0800131 (2015) [3] W. You, P. Tengdin, C. Chen, et. al., Phys. Rev. Lett., Vol. 121, p. 77204 (2018) [4] C. Donnelly, M. Guizar-Sicairos, V. Scagnoli, et al., Nature, Vol. 547, p. 328 (2017) [5] A. Pryor, Y. Yang, A. Rana, et al., Scientific Reports, Vol. 7, p. 10409 (2017)

**Images:**

 3D imaging of topologically stabilized spin textures in nanostructured materials – towards ultrafast imaging of spin transport

Antiferromagnets (AFMs) emerged as a versatile material science platform, which enabled numerous fundamental discoveries including the observation of monopole quasiparticles in frustrated systems and collective quantum effects, such as spin superfluidity and Bose–Einstein condensation of magnetic excitations [1]. Primary advantages of antiferromagnets are their terahertz operating frequencies, the absence of stray fields, magnetic field robustness, all of which result in numerous advantages including those in spintronics and spinorbitronics [2]. The key enabler of those applications is the presence of the Dzyaloshinskii-Moriya interaction (DMI). This, in turn, put stringent requirements on the magnetic symmetry of AFM, which should support weak ferromagnetism and chiral helimagnetism. This makes the portfolio of material systems available for these studies very limited that renders the progress in AFM-related fundamental and technological research to depend on time-consuming material screening and optimization of intrinsic chiral properties of AFMs. The field of curvilinear magnetism is well explored for ferromagnets, where magnetic responses are tailored by local curvatures [3]. By contrast, the topic of curvilinear AFMs is at its infancy [4-9]. The energy landscape of ring AFM and geometrically frustrated chains at non-zero temperature is characterized by a large number of metastable states including long-living noncollinear textures if anisotropy is strong enough [6]. In experiment, curvilinear AFMs are mainly represented by the molecular magnets [7] and metalized DNA molecules [8]. The shape anisotropy stemming from magnetostriction plays the major role in the ordering of the Neel vector in perovskite zig-zag stripes and nanodots determining the easy direction as the parallel or perpendicular to the boundary [9]. In this presentation, we will demonstrate that chiral responses of AFMs can be tailored by a geometrical curvature without the need to adjust material parameters. In a general case, an intrinsically achiral one-dimensional curvilinear AFM spin chain behaves as a chiral helimagnet with geometrically tunable DMI, orientation of the Neel vector and the helimagnetic phase transition, see Fig. 1 [4]. The helix-shaped spin chain possesses two ground states: the so-called homogeneous and periodic ones with respect to the motion along the chain. The energetically favorable state is determined by the direction of the geometry-driven DMI vector. In contrast to ferromagnets, there is no easy axis anisotropy competing with the geometry-driven one. Furthermore, the curvature-induced DMI results in the hybridization of spin wave modes. The low-frequency branch is gapless for straight chains and possesses the gap for any finite curvature. In addition, the DMI enables a geometrically-driven local minimum of the low frequency branch which increases for larger curvature and torsion, see Fig. 2. This opens exciting perspectives to study long-lived collective magnon states in AFMs. These findings position curvilinear 1D antiferromagnets as a novel platform for the realization of geometrically tunable chiral antiferromagnets for antiferromagnetic spinorbitronics and fundamental discoveries in the formation of coherent magnon condensates in the momentum space. The proposed description of vector fields living at curvilinear geometries can be applied for other systems with complex order parameters, such as ferroelectrics [10] or liquid crystals [11].

**References:**

[1] C. Castelnovo, R. Moessner, S. L. Sondhi, Nature, Vol. 451, p. 42 (2008); N. Nagaosa, Y. Tokura, Phys. Scr. Vol. T146, p. 014020 (2012); A. A. Zvyagin, Low Temp. Phys. Vol. 39, p. 901 (2013) [2] T. Jungwirth, J. Sinova, A. Manchon et al, Nat. Phys. Vol. 14, p. 200 (2018); V. Baltz, A. Manchon, M. Tsoi et al, Rev. Mod. Phys. Vol. 90, p. 015005 (2018) [3] R. Streubel, P. Fischer, F. Kronast et al, J. Phys. D: Appl. Phys. Vol. 49, p. 363001 (2016); A. Fernandez-Pacheko, R. Streubel, O. Fruchart et al, Nat. Comm. Vol. 8, p. 15756 (2016); D. D. Sheka, O. V. Pylypovskyi, P. Landeros et al., Comm. Phys. 3, 128 (2020) [4] O. V. Pylypovskyi, D. Y. Kononenko, K. V. Yershov et al, Nano Lett. Vol. 20, p. 8157 (2020) [5] K. K. Nanda, A. W. Addison, E. Sinn et al, Inorg. Chem. Vol 35, p. 5966 (1996); O. Cador, D. Gatteschi, R. Sessoli et al, Ang. Chem. Int. Ed. vol 43, p. 5196 (2004); M. Grzelczak, M. Correa-Duarte, V. Salgueirino-Maceira et al, Ang. Chem. Int. Ed. Vol. 46, p. 7026 (2007); T. Guidi, B. Gillon, S. A. Mason et al, Nat. Comm. Vol. 6, p. 7061 (2015) [6] S. Castillo-Sepulveda, R. A. Escobar, D. Altbir et al, Phys. Rev. B, Vol. 96, p. 024426 (2017) [7] S. J. Blundell, F. L. Pratt, J. Phys.: Cond. Matt. Vol. 16, p. R71 (2004) [8] K. Mizoguchi, S. Tanaka, M. Ojima et al, J. Phys. Soc. Jap. Vol. 76, p. 043801 (2007) [9] A. D. Bang, I. Hallsteinsen, R. V. Chopdekar et al, Appl. Phys. Lett. Vol. 115, p. 112403 (2019); M. S. Lee, P. Lyu, R. V. Chopdekar et al, J. Appl. Phys. Vol. 127, p. 203901 (2020) [10] M. Owczarek, K. A. Hujsak, D. P. Ferris et al, Nat. Comm. Vol. 7, p. 13108 (2016) [11] G. Napoli, L. Vergori, Phys. Rev. Lett. Vol. 108, p. 207803 (2012)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ae6c8e5f4d9443f9e0440eb83667f104.jpg)
Fig. 1. A general AFM spin chain and the helimagnetic phase transition of helix-shaped spin chain. The curvature-driven DMI and anisotropy modify the magnetic response as follows. The DMI is represented by vector d, while the equilibrium hard and easy axes for the Neel vectors are shown by e1 and e3, respectively. Adapted with permission from [4]. Copyright (2020) American Chemical Society.

![](https://assets.underline.io/uploads/markdown_image/1/image/0eef10e98fc40c90e95903e759da7ae1.jpg)
Fig. 2. The spin wave dispersion of the helix-shaped AFM spin chain and the respective depth δ of the local minimum at the dispersion curve. Adapted with permission from [4]. Copyright (2020) American Chemical Society.

 Curvilinear antiferromagnetism: Current state and perspectives INVITED

Spontaneous symmetry breaking is an ubiquitous ingredient in physics. It governs second order phase transitions whose statics are well described within the universal Ginzburg-Landau (GL) theory, as confirmed by a number of observations. However, the picture for the corresponding dynamics is less clear. Indeed, in the ordered phase, GL theory predicts two different low-energy excitations: the zero frequency Goldstone mode, corresponding to an oscillation of the phase of the order parameter, and the non-zero-frequency Higgs mode, corresponding to an oscillation of its amplitude.[1] In practice, the amplitude mode has remained very difficult to observe in condensed matter, either because it does not couple to the experimental probes or because it decays too fast into other types of excitations.[1] In this communication, we show that an archetypal micromagnetic texture, the so-called weak magnetic stripes,[2] constitutes an ideal platform to investigate such dynamics. Combining Brillouin light scattering with micromagnetic simulations and analytical modeling, we evidence spin wave features that can be directly identified to Goldstone/Higgs modes. The system is a 180nm thick amorphous CoFeB ferromagnetic magnetic film exhibiting a small perpendicular magnetic anisotropy K.[3] Under a strong enough in-plane magnetic field H, the film is homogeneously saturated. However, its spin-wave spectrum already displays a characteristic feature precursor of the nucleation of an inhomogeneous magnetic texture. This is shown in figure 1b, which displays a color plot of the spin-wave dispersion simulated in the Damon Eshbach configuration (wave-vector k⊥H). One recognizes a low frequency branch with a minimum around kc=21 rad/µm. Upon reducing the field, this minimum shifts to lower frequency and eventually reaches the horizontal axis (Fig. 1c), which is the behavior expected in the GL description: Upon approaching the critical field Hc, the curvature of the magnetic energy along a certain direction in the configuration space decreases to zero, which is associated to the softening of the corresponding normal mode. The mode profile at this point is shown in Fig. 1d: one recognizes an undulation of the vertical component of the magnetization in the middle of the film, together with quadrature undulations of the in-plane component of the magnetization on the top and bottom surfaces insuring magnetic flux closure. This pattern, and the critical fields and wavelength, correspond precisely to the weak magnetic stripes observed in this system. [3]. Indeed, upon further decreasing the field, the nucleation proceeds as a freeze of the soft spin-wave mode in a state with a given phase (or equivalently a given lateral position within the period), which constitutes the broken symmetry. Concomitantly, the characteristic cusp of the spin wave spectrum splits in two: One branch remains soft and a second one emerges with a gradual increase of its minimum frequency (Fig. 2a,b). Inspecting the spatial profile of these two modes (Fig. 2c,d) allows one to recognize them unambiguously as a Goldstone mode (rigid lateral oscillation of the stripe texture) and a Higgs mode (oscillation of the amplitude of the texture around its equilibrium value), respectively. We probe these modes experimentally using Brillouin light scattering onto thermally excited spin waves. The symbols in Fig.1b,c and Fig.2b show the position of the measured peaks. Although the limited wave-vector / frequency range accessible (&lt;20rad/µm, &gt;2GHz) does not allow to cover all the way down to the cusp, the agreement with the simulations is good. In particular, the measured dispersions display the very characteristic features predicted, namely the negative slopes (in the k⊥H configuration this can only happen due to the perpendicular anisotropy), the critical softening and the subsequent splitting in the broken symmetry phase. These essential features are visible around Hc and kc, which explains why they have been largely overlooked by the magnetism community, as previous dynamic studies of stripe domains had focused onto the k≈0 (ferromagnetic resonance) and/or deep supercritical (near remanence) regimes.[4,5] To support this identification, we shall present a Ginzburg Landau theory of weak stripe statics based on a stray-field-free ansatz [2,6] and extend it to the dynamics using the dipole-exchange spin wave formalism.[7] We shall also discuss the observability of these characteristic spin-wave modes for different measurements and compare them with Higgs/Goldstone modes occurring in other condensed-matter systems at completely different length/energy scales.[1,8]

**References:**

[1] D. Pekker and C.M. Varma, Annu. Rev. Condens. Matter Phys. 6, 269 (2015) [2] A. Hubert and R. Schäfer, Magnetic Domains, Springer (1998) [3] K. Ait Oukaci et al., Phys. Rev. Appl. 14, 024083 (2020) [4] N. Vukadinovic et al., Phys. Rev. Lett. 85, 2819 (2000) [5] C. Banerjee et al., Phys. Rev. B 96, 024421 (2017) [6] A. Hubert, Phys. Stat. Sol. 32, 519 (1969) [7] M. Grassi et al., Phys. Rev. Appl. 14, 024047 (2020) [8] Matsumoto M., J. Phys.: Conf. Ser. 592, 012123 (2015)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2f55dbdb13b46101fcd7656ce0220724.jpg)
Figure 1: Spin wave dispersions above Hc. (a) Geometry of the system. (b) Color plot: amplitude spectral density simulated for a 180nm thick CoFeB film in the Damon-Eshbach configuration (k⊥H) under a saturating in-plane field µ0H=30mT. Symbols: measured dispersion. (c) Idem at the critical field µ0Hc=14mT. (d) Vector plot of the dynamic magnetization distribution at the softening point [(kc,0) in panel c]. The first order magnetic anisotropy constant, saturation magnetization and exchange constant are K=36kJ/m3, µ0Ms=1.67T and A=16pJ/m, respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/8a3afb52035833ea4a8fcfc84a4b58ea.jpg)
Figure 2: Spin-wave dispersion below Hc (a) Simulated amplitude spectral density at µ0H=12mT. (b) Idem at µ0H=7mT. The symbols show the measured dispersions. (c) Color plot of the vertical component of the dynamic magnetization for the Goldstone mode superimposed onto the nucleated weak stripe texture. (g) Same for the Higgs mode.

Strongly nonlinear ferromagnetic resonance of Bi-doped YIG nanodisks

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Strongly nonlinear ferromagnetic resonance of Bi-doped YIG nanodisks

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