France



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.&lt;br/&gt;&lt;br/&gt;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].&lt;br/&gt;&lt;br/&gt;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.&lt;br/&gt;&lt;br/&gt;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.&lt;br/&gt;&lt;br/&gt;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)&lt;br/&gt;[2] M. Pomerantz, Phys. Rev. Lett. 7, 312 (1961)&lt;br/&gt;[3] J. Holanda, D. S. Maior, A. Azevedo et al., Nat. Phys. 14, 500 (2018)&lt;br/&gt;[4] K. An, A.N. Litvinenko, R. Kohno et al., Phys. Rev. B 101, 060407(R) (2020)&lt;br/&gt;[5] M. Harder, Y. Yang, B. M. Yao et al., Phys. Rev. Lett. 121, 137203 (2018)&lt;br/&gt;[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.



INTERMAG 2021

 Long range coupling of magnetic bilayers by coherent phonons

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AAMAS (International Conference on Autonomous Agents and Multiagent Systems) is the largest and most influential conference in the area of agents and multiagent systems. The aim of the conference is to bring together researchers and practitioners in all areas of agent technology and to provide a single, high-profile, internationally renowned forum for research in the theory and practice of autonomous agents and multiagent systems. AAMAS is the flagship conference of the non-profit International Foundation for Autonomous Agents and Multiagent Systems (IFAAMAS).

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.

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Frustrated systems with two or more competing interactions have a wide variety of phases with unconventional properties and complex symmetry. The pseudospin formalism is a powerful method to study complex magnetic systems with various degrees of freedom. Pseudospin models of anisotropic magnets are widely used to describe the properties of binary alloys, classical and quantum liquids, dilute magnets, superconductors, and many other physical systems [1]. In addition, disorder caused by introduction of impurities and defects into the system significantly affects the critical behavior and phase states of magnets [2]. This work is devoted to modelling and theoretical description of phase and critical properties of a low-dimensional site-dilute magnet with frustration caused by the competitive charge and spin orders. We consider the Blume-Emery-Griffiths-type spin-pseudospin model, which was initially proposed [3] to consider the competition between spin and charge orders in underdoped high-Tc cuprates like La2-xSrxCuO4 in the normal state. The model considers the CuO2 planes as a system of charge triplets consisting of three many-electron mixed-valence centers [CuO4]7-;6-;5- (corresponding nominally to the copper ion states Cu1+;2+;3+). Two charge states Cu1+;3+ are associated with two components of the pseudospin S=1 triplet with MS=±1. The Cu2+ state associated with the MS=0 component is magnetic and has a conventional spin σ=1/2. Within this pseudospin formalism, the Hamiltonian of the static spin-pseudospin model includes Ising spin exchange coupling (J), on-site (Δ) and inter-site (V) density-density correlations, which have the form of single-ion pseudospin anisotropy and Ising exchange coupling in the language of pseudospin operators. The chemical potential μ is necessary to take into account the constant charge density doped in the CuO2 plane, which gives the concentration of doped charged impurities n. The properties of the spin-pseudospin model in the ground state were studied with the mean-field approximation [3,4], and the temperature phase diagrams were obtained with the Bethe approximation [5]. We performed a computer simulation of a spin-pseudospin model using the classical Monte Carlo method and a modified Metropolis algorithm on a 2D square lattice with periodic boundary conditions. Phase transitions and critical properties of a dilute magnet with frustration were investigated in the framework of the theory of finite-size scaling. The system demonstrates the 2D Ising universality class as it is far from frustration, while as the system approaches the frustrated state, significant changes are observed in the critical behavior of the system: spin (charge) metastable phase states are formed, first-order phase transitions occur, and the critical exponents for second-order phase transitions depend on the parameters of the model, i.e. the critical behavior of the system becomes non-universal (for example, see Fig. 1) [6]. Also, the critical behavior becomes non-universal with an increase in the density of the doped charge n in the non-frustrated state. In addition, temperature and ground state phase diagrams were obtained, and the features of the formation of phase states with an increase in the density of doped impurities in frustration were revealed. There is a wide variety of phase states with different properties in the ground state. If the spin exchange coupling is greater than the one for pseudospin (V/J≤1), there are actually two phases in the ground state, namely the charge order (CO) and the antiferromagnetic (AFM) order, which is unstable with respect to the phase separation into charge and spin subsystems that behave like immiscible quantum liquids. Various configurations of phase separation are formed depending on the value of n, viz., a charge/AFM droplet or a charge/AFM stripe. There are 5 phases in the ground state for V/J&gt;1: AFM and “ferromagnetic” orders, and three types of CO. Analysis of the system in frustration has shown that the ground state of the system turns out to be degenerate not at one specific point, but in a whole range of parameters. Under identical calculation conditions, both AFM order and CO can form with a certain probability. Charge doping in a frustrated state leads to unusual effects. When the spin exchange coupling is strong (V/J≤1), AFM order changes to CO with decreasing temperature (or vice versa, depending on the value of n), as well as reentrant order-order phase transitions [7] occur, which is shown in Fig. 2. Thus, the competition between charge and magnetic orders, leading to frustration of the system, as well as charged impurities, significantly affect the properties of a two-dimensional Ising magnet. This work was prepared with the financial support of Competitiveness Enhancement Program - CEP 3.1.1.1-20.

**References:**

[1] Hovhannisyan V. V. et al., Physical Review E., Vol. 96, p. 062103 (2017) [2] Giacomin G., Springer, Vol. 2025 (2011) [3] Y. D. Panov et al., J. Supercond. Novel Magn., Vol. 29, p. 1077 (2016). [4] Y. D. Panov et al., J. Low Temp. Phys, Vol. 187, p. 646 (2017). [5] Y. D. Panov et al., JMMM, Vol. 477, p. 162 (2019). [6] D. N. Yasinskaya et al., Acta Phys. Pol. A., Vol. 137, p. 979 (2020) [7] D. N. Yasinskaya, V. A. Ulitko, Y. D. Panov, Phys. Solid State, Vol. 62, p. 1713 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fc9010fa0e9f0f0bf9f53632bb55fb8e.jpg)
Dependence of critical exponents for heat capacity and susceptibility on single-ion anisotropy Δ/J for V/J=0.2, n=0. Dark gray corresponds to the universality class of the 2D Ising model, light gray corresponds to non-universal critical behavior. White color corresponds to first-order phase transitions

![](https://assets.underline.io/uploads/markdown_image/1/image/33dac2d32c21ee256bf53ecc248d2123.jpg)
Temperature phase diagram for n = 0.15, V/J=1 near the frustration. The order type changes twice when the temperature decreases, i.e. the reentrant phase transition to AFM takes place

 Phase States and Critical Properties of a Dilute Magnet with Frustration



A common feature of many organic-based molecular magnets is that inter-molecular magnetic interactions are extremely weak compared to intra-molecular ones. As a result a bulk sample can be described in terms of independent individual molecular magnets made up of a finite cluster of coupled magnetic spins. Because of the large value of individual spins of several molecular magnetic compounds it turns out that such systems can be described to very high accuracy, at sufficiently high temperatures, by a classical Heisenberg spin model. Only for sufficiently low temperatures need one incorporate the quantum character of the spins. In this work we focus on the exact spin dynamics of small nanoscale molecular magnetic clusters of classical Heisenberg spins. The present study permits us to obtain analytical solutions for the time-dependent spin-spin autocorrelation function for certain clusters of classical Heisenberg spins coupled through exchange interaction. This study underscores the potential utility of a classical spin treatment of nanoscale magnetic clusters and provides results useful to interpret data obtained from neutron scattering experiments.

**References:**



**Images:**





 Exact Spin Dynamics of High Spin Nanoscale Molecular Magnetic Clusters

INTRODUCTION Phosphate fluoride-based cathode material is attracting great interest in secondary batteries due to its superior properties compared to phosphate-based cathode [1, 2]. The theoretical discharge capacity and redox voltage of LiFePO4F are 152 mAh/g and 2.75 V. In the case of LiFePO4F, unlike other intercalation cathode materials, stars in a state of charge and then discharge to form Li2FePO4F. In this paper, we synthesized the tavorite-structured LiFePO4F by two-step solid state reaction method with different molar ratios of LiF. We report on the structural and magnetic properties of LiFePO4F, especially as studied by Mössbauer spectroscopy. EXPERIMENT PROCEDURES The tavorite LiFePO4F cathode was synthesized by two-step solid state reaction method. First, the FePO4 was obtained using stoichiometric mixture of Fe2O3 and (NH4)2HPO4 as starting materials. The mixture was calcined at 300 oC for 7 h in air and sintered at 870 oC for 8 h in air. After that, the obtained FePO4 and LiF were mixed at different mole ratios of 1:1, 1:1.05, and 1:1.1, respectively. The mixture pressed into pellets and sintered at 625 oC for 1.5 h. The crystal structure of sample was determined with X-ray diffraction measurements (XRD) using Rigaku Ultima IV using Cu/Kα radiation. The magnetic properties of sample was measured by using a vibrating sample magnetometer (VSM) at various temperature. Mössbauer spectra were recorded from 4.2 and 295 K using a in transmission geometry employing a 57Co source and fitted by a least square fit programmed assuming Lorentzian line shapes. RESULTS AND DISCUSSION The XRD patterns of FePO4, which was pre-synthesized to make LiFePO4F, were confirmed the formation of hexagonal structure with P3121. The XRD patterns of LiFePO4F synthesized from above FePO4 and excess LiF were measured. Secondary phases from XRD patterns were found in the sample synthesized with LiF mole ratios of 1 and 1.1. The crystal structure of the refined LiFePO4F (LiF-excess 5 %) was determined to be triclinic with P space group and the lattice constants were a = 5.2973 Å, b = 7.2539 Å, c = 5.1506 Å, α = 107.92 °, β = 98.52 °, and γ = 107.36 ° (Fig. 1). The temperature dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization susceptibility χ(T) curves were measured at 100 Oe in the temperature range from 4.2 to 295 K. From ZFC-FC curve, the magnetic property of LiFePO4F showed antiferromagnetic behavior below the Néel temperature. The Mössbauer spectra of the LiFePO4F were obtained at various temperatures ranging from 4.2 to 295 K. The Mössbauer spectrum at 295 K was analyzed symmetric doublet with the electric quadrupole splitting of 1.10 mm/s and the isomer shift of 0.31 mm/s and, which is Fe3+ state (Fig. 2). Since LiFePO4F has two crystallographically unique Fe sites, Mössbauer for each temperature was measured to investigate detailed magnetic properties. The spectrum at 4.2 K were confirmed had two set of eight asymmetric line indicating that they have both magnetic dipole interaction and electric quadrupole interaction. The value of isomer shift at all temperature was analyzed as Fe3+ ions.

**References:**

[1] Y. Zhang, Q. Liang, C. Huang, et al., J. Solid State Electr., Vol. 22, p.1995 (2018). [2] N. Goubard-Bretesché, E. Kemnitz, and Nicola Pinna, Chem. Eur. J., Vol. 25, p.6189 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e039bb5160e298b54b22d9ae7af11ced.jpg)
Fig. 1. Refined XRD patterns of tavorite LiFePO4F.

![](https://assets.underline.io/uploads/markdown_image/1/image/16da341514aafef163ff9faa21efe26d.jpg)
Fig. 2. Mössbauer spectrum of tavorite LiFePO4F at 295 K.

 Synthesis, structural, and Mössbauer studies of tavorite LiFePO4F cathode material

At present, the study of physical properties of magnetic quasi-low-dimensional compounds intensively developed. Oxyborates with ludwigites structure are the members of such type of compounds due to the presence of quasi-two-dimensional elements in their structure. The unit cell of ludwigites includes 4 non-equivalent positions which can be occupied by magnetic cations. The magnetization reversal is occur in (Ni,Mn)3BO5 [1]. The exchange bias effects is occur in Ni5GeB2O10[2]. In this work, single crystals of (Ni,Cu)2MnBO5 with ludwigites structures were synthesized by flux method, using the fluxes based on bismuth trimolybdat. Structural and magnetic characterization of the samples was carried out. Magnetic transition occurs at the temperature at around 75 K (Fig. 1). M-H curves measured at 10 K are presented on Figure 2. Indirect exchange interactions calculation was held for the compound (Ni,Cu)2MnBO5 in frameworks of the Anderson-Zavadskiy indirect exchange interactions model. Calculation results showed that ferromagnetic and atiferromagnetic interaction competition presents between ions in plane and ions connected planes. Acknowledgments The research was funded by RFBR, Krasnoyarsk Territory and Krasnoyarsk Regional Fund of Science, project number 20-42-240011

**References:**

[1] L.N. Bezmaternykh, E.M. Kolesnikova, E.V. Eremin, et al. J. Magn. Magn. Mater. 364,55 (2014) [2] S.N. Sofronova, L.N. Bezmaternykh, E.V. Eremin,et al. J. Magn. Magn. Mater. 401, 217 (2016)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ffa2a493700f5078c7de96b9ab59e675.jpg)
Fig.1. The magnetization in the zero-field-cooled (ZFC), field-cooled (FC), and field-cooled-heating (FH) conditions measured at H=1 kOe.

![](https://assets.underline.io/uploads/markdown_image/1/image/2cfe810f1d7cb15d0679f2c6f1f3c436.jpg)
Fig. 2. M-H hysteresis loops measured at 10 K.

 Magnetic Properties of Quasi-Two-Dimensional Oxyborates (Ni,Cu)2MnBO5 with Ludwigite Structure



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

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).

Long range coupling of magnetic bilayers by coherent phonons

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Long range coupling of magnetic bilayers by coherent phonons

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