Magnon-polaron transport in magnetic insulators

Flebus, Benedetta; Shen, Ka; Kikkawa, Takashi; Uchida, K.; Qiu, Zhiyong; Saitoh, Eiji; Duine, R. A.; Bauer, G.

doi:10.1103/physrevb.95.144420

Cited by 113 publications

(155 citation statements)

References 33 publications

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“…To change delta, we fix f ζ = 1.09 and vary f σ . The value for f ζ is comparable to the enhancement in ζ calculated from theory for low temperatures 10 . Figure 6 shows a comparison between the distance dependence of V 0 nlSSE and V TA obtained from experiments ( Fig.…”

Section: Modellingsupporting

confidence: 80%

“…More experiments including thermal transport as well as an extension of the Boltzmann treatment presented in Ref. 10 to 2D geometries are necessary to fully come to grips with heat and spin transport in YIG.…”

Section: Discussionmentioning

confidence: 99%

“…We model the effect of the magnon-phonon hybridization by assuming a field-dependent magnon spin conductivity σ m (H) and bulk spin Seebeck coefficient ζ(H), which are both enhanced at the resonant field H TA . Note that the field-dependence only includes the contribution from the magnon-polarons 10 , and does not include the effect of magnons being frozen out by the magnetic field 24,44-46 since this is not the focus of this study. The parameter values used in the model are given in the Methods section of this paper.…”

Section: Modellingmentioning

confidence: 99%

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Nonlocal magnon-polaron transport in yttrium iron garnet

et al. 2017

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The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal bilayers as an inverse spin Hall effect voltage under a temperature gradient. The SSE can be detected nonlocally as well, viz. in terms of the voltage in a second metallic contact (detector) on the magnetic film, spatially separated from the first contact that is used to apply the temperature bias (injector). Magnon-polarons are hybridized lattice and spin waves in magnetic materials, generated by the magnetoelastic interaction. Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] interpreted a resonant enhancement of the local SSE in yttrium iron garnet (YIG) as a function of the magnetic field in terms of magnon-polaron formation. Here we report the observation of magnon-polarons in nonlocal magnon spin injection/detection devices for various injector-detector spacings and sample temperatures. Unexpectedly, we find that the magnon-polaron resonances can suppress rather than enhance the nonlocal SSE. Using finite element modelling we explain our observations as a competition between the SSE and spin diffusion in YIG. These results give unprecedented insights into the magnon-phonon interaction in a key magnetic material.When sound travels through a magnet the local distortions of the lattice exert torques on the magnetic order due to the magnetoelastic coupling 1 . By reciprocity, spin waves in a magnet affect the lattice dynamics. The coupling between spin and lattice waves (magnons and phonons) has been intensively researched in the last half century 2,3 . Yttrium iron garnet (YIG) has been a singularly useful material here, because it can be grown with exceptional magnetic and acoustic quality 2 . Magnons and phonons hybridize at the (anti)crossing of their dispersion relations, a regime that has attracted recent attention [4][5][6][7][8][9][10] . When the quasiparticle lifetimebroadening is smaller than the interaction strength, the strong coupling regime is reached; the resulting fully mixed quasiparticles have been referred to as magnonpolarons 6,7 .In spite of the long history and ubiquity of the magnonphonon interaction, it still leads to surprises. Evidence of a sizeable magnetoelastic coupling in YIG was recently found in experiments on spin caloritronic effects, i.e. the spin Peltier 11 and spin Seebeck effect 12,13 (SPE and SSE respectively). Recently, Kikkawa et al. showed that the hybridization of magnons and phonons can lead to a resonant enhancement of the local SSE in YIG 9 . Bozhko et al. found that this hybridization can play a role in the thermalization of parametrically excited magnons using Brillouin light scattering. They observed an accumulation of magnon-polarons in the spectral region near the anticrossing between the magnon and transverse acoustic phonon modes 14 . However, these previous experiments did not address the transport properties of magnon-polarons.Nonlocal spin injection and detection experiments are of great importance in probing the transport of spin in metals 15 , semiconductors 16 and graphene 1...

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Section: Modellingsupporting

confidence: 80%

Section: Discussionmentioning

confidence: 99%

Section: Modellingmentioning

confidence: 99%

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Nonlocal magnon-polaron transport in yttrium iron garnet

et al. 2017

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“…Furthermore, it ignores the effects of Kapitza resistances, which are important to the interpretations of the results. Next, 21 we compare the temperature dependences shown in Fig. S1 and S2 to the thermal conductivity of YIG and GGG, which we measured on bulk samples and reproduce in Fig.…”

Section: Supplementary Materialsmentioning

confidence: 99%

Evidence for the role of the magnon energy relaxation length in the spin Seebeck effect

et al. 2018

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Temperature-dependent spin-Seebeck effect data on Pt|YIG (Y3Fe5O12)|GGG (Gd3Ga5O12) are reported for YIG films of various thicknesses. The effect is reported as a spin-Seebeck resistivity (SSR), the inverse spin-Hall field divided by the heat flux, to circumvent uncertainties about temperature gradients inside the films. The SSR is a non-monotonic function of YIG thickness. A diffusive model for magnon transport demonstrates how these data give evidence for the existence of two distinct length scales in thermal spin transport, a spin diffusion length and a magnon energy relaxation length. 2Since the discovery of the (longitudinal) spin-Seebeck effect (SSE) 1 , much work has been done to identify the length scales involved in the phenomenon. 2 Using nonlocal detection, it has been shown that relaxation of thermal magnons in YIG is governed by a spin diffusion length. 3The latter is reported to be around 10 μm, and, in some studies, increases to up to 70 μm at low temperatures. 4,5 . This has led to a consensus that the micron-scale dependence of SSE observed in planar geometries corresponds to the generation and accumulation of the nonequilibrium magnondensity gradients in the bulk. These experiments have been modeled theoretically in terms of magnon spin transport only, while assuming that the magnon-phonon processes leading to the relaxation of the magnon energy occur over very short length scales; hence, their effects can be disregarded. 3,5 Nevertheless, it is clear that magnon energy relaxation mechanisms by the phononic environment must be invoked generally for a complete understanding of thermal spin transport, and particularly for the physics underlying the SSE. Indeed, while heaters and thermometers couple to phonons, these must in turn couple to magnons in order to give rise to the SSE in a magnon-based system like YIG. These relaxation processes can be parameterized by the length over which magnon-to-phonon thermalization occurs, and the latter is expected to be a much smaller length scale than magnon spin-diffusion lengths; a theoretical argument can be found in Ref. [6]. Early work 7 defines an energy relaxation length similar to the one invoked here, but without quantifying it. Additionally, phonon-magnon drag has been put into evidence in previous SSE experiments 8,9 , which, again, points to the importance of interactions between magnons and phonons. To the best of our knowledge, no explicit evidence for the effect of this length scale on SSE measurements has been reported to date.Previous articles on thin films using various growth techniques 10,11,12 have shown the SSE signal to increase with increasing YIG film thickness. In this study, we grow a series of 3 Pt|YIG|GGG heterostructures, with YIG thickness varying from 10 nm to 1 μm, using the same growth technique for all films. We measure the temperature-dependent spin-Seebeck effect on these structures and of bulk single-crystal Pt|YIG. The spin-Seebeck signal increases for film thicknesses from 10 to 250 nm and again for the bulk Y...

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“…When the applied field is larger than the critical field, the magnon dispersoin is separated from the TA/LA phonon modes. In the theory of hybrid magnon-phonon excitations [6,7], the SSE anomalies occur at magnetic fields and wave vectors at which the phonon dispersion curves are tangents to the magnon dispersion, where the effects of the magnon-phonon coupling are maximized [40].…”

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confidence: 99%

Direct observation of magnon-phonon coupling in yttrium iron garnet

Man

Shi

et al. 2017

Phys. Rev. B

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The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of ∼560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.Spin waves (magnons) and phonons are propagating disturbance of the ordered magnetic moment and lattice vibrations, respectively. They constitute two fundamental quasiparticles in a solid and can couple together to form a hybrid quasiparticle [1,2]. Since our current understandings of these quasiparticles are based on linearized models that ignore all the high-order terms than quadratic terms and neglect interactions among the quasiparticle themselves [3], magnons and phonons are believed to be stable and unlikely to interact and breakdown for most purposes [4]. Therefore, discovering and understanding how the otherwise stable magnons and phonons can couple and interact with each other to influence the electronic properties of solids are one of the central themes in modern condensed matter physics.In general, spin-lattice (magnon-phonon) coupling can modify magnon in two different ways. First, the static lattice distortion induced by the magnetic order may affect the anisotropy of magnon exchange couplings, as seen in the spin waves of iron pnictides with large inplane magnetic exchange anisotropy [5]. Second, the dynamic lattice vibrations interact with time-dependent spin waves may give rise to significant magnon-phonon coupling [6,7]. One possible consequence of such coupling is to create energy gaps in the magnon dispersion at the nominal intersections of the magnon and phonon modes [8,9], as seen in antiferromagnet (Y,Lu)MnO 3 [10]. Alternatively, magnon-phonon coupling may give rise to spin-wave broadening at the magnon-phonon crossing points [11]. In both cases, we expect the integrated intensity of hybridized excitations at the intersecting points to be the sum of separate magnon and phonon scattering intensity without spin-lattice coupling [8]. Finally, if magnon and phonon lifetime-broadening is smaller than their interaction strength, the resulting mixed quasiparticles can form magnon polarons [6,7].Here we use inelastic neutron scattering to study lowenergy ferromagnetic magnons and acoustic phonons in the ferrimagnetic insulator yttrium iron garnet (YIG) with chemical formula -14]. At zero field and 100 K, we confir...

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Magnon-polaron transport in magnetic insulators

Cited by 113 publications

References 33 publications

Nonlocal magnon-polaron transport in yttrium iron garnet

Nonlocal magnon-polaron transport in yttrium iron garnet

Evidence for the role of the magnon energy relaxation length in the spin Seebeck effect

Direct observation of magnon-phonon coupling in yttrium iron garnet

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