Microscopic spin-wave theory for yttrium-iron garnet films

Kreisel, Andreas; Sauli, Francesca; Bartosch, Lorenz; Kopietz, Peter

doi:10.1140/epjb/e2009-00279-y

Cited by 76 publications

(96 citation statements)

References 29 publications

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“…Therefore, the uniform mode can then in principle decay by splitting into magnons at the band minima assuming the kinematic requirements are met. Such magnon splitting through dipolar effects have been extensively studied in the similar compound YIG 70,[72][73][74] . Theoretical treatments which include dipolar effects are needed to determine if such effects can account for the observed decay of the uniform mode in Cu 2 OSeO 3 .…”

Section: Thz Dynamics Of the Uniform Modementioning

confidence: 99%

Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3

et al. 2017

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In this work, we present a comprehensive study of the low energy optical magnetic response of the skyrmionic Mott insulator Cu2OSeO3 via high resolution time-domain THz spectroscopy. In zero field, a new magnetic excitation (f0 = 2.03 THz) which has not been predicted by spin-wave theory is observed and shown, with accompanying time-of-flight neutron scattering experiments, to be a zone folded magnon from the R to Γ points of the Brillouin zone. Highly sensitive polarimetry experiments performed in weak magnetic fields, µ0H < 200 mT, observe Faraday and Kerr rotations which are proportional to the sample magnetization, allowing for optical detection of the skyrmion phase and construction of a magnetic phase diagram. From these measurements, we extract a critical exponent of β = 0.35 ± 0.04, in good agreement with the expected value for the 3D Heisenberg universality class of β = 0.367. In large magnetic fields, µ0H > 5 T, we observe the magnetically active uniform mode of the ferrimagnetic field polarized phase whose dynamics as a function of field and temperature are studied. In addition to extracting a g eff = 2.08 ± 0.03, we observe the uniform mode to decay through a non-Gilbert damping mechanism and to possesses a finite spontaneous decay rate, Γ0 ≈ 25 GHz, in the zero temperature limit. Our observations are attributed to Dzyaloshinkii-Moriya interactions, which have been proposed to be exceptionally strong in Cu2OSeO3 and are expected to impact the low energy magnetic response of such chiral magnets.

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Section: Thz Dynamics Of the Uniform Modementioning

confidence: 99%

Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3

et al. 2017

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“…The in-plane dispersion of spin waves in an infinite slab was treated long ago by Damon and Eshbach [3][4][5][6] where two principal modes were identified: a surface wave, now termed the DamonEshbach (DE) mode, and a so called backward volume (BV) mode. Here we report measurements of the variation of the velocity with wavevector resulting from an adjacent copper film for the DE mode in an insulating ferromagnet, yttrium iron garnet (YIG).…”

Section: Introductionmentioning

confidence: 99%

Effects of an adjacent metal surface on spin wave propagation

et al. 2018

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We experimentally investigate and model the effects of a copper surface adjacent to a surface on which spin waves propagate in a thin film of yttrium iron garnet (YIG). Investigation was performed using a phase detection method, which can map out the spin wave velocity as a function wavevector for small wavevector with high resolution. This velocity is in good agreement with a simple model and allows for extraction of the separation between the YIG film and the copper.

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“…The classical description is justified because the relevant magnon energies are much smaller than room temperature where the experiments are performed. Moreover, the saturation magnetization of YIG is rather large so that the spins can be approximated by three-component classical vectors S i (t) of length S ≈ 14 which are localized at the sites R i of a cubic lattice [32,33]. The coupling of the spins to the thermal phonon bath gives rise to randomness and dissipation.…”

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“…The fields are assumed to be oriented parallel to the longest axis of the stripe, which we identify with the z-axis. Aligning the x-axis perpendicular to the stripe, the wave-vectors of the inplane magnons are of the form k = k y e y + k z e z = |k| sin θ k e y + |k| cos θ k e z and the corresponding longwavelength magnon dispersion in the absence of the pumping field is [33,35] …”

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“…Here the energy ∆ = 4πµM S is determined by the saturation magnetization [33,34,45] 4πM S = 1750 G, the spin stiffness is given by ρ s = JSa 2 = 5.17 × 10 −9 Oe cm 2 × µ, and the form factor is [33,46] sion assumes then its minimal value E min = 2π ×4.9 GHz for k = ±k min e z with k min = 4.999 × 10 4 cm −1 . To fix the spin-phonon interaction and hence the damping kernel G ij (t, t ) we need the effective ionic mass [34,35,47] M = 5.17 g × a 3 /cm 3 , the velocities of the longitudinal and the transverse phonons, c = 7.209 × 10 5 cm/s, c ⊥ = 3.843 × 10 5 cm/s, and the magnetoelastic coupling tensor, which for the cubic lattice is of the form B αβ = δ αβ B + (1 − δ αβ )B ⊥ .…”

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Rayleigh-Jeans Condensation of Pumped Magnons in Thin-Film Ferromagnets

Rückriegel

Kopietz

2015

Phys. Rev. Lett.

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We show that the formation of a magnon condensate in thin ferromagnetic films can be explained within the framework of a classical stochastic non-Markovian Landau-Lifshitz-Gilbert equation where the properties of the random magnetic field and the dissipation are determined by the underlying phonon dynamics. We have numerically solved this equation for a tangentially magnetized yttrium-iron garnet film in the presence of a parallel parametric pumping field. We obtain a complete description of all stages of the nonequilibrium time evolution of the magnon gas which is in excellent agreement with experiments. Our calculation proves that the experimentally observed condensation of magnons in yttrium-iron garnet at room temperature is a purely classical phenomenon which should be called Rayleigh-Jeans rather than Bose-Einstein condensation.PACS numbers: 75.30. Ds, 75.10.Hk, 05.30.Jp In the past decade the nonequilibrium dynamics of parametrically pumped magnons in thin yttrium-iron garnet (YIG) films has been investigated by many experimental studies [1][2][3][4][5][6][7][8][9]. Very rich physics was found, including the overpopulation of the lowest energy state, which was interpreted as Bose-Einstein condensation (BEC) of magnons at room temperature and finite momentum. Using the technique of Brillouin light scattering it is even possible to measure the magnon distribution with momentum and time resolution [10]. This allows an observation of the parametric resonance and of the subsequent thermalization leading to the formation of the condensate in detail [6,7]. Unfortunately, a complete theoretical understanding of this phenomenon is still lacking and there is no theory that can simultaneously describe all stages of the experiment. While the so-called S-theory [11][12][13][14][15][16][17][18] is able to describe the parametric resonance used to populate certain magnon states, it does not properly take magnon-magnon scattering into account and therefore cannot describe the cascade of relaxation processes leading to the formation of a magnon condensate. On the other hand, theories that focus on the condensate usually do not take the pumping dynamics into account and start with some given quasiequilibrium state which can be identified with the ground state of some effective quantum mechanical Hamiltonian [19][20][21]. Phenomenological approaches of the Ginzburg-Landau type also have been used to study the condensation dynamics [22]. Finally, theories dealing with the relaxation processes and kinetics of excited magnons did not include the possibility of magnon condensation [23][24][25][26].Since BEC is a manifestation of quantum mechanics, it seems at the first sight reasonable that quantized magnons obeying Bose statistics are essential to obtain a satisfactory theoretical description of magnon condensation in YIG. However, since the experiments are performed at room temperature, which is large compared with the relevant magnon energies, the equilibrium distribution of the magnons is the Rayleigh-Jeans rather than t...

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Microscopic spin-wave theory for yttrium-iron garnet films

Cited by 76 publications

References 29 publications

Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3

Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3

Effects of an adjacent metal surface on spin wave propagation

Rayleigh-Jeans Condensation of Pumped Magnons in Thin-Film Ferromagnets

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