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arXiv:2009.12595 · Journal of Experimental and Theoretical Physics Letters

Bose—Einstein Condensation and Spin Superfluidity of Magnons in a Perpendicularly Magnetized Yttrium Iron Garnet Film

P. M. Vetoshko, Grigory A. Knyazev, A. N. Kuzmichev, A. A. Kholin, V. I. Belotelov, Yu. M. Bunkov

Published Sep 1, 2020• 23 citations
arXivHTMLPDF

Abstract

The formation of a Bose—Einstein condensate of magnons in a perpendicularly magnetized iron yttrium garnet film under radio-frequency pumping in a stripline has been studied experimentally. The characteristics of the nonlinear magnetic resonance and spatial distribution of the Bose—Einstein condensate of magnons in the gradient of a magnetic field have been analyzed. In these experiments, the system of bosonic magnons behaves in much the same way as the Bose—Einstein condensate of magnons in antiferromagnetic superfluid 3 He-B, which was previously studied in detail. The Bose—Einstein condensate of magnons forms a coherently precessing state having the properties of magnon superfluidity. Its stability is determined by the repulsion potential between excited magnons, which compensates the inhomogeneity of the magnetic field.

DOI
10.1134/s0021364020170105

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Reply to Comment on “Bose–Einstein Condensation and Spin Superfluidity of Magnons in a Perpendicularly Magnetized Yttrium Iron Garnet Film” (JETP Letters 112, 299 (2020))

Yu. M. Bunkov

The unity of the laws of physics makes it possible to apply a unified approach to various physical systems. Bose–Einstein condensation of magnons (mBEC) and related magnon superfluidity were discovered almost four decades ago in antiferromagnetic superfluid 3He. Bose–Einstein condensation was then detected in a gas of ultracold atoms, a gas of exciton-polaritons, etc. A magnon BEC was then detected in antiferromagnets with coupled nuclear–electron precession and in yttrium iron garnet (YIG) films. Moreover, the properties of the magnon BEC in films are fundamentally different when the magnetic field is oriented along and across the film plane. In this short article, the main properties of mBEC and, in particular, the difference between the “classical” mBEC for magnons with zero momentum and “exotic” mBEC for magnons with nonzero momentum are highlighted.

Journal of Experimental and Theoretical Physics Letters•Jun 1, 2022•4 citations

Magnon Supercurrent in a Magnon Bose-Einstein Condensate subject to a Thermal Gradient

Per Axel Clausen, B. Hillebrands

We report evidence for the existence of a supercurrent of magnons in a magnon Bose-Einstein condensate (BEC) prepared in a room temperature yttrium-iron-garnet (YIG) magnetic film. The magnon BEC is formed in a parametrically populated magnon gas, and its temporal evolution is studied by time-resolved Brillouin light scattering (BLS) spectroscopy in the area of the BLS laser focus. It has been found that local laser heating in the center of the condensate decreases the density of the magnon BEC in the steady-state pump regime and it enhances the temporal decrease of the freely evolving magnon condensate after the termination of the pumping pulse, but it does not alter the relaxation dynamics of the gaseous magnon phase. This phenomenon is understood as the appearance of a magnon supercurrent within the condensate due to a temperature-gradient induced phase gradient in the condensate. Bose-Einstein condensation [1] can be achieved either by decreasing the temperature of a boson gas [2] or by increasing its density. The latter method is especially applicable to g ases of weakly interacting quasi-particles such as excitons [3] , polaritons [4, 5], photons [6], and magnons [7, 8]. When a spin system is pumped, and when the injected magnons thermalize through scattering processes conserving both their number and the total energy, a Bose-Einstein condensate (BEC) may be formed at the lowest energy state of the energy-momentum spectrum even at room temperature conditions of the magnetic film carrying the magnons [8, 9]. As the condensed magnon phase is localized in the global energy minimum, its group velocity is exactly zero and no energy transport can be associated with the magnon BEC. The situation can change, when a magnon supercurrent driven by a gradient in the phase of the wavefunction of a magnon condensate can be excited. Such a phase gradient can be induced by, e.g., a potential gradient or a temperature gradient. However, the dynamics of magnon condensates in such a gradient is still terra incognita. Here we provide experimental insight into the evolution of a magnon BEC in a thermal gradient generated by local laser heating. We show that such a heating influences both the steady-state characteristics and the transitional behavior of the magno n BEC, which can be understood using the concept of a magnon supercurrent. We study the temporal evolution of a magnon BEC in a single-crystal yttrium iron garnet (YIG, Y3Fe5O12) film by

•Mar 2, 2015•1 citation
The formation of a Bose—Einstein condensate of magnons in a perpendicularly magnetized iron yttrium garnet film under radio-frequency pumping in a stripline has been studied experimentally. The characteristics of the nonlinear magnetic resonance and spatial distribution of the Bose—Einstein condensate of magnons in the gradient of a magnetic field have been analyzed. In these experiments, the system of bosonic magnons behaves in much the same way as the Bose—Einstein condensate of magnons in antiferromagnetic superfluid 3 He-B, which was previously studied in detail. The Bose—Einstein condensate of magnons forms a coherently precessing state having the properties of magnon superfluidity. Its stability is determined by the repulsion potential between excited magnons, which compensates the inhomogeneity of the magnetic field.
The conventional magnon Bose-Einstein condensation (BEC of magnons with k = 0) is a coherent state of excited magnons described by a common wave function. It was observed first in antiferromagnetic superfluid states of 3He. Here we report on the discovery of a very similar magnon BEC in ferrimagnetic film at room temperature. The experiments were performed in Yttrium Iron Garnet (YIG) films at a magnetic field oriented perpendicular to the film. The high-density quasiequilibrium state of excited magnon was formed by methods of pulse and/or Continuous Waves (CW) magnetic resonance. We have observed a Long Lived Induction Decay Signals (LLIDS), well known as a signature of spin superfluidity. We demonstrate that the BEC state may maintain permanently by continuous replenishment of magnons with a small radiofrequency (RF) field. Our finding opens the way for development of potential supermagnonic applications at an ambient conditions.
The Bose-Einstein condensation (BEC) of magnons created by a strong pumping in ferromagnetic thin films of yttrium iron garnet used as systems of finite size is considered analytically. Such a peculiarity, typical for this magnetic material, as the presence of a minimum in the spectrum of spin waves at a finite value of the wave vector is taken into account. The definition of high-temperature BEC is introduced and its characteristics are discussed. A role of boundary conditions for spin variables is analyzed, and it is shown that in the case of free spins on the boundary the magnon lattice can form in the system. The factors responsible for its appearance are discussed
Experimental evidence for Bose–Einstein condensation (BEC) of magnons at room temperature in a thin film of yttrium iron garnet (YIG) excited by parallel pumping is already available and different features of the experimental results have been explained qualitatively [Nature 443, 430 (2006)]. In the present work, we explain quantitatively different aspects of this experimental observation through spin wave treatment. In the case of parallel pumping field, we have developed a formula for the time required for the formation of magnon BEC in a thin film of ferromagnetic material. This relation is found to be in good agreement with known experimental results. In a similar treatment we predict the condition for the formation of BEC of magnons in the case of perpendicular pumping.
Parameters of the transition from classical dynamics of spin waves to the formation of a coherent magnon Bose–Einstein condensate have been obtained experimentally for the first time. The studies are performed on an yttrium iron garnet film beyond the radio frequency excitation region; thus, the coherent state of magnons is an eigenstate rather than a state induced by an external radio frequency field. The critical magnon density at the formation of the Bose–Einstein condensate is in good agreement with a theoretically predicted value. The transition is obtained at room temperature, which is possible owing to a small mass of magnons and their high density.
The unity of the laws of physics makes it possible to apply a unified approach to various physical systems. Bose–Einstein condensation of magnons (mBEC) and related magnon superfluidity were discovered almost four decades ago in antiferromagnetic superfluid 3He. Bose–Einstein condensation was then detected in a gas of ultracold atoms, a gas of exciton-polaritons, etc. A magnon BEC was then detected in antiferromagnets with coupled nuclear–electron precession and in yttrium iron garnet (YIG) films. Moreover, the properties of the magnon BEC in films are fundamentally different when the magnetic field is oriented along and across the film plane. In this short article, the main properties of mBEC and, in particular, the difference between the “classical” mBEC for magnons with zero momentum and “exotic” mBEC for magnons with nonzero momentum are highlighted.
We report evidence for the existence of a supercurrent of magnons in a magnon Bose-Einstein condensate (BEC) prepared in a room temperature yttrium-iron-garnet (YIG) magnetic film. The magnon BEC is formed in a parametrically populated magnon gas, and its temporal evolution is studied by time-resolved Brillouin light scattering (BLS) spectroscopy in the area of the BLS laser focus. It has been found that local laser heating in the center of the condensate decreases the density of the magnon BEC in the steady-state pump regime and it enhances the temporal decrease of the freely evolving magnon condensate after the termination of the pumping pulse, but it does not alter the relaxation dynamics of the gaseous magnon phase. This phenomenon is understood as the appearance of a magnon supercurrent within the condensate due to a temperature-gradient induced phase gradient in the condensate. Bose-Einstein condensation [1] can be achieved either by decreasing the temperature of a boson gas [2] or by increasing its density. The latter method is especially applicable to g ases of weakly interacting quasi-particles such as excitons [3] , polaritons [4, 5], photons [6], and magnons [7, 8]. When a spin system is pumped, and when the injected magnons thermalize through scattering processes conserving both their number and the total energy, a Bose-Einstein condensate (BEC) may be formed at the lowest energy state of the energy-momentum spectrum even at room temperature conditions of the magnetic film carrying the magnons [8, 9]. As the condensed magnon phase is localized in the global energy minimum, its group velocity is exactly zero and no energy transport can be associated with the magnon BEC. The situation can change, when a magnon supercurrent driven by a gradient in the phase of the wavefunction of a magnon condensate can be excited. Such a phase gradient can be induced by, e.g., a potential gradient or a temperature gradient. However, the dynamics of magnon condensates in such a gradient is still terra incognita. Here we provide experimental insight into the evolution of a magnon BEC in a thermal gradient generated by local laser heating. We show that such a heating influences both the steady-state characteristics and the transitional behavior of the magno n BEC, which can be understood using the concept of a magnon supercurrent. We study the temporal evolution of a magnon BEC in a single-crystal yttrium iron garnet (YIG, Y3Fe5O12) film by