A new phenomenon of momentum relaxation reversal has been discovered experimentally and explained theoretically for dipolar spin waves in magnetic garnet films. It is shown that the process of momentum relaxation, caused by the scattering of a signal wave on defects, can be reversed, and the signal can be restituted after it left the scattering region. The reversal of momentum relaxation is achieved by frequency-selective parametric amplification of a narrow band of scattered waves having low group velocities and frequencies close to the frequency of the original signal wave. The phenomenon can be used for the development of a new type of active microwave delay lines.
The damping of spin waves parametrically excited in the magnetic insulator Yttrium Iron Garnet (YIG) is controlled by a dc current passed through an adjacent normal-metal film. The experiment is performed on a macroscopically sized YIG(100 nm)/Pt(10 nm) bilayer of 4 × 2 mm 2 lateral dimensions. The spin-wave relaxation frequency is determined via the threshold of the parametric instability measured by Brillouin light scattering (BLS) spectroscopy. The application of a dc current to the Pt film leads to the formation of a spin-polarized electron current normal to the film plane due to the spin Hall effect (SHE). This spin current exerts a spin transfer torque (STT) in the YIG film and, thus, changes the spin-wave damping. Depending on the polarity of the applied dc current with respect to the magnetization direction, the damping can be increased or decreased. The magnitude of its variation is proportional to the applied current. A variation in the relaxation frequency of ±7.5% is achieved for an applied dc current density of 5 · 10 10 A/m 2 .The injection of a spin current into a magnetic film can generate a spin-transfer torque (STT) that acts on the magnetization collinearly to the damping torque. 1-3 Thus, it can be used for tuning of the damping of a magnetic film 4-7 as well as for the excitation of magnetization precession in the film. [8][9][10][11][12] In the first experimental realizations, a dc charge current was sent through an additional magnetic layer with a fixed magnetization direction in order to generate a spin-polarized current. 4,8-10 A different way to generate a spin current is based on the spin Hall effect (SHE) 13,14 caused by spin-dependent scattering of electrons in a non-magnetic metal with large spin-orbit interaction. 5,6,11 One of the advantages of the SHE is that it does not require a dc current in the magnetic layer and, thus, allows for the application of a STT to a magnetic dielectric such as yttrium iron garnet (YIG), 15 which is of particular interest for magnon spintronics 16-18 due to its extremely small damping parameter. 19-23 Moreover, a great advantage of the SHE is that a STT can be applied not only locally but to a large area of a magnetic film 5 and, thus, can be potentially used to compensate the damping in a whole complex magnonic circuit. 18 Up to now, the auto-oscillatory regime and the magnetization precession generation has only been reached in patterned structures. 11,12,24 Nonlinear multi-magnon scattering phenomena are assumed to be the reason that disturbs the generation process in un-patterned stuctures.At the same time, nonlinear scattering should not affect the SHE-STT-based damping compensation since spin-wave amplitudes in this case are much smaller. However, most experimental studies concerning this phenomena using metallic samples 4,6 as well as YIG structures 7,12 were performed with laterallyconfined nano-or micro-structures.Here, we use a YIG/Pt bilayer of macroscopic size to investigate SHE-STT damping variation. The measured variation of the damping...
Thermalization of a parametrically driven magnon gas leading to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum was studied by time-and wavevector-resolved Brillouin light scattering spectroscopy. Two distinct channels of the thermalization process related on dipolar and exchange parts of a magnon gas spectrum are clearly determined. It has been found that the magnon population in these thermalization channels strongly depends on applied microwave pumping power. The observed magnon redistribution between the channels is caused by the downward frequency shift of the magnon gas spectrum due to the decrease of the saturation magnetization in the course of injection of parametrically pumped magnons.
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