The fundamental phenomenon of Bose-Einstein Condensation (BEC) has been observed in different systems of real and quasi-particles. The condensation of real particles is achieved through a major reduction in temperature while for quasi-particles a mechanism of external injection of bosons by irradiation is required. Here, we present a novel and universal approach to enable BEC of quasi-particles and to corroborate it experimentally by using magnons as the Bose-particle model system. The critical point to this approach is the introduction of a disequilibrium of magnons with the phonon bath. After heating to an elevated temperature, a sudden decrease in the temperature of the phonons, which is approximately instant on the time scales of the magnon system, results in a large excess of incoherent magnons. The consequent spectral redistribution of these magnons triggers the Bose-Einstein condensation.

We studied the transient behavior of the spin current generated by the longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses. The LSSE was induced by means of pulsed microwave heating of the Pt layer and the spin currents were measured electrically using the inverse spin Hall effect in the same layer. We demonstrate that the time evolution of the LSSE is determined by the evolution of the thermal gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. These magnons move ballistically within the YIG film with a constant group velocity, while their number decays exponentially within an effective propagation length. The ballistic flight of the magnons with energies above 20 K is a result of their almost linear dispersion law, similar to that of acoustic phonons. By fitting the time-dependent LSSE signal for different film thicknesses varying by almost an order of magnitude, we found that the effective propagation length is practically independent of the YIG film thickness. We consider this fact as strong support of a ballistic transport scenario—the ballistic propagation of quasi-acoustic magnons in room temperature YIG.

Text The observation of the longitudinal spin Seebeck effect (LSSE) in magnetic insulators confirms the crucial role of magnons in spin caloric processes. In the case of the LSSE, the spin current carried by magnons is flowing along an applied thermal gradient. It can be measured as an electric signal in an adjacent nonmagnetic metal by means of the inverse spin Hall effect. Since the number of thermal magnons decays exponentially within an effective propagation length, a strong dependence of the LSSE temporal dynamics on the magnetic layer thickness is expected. We studied the transient behavior of the spin current generated by the LSSE in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses from 150 nm to 53 μm. A microwave pulse applied to the samples resulted in Joule heating of the Pt layer. The subsequent thermal gradient through the YIG thickness gives rise to a LSSE-voltage pulse. We found that the time evolution of the LSSE is determined by the temporal dynamics of the gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. The measured data have been fitted using a model that assumes ballistic motion of thermal magnons in a gradient. The average magnon propagation length of about 425 nm was found to be almost independent of the YIG film thickness. We consider this fact as a manifestation of the ballistic propagation of quasi-acoustic thermal magnons having almost linear dispersion in room temperature YIG.

The temporal evolution of pulsed spin Hall effect-spin transfer torque (SHE-STT) driven auto-oscillations in a yttrium iron garnet (YIG)-platinum (Pt) microdisk is studied experimentally using time-resolved Brillouin light scattering spectroscopy. The frequency of the auto-oscillations is different in the center when compared to the edge of the disk and is related to the simultaneous STT excitation of a bullet and a non-localized spin-wave mode. Furthermore, the magnetization precession intensity saturates on a time scale of 20 ns or longer, depending on the current density. For this reason, our findings suggest that a proper ratio between the current and the pulse duration is of crucial importance for future STT-based devices.

We quantify the relative weight of inverse spin Hall and spin rectification effects occurring in RF-sputtered polycrystalline permalloy, molecular beam epitaxy-grown epitaxial iron and liquid phase epitaxy-grown yttrium-iron-garnet bilayer systems with different capping materials. To distinguish the spin rectification signal from the inverse spin Hall voltage the external magnetic field is rotated in-plane to take advantage of the different angular dependencies of the prevailing effects. We prove that in permalloy anisotropic magnetoresistance is the dominant source for spin rectification while in epitaxial iron the anomalous Hall effect has an also comparable strength. The rectification in yttrium-iron-garnet/platinum bilayers reveals an angular dependence imitating the one seen for anisotropic magnetoresistance caused by spin Hall magnetoresistance.

The temporal evolution of pulsed Spin Hall Effect - Spin Transfer Torque (SHE-STT) driven auto-oscillations in a Yttrium Iron Garnet (YIG) / platinum (Pt) microdisc is studied experimentally using time-resolved Brillouin Light Scattering (BLS) spectroscopy. It is demonstrated that the frequency of the auto-oscillations is different in the center and at the edge of the investigated disc that is related to the simultaneous STT excitation of a bullet and a non-localized spin-wave mode. Furthermore, the magnetization precession intensity is found to saturate on a time scale of 20 ns or longer, depending on the current density. For this reason, our findings suggest that a proper ratio between the current and the pulse duration is of crucial importance for future STT-based devices.

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 mm2 lateral dimensions. The spin-wave relaxation frequency is determined via the threshold of the parametric instability measured by Brillouin light scattering 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. This spin current exerts a spin transfer torque 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 × 1010 A/m2.

We carried out a concerted effort to determine the absolute sign of the inverse spin Hall effect voltage generated by spin currents injected into a normal metal. We focus on yttrium iron garnet (YIG)∣platinum bilayers at room temperature, generating spin currents by microwaves and temperature gradients. We find consistent results for different samples and measurement setups that agree with theory. We suggest a right-hand-rule to define a positive spin Hall angle corresponding to the voltage expected for the simple case of scattering of free electrons from repulsive Coulomb charges.

The dependence of the spin-pumping effect on the yttrium iron garnet $({\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$, YIG) thickness detected by the inverse spin Hall effect (ISHE) has been investigated quantitatively. Due to the spin-pumping effect driven by the magnetization precession in the ferrimagnetic insulator ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film a spin-polarized electron current is injected into the Pt layer. This spin current is transformed into electrical charge current by means of the ISHE. An increase of the ISHE voltage with increasing film thickness is observed and compared to the theoretically expected behavior. The effective damping parameter of the YIG/Pt samples is found to be enhanced with decreasing ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film thickness. The investigated samples exhibit a spin mixing conductance of ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}=(3.87\ifmmode\pm\else\textpm\fi{}0.21)\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$ and a spin Hall angle between ${\ensuremath{\theta}}_{\mathrm{ISHE}}=0.013\ifmmode\pm\else\textpm\fi{}0.001$ and $0.045\ifmmode\pm\else\textpm\fi{}0.004$ depending on the used spin-diffusion length. Furthermore, the influence of nonlinear effects on the generated voltage and on the Gilbert damping parameter at high excitation powers is revealed. It is shown that for small YIG film thicknesses a broadening of the linewidth due to nonlinear effects at high excitation powers is suppressed because of a lack of nonlinear multimagnon scattering channels. We have found that the variation of the spin-pumping efficiency for thick YIG samples exhibiting pronounced nonlinear effects is much smaller than the nonlinear enhancement of the damping.

We present a study of the effective magnetization M-eff and the effective damping parameter alpha(eff) by means of ferromagnetic resonance spectroscopy on the ferromagnetic (FM) materials Ni81Fe19 (NiFe) and Co40Fe40B20 (CoFeB) in FM/Pt, FM/NM, and FM/NM/Pt systems with the non-magnetic (NM) materials Ru, Cr, Al, and MgO. Moreover, for NiFe layer systems, the influence of interface effects is studied by way of thickness dependent measurements of M-eff and alpha(eff). Additionally, spin pumping in NiFe/NM/Pt is investigated by means of inverse spin Hall effect (ISHE) measurements. We observe a large dependence of M-eff and alpha(eff) of the NiFe films on the adjacent NM layer. While Cr and Al do not induce a large change in the magnetic properties, Ru, Pt, and MgO affect M-eff and alpha(eff) in different degrees. In particular, NiFe/Ru and NiFe/Ru/Pt systems show a large perpendicular surface anisotropy and a significant enhancement of the damping. In contrast, the magnetic properties of CoFeB films do not have a large influence of the NM adjacent material and only CoFeB/Pt systems present an enhancement of alpha(eff). However, this enhancement is much more pronounced in NiFe/Pt. By the introduction of the NM spacer material, this enhancement is reduced. Furthermore, a difference in symmetry between NiFe/NM/Pt and NiFe/NM systems in the output voltage signal from the ISHE measurements reveals the presence of spin pumping into the Pt layer in all-metallic NiFe/NM/Pt and NiFe/Pt systems. (C) 2015 AIP Publishing LLC.

The investigation of the spin-pumping efficiency in yttrium iron garnet (YIG)/platinum (Pt) heterostructures revealed a strong dependence on the surface processing of YIG prior to the Pt deposition. A pure spin current was injected into the Pt layer using spin pumping and was consequently transformed into a charge current via the inverse spin Hall effect. The resultant electromotive force was used to calculate the spin-pumping efficiency, which varied by a factor of more then 150 for the different YIG surface treatments.

The spin pumping effect allows for the injection of a spin current from a ferromagnetic (FM) layer into an attached non-magnetic metal (NM) layer [1]. This spin current is subsequently transformed into a charge current by the inverse spin Hall effect (ISHE). Although many aspects of magnetization dynamics in FM | NM bilayers have been investigated by means of spin pumping and the inverse spin Hall effect (ISHE) in different materials [2,3], no systematic work on epitaxial samples has been performed. Epitaxial Þlms have the advantage of high quality interfaces and they offer additionally the possibility to study spin current effects in different crystallographic directions.

A microwave technique is employed to simultaneously examine the spin pumping and the spin Seebeck effect processes in a YIG|Pt bilayer system. The experimental results show that for these two processes, the spin current flows in opposite directions. The temporal dynamics of the longitudinal spin Seebeck effect exhibits that the effect depends on the diffusion of bulk thermal-magnons in the thermal gradient in the ferromagnetic-insulator|normal-metal system.

We examine the influence of crystal growth on the spin-pumping induced inverse spin Hall effect in Fe/Pt bilayers. The morphology of the Fe/Pt interface influences the effective spin mixing conductance. The increase of growth temperature leads to smoother and larger grains at the interface that enhance the effective spin mixing conductance. The spin current injection efficiency into Pt, measured by the inverse spin Hall effect, is maximized by optimizing the epitaxy of Pt on Fe. In magnetic field dependent measurements, the presence of a strong magnetic anisotropy gives rise to two distinct inverse spin Hall effect voltage peaks.