arXiv subjects

B. Hillebrands

Publications and source records attributed to B. Hillebrands.

At least 37 records · Page 2Linked to original sources

arXiv2013

Spin-wave excitation and propagation in microstructured waveguides of yttrium iron garnet (YIG)/Pt bilayers

We present an experimental study of spin-wave excitation and propagation in microstructured waveguides patterned from a 100 nm thick yttrium iron garnet (YIG)/platinum (Pt) bilayer. The life time of the spin waves is found to be more than an order of magnitude higher than in comparably sized metallic structures despite the fact that the Pt capping enhances the Gilbert damping. Utilizing microfocus Brillouin light scattering spectroscopy, we reveal the spin-wave mode structure for different excitation frequencies. An exponential spin-wave amplitude decay length of 31 μm is observed which is a significant step towards low damping, insulator based micro-magnonics.

cond-mat.mes-hall↗

arXiv2013

Sub-microsecond fast temporal evolution of the spin Seebeck effect

We present temporal evolution of the spin Seebeck effect in a YIG|Pt bilayer system. Our findings reveal that this effect is a sub-microseconds fast phenomenon governed by the temperature gradient and the thermal magnons diffusion in the magnetic materials. A comparison of experimental results with the thermal-driven magnon-diffusion model shows that the temporal behavior of this effect depends on the time development of the temperature gradient in the vicinity of the YIG|Pt interface. The effective thermal-magnon diffusion length for YIG|Pt systems is estimated to be around 700nm.

cond-mat.mtrl-sci↗

arXiv2013

Thickness and power dependence of the spin-pumping effect in Y3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect

The dependence of the spin-pumping effect on the yttrium iron garnet (Y3Fe5O12, 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 YIG 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 YIG film thickness. The investigated samples exhibit a spin mixing conductance of g=(7.43 \pm 0.36) \times 10^{18} m^{-2} and a spin Hall angle of theta_{ISHE} = 0.009 \pm 0.0008. Furthermore, the influence of nonlinear effects on the generated voltage and on the Gilbert damping parameter at high excitation powers are 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 multi-magnon 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.

cond-mat.mes-hall↗

arXiv2013

Optimization of the yttrium iron garnet/platinum interface for spin pumping-based applications

The dependence of the spin pumping efficiency and the spin mixing conductance on the surface processing of yttrium iron garnet (YIG) before the platinum (Pt) deposition has been investigated quantitatively. The ferromagnetic resonance driven spin pumping injects a spin polarized current into the Pt layer, which is transformed into an electromotive force by the inverse spin Hall effect. Our experiments show that the spin pumping effect indeed strongly depends on the YIG/Pt interface condition. We measure an enhancement of the inverse spin Hall voltage and the spin mixing conductance of more than two orders of magnitude with improved sample preparation.

cond-mat.mes-hall↗

arXiv2012

Mode conversion by symmetry breaking of propagating spin waves

We study spin-wave transport in a microstructured Ni81Fe19 waveguide exhibiting broken translational symmetry. We observe the conversion of a beam profile composed of symmetric spin-wave width modes with odd numbers of antinodes n=1,3,... into a mixed set of symmetric and asymmetric modes. Due to the spatial homogeneity of the exciting field along the used microstrip antenna, quantized spin-wave modes with an even number n of antinodes across the stripe's width cannot be directly excited. We show that a break in translational symmetry may result in a partial conversion of even spin-wave waveguide modes

cond-mat.mes-hall↗

arXiv2012

Nonlinear emission of spin-wave caustics from an edge mode of a micro-structured Co2Mn0.6Fe0.4Si waveguide

Magnetic Heusler materials with very low Gilbert damping are expected to show novel magnonic transport phenomena. We report nonlinear generation of higher harmonics leading to the emission of caustic spin-wave beams in a low-damping, micro-structured Co2Mn0.6Fe0.4Si Heusler waveguide. The source for the higher harmonic generation is a localized edge mode formed by the strongly inhomogeneous field distribution at the edges of the spin-wave waveguide. The radiation characteristics of the propagating caustic waves observed at twice and three times the excitation frequency are described by an analytical calculation based on the anisotropic dispersion of spin waves in a magnetic thin film.

cond-mat.mes-hall↗

arXiv2012

Heat-induced damping modification in YIG/Pt hetero-structures

We experimentally demonstrate the manipulation of magnetization relaxation utilizing a temperature difference across the thickness of an yttrium iron garnet/platinum (YIG/Pt) hetero-structure: the damping is either increased or decreased depending on the sign of the temperature gradient. This effect might be explained by a thermally-induced spin torque on the magnetization precession. The heat-induced variation of the damping is detected by microwave techniques as well as by a DC voltage caused by spin pumping into the adjacent Pt layer and the subsequent conversion into a charge current by the inverse spin Hall effect.

cond-mat.mes-hall↗

arXiv2012

Brillouin light scattering spectroscopy of parametrically excited dipole-exchange magnons

The spectral distribution of parametrically excited dipole-exchange magnons in an in-plane magnetized epitaxial film of yttrium-iron garnet was studied by means of frequency- and wavevector-resolved Brillouin light scattering spectroscopy. The experiment was performed in a parallel pumping geometry where an exciting microwave magnetic field was parallel to the magnetizing field. It was found that for both dipolar and exchange spectral areas parallel pumping excites the lowest volume magnon modes propagating in the film plane perpendicularly to the magnetization direction. In order to interpret the experimental observations, we used a microscopic Heisenberg model that includes exchange as well as dipole-dipole interactions to calculate the magnon spectrum and construct the eigenstates. As proven in our calculations, the observed magnons are characterized by having the highest possible ellipticity of precession which suggests the lowest threshold of parametric generation. Applying different pumping powers we observe modifications in the magnon spectrum that are described theoretically by a softening of the spin stiffness.

cond-mat.mes-hall↗

arXiv2012

Magnonic band gap design by the edge modulation of micro-sized waveguides

The potential to control the number of the spin-wave band gaps of a magnonic crystal (MC) by variation of its geometry is investigated by numerical simulations. The magnonic crystal is represented by a micro-sized planar ferromagnetic waveguide with periodically variable width. By choosing a step-like or sinusoidal variation of the width, the magnonic crystal reveals multiple or single band gaps, respectively. This allows for additional degrees of freedom in the design of MC based microwave filters and phase shifters with desired characteristics. The MCs band gaps have been studied in the space and frequency domains exploring the spin-wave spectrum dependence on the probing position inside the magnonic crystal.

cond-mat.mes-hall↗

arXiv2011

Direct detection of magnon spin transport by the inverse spin Hall effect

Conversion of traveling magnons into an electron carried spin current is demonstrated in a time resolved experiment using a spatially separated inductive spin-wave source and an inverse spin Hall effect (ISHE) detector. A short spin-wave packet is excited in a yttrium-iron garnet (YIG) waveguide by a microwave signal and is detected at a distance of 3 mm by an attached Pt layer as a delayed ISHE voltage pulse. The delay in the detection appears due to the finite spin-wave group velocity and proves the magnon spin transport. The experiment suggests utilization of spin waves for the information transfer over macroscopic distances in spintronic devices and circuits.

cond-mat.mes-hall↗

arXiv2011

Acoustic spin pumping: Direct generation of spin currents from sound waves in Pt/Y3Fe5O12 hybrid structures

Using a Pt/Y3Fe5O12 (YIG) hybrid structure attached to a piezoelectric actuator, we demonstrate the generation of spin currents from sound waves. This "acoustic spin pumping" (ASP) is caused by the sound wave generated by the piezoelectric actuator, which then modulates the distribution function of magnons in the YIG layer and results in a pure-spin-current injection into the Pt layer across the Pt/YIG interface. In the Pt layer, this injected spin current is converted into an electric voltage due to the inverse spin-Hall effect (ISHE). The ISHE voltage induced by the ASP is detected by measuring voltage in the Pt layer at the piezoelectric resonance frequency of the actuator coupled with the Pt/YIG system. The frequency-dependent measurements enable us to separate the ASP-induced signals from extrinsic heating effects. Our model calculation based on the linear response theory provides us with a qualitative and quantitative understanding of the ASP in the Pt/YIG system.

cond-mat.mtrl-sci↗

arXiv2011

Storage-recovery phenomenon in magnonic crystal

The phenomenon of wave trapping in an artificial crystal with limited number of periods is demonstrated experimentally using spin waves in a magnonic crystal. The information stored in the crystal is recovered afterwards by parametric amplification of the trapped wave. The storage process is based on the excitation of standing internal crystal modes and differs principally from the well-known phenomenon of deceleration of light in photonic crystals.

physics.optics↗

arXiv2011

Optical detection of spin transport in non-magnetic metals

We determine the dynamic magnetization induced in non-magnetic metal wedges composed of silver, copper and platinum by means of Brillouin light scattering (BLS) microscopy. The magnetization is transferred from a ferromagnetic Ni80Fe20 layer to the metal wedge via the spin pumping effect. The spin pumping efficiency can be controlled by adding an insulating but transparent interlayer between the magnetic and non-magnetic layer. By comparing the experimental results to a dynamical macroscopic spin-transport model we determine the transverse relaxation time of the pumped spin current which is much smaller than the longitudinal relaxation time.

cond-mat.other↗

arXiv2011

Temporal behavior of the inverse spin Hall voltage in a magnetic insulator-nonmagnetic metal structure

It is demonstrated that upon pulsed microwave excitation, the temporal behavior of a spin-wave induced inverse spin Hall voltage in a magnetic insulator-nonmagnetic metal structure is distinctly different from the temporal evolution of the directly excited spin-wave mode from which it originates. The difference in temporal behavior is attributed to the excitation of long-lived secondary spin-wave modes localized at the insulator-metal interface.

cond-mat.mes-hall↗

arXiv2011

Variable damping and coherence in a high-density magnon gas

We report on the fast relaxation behavior of a high-density magnon gas created by a parametric amplification process. The magnon gas is probed using the technique of spin-wave packet recovery by parallel parametric pumping. Experimental results show a damping behavior which is in disagreement with both the standard model of exponential decay and with earlier observations of non-linear damping. In particular, the inherent magnon damping is found to depend upon the presence of the parametric pumping field. A phenomenological model which accounts for the dephasing of the earlier injected magnons is in good agreement with the experimental data.

cond-mat.mtrl-sci↗

arXiv2011

Radiation of caustic beams from a collapsing bullet

Collapse of an intense (2+1)-dimensional wave packet in a medium with cubic nonlinearity and a two-dimensional dispersion of an order higher than parabolic is studied both theoretically and experimentally. The carrier waves are microwave backward volume spin waves which propagate in a stripe made from a thin ferrimagnetic film and the packet is a spin-wave bullet. We show that before being self-destroyed the bullet irradiates untrapped dispersive waves, which is in agreement with a previous theoretical prediction. Since, in addition, the ferromagnetic medium is characterized by an induced uniaxial anisotropy, this radiation takes the form of narrow beams of continuous waves at very specific angles to its propagation direction. Based on our theoretical calculations we find that these beams are caustic beams and the angles are the characteristic spin-wave caustic angles modified by the motion of the source.

nlin.PS↗

arXiv2010

Enhancement of the spin pumping efficiency by spin-wave mode selection

The spin pumping efficiency of lateral standing spin wave modes in a rectangular YIG/Pt sample has been investigated by means of the inverse spin-Hall effect (ISHE). The standing spin waves drive spin pumping, the generation of spin currents from magnetization precession, into the Pt layer which is converted into a detectable voltage due to the ISHE. We discovered that the spin pumping efficiency is significantly higher for lateral standing surface spin waves rather than for volume spin wave modes. The results suggest that the use of higher-mode surface spin waves allows for the fabrication of an efficient spin-current injector.

cond-mat.mes-hall↗

arXiv2010

Wide-range wavevector selectivity of magnon gases in Brillouin light scattering spectroscopy

Brillouin light scattering spectroscopy is a powerful technique for the study of fast magnetization dynamics with both frequency- and wavevector resolution. Here, we report on a distinct improvement of this spectroscopic technique towards two-dimensional wide-range wavevector selectivity in a backward scattering geometry. Spin-wave wavevectors oriented perpendicular to the bias magnetic field are investigated by tilting the sample within the magnet gap. Wavevectors which are oriented parallel to the applied magnetic field are analyzed by turning the entire setup, including the magnet system. The setup features a wide selectivity of wavevectors up to 2.04\cdot 10E5 rad/cm for both orientations, and allows selecting and measuring wavevectors of dipole- and exchange-dominated spin waves of any orientation to the magnetization simultaneously.

physics.ins-det↗