Showing papers by "Andrii V. Chumak published in 2015"

Optically reconfigurable magnetic materials

Abstract: The periodic modulation of the magnetic properties of magnonic crystals controls the flow of spin waves. An optical method is now shown that can produce such modulations by heating, which can be reprogrammed during operation. Structuring of materials is the most general approach for controlling waves in solids. As spin waves—eigen-excitations of the electrons’ spin system—are free from Joule heating, they are of interest for a range of applications, such as processing1,2,3,4,5, filtering6,7,8 and short-time data storage9. Whereas all these applications rely on predefined constant structures, a dynamic variation of the structures would provide additional, novel applications. Here, we present an approach for producing fully tunable, two-dimensionally structured magnetic materials. Using a laser, we create thermal landscapes in a magnetic medium that result in modulations of the saturation magnetization and in the control of spin-wave characteristics. This method is demonstrated by the realization of fully reconfigurable one- and two-dimensional magnonic crystals—artificial periodic magnetic lattices.

Measurements of the exchange stiffness of YIG films using broadband ferromagnetic resonance techniques

Abstract: Measurements of the exchange stiffness D and the exchange constant A of Yttrium Iron Garnet (YIG) films are presented. YIG films with thicknesses from 0.9 to 2.6 µm were investigated with a microwave setup in a wide frequency range from 5 to 40 GHz. The measurements were performed with the external static magnetic field applied in-plane and out-of-plane. The method of Schreiber and Frait (1996 Phys. Rev. B 54 6473), based on the analysis of the perpendicular standing spin wave mode frequency dependence on the applied out-of-plane magnetic field, was used to obtain the exchange stiffness D. This method was modified to avoid the influence of internal magnetic fields during the determination of the exchange stiffness. Furthermore, the method was also adapted for in-plane measurements. The results obtained using all methods are compared and values of D between (5.18 ± 0.01) 10−17 T m2 and (5.40 ± 0.02) 10−17 T m2 were obtained for different thicknesses. From this, the exchange constant was calculated to be A = (3.7 ± 0.4) pJm−1.

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

Abstract: 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.

Spin-wave logic devices based on isotropic forward volume magnetostatic waves

Abstract: We propose the utilization of isotropic forward volume magnetostatic spin waves in modern wave-based logic devices and suggest a concrete design for a spin-wave majority gate operating with these waves. We demonstrate by numerical simulations that the proposed out-of-plane magnetized majority gate overcomes the limitations of anisotropic in-plane magnetized majority gates due to the high spin-wave transmission through the gate, which enables a reduced energy consumption of these devices. Moreover, the functionality of the out-of-plane majority gate is increased due to the lack of parasitic generation of short-wavelength exchange spin waves.

Spin-wave logic devices based on isotropic forward volume magneto-static waves

Abstract: We propose the utilization of isotropic forward volume magneto-static spin waves in modern wave-based logic devices and suggest a concrete design for a spin-wave majority gate operating with these waves. We demonstrate by numerical simulations that the proposed out-of-plane magnetized majority gate overcomes the limitations of anisotropic in-plane magnetized majority gates due to the high spin-wave transmission through the gate, which enables a reduced energy consumption of these devices. Moreover, the functionality of the out-of-plane majority gate is increased due to the lack of parasitic generation of short-wavelength exchange spin waves.

A spin-wave logic gate based on a width-modulated dynamic magnonic crystal

Abstract: An electric current controlled spin-wave logic gate based on a width-modulated dynamic magnonic crystal is realized. The device utilizes a spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film and two conducting wires attached to the film surface. Application of electric currents to the wires provides a means for dynamic control of the effective geometry of waveguide and results in a suppression of the magnonic band gap. The performance of the magnonic crystal as an AND logic gate is demonstrated.

A spin-wave logic gate based on a width-modulated dynamic magnonic crystal

Abstract: An electric current controlled spin-wave logic gate based on a width-modulated dynamic magnonic crystal is realized. The device utilizes a spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film and two conducting wires attached to the film surface. Application of electric currents to the wires provides a means for dynamic control of the effective geometry of the waveguide and results in a suppression of the magnonic band gap. The performance of the magnonic crystal as an AND logic gate is demonstrated.

The role of the non-magnetic material in spin pumping and magnetization dynamics in NiFe and CoFeB multilayer systems

Abstract: We present a study of the effective magnetization Meff and the effective damping parameter αeff 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 Meff and αeff. Additionally, spin pumping in NiFe/NM/Pt is investigated by means of inverse spin Hall effect (ISHE) measurements. We observe a large dependence of Meff and αeff 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 Meff and αeff 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 t...

Sign of inverse spin Hall voltages generated by ferromagnetic resonance and temperature gradients in yttrium iron garnet platinum bilayers

Abstract: 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.

Formation of Bose–Einstein magnon condensate via dipolar and exchange thermalization channels

Abstract: 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.

Spin-transfer torque based damping control of parametrically excited spin waves in a magnetic insulator

Abstract: 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(100nm)/Pt(10nm) bilayer of 4x2 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.

Width-modulated magnonic crystal and its application for spin-wave logic

Abstract: Width-modulated dynamic magnonic crystal is developed. It utilizes a spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film and two conducting wires attached to the film surface between modulated and unmodulated areas. Application of electric currents to the wires produces Oersted fields that provides a means for dynamic control of the effective geometry of the waveguide and results in a suppression of the magnonic band gap. The performance of the magnonic crystal as an electric current controlled spin-wave AND logic gate is demonstrated.

Magnonen für den Computer von Übermorgen

Abstract: In der Magnonik wird die Information allein uber Spins in Spinwellen und ihren Quanten, den Magnonen, transportiert. Es fliest kein elektrischer Strom. Das vermeidet die Abwarme, die einer weiteren Miniaturisierung der herkommlichen Halbleitertechnologie allmahlich Grenzen setzt. Deshalb konnte die Magnonik im Computerbereich langfristig die ubliche Halbleitertechnologie erganzen oder teilweise ersetzen. Das Ausnutzen der Teilchen- und Welleneigenschaften der Magnonen eroffnet das neue Feld der Interferenzlogik. Mit ihr lassen sich magnonische Bauteile konstruieren. Das Funktionieren wesentlicher Bauteile wurde bereits erfolgreich im Labor demonstriert. Dazu zahlen Wellenleiter, Logikelemente, Transistoren und Speicher. Magnonen sind zudem verschrankbar. Damit konnte die Magnonik die Lucke zwischen Halbleitertechnologie und Quantencomputern fullen.

Magnetische Materialien nach Maß für die Spintronik

Abstract: In der Spintronik wird anstelle der elektrischen Elementarladung die zweite elementare Eigenschaft der Elektronen – der Spin – zur Informationsverarbeitung genutzt. Durch die Kopplung der Spins untereinander kann sich eine sogenannte Spinwelle – auch Magnon genannt – im Material ausbreiten. Physikern der TU Kaiserslautern ist jungst erstmals die gezielte optische Manipulation der Spinwellen-Propagation gelungen.