Chapter One - Spin-Wave Spin Current in Magnetic Insulators

Pure spin current, a flow of spin angular momentum without flow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin–orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law Tn with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.

The collective excitation of localized spins coupled by the magnetic dipole and quantum exchange interactions is called spin waves or, in quantized form, magnons. This quasiparticle, associated with the elementary magnetic excitation, is responsible for a flow of spin angular momentum, a spin current, in magnetic insulators. The spin-carrier can be converted from magnons to conduction electrons at magnetic/nonmagnetic interfaces through dynamical spin exchange coupling between magnetization and conduction electron spins. This spin-carrier conversion enables dynamical generation of spin currents from magnetic insulators, enabling to explore the physics of spin current in condensed matter [1], [2].

The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction. But in addition to charge, an electron also has spin, and the collective motion of spin can propagate-and so transfer a signal-in some insulators. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y(3)Fe(5)O(12) and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y(3)Fe(5)O(12) via spin-Hall effects. Second, making use of the transfer in a Pt/Y(3)Fe(5)O(12)/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y(3)Fe(5)O(12) layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping in Y(3)Fe(5)O(12) is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices.

Controlling spin currents, that is, the flow of spin angular momentum, in small magnetic devices, is the principal objective of spin electronics, a main contender for future energy-efficient information technologies1,2. A pure spin current has never been measured directly because the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport3,4. Here we report that resonant inelastic X-ray scattering (RIXS) can bridge this gap by measuring the spin current carried by magnons-the quanta of the spin wave excitations of the magnetic order-in the presence of temperature gradients across a magnetic insulator. This is possible due to the sensitivity of the momentum- and energy-resolved RIXS intensity to minute changes in the magnon distribution under non-equilibrium conditions. We use the Boltzmann equation in the relaxation time approximation to extract transport parameters, such as the magnon lifetime at finite momentum, essential for the realization of magnon spintronics.

A spin current - a flow of spin angular momentum - can be carried either by spin polarised free electrons or by magnons, the quanta of a moving collective oscillation of localised electron spins - a spin wave. Traditionally, it was assumed, that a spin wave in a magnetic film with spin-sink-free surfaces can transfer energy and angular momentum only along its propagation direction. In this work, using Brillouin light scattering spectroscopy in combination with a theory of dipole-exchange spin-wave spectra, we show that in obliquely magnetized free magnetic films the in-plane propagation of spin waves is accompanied by a transverse spin current along the film normal without any corresponding transverse transport of energy.

A spin current - a flow of spin angular momentum - can be carried either by spin-polarized free electrons or by magnons, the quanta of spin waves. Traditionally, it was assumed that a spin wave in a magnetic film with spin-sink-free surfaces could transfer energy and angular momentum only along its propagation direction. In this talk, using the data of Brillouin light scattering (BLS) spectroscopy in combination with an extended theory of dipole-exchange spin-wave spectra, I show that in obliquely magnetized magnetic films, the in-plane propagation of spin waves is accompanied by a transverse spin current without any corresponding transverse transport of energy [1].It is found that in the case of oblique magnetization, the transverse profiles of in-plane-propagating dipole-exchange spin waves are formed by two co-propagating partial waves with opposite group velocities. Their superposition results in a traveling wave pattern, which carries angular momentum and can be treated as a transversal spin current. Its existence is evidenced by the behavior of thermal spectra of magnons in a yttrium iron garnet film measured in a wide range of wavenumbers using our novel wavevector-resolved BLS setup.In the case of an external excitation of the spin-wave modes carrying the transversal spin current, the angular momentum conservation law would lead to mechanical deformations or/and to the rotation of the magnetic film as a whole, which could be detected using the existing methods of spin mechatronics [2]. Furthermore, the presence of the unconventional spin current may be directly electrically detected [1, 3].We believe that these non-trivial properties of spin-wave modes may be used in the future to control the effects of spin pumping or/and to generate spin currents in nano-scale spintronic signal processing and signal-generating devices.Funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the Research Unit TRR 173–268565370 “Spin+X” (projects B01 and B04), as well as financial support by the European Research Council within the AdG SuperMagnonics, is gratefully acknowledged. The work was also supported in part by the US National Science Foundation, by the US Air Force Office of Scientific Research, and by the Oakland University Foundation. **

Spin current represents a flow of spin angular momentum and does not require movement of charges. Magnetic insulators can therefore work as a source as well as a medium of spin currents, which has been established in ferrimagnetic insulators. Here, we report recent progress in the generation and electrical detection of spin currents in uniaxial antiferromagnetic insulators carried by antiferromagnetic magnons, both thermally and resonantly excited, respectively, via the spin Seebeck effect and antiferromagnetic spin pumping by subterahertz microwaves. These findings open up many exciting possibilities of potential antiferromagnetic applications.

In magnetic insulators, transport of charge is prohibited due to the large bandgap. Spin can still be transported however by spin waves (magnons), the excitations of magnetic systems. The field that studies the properties of spin waves in magnetic insulators is known as magnon spintronics [1]. In the past years, research in the field has been focused on dipolar magnons, which are low-energy spin waves. We have shown [2] that magnons with energy comparable to the thermal energy (exchange magnons) can also transport spin over long distances, characterized by a spin diffusion length λ ≈ 9.5 μm. We have developed a non-local measurement scheme in which exchange magnons are excited and detected making use of the spin Hall- and inverse spin Hall-effect, respectively. This enables the conversion from electronic charge, to electron spin current, to magnonic spin current and vice-versa, using DC electronic signals. This provides a direct interface with conventional electronics and opens up new magnonic device functionalities. Additionally, it allows us to gain insight in the transport of magnons by studying the non-local signal as a function of various parameters, such as an external magnetic field [3] or sample temperature. Finally, studying the long-distance transport of thermal magnons can increase our understanding of the spin Seebeck effect in both the longitudinal and the non-local geometry. [1] A.V. Chumak et al., Nat. Phys. 11, 453-461 (2015) [2] L.J. Cornelissen et al., Nat. Phys. 11, 1022-1026 (2015) [3] L.J. Cornelissen and B.J. van Wees, Phys. Rev. B 93, 020403(R) (2016)

Spin waves offer intriguing possibilities for transmitting and processing information in future low-power electronics. Most proposed devices, however, require the efficient excitation and detection of spin waves in the sub-micrometer range, that is a rather challenging task. In fact, coplanar and microstrip waveguides have been widely used in the past to excite and detect spin waves with wavelengths of tens of microns in thin films of both metallic ferromagnets and on magnetic insulators, but the scalability of these structures micrometer or sub-micrometer have not been investigated in detail. In this study, we present a combined experimental/computational study of a few possible input structures consisting of either symmetrical or asymmetrical coplanar waveguides on top of CoFe films, with widths going all the way down to 250 nm. The primary goal of this work is to present a case study, aiming to explore the limitations of waveguides in creating short-wavelength spin waves for future nanoelectronic applications. We use micro-focused Brillouin light scattering measurements and micromagnetic simulations to analyze the characteristics of the emitted spin waves, achieving reasonable agreement between experiment and simulations. We find that due to the inherently delocalized field distributions of waveguides, and also to the relatively high resistivity of narrow waveguides, they all show poor efficiency for generating spin waves with wavelength below about 2 μm, corresponding to frequencies above 10 GHz in a moderate external field. This means that the intensity of the generated spin waves for a given input power drops quickly for the frequency/wavelength range which is most relevant for emerging applications. This case study demonstrates many of the inherent inefficiencies and limitations of waveguide-based spin wave generation in this regime. Our work supports the conclusion that one may have to use a different mechanism for spin wave generation, exploiting multiferroic structures, spin-orbit torques or nanopatterned, multi-layered magnetic materials, all being the subject of intense current research.

Converting angular momentum between different degrees of freedom within a magnetic material results from a dynamic interplay between electrons, magnons, and phonons. This interplay is pivotal to implementing spintronic device concepts that rely on spin angular momentum transport. We establish a new concept for long-range angular momentum transport that further allows us to address and isolate the magnonic contribution to angular momentum transport in a nanostructured metallic ferromagnet. To this end, we electrically excite and detect spin transport between two parallel and electrically insulated ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin current conversion within the ferromagnetic strip generates electronic spin angular momentum that is transferred to magnons via electron-magnon coupling. We observe a finite angular momentum flow to the second ferromagnetic strip across a diamagnetic substrate over micron distances, which is electrically detected in the second strip by the inverse charge-to-spin current conversion process. We discuss phononic and dipolar interactions as the likely cause to transfer angular momentum between the two strips. Moreover, our Letter provides the experimental basis to separate the electronic and magnonic spin transport and thereby paves the way towards magnonic device concepts that do not rely on magnetic insulators.

In the last decades, the investigation of microwave spin waves (SW) in thin magnetic films has been an attractive field of research due to their very short wavelength reaching down to nanometers at GHz frequencies. SW also have a rich dispersion relation, that depends on their propagation direction with respect to a magnetic field. Hence, SW -based devices are promising candidates for microwave information processing, and eventually for overcoming the limitations encountered with CMOS-devices such as power consumption. In magnonic devices, a key challenge is to achieve long propagation distances of the spin waves. This requires to work with magnetic materials with small magnetic damping such as Yttrium Iron Garnet (YIG).(YIG), has by far the lowest magnetic damping which allows the spin waves propagation to be spread over millimeter-scale distances making it a reference material for spin-wave dynamics studies. Kajiwara and al. [1] confirmed that an electric current injection in a YIG film can be converted into spin-waves. In fact, when placing a non-magnetic metal with a high spin orbit coupling such as platinum on top of a magnetic insulator such as YIG, flow of charge current in the metal leads to its conversion into a pure spin current (via the Spin Hall Effect), which is then injected in the magnetic insulator where it adds up as an additional torque on the YIG’s magnetization. The above-mentioned mechanism of STT generation originating from pure spin-current obtained through the spin-orbit interaction of a heavy metal is called spin orbit torque (SOT).Recent studies [2], [3] with Platinum stripes placed on top of a 20 nm thick YIG waveguide (Pt/YIG) showed that SOT could generate a full compensation of the damping, leading to auto-oscillations of the magnetization above a critical injected current density. When excited with a microwave field, spin waves propagation length was increased by a factor of 10 in the bilayer. However, the possibility to achieve an amplification of propagating spin-waves was not observed yet, due to the onset of nonlinear dissipative processes above the critical current. More recently, materials having perpendicular magnetic anisotropy have demonstrated a full damping compensation along with SW emission, such behavior is accomplished by substituting Bismuth in YIG [4].Here, we report on a study of SW propagation driven by SOT in a Pt/Bi-YIG waveguide where SW are detected using micro-focused Brillouin Light Scattering spectroscopy (BLS). The 20 nm thick and 21 µm long Bi-YIG waveguide is covered with 7 nm of Platinum. The experiments show the lossless propagation of a pulse of spin-waves excited by an rf-field generated by an antenna placed on top of the waveguide, under zero or positive damping during the course of 200 ns. Here, we show the full scale micromagnetic simulations done to emulate this experiment. The result shows a verification of the lossless propagation of SWs as seen in the experiment. By including a realistic Gilbert damping parameter and the Slonczewski torque to the Bi-YIG structure to compensate the magnetic losses, the results reproduce not only the steady state dynamics but also transitional dynamics when the rf-excitation is turned on and off at the ns time-scale.In summary, the inclusion of STT in the micromagnetic simulations showed an accordance with the experimental results, which verifies the ability to obtain a lossless propagation of SWs in the Bi-YIG waveguide and their non-suppression even after a hundred of nanoseconds after cutting off the excitation field. **

We synthesized epitaxial BTO-BFO heterostructure with decreased leakage and simultaneously improved the multiferroic properties. This study provides new direction for ferromagnetic resonance studies, in high quality BTO-BFO films grown on LSMO. We observed small Gilbert damping (α=0.004) and the absence of large inhomogeneous broadening, in a film with 80 nm thickness of BTO-BFO on LSMO (110). This fact offers opportunities for employing this material system for spin transfer in multifunctional materials where controlling magnetization by a flow of spin angular momentum, or spin current, is crucial toward developing nanoscale spin-based memory and devices. Magnetic insulators, such as BTO-BFO on LSMO, are potentially excellent candidates for pure spin current without the existence of charge current.

Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.

Conduction electron can carry a flow spin angular momentum, a spin current. This paper describes another type of spin current: a spin current carried by spin waves. Some types of spin waves can carry a spin current, which can propagate both in insulators and metals. These two types of spin currents were found to exchange each other at a Pt/Y3Fe5O12 interface.

Part 1: Linear Microwave Processes in Magnetic Materials: Ferromagnetic Resonance in Magnetic Insulators (P Kabos & C E Patton) Submilimeter Spectroscopy of Antiferromagnetic Dielectrics. Rare Earth Orthoferrites (A M Balbashov et al) Magnetic Ordering in Amorphous Insulators (G Srinivasan) Ferromagnetic Resonance Studies in Metallic Multilayers (R Krishnan & S Prasad) Propagation and Interaction of Magnetostatic Waves in Planar Magnetic Structures with Non-Stationary Parameters (Yu Fetisov) Spectrum of Microwave Spin Waves in Magnetic Multilayers (B A Kalinikos & P A Kolodin) Excitation of Propagating Spin Waves in Magnetic Films and Layered Structures (B A Kalinikos et al). Part 2: Nonlinear Microwave Processes in Magnetic Materials: Size Effects in Parametric Excitation of Spin Waves in Ferrites (V B Cherepanov & A N Slavin) Multistability and Temperature-Induced Nonlinearity of Parametrically Excited Spin Waves (H Benner & G Wiese) Experimental Observation of Bright and Dark Spin Wave Envelope Solitons in Magnetic Films (M Chen et al) Part 3: Interaction of Microwave Spin Waves with Light: Collinear Interaction of Optical Guided Modes with Microwave Spin Waves in Magnetic Films (D D Stancil) Brillouin Light Scattering from Dipole-Exchange Microwave Spin Waves in Magnetic Films (I V Rojdestvensky et al) Photothermal Characterization of Microwave Magnetic Excitations in Ferrites (J Pelzl & 0 von Gelsau).