Coherent Spin Waves Research Articles

Magnetic interfaces as sources of coherent spin waves

We have developed a simple but general analytical theory that elucidates the mechanism of spin-wave generation from interfaces between ferromagnetic media pumped by a uniform microwave magnetic field. Our calculations show that, provided there is a finite coupling between the two media, the amplitude of the emitted spin waves depends linearly on the difference between their magnetic susceptibilities. The theory is successfully applied to interpret qualitatively three recent experimental studies in which such a spin-wave emission was observed. Furthermore, we describe how our approach can be extended to several more complicated spin-wave excitation schemes employing electric, elastic, and optical stimuli.

Dual Nature of Magnetism in a Uranium Heavy-Fermion System.

The duality between the localized and itinerant nature of magnetism in 5f-electron systems has been a long-standing puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single-crystalline system of UPt_{2}Si_{2}. In the antiferromagnetic state, we observe a broad continuum of diffuse magnetic scattering with a resonancelike gap of ≈7 meV and the surprising absence of coherent spin waves, suggestive of itinerant magnetism. While the gap closes above the Néel temperature, strong dynamic spin correlations persist to a high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by a local moment with J=4. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand UPt_{2}Si_{2} and, by extension, other 5f systems, in general.

Spinwaves transmittance through antiferromagnetic impurities insured in perfect ordered ferromagnetic thin films

In this paper, we introduce a theoretical approach, based on the matching technique, to study the magnetic properties and the spin dynamics in ferromagnetic thin film A containing an ordered magnetic impurities B with antiferromagnetic coupling between A and B sites. We examine, in particular, the coherent spinwaves diffusion via the A/B nanocontact.The total spinwaves transmittance by the B slab, taken as an impurity sites, in the system is calculated and analyzed for different cases of the spin intensity and the magnetic exchange constant values. The obtained results are compared, also, to those of the perfect ferromagnetic thin film A. The calculated properties are presented for all accessible frequencies in the propagating bands.The results highlight the localized spin states on the interface zone and their interactions with incident spinwaves and demonstrate the dependence of the magnetic properties on the interface parameters.

Brillouin-Mandelstam spectroscopy of standing spin waves in a ferrite waveguide

This article reports results of experimental investigation of the spin wave interference over large distances in the Y3Fe2(FeO4)3 waveguide using Brillouin-Mandelstam spectroscopy. Two coherent spin waves are excited by the micro-antennas fabricated at the edges of the waveguide. The amplitudes of the input spin waves are adjusted to provide approximately the same intensity in the central region of the waveguide. The relative phase between the excited spin waves is controlled by the phase shifter. The change of the local intensity distribution in the standing spin wave is monitored using Brillouin-Mandelstam light scattering spectroscopy. Experimental data demonstrate the oscillation of the scattered light intensity depending on the relative phase of the interfering spin waves. The oscillations of the intensity, tunable via the relative phase shift, are observed as far as 7.5 mm away from the spin-wave generating antennas at room temperature. The obtained results are important for developing techniques for remote control of spin currents, with potential applications in spin-based memory and logic devices.

Pinned domain wall oscillator as a tuneable direct current spin wave emitter

Local perturbations in the relative orientation of the magnetic moments in a continuous magnetic system can propagate in the form of waves. These so-called spin waves represent a promising candidate as an information carrier for spin-based low-power applications. A localized, energy-efficient excitation of coherent and short-wavelength spin waves is a crucial technological requirement, and alternatives to excitation via the Oersted field of an alternating current must be explored. Here, we show how a domain wall pinned at a geometrical constriction in a perpendicularly magnetized thin nanowire emits spin waves when forced to rotate by the application of a low direct current flowing along the wire. Spin waves are excited by the in-plane stray field of the rotating domain wall and propagate at an odd harmonic of the domain wall rotation frequency in the direction of the electron’s flow. The application of an external field, opposing domain wall depinning induced by the current, breaks the symmetry for spin wave propagation in the two domains, allowing emission in both directions but at different frequencies. The results presented define a new approach to manufacture tuneable high-frequency spin wave emitters of easy fabrication and low power consumption.

Isotropic transmission of magnon spin information without a magnetic field.

Spin-wave devices (SWD), which use collective excitations of electronic spins as a carrier of information, are rapidly emerging as potential candidates for post-semiconductor non-charge-based technology. Isotropic in-plane propagating coherent spin waves (magnons), which require magnetization to be out of plane, is desirable in an SWD. However, because of lack of availability of low-damping perpendicular magnetic material, a usually well-known in-plane ferrimagnet yttrium iron garnet (YIG) is used with a large out-of-plane bias magnetic field, which tends to hinder the benefits of isotropic spin waves. We experimentally demonstrate an SWD that eliminates the requirement of external magnetic field to obtain perpendicular magnetization in an otherwise in-plane ferromagnet, Ni80Fe20 or permalloy (Py), a typical choice for spin-wave microconduits. Perpendicular anisotropy in Py, as established by magnetic hysteresis measurements, was induced by the exchange-coupled Co/Pd multilayer. Isotropic propagation of magnon spin information has been experimentally shown in microconduits with three channels patterned at arbitrary angles.

Spin superfluidity and spin waves in YIG films

Recently it was suggested that stationary spin supercurrents (spin superfluidity) are possible in the magnon condensate observed in yttrium-iron-garnet (YIG) magnetic films under strong external pumping. Here we analyze this suggestion. From topology of the equilibrium order parameter in YIG one must not expect energetic barriers making spin supercurrents metastable. However some small barriers of dynamical origin are possible nevertheless. The critical phase gradient (analog of the Landau critical velocity in superfluids) is proportional to intensity of the coherent spin wave (number of condensed magnons). The conclusion is that although spin superfluidity in YIG films is possible in principle, the published claim of its observation is not justified. The analysis revealed that the widely accepted spin-wave spectrum in YIG films with magnetostatic and exchange interaction required revision. This led to revision of non-linear corrections, which determine stability of the magnon condensate with and without spin supercurrents.

Spin-Cherenkov effect in a magnetic nanostrip with interfacial Dzyaloshinskii-Moriya interaction.

Spin-Cherenkov effect enables strong excitations of spin waves (SWs) with nonlinear wave dispersions. The Dzyaloshinskii-Moriya interaction (DMI) results in anisotropy and nonreciprocity of SWs propagation. In this work, we study the effect of the interfacial DMI on SW Cherenkov excitations in permalloy thin-film strips within the framework of micromagnetism. By performing micromagnetic simulations, it is shown that coherent SWs are excited when the velocity of a moving magnetic source exceeds the propagation velocity of the SWs. Moreover, the threshold velocity of the moving magnetic source with finite DMI can be reduced compared to the case of zero DMI. It thereby provides a promising route towards efficient spin wave generation and propagation, with potential applications in spintronic and magnonic devices.

High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque

We study experimentally with submicrometer spatial resolution the propagation of spin waves in microscopic waveguides based on the nanometer-thick yttrium iron garnet and Pt layers. We demonstrate that by using the spin-orbit torque, the propagation length of the spin waves in such systems can be increased by nearly a factor of 10, which corresponds to the increase in the spin-wave intensity at the output of a 10 μm long transmission line by three orders of magnitude. We also show that, in the regime, where the magnetic damping is completely compensated by the spin-orbit torque, the spin-wave amplification is suppressed by the nonlinear scattering of the coherent spin waves from current-induced excitations.

Excitation of coherent propagating spin waves by pure spin currents.

Utilization of pure spin currents not accompanied by the flow of electrical charge provides unprecedented opportunities for the emerging technologies based on the electron's spin degree of freedom, such as spintronics and magnonics. It was recently shown that pure spin currents can be used to excite coherent magnetization dynamics in magnetic nanostructures. However, because of the intrinsic nonlinear self-localization effects, magnetic auto-oscillations in the demonstrated devices were spatially confined, preventing their applications as sources of propagating spin waves in magnonic circuits using these waves as signal carriers. Here, we experimentally demonstrate efficient excitation and directional propagation of coherent spin waves generated by pure spin current. We show that this can be achieved by using the nonlocal spin injection mechanism, which enables flexible design of magnetic nanosystems and allows one to efficiently control their dynamic characteristics.

NONLINEAR SPIN WAVES IN GRAPHENE STRUCTURES

Basing on Dirac equation for interacting massless fermions, we propose a nonlinear model that describes a possible mechanism of ferromagnetism in graphene structures, resulting from electron–electron interaction and spontaneous breaking of spin symmetry of valence electrons. Qualitative predictions of the model are important for practical applications in spintronics. Localized kink–antikink patterns of valence electron spin density on the graphene surface are calculated, their interaction is described, and, finally, the formation of their quasi-bound metastable states (breathers) is investigated. The spectrum of breathers is calculated in both the analytical and the numerical form. Possible inverted population of the appropriate states may be used to generate quantum coherent nonlinear spin waves that can find practical applications in nanoelectronics and spintronics.

Controlling coherent and incoherent spin dynamics by steering the photoinduced energy flow

We present a femtosecond spectroscopic magneto-optical investigation of the coherent and incoherent spin dynamics in the antiferromagnetic dielectric ${\text{KNiF}}_{3}$. The pathways of the photoinduced energy flow to spins were controlled by tuning the pump photon energy. In particular, we demonstrate that laser pulses, with photon energy tuned to a nearly-zero-absorption region, excite the spin system without any signatures of heating of electrons or phonons. In this regime the ultrafast excitation of coherent spin waves is followed by a gradual increase of the spin temperature solely due to decoherence of the laser-generated magnons, as revealed by our simultaneous measurement of both the transversal and the longitudinal component of the spin dynamics.

Selective excitation of spin resonance in orthoferrite PrFeO3 with impulsive polarized terahertz radiation

Single cycle terahertz (THz) pulses were employed to excite coherent spin waves in (110)-oriented PrFeO3 single crystal. The free induction decay radiations at frequency of 0.34 THz (quasi-ferromagnetic mode, FM mode) and 0.41 THz (quasi-antiferromagnetic mode, AFM mode) were observed arising from the coupling of magnetic moment with the impulsive magnetic field of polarized terahertz radiation. These two spin modes in PrFeO3 can be excited and modulated by the magnetic field of THz pulse with a specific polarization with respect to the crystal axis. The extracted complex dielectric permittivity and magnetic permeability dispersion in THz range suggest a higher efficiency of energy transfer from the impulsive THz pulse into the AFM than the FM spin system.

Resonant microwave-to-spin-wave transducer

We use time resolved scanning Kerr microscopy and analytical and numerical calculations to demonstrate coupling of uniform global microwave field to propagating spin waves for emerging magnonic architectures. The coupling is mediated by the local dynamic dipolar field produced by the magnetization of a resonantly driven all-metallic magnetic microwave-to-spin-wave transducer. The local dipolar field can exceed that of the incident microwave field by one order of magnitude. Our numerical simulations demonstrate the ability of the transducer to unidirectionally emit coherent exchange spin waves of nanoscale wavelengths with the emission direction programmed by the magnetic state of the transducer.

Erratum: Electrical current driven by a coherent spin wave in a bulk ferromagnetic semiconductor [Phys. Rev. B84, 012402 (2011)

Erratum: Electrical current driven by a coherent spin wave in a bulk ferromagnetic semiconductor [Phys. Rev. B<b>84</b>, 012402 (2011)

Direct observation of a propagating spin wave induced by spin-transfer torque

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Superconductivity mediated by the antiferromagnetic spin wave in chalcogenide iron-based superconductors

The ground state of K${}_{0.8+x}$Fe${}_{1.6+y}$Se${}_{2}$ and other iron-based selenide superconductors are doped antiferromagnetic semiconductors. We propose that the low energy physics of this system is governed by a model Hamiltonian of interacting electrons with onsite ferromagnetic exchange interactions and intersite superexchange interactions. We have derived the effective pairing potential of electrons under the linear spin-wave approximation and shown that the superconductivity can be driven by mediating coherent spin-wave excitations in these materials. Our work provides a natural account for the coexistence of superconducting and antiferromagnetic long range orders observed by neutron scattering and other experiments.

Electrical current driven by a coherent spin wave in a bulk ferromagnetic semiconductor

Electrical current driven by a coherent spin wave in a bulk ferromagnetic semiconductor

Excitation of propagating spin waves with global uniform microwave fields

We demonstrate a magnonic architecture that converts global free-space uniform microwaves into spin waves propagating in a stripe magnonic waveguide. The architecture is based upon dispersion mismatch between the narrow magnonic waveguide and a wide “antenna” patch, both patterned from the same magnetic film. The spin waves injected into the waveguide travel to distances as large as several tens of micrometers. The antennas can be placed at multiple positions on a magnonic chip and used to excite mutually coherent multiple spin waves for magnonic logic operations. This demonstration paves way for “magnonics” to become a pervasive technology for information processing.

Coherent terahertz control

Spin and charge terahertz excitations in solids are promising for implementing future technologies such as spintronics and quantum computation, but coherently controlling them has been a significant challenge. Researchers have now manipulated coherent spin waves in an antiferromagnet using the intense magnetic field of ultrashort terahertz pulses.