Magnon Generation Research Articles

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal–insulator interface. Analytical modeling shows that the electrons’ dynamics are almost instantaneously imprinted onto js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

Organic-based magnon spintronics.

Magnonics concepts utilize spin-wave quanta (magnons) for information transmission, processing and storage. To convert information carried by magnons into an electric signal promises compatibility of magnonic devices with conventional electronic devices, that is, magnon spintronics 1 . Magnons in inorganic materials have been studied widely with respect to their generation2,3, transport4,5 and detection 6 . In contrast, resonant spin waves in the room-temperature organic-based ferrimagnet vanadium tetracyanoethylene (V(TCNE) x (x ≈ 2)), were detected only recently 7 . Herein we report room-temperature coherent magnon generation, transport and detection in films and devices based on V(TCNE) x using three different techniques, which include broadband ferromagnetic resonance (FMR), Brillouin light scattering (BLS) and spin pumping into a Pt adjacent layer. V(TCNE) x can be grown as neat films on a large variety of substrates, and it exhibits extremely low Gilbert damping comparable to that in yttrium iron garnet. Our studies establish an alternative use for organic-based magnets, which, because of their synthetic versatility, may substantially enrich the field of magnon spintronics.

Generation of coherent magnons in NiO stimulated by EUV pulses from a seeded free-electron laser

The recent development of seeded free-electron lasers made possible the implementation of a new class of time-resolved experiments, strictly requiring pulses characterized by quasi full coherence. Here, circularly polarized EUV pulses have been used to launch magnetic excitations (magnons) in a thin film of nickel oxide (NiO), subsequently revealed via optical techniques. The main novelty of this work is that these collective modes have been excited using EUV pulses resonant at core electrons energies. Switching the EUV polarization, a flip equal to Pi in the mode polarity has been observed, which can be ascribed to an optomagnetic effect, as well as a mode frequency shift, scaling as a function of the absorption cross section between core electrons and circularly polarized photons. A phenomenological model supports these findings, too.

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film's magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

Stoner vs. Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Stoner vs. Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Microwave and dc response of an Abrikosov vortex lattice in ferromagnetic superconductors

In magnetic superconductors the magnetic flux dynamics is influenced by the interaction of vortices with the magnetization. This interaction leads to the appearance of an additional damping force acting on the vortices. By solving the London and Landau–Lifshitz–Gilbert equations we analyze the ac and dc responses of a ferromagnetic superconductor in the mixed state. If the vortices are driven by an ac force, their viscosity is enhanced due to the generation of magnons. This viscosity enhancement affects the surface impedance of the sample. In the case of a dc driving current vortices start to radiate magnons when their velocity exceeds a threshold value. As a result, either a step-like feature or a series of peaks appear on the I–V curve.

Magnon radiation by moving Abrikosov vortices in ferromagnetic superconductors and superconductor-ferromagnet multilayers

In systems combining type-II superconductivity and magnetism the non-stationary magnetic field of moving Abrikosov vortices may excite spin waves, or magnons. This effect leads to the appearance of an additional damping force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert equations we calculate the magnetic moment induced force acting on vortices in ferromagnetic superconductors and superconductor/ferromagnet superlattices. If the vortices are driven by a dc force, magnon generation due to the Cherenkov resonance starts as the vortex velocity exceeds some threshold value. For an ideal vortex lattice this leads to an anisotropic contribution to the resistivity and to the appearance of resonance peaks on the current voltage characteristics. For a disordered vortex array the current will exhibit a step-like increase at some critical voltage. If the vortices are driven by an ac force with a frequency \omega, the interaction with magnetic moments will lead to a frequency-dependent magnetic contribution \eta_M to the vortex viscosity. If \omega is below the ferromagnetic resonance frequency \omega_F, vortices acquire additional inertia. For \omega > \omega_F dissipation is enhanced due to magnon generation. The viscosity \eta_M can be extracted from the surface impedance of the ferromagnetic superconductor. Estimates of the magnetic force acting on vortices for the U-based ferromagnetic superconductors and cuprate/manganite superlattices are given.

A scheme for tunable magnon laser based on a hybrid quantum dot-ferromagnetic waveguide system

We have theoretically demonstrated the generation of coherent magnons with the help of a microwave pump field in a hybrid semiconductor quantum dot-ferromagnetic waveguide system. It is shown that this coherent amplification will occur due to the quantum dot-magnon coupling via a strong pump beam. The intensity of the magnon laser could be drastically enhanced by increasing the intensity of the pump beam. The scheme proposed here will have its potential applications in spin-based devices and quantum information processing.

S-f(d) EXCHANGE MECHANISM OF MAGNON GENERATION BY SLOW SPINPOLARONS

It is shown that in a single-axis antiferromagnetic semiconductor placed in a strong magnetic field, dispersionless magnons start emitting at any arbitrarily small velocity of an electron occurring in a spinpolaron state. If magnons are dispersed they are generated when the spinpolaron velocity exceeds the minimum phase velocity of magnons. The maximum power of magnon generation caused by the drift of spinpolarons is estimated.

Novel spintronic device: terahertz magnon-photon laser

A novel method of generating THz radiation is proposed. The method is based on pumping of non-equilibrium electrons into the upper (spin-down) subband of spin-polarized half-metallic ferromagnets or ferromagnetic semiconductors. The electrons rapidly emit magnons with THz frequency, pass into highly excited states of the spin-up subband, and fall into the ground state due to interaction with the equilibrium spin-up electrons or emitting optical phonons. The mechanism of magnon generation is similar to a three-level laser, and at a critical pumping intensive magnon lasing begins. Merging of two magnons generates a THz photon. Thus, a magnon laser becomes a THz photon laser, the generated power being of orders of milliwatt vs. several microwatt power generated by contemporary devices. The generated frequency is material dependent, and can be changed by tuning the magnetic field and the bias. The proposed device may have many applications in different fields including medicine (cancer surgery), molecular imaging, security problems etc.

Phenomenological theory for coherent magnon generation through impulsive stimulated Raman scattering

Phenomenological theory for coherent magnon generation through impulsive stimulated Raman scattering

Spin Hall Effect Without Spin-Orbit Coupling in a Homogeneous Ferromagnet

Spin (magnetization) dynamics defines functionality of spintronic devices. Here we discuss two effects in magnetic dynamics-the spin Hall effect and the current-driven magnon generation. The spin Hall effect without spin-orbit coupling is considered in a monodomain ferromagnet where an electric current flow can induce a so-called anomalous or spontaneous current similar to the Hall current. Because the induced current is spin-polarized, a non-equilibrium magnetization accumulates near the ferromagnet's boundaries. We determine the basic characteristics of the spin Hall effect without spin-orbit coupling using a simple model. This effect contributes, particularly, in the anomalous Hall effect. We analyze the possibility of magnon generation in a monodomain ferromagnet by means of this new type of the spin Hall effect.

Envelope solitons in a medium with strong nonlinear damping

The formation of microwave spin-wave envelope solitons in the ferrite film of yttrium iron garnet in the presence of strong nonlinear damping has been experimentally found and investigated. The solitons are formed due to the four-wave interactions of spin waves when the characteristic time of wave modulation-instability development is much shorter than the time for establishing the generation of short-wavelength magnons. The presence of nonlinear damping allows the excitation of spin-wave envelope solitons by means of long microwave pulses.

Generation of current-driven magnons inCo∕Cumultilayers with antiferromagnetic alignment of adjacentColayers

We study experimentally the current-driven magnon generation (CDMG) by a flux of spin polarized conduction electrons in magnetic multilayers. The usual prerequisite for CDMG in magnetic nanostructures is a sufficiently high spin polarization of the flux. Here we report observation of CDMG in $\mathrm{Co}∕\mathrm{Cu}$ multilayers by an electron flux with negligible spin polarization. The latter is mediated by antiferromagnetic alignment of adjacent $\mathrm{Co}$ layer magnetizations in our multilayers in zero applied magnetic field. We propose that in this case CDMG is produced by the Cherenkov radiation of magnons. By applying magnetic field perpendicular to the layers of our multilayer we could vary the direction of magnetization in the excited ferromagnet with respect to the magnon wave vector in the plane of the layers. The latter offers novel possibilities for calibration of microcontact spectrometer of magnetic excitations in situ. The Cherenkov radiation of magnons can be used for studying Fermi surface topology.

CURRENT-DRIVEN MAGNONS IN MAGNETIC MULTILAYERS

Spin waves, or magnons, are intrinsic excitations in magnetic materials and have attracted considerable attention since introduced by Bloch in 1930.1 Recently ideas of spintronics have impacted this field dramatically. Berger2 and Sloczewski3 introduced completely new aspects of the physics of metallic ferromagnets in the presence of an electric current of high density. Excitation of magnons by an electric current has been predicted and seen in magnetic multilayers,4-6 where the driving force for the excitations is the spin-momentum transfer associated with an electric current flow. We have developed a microcontact technique (magnetic microcontact spectroscopy) to investigate the current-driven magnon generation.4,5 In our experiments we inject current densities as high as 109 A/cm2 into a Co/Cu multi-layer through a point contact made using a sharpened Ag wire carefully brought into contact with a multi-layer file. We observe current-driven magnons in multi-layers with both ferromagnetic4,5 and antiferromagnetic7 configuration of the adjacent layer magnetizations. Some further interesting applications of the technique, particularly, for studying fluctuation phenomena, resonant magnon generation, acoustic wave generation, etc., will be presented.

Collective relaxation of spins and 'superradiance' of magnons

The Heisenberg model of a ferromagnet that contains weakly bound impurity or nuclear spins is considered. The new nonlinear kinetic equation for collective spin operator Z-components of these selected spins (equal to 1/2) in the initial condition with inverted spins and temperature T not=0 is obtained. This equation has an exact solution. It is shown that the collective relaxation of these selected spins under the considered nonequilibrium initial conditions reveals cooperative behaviour of the superradiant type accompanied by coherent magnon generation.

Magnon generation by laser beam scanning along a ferromagnet surface

A new method of spin wave generation by laser beam scanning along a ferromagnet surface is proposed. It is shown that due to the action of the beam a local magnetic inhomogeneity appears that can be considered as a source of Cherenkov radiation. Depending on the form of the beam cross section and properties of the ferromagnet, the formation of a one-, two- or three-dimensional source is possible.

Optical generation and bottlenecking of nonzero-wave-vector magnons via Er3+ centers in antiferromagnetic MnF2.

Optical generation and bottlenecking of nonzero-wave-vector magnons in ${\mathrm{MnF}}_{2}$ is accomplished by mediation of the excited $^{4}\mathrm{F}_{9/2}$(1) Kramers doublet of ${\mathrm{Er}}^{3+}$ centers. Following pulsed optical pumping into the upper doublet level, the generation of resonant magnons is manifest via one-magnon relaxation to the lower level in the event the doublet splitting is made to exceed the magnon gap with an external field. This relaxation, in parallel with the slower spin-phonon relaxation, is in turn observed via the time evolution of the luminescence. Upon severe optical pumping, the magnons become imprisoned under bottlenecking conditions by excited ${\mathrm{Er}}^{3+}$ maintained in the lower $^{4}\mathrm{F}_{9/2}$(1) component, which then may serve as detectors. These experiments allow one to deduce the magnon lifetime, which indicates that spatial diffusion is a major mechanism of removal out of the excited zone. Additionally, the Zeeman effect of the luminescence is investigated, yielding the splittings of the $^{4}\mathrm{F}_{9/2}$(1) and $^{4}\mathrm{I}_{15/2}$(1) doublets, and inferring that the ${\mathrm{Er}}^{3+}$ moments occur in eight different orientations in conformity with the magnetic symmetry.

Spin wave amplification by electrons due to interband absorption of laser radiation in magnetic semiconductors

We investigate here the possibility of generation of nonequilibrium magnons by electrons due to interband absorption of a laser field in magnetic semiconductors. It is found that under certain conditions the magnon excitation rate may become greater than the rate of relaxation and the magnon system may reach instability.

Generation of short wavelength magnons in ferromagnetic semiconductors

We study the conditions of generation of magnons in ferromagnetic semiconductors excited by both an electromagnetic and a d.c. electric field. We conclude that the rate of generation of magnons may become greater than the rate of relaxation and the magnon system may reach instability.