Spin-wave Instability Research Articles

Spin density wave instabilities in theNbS2monolayer

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDC) are a family of laminar materials presenting exciting properties such as charge density waves (CDW), superconductivity and metal-insulating transitions among others. 2H-NbS2 is a particular case within the family, because it is the only one that is superconductor without exhibiting a CDW order. Although no long range magnetic order was experimentally observed in the TMDC, we show here that the single monolayer of NbS2 is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wave-like magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zig-zag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibilty and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wavevector asociated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi 2D character of this system, and the rather strong Coulomb interactions of the 4d band of the Nb atom. We also estimate the amplitude of the spin-fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition.

Parallel Pumping of Spin Waves for Ferromagnetic Nanowires and Nanotubes With Circular Cross Sections

Calculations are reported for the spin-wave instabilities in a ferromagnetic nanowire with circular cross section, as well as nanotubes, under conditions of parallel microwave pumping. The quantized spin waves are characterized by a one-dimensional wave vector along the nanowire and the external applied magnetic field is taken to be parallel to the longitudinal axis. In nanowires with lateral dimensions less than about 200 nm, the spatial quantization of the eigenmodes become predominant and our theory employs a microscopic (or Hamiltonian-based) dipole-exchange method analogous to that used recently for ultrathin films and nanowire stripes with rectangular cross section. Numerical applications are made to wires with different radii and comparisons are presented with nanowires having square cross sections. When our nanowire results are compared with those for macroscopic samples or for ultrathin films, it is found that the “butterfly curves” (threshold field vs. applied field) are significantly modified and show more structural features.

Growth and spin-wave properties of thin Y3Fe5O12 films on Si substrates

We describe synthesis of submicron Y3Fe5O12 (YIG) films sputtered on Si substrates and present results of the investigation of ferromagnetic resonance (FMR) and spin waves in YIG/SiO2/Si structures. It is found that decrease of the annealing time leads to essential reduction of the FMR linewidth ΔH and, consequently, to reduction of relaxation losses of spin waves. Spin-wave propagation in in-plane magnetized YIG/SiO2/Si structures is studied. We observe the asymmetry of amplitude-frequency characteristics of the Damon-Eshbach spin waves caused by different localizations of spin waves at the free YIG surface and at the YIG/SiO2 interface. Growth of the generating microwave power leads to spin-wave instability and changes amplitude-frequency characteristics of spin waves.

Renormalization group theory for Kondo breakdown in Kondo lattice systems

We present a renormalization group (RG) theory for the breakdown of Kondo screening in the Kondo lattice model (KLM) without pre-assumptions about the competition between Kondo effect and magnetic ordering or Fermi surface criticality. We show that the vertex between a single, local Kondo spin and the extended conduction electrons obtains RKKY- induced, non-local contributions in the in-and out-going coordinates of scattering electrons due to scattering at surrounding Kondo sites, but it remains local in the Kondo spin position. This enables the existence of a local Kondo screening scale TK(y) in the KLM, controlled by the effective RKKY coupling parameter y. TK(y) is determined by the RG flow of the local spin exchange coupling in the presence of the self-consistent spin response on surrounding Kondo sites. We show that TK(y) exhibits universal behavior and is suppressed by the antiferromagnetic RKKY coupling. Beyond a maximal RKKY parameter value ymax Kondo screening ceases to exist even without magnetic ordering. The theory opens up the possibility of describing quantum critical scenarios involving spin wave instabilities or local Kondo breakdown on the same footing.

Spin-wave Instability Theory for Ferromagnetic Nanostructures

A microscopic, or Hamiltonian-based, theory is outlined for studying the spin-wave instability thresholds of the parametric processes occurring in ferromagnetic nanostructures under conditions of pumping with a microwave eld. Most previous work has concentrated on spheres or lms with dimensions of order several μm or more, with the theoretical interpretation being made in terms of macroscopic (or continuum) methods. At smaller length scales, as in ultrathin lms and nanowires with thickness or lateral dimensions less than about 100 nm, the discreteness of the quantized spin waves and their spatial distributions become modi ed, making it more appropriate to employ a microscopic approach to the nonlinear dynamics with a lattice of e ective spins interacting through the magnetic dipole dipole and exchange interactions. E ects of microwave pumping (in either the parallel or perpendicular con guration) are incorporated in calculations for the instability thresholds of the quantized spin-wave bands in di erent nanostructures and materials.

Theory of spin-wave instability thresholds for ultrathin ferromagnetic films under parallel pumping

A microscopic, or Hamiltonian-based, theory is developed to study the spin-wave instability thresholds for the parametric processes occurring in ultrathin ferromagnetic films under conditions of parallel pumping with a microwave field. Most previous work has focused on spheres or films with dimensions of order several microns or more, and the theoretical interpretation has been in terms of macroscopic (continuum) methods. At smaller size scales, as in ultrathin films with thickness less than about 100 nm, the discreteness of the quantized spin waves and their spatial distributions become modified, making it more appropriate (particularly for the nonlinear dynamics) to employ a microscopic or Hamiltonian-based approach with a lattice of effective spins interacting with one another through the magnetic dipole-dipole and exchange interactions. This work utilizes a similar microscopic approach, but with the effects of microwave pumping included, to calculate the spin-wave instability thresholds for the bands of quantized spin waves in ultrathin films with the saturation magnetization parallel to their surface. Magnetic materials with different ratios of the exchange to dipole-dipole strengths are considered, taking examples for relatively strong, intermediate, and weak exchange. In the case of parallel pumping it is found that the shape of the typical ``butterfly curve,'' as conventionally obtained with macroscopic samples of YIG, becomes modified in these ultrathin films due to the occurrence of several quantized spin-wave bands and their spatial properties.

Dynamics of antiferromagnets driven by spin current

When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the background magnetization via spin-transfer torques. In antiferromagnetic (AFM) materials, however, the corresponding problem is unsolved. We derive microscopically the dynamics of an AFM system driven by spin current generated by an attached FM polarizer, and find that the spin current exerts a driving force on the local staggered order parameter. The mechanism does not rely on the conservation of spin angular momentum, nor does it depend on the induced FM moments on top the AFM background. Two examples are studied: (i) A domain wall is accelerated to a terminal velocity by purely adiabatic effect where the Walker's break-down is avoided; and (ii) Spin injection modifies the AFM resonance frequency, and spin current injection triggers spin wave instability of local moments above a threshold.

Current-induced magnetization reversal in SrRuO3

We inject current pulses into uniformly magnetized patterns of thin films of the itinerant ferromagnet SrRuO3, while monitoring the effective temperature of the patterns during the current injection. We gradually increase the amplitude of the pulses until magnetization reversal occurs. We observe magnetization reversal induced by current above a temperature-dependent threshold and show that this effect is not simply due to sample heating or Oersted fields. We discuss the applicability of the current-induced spin-wave instability scenario.

Passage of two-frequency signals in the Bragg resonance band of a one-dimensional magnon crystal

Passage of a two-frequency signal with high and low power levels through a spin-wave transmission line with a one-dimensional periodic microstructure formed on the surface of a ferromagnetic film is experimentally studied. Experiments are carried out under the condition of three-wave parametric instability of spin waves. It is shown that, near the central frequency of the first Bragg resonance at certain power levels of the large and small signals, the nonlinear spin-wave transmission line can operate as either a power limiter or a signal-to-noise enhancer.

Observation of self-modulation spin-wave instability in a one-dimensional magnonic crystal

The emergence of the self-modulation instability of monochromatic microwave spin waves excited in a periodic magnetic film structure, a magnonic crystal, has been observed. The magnonic crystal was fabricated by chemically etching the surface a single-crystal yttrium-iron garnet film. The self-modulation instability has been observed at the frequencies corresponding to the band gaps of the spin-wave spectrum of the magnonic crystal caused by Bragg resonances. Above a certain threshold value of the input microwave signal, the time profile of the output pulses corresponds to envelope solitons.

Electrical detection of nonlinear ferromagnetic resonance in single elliptical permalloy thin film using a magnetic tunnel junction

A quantitative method to detect ferromagnetic resonance using magnetic tunnel junction structure has been developed. Experimental results reveal three distinct regions for single elliptical permalloy film of micrometer lateral size. Above the spin wave instability threshold, the experimental results show a linear response of the longitudinal magnetization component to the microwave field amplitude over a large range rather than a lock-up phenomenon appeared in macroscopic permalloy films and then a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance.

Observation of the Chaotic Spin-Wave Soliton Trains in Magnetic Films

The transition from stationary to chaotic spin-wave soliton trains has been observed. The experiment utilized cw excitation of envelope solitons through self-modulation instability of spin waves. By increasing the spin-wave power, the secondary self-modulation instability succeeded the primary modulation instability, resulting in after-modulation of the soliton train amplitude. Further increase of the spin-wave power led to development of the higher-order instabilities, resulting in formation of the chaotic soliton train.

Hamiltonian formulation of nonlinear spin-wave dynamics: Theory and applications

A systematic step-by-step development of nonlinear spin-wave theory within the framework of a Hamiltonian formalism is given. The expansion coefficients in the spin-wave Hamiltonian up to the fourth order in the canonical spin-wave amplitudes for a uniformly magnetized ellipsoidal sample are given in general form. This is done for a general orientation of the external field and for a general wave vector. Applications are given for (A) the second-order Suhl spin-wave instability coupling coefficients, (B) the spin-wave wave-vector-dependent nonlinear frequency shift for an in-plane magnetized thin film, and (C) the uniform magnetization mode (ferromagnetic resonance) nonlinear frequency shift for an obliquely magnetized thin film. The analytical results for (C) are compared with results obtained from a time forward numerical analysis of the torque equation of motion and show a near-perfect match.

Magnetization reversal via internal spin waves in magnetic nanoparticles

By numerically solving the equations of motion for atomic spins we show that internal spin-wave processes in large enough magnetic particles, initially in unstable states, lead to complete magnetization reversal and thermalization. The particle's magnetization $m$ strongly decreases in the middle of reversal and then recovers. The closer is the initial orientation of $\mathbf{m}$ to the energy minimum, the slower is the relaxation toward it and the smaller is the decrease in $m$ in the course of relaxation. We identify two main scenarios, exponential and linear spin-wave instabilities. For the latter, the longitudinal and transverse relaxation rates have been obtained analytically. Orientation dependence of these rates leads to a nonexponential relaxation of the particle's magnetization at long times.

Modeling of microwave-assisted switching in micron-sized magnetic ellipsoids

The microwave assisted magnetisation reversal is modelled in a permalloy micron-sized magnetic ellipsoid. Our simulations confirm that this process requires less field than magnetisation reversal under a static field. This is due to a different reversal mode which in case of the microwave-assisted process is always a ripple structure. During the magnetisation reversal two stages: nucleation and relaxation are distinguished. The nucleation process is governed by spinwave instabilities. The relaxation process is related to the domain expansion through domain wall propagation determined by the precessional motion of magnetic moments in the center of the domain walls. As a consequence, the switching time is a complex oscillating function of the microwave frequency.

Spin-Wave Instabilities and Noncollinear Magnetic Phases of a Geometrically Frustrated Triangular-Lattice Antiferromagnet

This Letter examines the relation between the spin-wave instabilities of collinear magnetic phases and the resulting noncollinear phases for a geometrically frustrated triangular-lattice antiferromagnet in the high-spin limit. Using a combination of phenomenological and Monte Carlo techniques, we demonstrate that the instability wave vector with the strongest intensity in the collinear phase determines the wave vector of a cycloid or the dominant elastic peak of a more complex noncollinear phase. Our results are related to the observed multiferroic phase of Al-doped CuFeO2.

Simulation of a spin-wave instability from atomistic spin dynamics

We study the spin dynamics of a Heisenberg model at finite temperature in the presence of an external field or a uniaxial anisotropy. For the case of the uniaxial anisotropy our simulations show that the macro moment picture breaks down. An effect which we refer to as a spin-wave instability (SWI) results in a non-dissipative Bloch-Bloembergen type relaxation of the macro moment where the size of the macro moment changes, and can even be made to disappear. This relaxation mechanism is studied in detail by means of atomistic spin dynamics simulations.

Field-induced transition from parallel to perpendicular parametric pumping for a microstrip transducer

Microstrip transducers used for the excitation of spin waves in magnetic films possess two characteristic properties: high spatial localization of the microwave magnetic field and the presence of field components parallel and perpendicular to the bias field. Here, the effects of these features on the process of parametric pumping are presented. By microwave measurements of the spin-wave instability threshold a transition from parallel pumping to perpendicular pumping at the critical field with the minimal threshold is observed. This transition is accompanied by a sharp threshold increase above the critical field due to the spatial confinement of the pump region.

Emission of noise-like spin wave packets in the presence of three-magnon decay processes and the kinetic instability of waves in a ferromagnetic film

The features of spin wave emission from a ferromagnetic film in the direction of the propagation of a surface magnetostatic wave have been experimentally investigated at various input signal powers. Radiation in the form of two noise-like spin wave packets has been detected near the frequency corresponding to half the pump frequency. This radiation is caused by three-magnon processes of the decay of a surface magnetostatic wave and by the kinetic instability of spin waves.

Spin-wave analysis of uniaxial nanopillar devices

The spin-wave instability of self-oscillations in uniaxial nanopillar devices is studied. It is demonstrated that under certain conditions these self-oscillations are stable with respect to spin-wave perturbations. This theoretical result is consistent with experimentally observed stability of large angle self-oscillations driven by spin-polarized current injection.