Showing papers on "Spin wave published in 2004"

Observation of Coherent Oscillations in a Single Electron Spin

Abstract: Rabi nutations and Hahn echo modulation of a single electron spin in a single defect center have been observed. The coherent evolution of the spin quantum state is followed via optical detection of the spin state. Coherence times up to several microseconds at room temperature have been measured. Optical excitation of the spin states leads to decoherence. Quantum beats between electron spin transitions in a single spin Hahn echo experiment are observed. A closer analysis reveals that beats also result from the hyperfine coupling of the electron spin to a single $^{14}\mathrm{N}$ nuclear spin. The results are analyzed in terms of a density matrix approach of an electron spin interacting with two oscillating fields.

Lieb-Schultz-Mattis in higher dimensions

Abstract: A generalization of the Lieb-Schultz-Mattis theorem to higher-dimensional spin systems is shown. The physical motivation for the result is that such spin systems typically either have long-range order, in which case there are gapless modes, or have only short-range correlations, in which case there are topological excitations. The result uses a set of loop operators, analogous to those used in gauge theories, defined in terms of the spin operators of the theory. We also obtain various cluster bounds on expectation values for gapped systems. These bounds are used, under the assumption of a gap, to rule out the first case of long-range order, after which we show the existence of a topological excitation. Compared to the ground state, the topologically excited state has, up to a small error, the same expectation values for all operators acting within any local region, but it has a different momentum.

Quantum magnetic excitations from stripes in copper oxide superconductors

Abstract: In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized--one per copper site--by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

Hyperfine interaction in a quantum dot: Non-Markovian electron spin dynamics

Abstract: We have performed a systematic calculation for the non-Markovian dynamics of a localized electron spin interacting with an environment of nuclear spins via the Fermi contact hyperfine interaction. This work applies to an electron in the $s$-type orbital ground state of a quantum dot or bound to a donor impurity, and is valid for arbitrary polarization $p$ of the nuclear spin system, and arbitrary nuclear spin $I$ in high magnetic fields. In the limit of $p=1$ and $I=\frac{1}{2}$, the Born approximation of our perturbative theory recovers the exact electron spin dynamics. We have found the form of the generalized master equation (GME) for the longitudinal and transverse components of the electron spin to all orders in the electron spin-nuclear spin flip-flop terms. Our perturbative expansion is regular, unlike standard time-dependent perturbation theory, and can be carried out to higher orders. We show this explicitly with a fourth-order calculation of the longitudinal spin dynamics. In zero magnetic field, the fraction of the electron spin that decays is bounded by the smallness parameter $\ensuremath{\delta}=1∕{p}^{2}N$, where $N$ is the number of nuclear spins within the extent of the electron wave function. However, the form of the decay can only be determined in a high magnetic field, much larger than the maximum Overhauser field. In general the electron spin shows rich dynamics, described by a sum of contributions with nonexponential decay, exponential decay, and undamped oscillations. There is an abrupt crossover in the electron spin asymptotics at a critical dimensionality and shape of the electron envelope wave function. We propose a scheme that could be used to measure the non-Markovian dynamics using a standard spin-echo technique, even when the fraction that undergoes non-Markovian dynamics is small.

The structure of the high-energy spin excitations in a high-transition-temperature superconductor

Abstract: In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity1. The high transition temperatures (high-Tc) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials2. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at Tc. The most obvious feature in the magnetic excitations of high-Tc superconductors such as YBa2Cu3O6+x is the so-called ‘resonance’3,4,5,6. Although the resonance may be strongly coupled to the superconductivity3,4,5,6,7,8, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)xCuO4 family and is not universally present in Bi2Sr2CaCu2O8+δ (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at Tc, and have spectral weight far exceeding that of the ‘resonance’. The discovery of similar excitations in La2–xBaxCuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

Domain-wall induced phase shifts in spin waves.

Abstract: We study the interaction between two important features of ferromagnetic nanoparticles: magnetic domain walls and spin waves Micromagnetic simulations reveal that magnetostatic spin waves change their phase as they pass through domain walls Similar to an Aharonov-Bohm experiment, we suggest to probe this effect by splitting the waves on different branches of a ring The interference of merging waves depends on the domain walls in the branches A controlled manipulation of spin-wave phases could be the first step towards nanoscaled ferromagnetic devices performing logical operations based on spin-wave propagation

Domain-Wall Dynamics and Spin-Wave Excitations with Spin-Transfer Torques

Abstract: A generalization of spin-transfer torques in ferromagnetic structures is proposed. For a spatially nonuniform magnetization, the spin torque has a form nearly identical to that in magnetic multilayers. We show that the domain-wall motion driven by the current has many unique features that do not exist in the conventional domain-wall motion driven by a magnetic field. We also demonstrate that the spin torque can generate bulk and surface spin excitations that have been seen in point-contact experiments.

Excitations of incoherent spin-waves due to spin-transfer torque.

Abstract: The possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current, as predicted by Slonczewski and Berger, has recently been demonstrated. This observation opens important prospects of applications in radiofrequency components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics within the coherent spin-torque model. In some cases, the telegraph noise caused by spin-currents could not be quantitatively described by that model. This has led to controversy about the need for an effective magnetic temperature model. Here we interpret the experimental results of Kiselev et al. using micromagnetic simulations. We point out the key role played by incoherent spin-wave excitation due to spin-transfer torque. The incoherence is caused by spatial inhomogeneities in local fields generating distributions of local precession frequencies. We observe telegraph noise with gigahertz frequencies at zero temperature. This is a consequence of the chaotic dynamics and is associated with transitions between attraction wells in phase space.

Domain-wall dynamics driven by adiabatic spin-transfer torques

Abstract: In a first approximation, known as the adiabatic process, the direction of the spin polarization of currents is parallel to the local magnetization vector in a domain wall. Thus the spatial variation of the direction of the spin current inside the domain wall results in an adiabatic spin-transfer torque on the magnetization. We show that domain-wall motion driven by this spin torque has many unique features that do not exist in the conventional wall motion driven by a magnetic field. By analytically and numerically solving the Landau-Lifshitz-Gilbert equation along with the adiabatic spin torque in magnetic nanowires, we find that the domain wall has its maximum velocity at the initial application of the current but the velocity decreases to zero as the domain wall begins to deform during its motion. We have computed domain-wall displacement and domain-wall deformation of nanowires, and concluded that the spin torque based on the adiabatic propagation of the spin current in the domain wall is unable to maintain wall movement. We also introduce the concept of domain-wall inductance to characterize the capacity of the spin-torque-induced magnetic energy stored in a domain wall. In the presence of domain-wall pinning centers, we construct a phase diagram for the domain-wall depinning by the combined action of the magnetic field and the spin current.

Excitations of incoherent spin-waves due to spin-transfer torque

Abstract: As predicted by Slonczewski and Berger, the possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current has been recently demonstrated. This observation opens very important perspectives of applications in RF components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics results within the coherent spin-torque model (CSM). In some cases, the telegraph noise caused by spin-currents could not be described quantitatively by the CSM. This led to controversies about the need of an effective magnetic temperature model (ETM). Here we interpret the experimental results of Kiselev et al. [Nature 425, 380 (2003)] using micromagnetic simulations. We point out the key role played by incoherent spin-waves excitation due to spin-transfer effects. The incoherence is caused by the spatial inhomogeneities of the local fields, generating a distribution of local precession frequencies. It results in telegraph noise at zero temperature associated with transitions between attraction wells in phase space.

Stripe order, depinning, and fluctuations in La 1.875 Ba 0.125 CuO 4 and La 1.875 Ba 0.075 Sr 0.050 CuO 4

Abstract: We present a neutron scattering study of stripe correlations measured on a single crystal of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. Within the low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin and charge stripe order are observed below 50 K. For excitation energies $\hbar\omega\le12$ meV, we have characterized the magnetic excitations that emerge from the incommensurate magnetic superlattice peaks. In the ordered state, these excitations are similar to spin waves. Following these excitations as a function of temperature, we find that there is relatively little change in the {\bf Q}-integrated dynamical spin susceptibility for $\hbar\omega\sim10$ meV as stripe order disappears and then as the structure transforms from LTT to the low-temperature-orthorhombic (LTO) phase. The {\bf Q}-integrated signal at lower energies changes more dramatically through these transitions, as it must in a transformation from an ordered to a disordered state. We argue that the continuous evolution through the transitions provides direct evidence that the incommensurate spin excitations in the disordered state are an indicator of dynamical charge stripes. An interesting feature of the thermal evolution is a variation in the incommensurability of the magnetic scattering. Similar behavior is observed in measurements on a single crystal of La$_{1.875}$Ba$_{0.075}$Sr$_{0.050}$CuO$_{4}$; maps of the scattered intensity in a region centered on the antiferromagnetic wave vector and measured at $\hbar\omega=4$ meV are well reproduced by a model of disordered stripes with a temperature-dependent mixture of stripe spacings. We discuss the relevance of our results to understanding the magnetic excitations in cuprate superconductors.

Spin wave contributions to the high-frequency magnetic response of thin films obtained with inductive methods

Abstract: The high-frequency magnetic response of Permalloy thin films have been measured using network-analyzer ferromagnetic resonance. We demonstrate that the excitation of spin waves by the coplanar wave-guide modify the magnetic response appreciably, in particular, by causing a frequency shift and broadening of the resonance peak. An analytic theory is presented to account for the experimental observations and provides a quantitative tool to accurately determine the Gilbert damping constant.

Size Induced Variations in Structural and Magnetic Properties of Double Exchange La0.8Sr0.2MnO3−δ Nano-Ferromagnet

Abstract: A detailed study on the influence of particle size varied from 8 nm to 53 nm on the structural and magnetic properties of La0.8Sr0.2MnO3−δ has been done. The unit cell volume increases and the microstrain in the compound shows peak formation as the particle size decreases. Nano particles of La0.8Sr0.2MnO3−δ exhibit superparamagnetism whose blocking temperature has a nonlinear and logarithmic decreasing tendency as function of particle size and applied magnetic field, respectively. Evidence of formation of a magnetically dead layer at the surface has been found and the ratio of the thickness of the dead layer to the particle size increases exponentially with particle size. The coercivity of the nanoparicles increases manifold as particle size varies from 53 nm to 21 nm. In the single domain region the coercivity exhibits a d−1.125 behavior. The temperature dependence of the saturation magnetization shows strong collective excitation due to the spin wave that varies as Tα with α>αbulk of 3/2. Thus the spin ...

Spin polarization of electrons by nonmagnetic heterostructures: the basics of spin optics.

Abstract: We propose to use the lateral interface between two regions with different strengths of the spin-orbit interaction(s) to spin polarize the electrons in gated two-dimensional semiconductor heterostructures. For a beam with a nonzero angle of incidence, the transmitted electrons will split into two spin polarization components propagating at different angles. We analyze the refraction at such an interface and outline the basic schemes for filtration and control of the electron spin.

Two-magnon scattering in a self-assembled nanoscale network of misfit dislocations

Abstract: The static and dynamic properties of magnetic Pd/Fe(001) ultrathin film crystalline structures prepared on GaAs(001) templates were investigated by ferromagnetic resonance (FMR) from 10 to 73 GHz. It will be shown that the formation of a self-assembled nanoscale network of misfit dislocations in crystalline structures can be detected during the growth by fan-out diffraction features in reflection high electron energy diffraction. This network of defects leads to a strong extrinsic magnetic damping. The out-of-plane measurements of the FMR linewidth have revealed that the extrinsic damping is caused by two-magnon scattering. The contribution to the FMR linewidth from two-magnon scattering is strongly anisotropic and follows the rectangular symmetry of the glide planes of the misfit dislocation network. It will be shown that the observed strong anisotropy in two-magnon scattering can be interpreted by Fourier components of magnetic defects. The angular dependence of the Fourier components results in an effective channelling of scattered spin waves.

Dispersive excitations in the high-temperature superconductor La2-xSrxCuO4

Abstract: High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.

Spin excitations of nanometric cylindrical dots in vortex and saturated magnetic states

Abstract: The magnetic normal modes of nanometric magnetic disks are calculated using a recently developed, hybrid micromagnetic method. The method is based on the subdivision of a particle into small cells and the development of a ``dynamical matrix'' that contains the restoring torques acting on the magnetization of each cell. The method includes dipolar, Zeeman, and exchange interactions. The results of these calculations are used to interpret the spin wave spectra measured by Brillouin scattering in an array of nanometric Permalloy™ disks over a wide range of applied fields encompassing both the vortex and saturated states.

Finite temperature properties and frustrated ferromagnetism in a square lattice Heisenberg model

Abstract: The spin 1/2 Heisenberg model on a square lattice with antiferromagnetic nearest- and next-nearest neighbour interactions (the J 1-J 2 model) has long been studied as a paradigm of a two-dimensional frustrated quantum magnet. Only very recently, however, have the first experimental realisations of such systems been synthesized. The newest material, Pb2VO(PO4)2 seems to have mixed ferro- and antiferromagnetic exchange couplings. In the light of this, we extend the semiclassical treatment of the J 1-J 2 model to include ferromagnetic interactions, and present an analysis of the finite temperature properties of the model based on the exact diagonalization of 8, 16 and 20 site clusters. We propose that diffuse neutron scattering can be used to resolve the ambiguity inherent in determining the ratio and sign of J 1 and J 2 from thermodynamic properties alone, and use a finite temperature Lanczos algorithm to make predictions for the relevant high temperature spin-spin correlation functions. The possibility of a spin-liquid phase occurring for ferromagnetic J 1 is also briefly discussed.

Are half-metallic ferromagnets half metals? (invited)

Abstract: Several classes of materials are currently under investigation as potential high-spin-polarization materials. Unfortunately, the proposed half-metallic materials, including the semi-Heusler alloys, the manganese perovskites, and the “simpler” oxides such as chromium dioxide and magnetite, suffer from fundamental limitations. First, the postulated half-metallic systems lose their full (T=0) spin polarization at finite temperatures and, second, surfaces, interfaces, and structural inhomogenities destroy the complete spin polarization of half-metallic systems even at zero temperature. In a strict sense, half-metallic ferromagnetism is limited to zero temperature since magnon and phonon effects lead to reductions in polarization at finite temperatures.

Spin-wave spectra of perpendicularly magnetized circular submicron dot arrays

Abstract: Dynamic microwave properties of arrays of circular Ni and Ni81Fe19 dots were studied by X-band ferromagnetic resonance (FMR) technique. All of the dots had the same radius 0.5μm, thickness 50–70nm, and were arranged into rectangular or square array with different interdot separations. In the case of perpendicular magnetization multiple (up to 8) sharp resonance peaks were observed below the main FMR peak, and the relative positions of these peaks were independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account.

Phenomenological theory of current-induced magnetization precession

Abstract: We solve appropriate drift-diffusion and Landau-Lifshitz-Gilbert equations to demonstrate that unpolarized current flow from a nonmagnet into a ferromagnet can produce a precession-type instability of the magnetization. The fundamental origin of the instability is the difference in conductivity between majority spins and minority spins in the ferromagnet. This leads to spin accumulation and spin currents that carry angular momentum across the interface. The component of this angular momentum perpendicular to the magnetization drives precessional motion that is opposed by Gilbert damping. Neglecting magnetic anisotropy and magnetostatics, our approximate analytic and exact numerical solutions using realistic values for the material parameters show (for both semi-infinite and thin-film geometries) that a linear instability occurs when both the current density and the excitation wave vector parallel to the interface are neither too small nor too large. For many aspects of the problem, the variation of the magnetization in the direction of the current flows makes an important contribution.

Radiation of spin waves by a single micrometer-sized magnetic element

Abstract: Dynamic magnetic properties of a single micrometer-sized magnetic element consisting of a permalloy and a partially patterned CoFe layer separated by an intervening Cu spacer layer have been studied by means of a micro-focus Brillouin light scattering setup, which allows for local measurements of the magnetization dynamics on the submicrometer scale. It is shown that quantized spin-wave modes excited in the magnetic element act as radiation sources for spin waves in the surrounding magnetic film. It is found that the intensities of spin waves excited by different quantized modes follow different distance laws when traveling away from the region of excitation.

Tunneling of Dipolar Spin Waves through a Region of Inhomogeneous Magnetic Field

Abstract: We show experimentally and by numerical simulations that spin waves propagating in a magnetic film can pass through a region of a magnetic field inhomogeneity or they can be reflected by the region depending on the sign of the inhomogeneity. If the reflecting region is narrow enough, spin-wave tunneling takes place. We investigate the tunneling mechanism and demonstrate that it has a magnetic dipole origin.

Phase diagram and magnetic collective excitations of the Hubbard model for graphene sheets and layers

Abstract: We discuss the magnetic phases of the Hubbard model for the honeycomb lattice both in two and three spatial dimensions. A ground state phase diagram is obtained depending on the interaction strength $U$ and electronic density $n$. We find a first order phase transition between ferromagnetic regions where the spin is maximally polarized (Nagaoka ferromagnetism) and regions with smaller magnetization (weak ferromagnetism). When taking into account the possibility of spiral states, we find that the lowest critical $U$ is obtained for an ordering momentum different from zero. The evolution of the ordering momentum with doping is discussed. The magnetic excitations (spin waves) in the antiferromagnetic insulating phase are calculated from the random-phase approximation for the spin susceptibility. We also compute the spin fluctuation correction to the mean field magnetization by virtual emission/absorption of spin waves. In the large $U$ limit, the renormalized magnetization agrees qualitatively with the Holstein-Primakoff theory of the Heisenberg antiferromagnet, although the latter approach produces a larger renormalization.

Influence of a uniform current on collective magnetization dynamics in a ferromagnetic metal

Abstract: We discuss the influence of a uniform current j on the magnetization dynamics of a ferromagnetic metal We find that the magnon energye(q ) has a current-induced contribution proportional to qiJ W , where J W is the spin current, and predict that collective dynamics will be more strongly damped at finite j We obtain similar results for models with and without local moment participation in the magnetic order For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for j*10 9 Ac m 22 We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems

Effect of Electron-Electron Interaction on Spin Relaxation of Charge Carriers in Semiconductors

Abstract: An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire

Current-induced transverse spin-wave instability in a thin nanomagnet.

Abstract: We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced "spin-transfer torque," exists for one current direction only.

Spin anisotropy and slow dynamics in spin glasses.

Abstract: We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.

Pressure-induced quantum phase transition in the spin-liquid TlCuCl3.

Abstract: The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.