Showing papers on "Spin wave published in 2006"

Current-induced magnetization reversal in nanopillars with perpendicular anisotropy

Abstract: Devices that show a magnetic anisotropy normal to the film surface hold great promise towards faster and smaller magnetic bits in data-storage applications. We describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular anisotropy and high coercive fields. The best results are observed for Co/Ni multilayers, which have higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayers. The reference layers were designed to have significantly higher anisotropy allowing a complete current-field phase diagram of the free-layer reversal to be explored. The results are compared to micromagnetic modelling of the free layer that, depending on the bias current and applied field, details regions of irreversible magnetic switching, coherent and incoherent spin waves, or static non-uniform magnetization states. This ability to manipulate high-anisotropy magnetic elements could prove useful for a range of spintronic applications.

A spin triplet supercurrent through the half-metallic ferromagnet CrO2.

Abstract: In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur if spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental set-up is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet pairs to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.

Exact SU(2) symmetry and persistent spin helix in a spin-orbit coupled system.

Abstract: Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constants, and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions and is generated by operators whose wave vector depends on the coupling strength. It renders the spin lifetime infinite at this wave vector, giving rise to a persistent spin helix. We obtain the spin fluctuation dynamics at, and away from, the symmetry point and suggest experiments to observe the persistent spin helix.

Possible evidence for electromagnons in multiferroic manganites

Abstract: Magnetodielectric materials are characterized by a strong coupling of the magnetic and dielectric properties and, in rare cases, simultaneously show both magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto–dielectric coupling and as a consequence fundamentally different spin excitations exist: electro-active magnons (or electromagnons), spin waves that can be excited by a.c. electric fields. Here we provide evidence that these excitations appear in the phase with an incommensurate magnetic structure of the manganese spins. In external magnetic fields this incommensurate structure can be suppressed and the electromagnons wiped out, thereby inducing considerable changes in the index of refraction from d.c. up to terahertz frequencies. Hence, besides adding a creature to the zoo of fundamental excitations, the refractive index can be tuned by moderate magnetic fields, which enables the design of the next generation of optical switches and optoelectronic devices.

Spin-filtered edge states and quantum Hall effect in graphene.

Abstract: Electron edge states in graphene in the quantum Hall effect regime can carry both charge and spin. We show that spin splitting of the zeroth Landau level gives rise to counterpropagating modes with opposite spin polarization. These chiral spin modes lead to a rich variety of spin current states, depending on the spin-flip rate. A method to control the latter locally is proposed. We estimate Zeeman spin splitting enhanced by exchange, and obtain a spin gap of a few hundred Kelvin.

Spin-transfer-driven ferromagnetic resonance of individual nanomagnets.

Abstract: We demonstrate a technique that enables ferromagnetic resonance measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by more than a factor of 50 compared to individual ferromagnetic samples measured by other resonance techniques. Studies of the resonance frequencies, amplitudes, linewidths, and line shapes as a function of microwave power, dc current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.

Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet

Abstract: We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.

The effect of collective spin-1 excitations on electronic spectra in high- Tc superconductors

Abstract: We review recent experimental and theoretical results on the interaction between single-particle excitations and collective spin excitations in the superconducting state of high-Tc cuprates. We con...

Theory of spin torques and giant magnetoresistance in antiferromagnetic metals

Abstract: Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. Here, we propose that corresponding effects occur in circuits containing antiferromagnetic metals. The critical current for order parameter orientation switching can be smaller in the antiferromagnetic case because of the absence of shape anisotropy and because spin torques can act through the entire volume of an antiferromagnet. We discuss possible applications of antiferromagnetic metal spintronics.

Current-Induced Polarization and the Spin Hall Effect at Room Temperature

Abstract: Electrically induced electron spin polarization is imaged in $n$-type ZnSe epilayers using Kerr rotation spectroscopy. Despite no evidence for an electrically induced internal magnetic field, current-induced in-plane spin polarization is observed with characteristic spin lifetimes that decrease with doping density. The spin Hall effect is also observed, indicated by an electrically induced out-of-plane spin polarization with opposite sign for spins accumulating on opposite edges of the sample. The spin Hall conductivity is estimated as $3\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }{\ensuremath{\Omega}}^{\ensuremath{-}1}\text{ }{\mathrm{m}}^{\ensuremath{-}1}/|e|$ at 20 K, which is consistent with the extrinsic mechanism. Both the current-induced spin polarization and the spin Hall effect are observed at temperatures from 10 to 295 K.

Exchange explosions: Magnetization dynamics during vortex-antivortex annihilation.

Abstract: A magnetic vortex and an antivortex can annihilate, resulting in a homogeneous magnetization. A detailed description of the magnetization dynamics of such annihilation processes is obtained by micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation. We show that, depending on the relative polarization of the vortex-antivortex pair, the annihilation process is either a continuous transformation of the magnetic structure or it involves the propagation of a micromagnetic singularity (Bloch point) causing a burstlike emission of spin waves. These results provide new insight into a fundamental micromagnetic process that has recently been proposed for a controlled generation of spin waves.

Spin Nematic Phase in S = 1 Triangular Antiferromagnets

Abstract: Spin nematic order is investigated for an S =1 spin model on a triangular lattice with bilinear–biquadratic interactions. We particularly studied an antiferro nematic order phase with a three-sublattice structure, and magnetic properties are calculated at zero temperature by bosonization. Two types of bosonic excitations are found and we calculated dynamic and static spin correlations. One is a gapless excitation with linear energy dispersion around k ∼ 0 , and this leads to a finite spin susceptibility at T =0 and would have a specific heat C ( T ) ∼ T 2 at low temperatures. These behaviors can explain many of the characteristic features of a recently discovered spin liquid state in the triangular magnet, NiGa 2 S 4 .

Spin-transfer torque for continuously variable magnetization

Abstract: We report quantum and semiclassical calculations of spin current and spin-transfer torque in a free-electron Stoner model for systems where the magnetization varies continuously in one dimension Analytic results are obtained for an infinite spin spiral and numerical results are obtained for realistic domain wall profiles The adiabatic limit describes conduction electron spins that follow the sum of the exchange field and an effective, velocity-dependent field produced by the gradient of the magnetization in the wall Nonadiabatic effects arise for short domain walls but their magnitude decreases exponentially as the wall width increases Our results cast doubt on the existence of a recently proposed nonadiabatic contribution to the spin-transfer torque due to spin-flip scattering

High-temperature ferromagnetism of $sp$ electrons in narrow impurity bands: Application to CaB$_6$

Abstract: Ferromagnetism with high Curie temperature $T_c$, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high $T_c$ within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering $T_c$ below its value given by Stoner theory. These ideas are applied to CaB$_6$ and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B$_2$ and O$_2$ dimers might be exploited in suitable structures containing these elements.

High-temperature ferromagnetism of sp electrons in narrow impurity bands: application to CaB6

Abstract: Ferromagnetism with high Curie temperature Tc, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high Tc within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering Tc below its value given by Stoner theory. These ideas are applied to CaB6 and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B2 and O2 dimers might be exploited in suitable structures containing these elements.

Electrical measurement of spin-wave interactions of proximate spin transfer nanooscillators.

Abstract: We have investigated the interaction mechanism between two nanocontact spin transfer oscillators made on the same magnetic spin valve multilayer. The oscillators phase lock when their precession frequencies are made similar, and a giant magnetoresistance signal is detectable at one contact due to precession at the other. Cutting the magnetic mesa between the contacts with a focused-ion beam modifies the contact outputs, eliminates the phase locking, and strongly attenuates the magnetoresistance coupling, which indicates that spin waves rather than magnetic fields are the primary interaction mechanism.

Suppression of spin relaxation in submicron InGaAs wires.

Abstract: We investigate electron-spin dynamics in narrow two-dimensional $n$-InGaAs channels as a function of the channel width The spin relaxation times increase with decreasing channel width, in accordance with recent theoretical predictions based on the dimensionally constrained D'yakonov-Perel' mechanism Surprisingly, the suppression of the relaxation rate, which is anticipated for the one-dimensional limit, is observed for widths that are an order of magnitude larger than the electron mean free path We find the spin precession length and the channel width to be the relevant length scales for interpreting these results

Spin amplification, reading, and writing in transport through anisotropic magnetic molecules

Abstract: Inelastic transport through a single magnetic molecule weakly coupled to metallic leads is studied theoretically. We consider dynamical processes that are relevant for writing, storing, and reading spin information in molecular memory devices. Magnetic anisotropy is found to be crucial for slow spin relaxation. In the presence of anisotropy we find giant spin amplification: The spin accumulated in the leads if a bias voltage is applied to a molecule prepared in a spin-polarized state can be made exponentially large in a characteristic energy divided by temperature. For one ferromagnetic and one paramagnetic lead the molecular spin can be reversed by applying a bias voltage even in the absence of a magnetic field. We propose schemes for reading and writing spin information based on our findings.

Theory of mutual phase locking of spin-torque nanosized oscillators

Abstract: A theory of mutual phase locking of a pair of generating magnetic nanocontacts driven by a spin-polarized current is developed using the general theory of coupled nonlinear oscillators. Expressions for the phase-locking band and for the frequency of phase-locked oscillations are derived and compared to result of experiments. It is shown that in typical experiments, where the separation $a$ between the nanocontacts lies in the interval $0.2lal1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the coupling between the nanocontacts leading to the phase-locking effect is mainly caused by propagating spin waves radiating into the common free layer, rather than by the direct magnetodipole interaction.

Spin-Wave Interference in Microscopic Rings

Abstract: We have studied the spin excitations of ferromagnetic rings and observed a distinct series of quantized modes in the vortex state. We attribute them to spin waves that circulate around the ring and interfere constructively. They form azimuthal eigenmodes of a magnetic ring resonator which we resolve up to the fourth order. The eigenfrequencies are calculated semianalytically and classified as a function of magnetic field by a quantization rule which takes into account a periodic boundary condition. Strikingly each mode exists only below a characteristic field.

Exchange interactions, spin waves, and transition temperatures in itinerant magnets

Abstract: This contribution reviews an ab initio two-step procedure to determine exchange interactions, spin-wave spectra, and thermodynamic properties of itinerant magnets. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature. In the second step, parameters of an effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and the one-electron Green functions. The Heisenberg Hamiltonian and methods of statistical physics are employed in subsequent evaluation of magnon dispersion laws, spin-wave stiffness constants, and Curie/Neel temperatures. The applicability of the developed scheme is illustrated by selected properties of various systems such as transition and rare-earth metals, disordered alloys including diluted magnetic semiconductors, ultrathin films, and surfaces. A comparison to other ab initio approaches is presented as well.

Spin Injection and Detection in Silicon

Abstract: Spin injection and detection in silicon is a difficult problem, in part because the weak spin-orbit coupling and indirect gap preclude using standard optical techniques. Two ways to overcome this difficulty are proposed, both based on spin-polarized transport across a heterojunction. Using a realistic transport model incorporating the relevant spin dynamics of both electrons and holes, it is argued that symmetry properties of the charge current can be exploited to detect electrical spin injection in silicon using currently available techniques.

Imaging mesoscopic spin Hall flow: Spatial distribution of local spin currents and spin densities in and out of multiterminal spin-orbit coupled semiconductor nanostructures

Abstract: We introduce the concept of bond spin currents, which describe the spin transport between two sites of the lattice model of a multiterminal spin-orbit (SO) coupled semiconductor nanostructure, and express them in terms of the spin-dependent nonequilibrium (Keldysh) Green functions for the Landauer setup where the nanostructure is attached to many semi-infinite ideal leads terminating in macroscopic thermalizing reservoirs. This formalism is applied to obtain the spatial distribution of microscopic spin currents in a clean phase-coherent two-dimensional electron (2DEG) gas with the Rashba type of SO coupling attached to four external leads. Together with the corresponding spatial profiles of the steady-state spin density, such visualization of the phase-coherent spin flow allow us to resolve several key issues for the understanding of microscopic mechanisms which generate pure spin Hall currents in the transverse leads of ballistic devices due to the flow of unpolarized charge current through their longitudinal leads: (i) while bond spin currents are nonzero locally within the SO coupled sample and neighboring region of the leads even in equilibrium (when all leads are at the same potential), the total spin currents obtained by summing the bond spin currents over any cross section within the leads are zero, so that no spin is actually transported by such equilibrium spin currents; (ii) when the device is brought into a nonequilibrium state (supporting steady-state charge current) by applying the external voltage difference between its longitudinal leads, only the wave functions (or Green functions) around the Fermi energy contribute to the total spin current through a given transverse cross section; (iii) the total spin Hall current is not conserved within the SO coupled region---however, it becomes conserved and physically well-defined quantity in the ideal leads where it is, furthermore, equal to the spin current obtained within the multiprobe Landauer-B\"uttiker scattering formalism in linear response regime. The spatial profiles of the local spin currents and stationary flowing spin densities crucially depend on whether the sample is smaller or greater than the spin precession length, thereby demonstrating its essential role as the characteristic mesoscale for the spin Hall effect in ballistic multiterminal semiconductor nanostructures. Although the static spin-independent disorder reduces the magnitude of the total spin current in the leads, the bond spin currents and spin densities remain nonzero throughout the whole diffusive 2DEG sample.

Spin Waves in the Frustrated Kagomé Lattice Antiferromagnet KFe 3 (OH) 6 (SO 4 ) 2

Abstract: The spin wave excitations of the $S=5/2$ kagom\'e lattice antiferromagnet ${\mathrm{KFe}}_{3}(\mathrm{OH}{)}_{6}({\mathrm{SO}}_{4}{)}_{2}$ have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted ``zero energy mode,'' verifying a fundamental prediction for the kagom\'e lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin wave data. The antisymmetric Dzyaloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure.

Generating spin currents in semiconductors with the spin Hall effect.

Abstract: We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.

Microscopic Calculation of Spin Torques in Disordered Ferromagnets(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

Abstract: Effects of conduction electrons on magnetization dynamics, represented by spin torques, are calculated microscopically in the first order in spatial gradient and time derivative of magnetization. Special attention is paid to the so-called β-term and the Gilbert damping, α, in the presence of electrons' spin-relaxation processes, which are modeled by quenched magnetic (and spin–orbit) impurities. It is shown that these two torque terms actually arise from the spin relaxation processes, both transverse and longitudinal, and their coefficients are different, α≠β, in general. These features hold for both localized and itinerant models for ferromagnetism.

Dynamics of composite haldane spin chains in IPA-CuCl3.

Abstract: Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl{sub 3} are studied by cold neutron inelastic scattering. Strongly dispersive gap excitations are observed. Contrary to previously proposed models, the system is best described as an asymmetric quantum spin ladder. The observed spectrum is interpreted in terms of composite Haldane spin chains. The key difference from actual S = 1 chains is a sharp cutoff of the single-magnon spectrum at a certain critical wave vector.

Enhanced spin injection polarization in Co/Cu/Co nonlocal lateral spin valves

Abstract: Nonlocal spin injection is demonstrated in Co∕Cu∕Co lateral spin valves Measurement of a series of structures with varying Cu length between the Co electrodes permits a direct determination of both the Cu spin diffusion length λs and the spin injection polarization P at the Co∕Cu interface The value of λs is 200±20nm at 10K and ⩾110nm at 300K, which is shorter than previously reported, due to the possible presence of impurities in the Cu However, the value of P is >7%, which is improved compared with previous reports of 2%, which is attributed to a higher interfacial quality