Theory of the angular magnetoresistance in CPP spin valves

The resistance of CPP spin valve is a continuous function of the angle θ between the magnetizations of both ferromagnets. We use the circuit theory for non-collinear magnetoelectronics to compute the angular magnetoresistance of CPP spin valves taking the spin accumulation in the ferromagnetic layers into account.     

Spin diffusion in a Heisenberg spin glass

Using a combination of analytic and Monte Carlo techniques we obtain estimates for the spin diffusion constant and spin conductivity in a classical simple cubic Heisenberg spin glass which has a Gaussian distribution of exchange interactions between nearest neighbors.     

Spin hall effect in the presence of spin diffusion

Hirsch [Phys. Rev. Lett. 83, 1834 (1999)] recently proposed a spin Hall effect based on the anomalous scattering mechanism in the absence of spin-flip scattering. Since the anomalous scattering causes both anomalous currents and a finite spin-diffusion length, we derive the spin Hall effect in the presence of spin diffusion from a semiclassical Boltzmann equation. When the formulation is applied to certain metals and semiconductors, the magnitude of the spin Hall voltage due to the spin accumulation is found to be much larger than that of magnetic multilayers. An experiment is proposed to measure this spin Hall effect.     

Interaction-driven spin precession in quantum-dot spin valves

We analyze spin-dependent transport through spin valves composed of an interacting quantum dot coupled to two ferromagnetic leads. The spin on the quantum dot and the linear conductance as a function of the relative angle theta of the leads' magnetization directions is derived to lowest order in the dot-lead coupling strength. Because of the applied bias voltage spin accumulates on the quantum dot, which for finite charging energy experiences a torque, resulting in spin precession. The latter leads to a nontrivial, interaction-dependent, theta dependence of the conductance. In particular, we find that the spin-valve effect is reduced for all theta not equal pi.     

自旋密度光栅和本征GaAs量子阱中的电子自旋双极扩散

为研究空穴对自旋极化电子扩散的影响,提出用自旋密度光栅方法来观察电子自旋扩散过程。由飞秒激光在本征GaAs多量子阱中激发产生瞬态自旋光栅和瞬态自旋密度光栅,并用于研究电子自旋扩散和电子自旋双极扩散。实验测得自旋双极扩散系数Das ＝25.4 cm 2/s,低于自旋扩散系数Ds ＝113.0 cm 2/s,表明自旋密度光栅中电子自旋扩散受到空穴的显著影响。     

Spin diffusion in semiconductors

The behavior of spin diffusion in doped semiconductors is shown to be qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet in an intrinsic semiconductor must be a multiple-band disturbance, involving inhomogeneous distributions of both electrons and holes, in a doped semiconductor a single-band disturbance is possible. For n-doped nonmagnetic semiconductors the enhancement of diffusion due to a degenerate electron sea in the conduction band is much larger for these single-band spin packets than for charge packets-this explains the anomalously large spin diffusion recently observed in n-doped GaAs at 1.6 K. In n-doped ferromagnetic and semimagnetic semiconductors the motion of spin packets polarized antiparallel to the equilibrium carrier spin polarization is predicted to be an order of magnitude faster than for parallel polarized spin packets. These results are reversed for p-doped semiconductors.     

Temporal evolution of spin accumulation in nonlocal spin valves

A temporal evolution of spin signals has been observed in nanoscale Co/Cu nonlocal spin valves. The spin diffusion length of Cu increases from ∼300 to ∼350 nm measured at 295 K over a 50-day period in the ambient environment after the fabrication of the devices. This is attributed to a gradual oxidation in the regions near the side surfaces of the Cu channel, where Co impurities are implanted during the fabrication. An increase of spin injection polarization is also found and attributed to the change of oxidation states at the interfaces.     

自旋阀中电流诱导磁化翻转行为的研究

通过实验研究了一种特殊的反对称自旋阀结构．研究发现,随着外加磁场的增大,该结构纳米器件表现出了一种由“逆CIMS”向“正常CIMS”的转变．这种现象是因为：该反对称自旋阀在不同的外加磁场下有不同的磁化取向,因而引起不同的CIMS行为． 关键词： CPP ESPV CIMS     

Spin waves in quasiequilibrium spin systems

Using the Landau Fermi liquid theory we discovered a new propagating transverse spin wave in a paramagnetic system which is driven slightly out of equilibrium without applying an external magnetic field. We find a gapless mode which describes the uniform precession of the magnetization in the absence of a magnetic field. We also find a gapped mode associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to a T3/2 contribution to the specific heat. These modes significantly contribute to the dynamic structure function.     

Spin diffusion in double-exchange manganites

The theoretical study of spin diffusion in double-exchange magnets by means of dynamical mean-field theory is presented. We demonstrate that the spin-diffusion coefficient becomes independent of the Hund's coupling J(H) in the range of parameters J(H)S>W>T, W being the bandwidth, relevant to colossal magnetoresistive manganites in the metallic part of their phase diagram. Our study reveals a close correspondence as well as some counterintuitive differences between the results on Bethe and hypercubic lattices. Our results are in accord with neutron-scattering data and with previous theoretical work for high temperatures.     

Dual spin valves with nano-oxide layers

A novel method is proposed for increasing the giant magnetoresistance (GMR) of dual spin valves of the CoFe/Cu/CoFe/Cu/CoFe type by inserting nano-oxide layers (NOLs) into the pinned layers. Using this method, MR ratio of 23.5% was obtained, a value equal to those of specular spin valves with antiferromagnetic oxide, e.g., NiO. This method allows the selection of metallic materials for the antiferromagnetic layer. In addition, we obtained the specularity factor of upper NOLs, 0.8, and that of lower NOLs, 0.7, by calculating Boltzmann equations taking into account the roughness of each interlayer. This model shows that the MR ratio would be 27.5% for dual spin valves with ideal NOLs.     

Spin current in quantum XXZ spin chain

The spin current in the one-dimensional quantum XXZ spin chain is studied based on the exact solutions. It is found that the spin voltage controlled by the unparallel boundary magnetic fields can induce the pure longitudinal spin current in the system. By using Wiener–Hopf and generalized algebraic Bethe ansatz methods, the analytic expressions for the spin current and the spin conductance are obtained. The spin current is proportional to the inverse of the length of the spin chain. The magnitude of spin current can be manipulated by the strength and the twist angle of two boundary magnetic fields. The exact analysis also shows that there exist an Ohm law or London equation type relation between the spin current and the spin conductance.     

Spin waves and spin polarizability in atoms

Linearized hydrodynamic equations for spin-up and -down fluids oscillating about the Thomas-Fermi ground state are derived variationally and estimates for the lowest-lying spin waves obtained. Static solutions in an external magnetic field yield a model spin susceptibility.     

Spin filter,spin amplifier and spin diode in graphene nanodisk

Graphene nanodisk is a graphene derivative with a closed edge. The trigonal zigzag nanodisk with size N has N-fold degenerated zero-energy states. It can be interpreted as a quantum dot with an internal degree of freedom. The ground state of nanodisk is a quasi-ferromagnet, which is a ferromagnetic-like state with a finite but very long life time. We investigate spin-filter effects in the system made of nanodisks and leads. A novel feature of the nanodisk spin filter is that its spin can be controlled by the spin current. We propose some applications for spintronics, such as spin memory, spin amplifier and spin diode. It is argued that a spin current is reinforced (rectified) by feeding it into a nanodisk spin amplifier (diode). Graphene nanodisk would be a promising candidate of future electronic and spintronic nanodevices.     

Phonon-assisted spin diffusion in solids

The spin flip-flop transition rate is calculated for the case of spectral spin diffusion within a system of dipolarly coupled spins in a solid where the lattice vibrations are present. Long-wavelength acoustic phonons time-modulate the interspin distance r_ij and enhance the transition rate via the change of the 1/r³_ij term in the coupling dipolar Hamiltonian. The phonon-assisted spin diffusion rate is calculated by the golden rule in the Debye approximation of the phonon density of states. The coupling of the spins to the phonons introduces temperature dependence into the transition rate, in contrast to the spin diffusion in a rigid lattice, where the rate is temperature-independent. The direct (one-phonon absorption or emission) processes introduce a linear temperature dependence into the rate at temperatures not too close to T = 0. Two-phonon processes introduce a more complicated temperature dependence that again becomes simple analytical for temperatures higher than the Debye temperature, where the rate is proportional to T², and in the limit T → 0, where the rate varies as T⁷. Raman processes (one-phonon absorption and another phonon emission) dominate by far the phonon-assisted spin flip-flop transitions.     

Spin squeezing inequalities for arbitrary spin

We determine the complete set of generalized spin squeezing inequalities, given in terms of the collective angular momentum components, for particles with an arbitrary spin. They can be used for the experimental detection of entanglement in an ensemble in which the particles cannot be individually addressed. We also present a large set of criteria involving collective observables different from the angular momentum coordinates. We show that some of the inequalities can be used to detect k-particle entanglement and bound entanglement.     

Spin guides and spin splitters: waveguide analogies in one-dimensional spin chains

Here we show a mapping between waveguide theory and spin-chain transport, opening an alternative approach to solid-state quantum information transport. By applying temporally varying control profiles to a spin chain, we design a virtual waveguide or "spin guide" to conduct spin excitations along defined space-time trajectories of the chain. We show that the concepts of confinement, adiabatic bend loss, and beam splitting can be mapped from optical waveguide theory to spin guides, and hence to "spin splitters." Importantly, the spatial scale of applied control pulses is required to be large compared to the interspin spacing, thereby allowing the design of scalable control architectures.     

Spin anisotropy and slow dynamics in spin glasses

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.