Nanometer scale observation of high efficiency thermally assisted current-driven domain wall depinning

Nanometer scale observation of the depinning of a narrow domain wall (DW) under a spin current is reported. We studied approximately 12 nm wide 1D Bloch DWs created in thin films exhibiting perpendicular magnetic anisotropy. Magnetotransport measurements reveal thermally assisted current-driven DW motion between pinning sites separated by as little as 20 nm. The efficiency of current-driven DW motion assisted by thermal fluctuations is measured to be orders of magnitude higher than has been found for in-plane magnetized films, allowing us to control DW motion on a nanometer scale at low current densities.     

On bound state computations in three- and four-electron atomic systems

A variational approach is developed for bound state calculations in three- and four-electron atomic systems. This approach can be applied to determine, in principle, an arbitrary bound state in three- and four-electron ions and atoms. Our variational wave functions are constructed from four- and five-body Gaussoids that respectively depend on six (r ₁₂, r ₁₃, r ₁₄, r ₂₃, r ₂₄, r ₃₄) and ten (r ₁₂, r ₁₃, r ₁₄, r ₁₅, r ₂₃, r ₂₄, r ₂₅, r ₃₄, r ₃₅ and r ₄₅) relative coordinates. The approach allows operating with the more than one electron spin functions. In particular, the trial wave functions for the ¹ S states in four-electron atomic systems include the two independent spin functions χ₁ = αβαβ + βαβα − βααβ − αββα and χ₂ = 2ααββ + 2ββαα − βααβ − αββα − βαβα − αβαβ. We also discuss the construction of variational wave functions for the excited 2³ S states in four- electron atomic systems.     

Spin filtering through ballistic nanojunctions, the role of geometry and of spin orbit interaction

We suggest a spin filter scheme using T-stub nanometric crossjunctions patterned in two dimensional electron gases (2DEGs) in the presence of spin orbit interaction (SOI). We compare the effects of SOI arising from vertical confinement of charge carriers in the well, Rashba or α-SOI, with SOI generated by lateral confinement of the wire, β-SOI. We show that β coupling can be more effective in generating a spin polarized current as compared to α-SOI. We also compare the efficiency of the T-stub filter with the one of the X shaped cross junction.     

Magnetic properties of ordered nanowires of the quasi-two-dimensional antiferromagnet SpFeMn(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>

Ordered arrays of nanowires of the photochromic antiferromagnet SpFeMn(C₂O₄)₃ (where Sp is 1-{(1′,3′,3′-trimethyl-6-nitro-5′-chlorospiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) have been fabricated in anodized aluminum oxide pores with diameters of 20 and 200 nm. It has been revealed that the growth of the spin-glass phase with noncollinear ordering of spins in nanowires is suppressed in favor of the uniaxial antiferromagnetic phase. A decrease in the nanowire diameter leads to an increase in the anisotropy of the magnetic resonance spectra. This is associated with the magnetocrystalline anisotropy that considerably exceeds the anisotropy of the nanowire shape.     

Current-driven magnetization switching and domain wall motion in nanostructures—Survey of recent experiments

An overview of recent experimental studies and new routes in the field of current-driven magnetization dynamics in nanostructured materials is given. The review introduces the basic concepts (Landau–Lifshitz phenomenology, critical current, spin currents in relation to spin accumulation, adiabatic/non-adiabatic spin-torque) and describes the main results of recent experiments on current-driven magnetization reversal within vertical pillar-like nanostructures and current-driven domain wall motion within laterally confined specimens. While for the pillar systems a discussion is provided of how the introduction of layers with perpendicular magnetic anisotropy, tunnel barriers and exchange bias and(or) oxide layers can be used to reduce the critical current densities for current-induced switching, the role of perpendicular anisotropy, use of spin valve structures, diluted magnetic semiconductors and epitaxial materials to increase the domain wall velocities are reviewed in the case of current-driven domain wall movement within lateral systems.     

Influence of exchange coupling on current-driven domain wall motion in a nanowire

In this study, the effect of exchange stiffness constant on current-driven domain wall motion in nanowires with in-plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA) has been investigated using micromagnetic simulation. The critical current density in a nanowire with IMA decreases as the exchange stiffness constant decreases because the domain wall width at the upper edge of the nanowire narrows according to the decrease of the exchange stiffness constant. On the other hand, the critical current density in a nanowire with PMA slightly decreases contrary to that of IMA although the domain wall width reasonably decreases as the exchange stiffness constant decreases. The slight reduction rate of the critical current density is due to the increase of the effective hard-axis anisotropy of PMA nanowire.     

Third-order elastic moduli of single-layer graphene

In the framework of the Keating model with allowance made for the anharmonic constant of the central interaction between the nearest neighbors μ, analytical expressions have been obtained for three third-order independent elastic constants c _ijk (μ, ζ) of single-layer graphene, where ζ = (2α − β)/(4α + β) is the Kleinman internal displacement parameter and α and β are the harmonic constants of the central interaction between the nearest neighbors and the noncentral interaction between the next-nearest neighbors, respectively. The dependences of the second-order elastic constants on the pressure p have been determined. It has been shown that the moduli c ₁₁ and c ₂₂ differently respond to the pressure. Therefore, graphene is isotropic in the harmonic approximation, whereas the inclusion of anharmonicity leads to the appearance of the anisotropy.     

Sign of the quadrupole moment of proton drip-line nucleus 8B

The sign of the quadrupole coupling constant eqQ of ⁸B in ZrB₂ is determined to be positive through the β-delayed α-ray anisotropy measurement combined with the spin manipulation technique by use of the β-NMR. According to the theoretical calculation of the electric field gradient in ZrB₂, the sign of the quadrupole moment of ⁸B is deduced to be positive. This revised version was published online in August 2006 with corrections to the Cover Date.     

Two-particle dispersion in model two-dimensional velocity fields

We consider two-particle dispersion in a velocity field, where the relative two-point velocity scales according to v ²(r) ∝r ^α and the corresponding correlation time scales as τ(r) ∝r ^β, and fix α = 2/3, as typical for turbulent flows. We show that two generic types of dispersion behavior arize: For α/2 + β < 1 the correlations in relative velocities decouple and the diffusion approximation holds. In the opposite case, α/2 + β > 1, the relative motion is strongly correlated. The case of Kolmogorov flows corresponds to a marginal, nongeneric situation. In this case, depending on the particular parameters of the flow, the dispersion behavior can be rather diffusive or rather ballistic. Received 13 March 2001     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Domain wall dynamics driven by spin transfer torque and the spin-orbit field

We have studied current-driven dynamics of domain walls when an in-plane magnetic field is present in perpendicularly magnetized nanowires using an analytical model and micromagnetic simulations. We model an experimentally studied system, ultrathin magnetic nanowires with perpendicular anisotropy, where an effective in-plane magnetic field is developed when current is passed along the nanowire due to the Rashba-like spin-orbit coupling. Using a one-dimensional model of a domain wall together with micromagnetic simulations, we show that the existence of such in-plane magnetic fields can either lower or raise the threshold current needed to cause domain wall motion. In the presence of the in-plane field, the threshold current differs for positive and negative currents for a given wall chirality, and the wall motion becomes sensitive to out-of-plane magnetic fields. We show that large non-adiabatic spin torque can counteract the effect of the in-plane field.     

Thermal depolarisation current study of electron irradiated fluorinated ethylene propylene

Thermally stimulated current (TSC) studies have been reported in the co-polymer of tetrafluoro ethylene and hexafluoropropylene films. Depolarisation current peaks are obtained atα ₁,α ₂ andβ relaxation temperatures of the polymer and the detrapping process is explained on the basis of its molecular motion. A cross-over electron energy of 18 keV is observed where the nature of TSC spectra undergoes a remarkable change. This is explained in relation to the surface states in FEP. Five groups of trapping levels, 0.25±0.08, 0.57±0.10, 1.07±0.1, 1.3±0.25 and 2.3±0.4 eV are obtained.     

Magneto-optical and micromagnetic simulation study of the current-driven domain wall motion in ferromagnetic (Ga,Mn)As

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved polar magneto-optical Kerr effect microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1-2×10⁵ A/cm². Considering the spin-transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current-driven domain wall motion in this material.     

Electrical properties of the conducting polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine

The electrical properties of the conducting polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine (poly-THD) are investigated for the first time. The stability of the conductivity with time, the current-voltage characteristics over a broad range of applied voltages, the pressure dependence of the conductivity, and the low-frequency conductivity fluctuations in the range of analysis frequencies 20 Hz–10 kHz are studied. Appreciable anisotropy of the conductivity is discovered under strong uniaxial compression. Hooge’s parameter α is evaluated (α∼20 at 300 K). Fiz. Tverd. Tela (St. Petersburg) 39, 778–782 (April 1997)     

Analytic Perturbation Theory for Bound States in the Transfer Matrix Spectrum of Weakly Correlated Lattice Ferromagnetic Spin Systems

We consider general d-dimensional lattice ferromagnetic spin systems with nearest neighbor interactions in the high temperature region (β≪1). Each model is characterized by a single site apriori spin distribution taken to be even. We also take the parameter α=〈s ⁴〉−3〈s ²〉²>0, i.e. in the region which we call Gaussian subjugation, where 〈s ^k 〉 denotes the kth moment of the apriori distribution. Associated with the model is a lattice quantum field theory known to contain a particle of asymptotic mass −lnβ and a bound state below the two-particle threshold. We develop a β analytic perturbation theory for the binding energy of this bound state. As a key ingredient in obtaining our result we show that the Fourier transform of the two-point function is a meromorphic function, with a simple pole, in a suitable complex spectral parameter and the coefficients of its Laurent expansion are analytic in β.     

On the mechanism of phase transformation in Ag2Se

A differential thermal analysis ΔT _y (T) in vacuum has been performed, and the temperature gradient ΔT _x (T) along the Ag₂Se sample during the transition α → β has been studied. It has been shown that the transitions α → α′ and β′ → β are displacive transitions and that the transition α′ → β′ is a reconstructive transition. It has been found that the temperature gradient along the sample during the transition α′ → β′ passes through a deep minimum due to a strong increase in the specific heat capacity.     

Real-space observation of current-driven domain wall motion in submicron magnetic wires

We report direct observation of current-driven magnetic domain wall (DW) displacement by using a well-defined single DW in a microfabricated magnetic wire with submicron width. Magnetic force microscopy visualizes that a single DW introduced in a wire is displaced back and forth by positive and negative pulsed current, respectively. The direct observation gives quantitative information on the DW displacement as a function of the intensity and the duration of the pulsed current. The result is discussed in terms of the spin-transfer mechanism.     

Effect of the Dzyaloshinskii-Moriya interaction on heat conductivity in one-dimensional quantum Ising chains

The effect of the Dzyaloshinskii-Moriya (DM) interaction on the heat conduction in the quantum Ising chain has been studied by solving the Lindblad master equation. The chain is subject to a uniform transverse field h, while the exchange couplings {J _m } between the nearest-neighbor spins are either uniform, random or quasi-periodic. The average energy-density profile and the average energy current in the non-equilibrium steady state have been numerically calculated. The ballistic transport is observed in the uniform Ising chain with DM interaction. For the random Ising chain with DM interaction, the energy gradient is observed in the bulk of the spin chain whose energy current appears to scale as the system size ⟨Q⟩ ∼ exp(βN) with β < 0. For the quasi-periodic Ising chain with DM interaction, the J _m takes the two values J _A and J _B arranged in the Fibonacci sequence. The energy gradient also exists in the spin chain and the energy current behaves as ⟨Q⟩ ∼ N ^α with α < 0. By increasing the strength of the DM interaction D, a non-trivial transition from the thermal insulator heat transport to anomalous heat conduction is found in the Fibonacci Ising chain with large ratio of couplings λ = J _A /J _B . A rough phase diagram of λ vs. D is given in this paper as well.     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

Uniform spin wave modes in antiferromagnetic nanoparticles with uncompensated moments

In magnetic nanoparticles the uniform precession (q = 0 spin wave) mode gives the predominant contribution to the magnetic excitations. We have calculated the energy of the uniform mode in antiferromagnetic nanoparticles with uncompensated magnetic moments, using the coherent potential approximation. In the presence of uncompensated moments, an antiferromagnetic nanoparticle must be considered as a kind of a ferrimagnet. Two magnetic anisotropy terms are considered, a planar term confining the spins to the basal plane, and an axial term determining an easy axis in this plane. Excitation energies are calculated for various combinations of these two anisotropy terms, ranging from the simple uniaxial case to the planar case with a strong out-of-plane anisotropy. In the simple uniaxial case, the uncompensated moment has a large influence on the excitation energy, but in the planar case it is much less important. The calculations explain recent neutron scattering measurements on nanoparticles of antiferromagnetic α-Fe₂O₃ and NiO.