Ryoko Sugano

Two-stage melting of disordered vortex-line lattice in Bi2Sr2CaCu2O8+δ

Abstract We study vortex matter phase diagram of Bi2Sr2CaCu2O8+δ, by using Monte Carlo simulation based on Lawrence–Doniach model. Introducing strong point-like disorder, we find two successive phase transformations from the strongly pinned vortex glass to pancake vortex gas via an intermediate weakly pinned glass, when temperature is increased. The first one is identified as an intra-layer depinning transition as pre-melting, which subdivides the glass phase into two phases. The intermediate glass melts with inter-layer decoupling of vortex lines, implying the loss of interlayer long-range coherence. These two-phase boundaries agree with recent experimental data.

Power-law resistive behavior of two-dimensional Josephson junction array with bond defects: Dynamic simulation and its relevance to high-Tc oxides

Abstract Nonlinear electric resistivity of the two-dimensional(2D) Josephson junction array with bond defects (defect density p ) is calculated directly using the Langevin simulation technique. It is found that both the current-voltage ( I-V ) characteristics and the p-V resistive behavior at low temperatures obey power-laws with continuously varying exponents: V ∝ I α p and V ∝ p β I . Computational results are quite similar to those observed experimentally in the ion irradiation effect on high- T c oxides. The ion irradiation is thought to create random defects in the 2D intrinsic weak-link structure which is peculiar to the high- T c oxide superconductor. It is suggested that the vortex-antivortex pair excitation and its transport property in 2D media with bond disorder are crucial for the defect-dependent power-law.

Monte Carlo study of vortex-line dynamics with correlated disorders

Abstract We have studied single vortex-line dynamics with multiple point- and columnar-pins under weak magnetic fields ( H || c -axis) in a model high- T c superconductor, via Monte Carlo simulations in terms of vortex variables. Under various disorder configurations, the I – V characteristics and thermal averages of localization lenght l ⊥ of a single vortex line for various external currents are calculated by introducing the tilted potential. It is found that the non-Ohmic I – V characteristic R ∝ exp[−( E k / T )( I l / I )] appears in the case with multiple columnar-pins. We will discuss this behavior in association with tunneling via half-loop excitation.

Peculiar `from-Edge-to-Interior' Spin Freezing in a Magnetic Dipolar Cube

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipole–dipole interaction, and have found its peculiar from-edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edge grow domains with short-ranged orders similar to the corresponding bulk orders, and finally the system is expected to end up with a unique multidomain ground state at the lowest temperature. We infer that these freezing characteristics are attributed to the anisotropic and long-range nature of the dipole–dipole interaction combined with a finite-size effect.

The effect of pointlike pinning on vortex phase diagram of Bi2Sr2CaCu2O8+δ

Abstract We numerically examine a possible vortex-matter phase diagram of Bi 2 Sr 2 CaCu 2 O 8+ δ in the presence of dense pointlike defects, by using a Monte Carlo simulation based on the Lawrence–Doniach model. Distinct phases of the Bragg glass, vortex glass, and pancake vortex gas, together with a depinning line within the glass phases, manifest themselves on the field ( B )-temperature ( T ) plane. While the locations of these phase boundaries are sensitive to pointlike pinning energy U p ( T ), we phenomenologically obtain a universal B – T phase diagram depending on a single parameter U p ( T ). We also discuss the relevance of numerical simulation to recent experiments.

Landau–Lifshitz–Gilbert study of the effect of pulse width on spin-transfer torque magnetization switching

We studied the effect of a current pulse width on current-induced magnetization switching in magnetic tunnel junctions based on a macrospin model of the free layer. We performed finite temperature Langevin simulations of the Landau–Lifshitz–Gilbert–Slonczewski equation with an additional spin-torque term. By evaluating the switching current density, we obtained the diagram in the plane of the critical current density and the pulse width at 300 K. As the pulse width increased, we observed an adiabatic regime in the shorter pulse widths, an intermediate crossover regime, and a thermally activated regime in long pulse widths. We found that the easy-plane anisotropy field shifts the crossover pulse width to the lower pulse width, suggesting that the reversed region is enhanced by controlling the device shape. Our results are consistent with those of recent experiments over the pulse widths ranging from 10−1 to 105 ns.

Magnetic Properties of Two-Dimensional Dipolar Squares: Boundary Geometry Dependence

By means of the molecular dynamics simulation of gradual cooling processes, we investigate magnetic properties of classical spin systems only with the magnetic dipole–dipole interaction, which we call dipolar systems. Focusing on their finite-size effect, particularly their boundary geometry dependence, we study two finite dipolar squares cut out from a square lattice with Φ=0 and π/4, where Φ is the angle between the direction of the lattice axis and that of the square boundary. Distinctly different results are obtained in the two dipolar squares. In the Φ=0 square, the “from-edge-to-interior freezing” of spins is observed. Its ground state has a multidomain structure whose domains consist of two among infinitely (continuously) degenerated Luttinger–Tisza (LT) ground-state orders on a bulk square lattice, i.e., the two antiferromagnetically aligned ferromagnetic chains (af-FMC) orders directed parallel to the two lattice axes. In the Φ=π/4 square, on the other hand, the freezing starts from the interior ...

Point-defects-induced vortex phase diagram in high-Tc superconductors: Monte Carlo simulation study

Abstract We investigated the vortex phase diagram of high- T c copper-oxide superconductors in the presence of dense point defects (PDs), by using a Monte Carlo simulation based on the Lawrence–Doniach model. We found a temperature-driven depinning line within the Bragg and vortex glass phases, well below the melting line in the case of high anisotropy. We also found a field-driven transition line from the Bragg-to-vortex glass at low temperatures, accompanied by an abrupt reduction in the interlayer vortex correlation. The complex phase boundaries are drastically controlled by the pinning force of PDs, defect density, and anisotropy.

Power-law resistive behavior and KT transition of a weakly-frustrated 2D Josephson-junction array

Abstract Current-voltage ( I – V ) characteristics and magnetoresistance are calculated for a frustrated 2D Josephson-junction array in weak frustration( f ) regime via finite-temperature dynamic simulations. There are two co-existing non-linear power-laws, V ∝ I α and V ∝ f s , in which the universal jump of Kosterlitz-Thouless transition appears in the temperature dependence of both exponents, at α ≅ 3 and at s ≅ 2. Numerical and experimental results are in accord in high z . sbnd ; T c oxides under magnetic fields.

Three dimensional simulations of spin Hall effect in magnetic nanostructures

We investigate the spatial distribution of spin/charge current in a nonlocal geometry of a magnetic nanostructure with Py∕I∕Cu and Cu∕Pt contacts and analyze the spin Hall effects by applying finite element method in three dimensions. We find that the inhomogeneous current distribution appears in the Cu∕Pt connection area, suggesting the possibility of the spin-signal enhancement by controlling the contact shape and the width of the electrodes. The calculated spin Hall voltage is consistent with the experimentally observed Hall resistance.