Munetaka Sasaki

Temperature chaos and bond chaos in Edwards-Anderson Ising spin glasses: domain-wall free-energy measurements.

Domain-wall free energy sigmaF, entropy sigmaS, and the correlation function C(temp) of sigmaF are measured independently in the four-dimensional +/-J Edwards-Anderson (EA) Ising spin glass. The stiffness exponent theta, the fractal dimension of domain walls d(s), and the chaos exponent zeta are extracted from the finite-size scaling analysis of sigmaF, sigmaS, and C(temp), respectively, well inside the spin-glass phase. The three exponents are confirmed to satisfy the scaling relation zeta = d(s)/2 - theta derived by the droplet theory within our numerical accuracy. We also study bond chaos induced by random variation of bonds, and find that the bond and temperature perturbations yield the universal chaos effects described by a common scaling function and the chaos exponent. These results strongly support the appropriateness of the droplet theory for the description of chaos effect in the EA Ising spin glasses.

Strain-Induced Néel Temperature Enhancement in Corundum-Type Cr2O3 and Fe2O3

The effect of strain on Neel temperature in corundum-type Cr2O3 and Fe2O3 was theoretically studied. We calculated the exchange coupling constants up to the fifth-nearest neighbors using the first-principles density functional method and evaluated the Neel temperature using Monte Carlo simulation of the classical Heisenberg model. We showed that the Neel temperature is enhanced (decreased) by tensile (compressive) strain along the c-axis. The Neel temperatures of Cr2O3 and Fe2O3 are enhanced more than 20 and 7% by 5% strain of the crystal lattice, respectively.

Memory effect, rejuvenation and chaos effect in the multi-layer Random Energy Model

We introduce magnetization to the Multi-layer Random Energy Model which has a hierarchical structure, and perform Monte Carlo simulations to observe behavior of ac-susceptibility. We find that this model is able to reproduce three prominent features of spin glasses, i.e., memory effect, rejuvenation and chaos effect, which were found recently by various experiments on aging phenomena with temperature variations.

Scaling analysis of domain-wall free energy in the Edwards-Anderson Ising spin glass in a magnetic field.

The stability of the spin-glass phase against a magnetic field is studied in the three- and four-dimensional Edwards-Anderson Ising spin glasses. Effective couplings J(eff) and effective fields H(eff) associated with length scale L are measured by a numerical domain-wall renormalization-group method. The results obtained by scaling analysis of the data strongly indicate the existence of a crossover length beyond which the spin-glass order is destroyed by field H. The crossover length well obeys a power law of H which diverges as H --> 0 but remains finite for any nonzero H, implying that the spin-glass phase is absent even in an infinitesimal field. These results are well consistent with the droplet theory for short-range spin glasses.

Stochastic Cutoff Method for Long-Range Interacting Systems

A new Monte Carlo method for long-range interacting systems is presented. This method involves eliminating interactions stochastically with the detailed balance condition satisfied. When pairwise interactions V i j of an N -particle system decrease with the distance as r i j -α , computational time per Monte Carlo step is O ( N ) for α≥ d and O ( N 2-α/ d ) for α< d , where d is the spatial dimension. We apply the method to a two-dimensional magnetic dipolar system. The method enables us to treat a huge system of 256 2 spins within a reasonable computational time, and reproduces a circular order originating from long-range dipolar interactions.

Microwave Generation on Geometrically Constrained Magnetic Wall: Effect of Twist Angle

We studied the current-induced magnetization dynamics of geometrically constrained magnetic walls (GCMWs) with twist angles of 90–180°, in order to examine their possibility as microwave generators using the effects of the twist angles. We numerically solved the Landau–Lifshitz–Gilbert equation with a spin-transfer torque term and found that the magnetic structures of GCMWs with certain twist angles oscillate between the Bloch and Neel walls (OBN) with microwave frequency, even though the twist angle is not 180°. We also found the critical twist angle below which the oscillation of the magnetic structure is inhibited. We showed that the oscillation frequency shows a hysteresis when we vary the twist angle of a GCMW or the corresponding applied magnetic field. Then, we also discussed the microwave generation induced by the periodic-deformation dynamics of GCMWs in the OBN.

Analysis on Aging in the Generalized Random Energy Model

A new dynamics more natural than that proposed by Bouchaud and Dean is introduced to the Generalized Random Energy Model, and the master equation for the dynamics is solved exactly to calculate the time correlation function C ( t + t w , t w ) . Although our results are very similar to those obtained by Bouchaud and Dean qualitatively, the exponents for power law relaxation are different. The Zero-Field-Cooled magnetization is also calculated with a relation between the correlation function and the response function which holds even if the relaxation is non-equilibrium. The validity of these analytic results are confirmed by numerical simulations.

Effect of lattice deformation on exchange coupling constants in Cr2O3

We studied lattice deformation effect on exchange interaction in the corundum-type Cr2O3 theoretically. First-principles electronic structure calculations were performed to evaluate the total energy and exchange coupling constants of Cr2O3 under lattice deformation. We found that a few percent elastic deformation is expected via misfit strain and that the first- and second-nearest neighbor exchange coupling constants of Cr2O3 strongly depend on the lattice deformation. These results imply a possibility for improving the thermal stability of Cr2O3 based magnetoelectric devices by lattice deformation.

An Efficient Monte-Carlo Method for Calculating Free Energy in Long-Range Interacting Systems

We present an efficient Monte-Carlo method for long-range interacting systems to calculate free energy as a function of an order parameter. In this method, a variant of the Wang–Landau method regarding the order parameter is combined with the stochastic cutoff method, which has recently been developed for long-range interacting systems. This method enables us to calculate free energy in long-range interacting systems with reasonable computational time despite the fact that no approximation is involved. This method is applied to a three-dimensional magnetic dipolar system to measure free energy as a function of magnetization. By using the present method, we can calculate free energy for a large system size of 16 3 spins despite the presence of long-range magnetic dipolar interactions. We also discuss the merits and demerits of the present method in comparison with the conventional Wang–Landau method in which free energy is calculated from the joint density of states of energy and order parameter.

Reformulation of the stochastic potential switching algorithm and a generalized Fourtuin-Kasteleyn representation.

A new formulation of the stochastic potential switching algorithm is presented. This reformulation naturally leads us to a generalized Fourtuin-Kasteleyn representation of the partition function Z. A formula for internal energy E and that of heat capacity C are derived from derivatives of the partition function. We also derive a formula for the exchange probability in the replica exchange Monte Carlo method. By combining the formulas with the Stochastic cutoff method, we can greatly reduce the computational time to perform internal energy and heat capacity measurements and the replica exchange Monte Carlo method in long-range interacting systems. Numerical simulations in three-dimensional magnetic dipolar systems show the validity of the method.