Nonmonotonic zero-point entropy in diluted spin ice

Water ice and spin ice are important model systems in which theory can directly account for "zero-point" entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with nonmagnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90%. Investigations of both Ho and Dy spin ices reveal that the zero-point entropy depends nonmonotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice.     

Nonextensive statistical mechanics of q-bosons based on the q-deformed entropy

Based on the q ↔ q⁻¹ symmetric deformed entropy, we develop a general framework for nonextensive statistical mechanics of ensembles of q-deformed systems. Applying this doubly deformed formalism to q-bosons, a correction to the Planck law is evaluated in the weak deformation regime and its properties are discussed. It is found that at high temperature the dominant part of the correction comes from the deformation of the oscillator dynamics, whereas at low temperature the deformation of the entropy gives a leading contribution. This suggests the nonextensive approach to q-deformed ensembles might be important at low temperature.     

Probability distribution function of dipolar field in two-dimensional spin ensemble

We theoretically determine the probability distribution function of the net field of the random planar structure of dipoles which represent polarized particles. At small surface concentrations c of the point dipoles this distribution is expressed in terms of special functions. At the surface concentrations of the dipoles as high as 0.6 the dipolar field obey the Gaussian law. To obtain the distribution function within transitional region c < 0.6, we propose the method based on the cumulant expansion. We calculate the parameters of the distributions for some specific configurations of the dipoles. The distribution functions of the ordered ensembles of the dipoles at the low and moderate surface concentrations have asymmetric shape with respect to distribution medians. The distribution functions allow to calculate various physical parameters of two-dimensional interacting nanoparticle ensembles.     

Residual entropy of ice nanotubes and ice layers

A relatively simple algorithm is presented for the complete enumeration of all H-bond networks in finite fragments of ice nanotubes and ice layers with periodic boundary conditions. This algorithm is based on the well-known transfer matrix method and it includes a convenient procedure for calculation of the elements of transfer matrices themselves. To facilitate this, it is necessary to specify only very small local matrices of sizes 2×2 or 4×4. We present exhaustive statistics of H-bonds arrangements for finite-size zigzag- and armchair-like ice nanotubes, for the fragments of hexagonal monolayer and bilayer and also for ice nanotubes consisting of stacked n$n$-membered rings. Using the new algorithm, we have also calculated the specific residual entropy for the infinite two-dimensional lattices. The agreement with the well-known solution for a square ice model demonstrates the reliability of the obtained results.     

Why spin ice obeys the ice rules

The low-temperature entropy of the spin ice compounds, such as and , is well described by the nearest-neighbor antiferromagnetic Ising model on the pyrochlore lattice, i.e., by the "ice rules." This is surprising since the dominant coupling between the spins is their long ranged dipole interaction. We show that this phenomenon can be understood rather elegantly: one can construct a model dipole interaction, by adding terms of shorter range, which yields precisely the same ground states, and hence entropy, as the nearest-neighbor interaction. A treatment of the small difference between the model and true dipole interactions reproduces the numerical work by Gingras et al. in detail. We are also led to a more general concept of projective equivalence between interactions.     

非广延统计力学与完全开放系统的统计分布

简介了非广延统计力学的Tsallis统计,用其计算了理想气体;推导出了以含有非广延熵常数的Shannon熵为基础和以Tsallis熵为基础的非广延统计力学的完全开放系统的统计分布及计算热力学量的公式;讨论表明:Tsallis熵对应的统计分布及计算热力学量的公式在非广延参量q→1时,完全过渡到了Shannon熵对应的形式.     

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     

Self-consistent treatment of spin and magnetization dynamic effect in spin transfer switching

The effect of itinerant spin moment (m) dynamic in spin transfer switching has been ignored in most previous theoretical studies of the magnetization (M) dynamics. Thus in this paper, we proposed a more refined micromagnetic model of spin transfer switching that takes into account in a self-consistent manner of the coupled m and M dynamics. The numerical results obtained from this model further shed insight on the switching profiles of m and M, both of which show particular sensitivity to parameters such as the anisotropy field, the spin torque field, and the initial deviation between m and M.     

A novel type of domain walls with two-dimensional magnetization distribution in magnetic triaxial films

On the basis of numerical minimization of total energy in magnetic triaxial ferromagnetic films with a surface of a (1 1 0)-type, we investigated two-dimensional structures of domain walls within a rigorous micromagnetic approach that takes into account all the main interactions including the dipole–dipole one. Novel two-vortex and three-vortex domain wall structures are established to exist. The profiles of domain wall structures and their stability regions are studied.     

Exact valence bond entanglement entropy and probability distribution in the XXX spin chain and the potts model

We determine exactly the probability distribution of the number N_(c) of valence bonds connecting a subsystem of length L>1 to the rest of the system in the ground state of the XXX antiferromagnetic spin chain. This provides, in particular, the asymptotic behavior of the valence-bond entanglement entropy S_(VB)=N_(c)ln2=4ln2/pi(2)lnL disproving a recent conjecture that this should be related with the von Neumann entropy, and thus equal to 1/3lnL. Our results generalize to the Q-state Potts model.     

Decay of the remanent magnetization in the asymmetric spin chain

The dynamics of the one-dimensional spin glass with asymmetric interactions between neighboring spins is considered. We confine ourselves to discrete couplings with values ±J. We show that the algebraic decay of the remanent magnetization of the infinite ±J-spin chain at zero temperature is only valid for symmetric couplings. Our analytical investigations as well as computer simulations show stretched exponential decay for any finite concentration of antisymmetric bonds. Thus, the asymmetric ±J-spin chain shows an asymmetry-induced phase transition at zero temperature.     

Nonextensive black-body distribution function and Einstein's coefficients A and B

The nonextensive statistics of a boson gas within the dilute gas approximation is applied to black-body radiation. Einstein's spontaneous emission coefficient A₂₁, stimulated emission coefficient B₂₁, and absorption coefficient B₁₂ are studied in connection with this new statistical mechanics. Relations including nonextensivity between these coefficients are obtained.     

Oscillatory magnetization of two-dimensional electron gas

Diamagnetic moment of the two-dimensional electron gas is calculated for finite temperature using a simple energy band model and assuming no collisional broadening of Landau levels. Numerical results are presented for GaAs band parameters. The diamagnetic moment oscillates around zero value as a function of magnetic field strength and tends to zero at low fields. It is concluded that both the diamagnetic and the paramagnetic susceptibilities of the 2D electron gas vanish in the low field limit.     

Self-consistent calculation of spin transport and magnetization dynamics

A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting various types of current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, simultaneous, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization after the spin accumulation has been integrated out. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can make stationary states unstable and induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. A key aspect of this reduction is the suppression of the excitation of short wavelength spin waves by the non-local feedback. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque. These examples show that the self-consistent treatment of spin transport and magnetization dynamics is important for understanding the physics of the coupled dynamics and for providing a bridge between the ongoing research fields of current-induced magnetization dynamics and the newly emerging fields of magnetization-dynamics-induced generation of charge and spin currents.     

Anomalous sensitivity to initial conditions and entropy production in standard maps: Nonextensive approach

We perform a throughout numerical study of the average sensitivity to initial conditions and entropy production for two symplectically coupled standard maps focusing on the control-parameter region close to regularity. Although the system is ultimately strongly chaotic (positive Lyapunov exponents), it first stays lengthily in weak-chaotic regions (zero Lyapunov exponents). We argue that the nonextensive generalization of the classical formalism is an adequate tool in order to get nontrivial information about the first stage of this crossover phenomenon. Within this context we analyze the relation between the power-law sensitivity to initial conditions and the entropy production.     

Susceptibility,hysteresis and magnetization states in the CuMn spin glass

AC susceptibilities have been measured at T ⪡ T_f on a quenched Cu-8.25 at% Mn sample along the hysteresis curve and especially in remanent magnetization states. Along the M(H)-curve _ϰ∥ starts to decrease already below the S-point field H_w, when dM/dM is still increasing. After reduction of the external field from H ⪢ H_w to zero ϰ_⊥ by far exceeds ϰ _∥, which itself is a little increased above its starting (ZFC) value. Reversing the external field ϰ_∥ develops distinct maxima at the step fields H_st. A model is developed considering the spin glass at M_r to consist of magnetic moments bound by different anisotropies to preferential directions which are oriented partly at random (anisotropy field H¹_w), partly along M_r (anisotropy field H H¹_st ⪡ H¹_w). The model connects both susceptibilities with characteristic parameters of the hysteresis and quantitatively explains the data.     

Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.