Detection of current-induced resonance of geometrically confined domain walls

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10(10) A/m2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.     

Pb(Zr0.52Ti0.48)O3陶瓷畴界粘滞运动的介电损耗模拟

对不同频率下(1—10kHz)Pb(Zr_0.52Ti_0.48)O₃多晶陶瓷的介电性能的测量表明：在接近铁电相变温度T_c以下存在一介电损耗峰,该峰具有弛豫特征但不满足Arrhenius关系．这一损耗峰被认为是由于畴界与晶格、缺陷钉扎的互作用引起的．用畴界粘滞运动的动力学方程,考虑陶瓷样品中T_c离散分布的情况,模拟了该介电损耗峰在不同频率下的行为,得到了与实验数据一致的结果．并由拟合参数计算了畴界运动     

Pb(Zr0.52Ti0.48)O3陶瓷畴界粘滞运动的介电损耗模拟

对不同频率下（１ —１０ ｋ Ｈｚ） Ｐｂ（ Ｚｒ０５２ Ｔｉ０４８） Ｏ３ 多晶陶瓷的介电性能的测量表明：在接近铁电相变温度 Ｔｃ 以下存在一介电损耗峰,该峰具有弛豫特征但不满足 Ａｒｒｈｅｎｉｕｓ 关系．这一损耗峰被认为是由于畴界与晶格、缺陷钉扎的互作用引起的．用畴界粘滞运动的动力学方程,考虑陶瓷样品中 Ｔｃ 离散分布的情况,模拟了该介电损耗峰在不同频率下的行为,得到了与实验数据一致的结果．并由拟合参数计算了畴界运动的粘滞系数和缺陷钉扎引起的回复力常量．     

Structure and interaction of antiferromagnetic domain walls in hexagonal YMnO3

The structure of antiferromagnetic (AFM) domain walls and their interaction with lattice strain are derived taking the multiple-order-parameter compound YMnO3 as a model example. Contrary to the conviction that AFM domain walls are energetically unfavorable, their interaction with lattice strain lowers the total energy of the system and leads to a piezomagnetic clamping of the electric and magnetic order parameters in good agreement with the available experimental data.     

Coercivity of precipitation hardened cobalt rare earth 17:2 permanent magnets

The influence of the microstructure on the coercivity has been investigated by means of transmission electron microscopy. It is shown that a thin coherent (CoCu)₅Sm-cell boumdary phase, separating cells of 17:2-crystal structure, acts as a pinning centre for magnetic domain walls. The attractive interaction force is interpreted in terms of a micromagnetic theory for domain wall pinning. The coercive force is determined by the domain wall energy gradient and by the magnetoelastic coupling energy between domain wall stresses and lattice deformation strains. The calculated coercive force due to the lattice mismatch, originated by the cellular coherent precipitation structure, is comparable to the experimentally obtained values.     

Ferroelectrics

A diatomic linear chain model is used to describe the dynamical properties of displacive type ferroelectric compounds. For these materials the non-bondingp-orbitals of the chalcogen ions play an essential role, which is taken into account by assuming a nonlinear fourth-order polarizability at the chalcogen-ion lattice site. Within the self-consistent phonon approximation soft modes, phonon dispersion curves, phonon anomalies and related quantities can be calculated for all temperatures. Going beyond the self-consistent phonon approximation, the model yields, in the continuum limit, interesting new solutions such as periodic non-linear waves, kinks, which describe the statics and dynamics of ferroelectric domain walls, and pulse solutions.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Domino mechanisms in photoinduced phase transitions

Photoinduced structural phase transitions via excited electronic states are discussed theoretically using a one-dimensional model composed of localized electrons and lattices under the adiabatic or diabatic approximation. We show that the global structural change by photoexcitation only at a site is possible, and we clarify conditions for the occurrence of such phenomena. Spatiotemporal dynamics of nonequilibrium first-order phase transitions is also investigated in detail in terms of photoinduced nucleations and domino processes of the domain boundaries (domain walls), which are in striking contrast to the mean-field dynamics. In the adiabatic regime, after the spontaneous emission of a photon, an initial local structural change (i) remains locally, (ii) induces cooperatively a global structural change, or (iii) disappears and returns to the initial phase. Dynamical features of the case (ii) are characterized by the deterministic (semichaotic) domino process; domain walls between the two phases move determinis-tically at a constant velocity (with changing speed) without further spontaneous emissions in the case of strong (weak) dissipation. In the diabatic regime, similar three types of structural change exist. The domain-wall dynamics is described as the stochastic domino process, which is accompanied by the successive radiative transitions. A new theoretical treatment is also proposed to study crossover between the adiabatic and diabatic regimes.     

Effect of piezomagnetism on coupling of electric and magnetic domain walls in hexagonal manganites

The profiles of antiferromagnetic domain walls in hexagonal manganites RMnO₃ are obtained numerically depending on anisotropy and internal strain due to the lattice distortion at the ferroelectric domain walls. It is found that the piezomagnetism can lower the free energy of the system thus it favors the coupling between electric and magnetic domain walls. Due to the piezomagnetic effect, the clamped antiferromagnetic domain walls with spin orientation angle ψ changing from 0 to π have different profiles comparing with those of ψ changing from 0 to -π, and the former is energetically more favorable than the latter when the internal strain is tensile at the FEL domain walls while it is the contrary for compressive strain. Moreover, the strongest coupling between the FEL domain walls and the favorable AFM domain walls can be achieved at an optimized internal strain.     

Study of the specific features of lithium niobate crystals near the domain walls

The domain walls in LiNbO₃ are studied using X-ray topography, X-ray diffraction, and scanning electron microscopy. It is shown that the regions of distortions of the crystal lattice near the domain walls of different types are different and have lateral sizes of 100–200 μm. The character and the magnitude of the distortions are substantially dependent on the method of formation of the domain structure. Under irradiation in a scanning electron microscope, the crystal region up to 25 μm in width near the “tail-to-tail” domain walls is charged more slowly and, unlike the “head-to-head” domain walls, exhibits a dynamic charge image in the secondary-electron regime.     

On the thickness of domain walls in ferroelectrics and ferroelastics

Ferroelastic and ferroelectric domain walls are commonly described by wall profiles of the tanh(x/w)-type. We argue that this profile is still a good approximation if higher-order gradient energies are considered. Such energies are relevant for phase transitions close to structural incommensurations and also for phase transitions with dominant elastic interactions. Their effect on the wall profile is to influence the effective wall thickness. Positive gradient energies tend to widen domain walls beyond the values predicted in classic Landau-Ginzburg theory.     

Electroacoustic waves confined by a moving domain wall superlattice of a ferroelectric crystal

The dispersion properties of electroacoustic wave modes confined by a superlattice of uniformly moving 180° domain walls in a tetragonal ferroelectric crystal are considered. It is shown that the manifold of partial electroacoustic interfacial waves in the lattice is restricted to the first allowed band, the configuration of which in the plane of spectral variables can significantly vary under the action of the moving domain walls.     

Self-consistent treatment of superconducting hole systems and anharmonic lattices in oxides

An extention of Hirsch's Hamiltonian describing the hole system is proposed by including a coupling to the anharmonic lattice and a modulation of the hopping interaction due to lattice displacements. The model is discussed within the variational Bogoliubov's approach, assuming a BCS-like trial Hamiltonian for the hole-like subsystem and a self-consistent phonon approximation for the lattice. The model allows in general to discuss an interplay between superconductivity and ferroelectricity. The results for the critical temperature and the isotope effect coefficient in the superconducting paraelectric phase in one dimension are presented. In particular, the strong influence of the phonon softening on the superconducting transition is widely discussed.     

Dissipation of longitudinal elastic waves in magnets considering displacement and rotation processes

The amplitude-independent component of magnetic losses (the internal friction) and the dynamic defect of Young's modulus associated with processes of reversible displacements of domain walls and rotations of spontaneous magnetization vectors are described theoretically for idealized triaxial magnets considering the actual domain structure, the domain wall type, and the concentrations of magnetic phases. From formulas obtained by the perturbation method it follows that the contribution of rotation processes to the effects examined is rather noticeable. Kursk State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 3–8, February, 2000.     

Analysis of the displacement field of a moving dislocation in a crystal

The method of stationary phase is applied to the asymptotic evaluation of the integrals that give the atomic displacements for a high velocity screw dislocation in a simple cubic lattice. The moving dislocation creates a wake behind it. Inside the wake there are large oscillatory displacements due to the emission of sound waves, while in front of the dislocation the displacements are given to a good approximation by continuum elastic theory. The strain field inside the wake is oscillatory and falls off as the square root of the distance from the dislocation core in the absence of dissipation; an exponential damping occurs if the phonons are assigned a finite lifetime. Special attention is given to the edge of the wake where the strain field decreases as the inverse cube root in the absence of damping. The comparison with the numerical evaluation of the atomic displacement is very good over the range of validity of the asymptotic formulae     

Dynamical evolution of wall-string systems

The dynamical evolution of wall-string systems is investigated. Walls intersect each other and collapse with few oscillation so that the lost energy radiates as a scalar mode. Such moving walls attract matter in contrast to static walls. As for the N = 1 axion domain wall model, the walls prove to be harmless in the standard cosmology.     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

Models for Gapped Boundaries and Domain Walls

We define a class of lattice models for two-dimensional topological phases with boundary such that both the bulk and the boundary excitations are gapped. The bulk part is constructed using a unitary tensor category C{\mathcal C} as in the Levin-Wen model, whereas the boundary is associated with a module category over C{\mathcal C} . We also consider domain walls (or defect lines) between different bulk phases. A domain wall is transparent to bulk excitations if the corresponding unitary tensor categories are Morita equivalent. Defects of higher codimension will also be studied. In summary, we give a dictionary between physical ingredients of lattice models and tensor-categorical notions.     

Structural transformations on Cu(110) under molecular iodine action

Interaction of molecular iodine with the Cu(110) surface is investigated by the methods of ultrahigh vacuum scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). It is found that at the coverage θ =0.5 monolayer (ML) iodine forms a simple commensurate c(2×2) lattice. Further exposure of iodine leads to uniaxial compression of the c(2×2) lattice along the 〈110〉 direction of the substrate. The STM data indicate that compression of the iodine layer proceeds through formation of striped domain walls. As the coverage is saturated at θ = 0.63 ML, iodine forms a uniformly compressed quasi-hexagonal structure. Further exposure of iodine on the Cu(110) surface results in growth of a copper iodide film. STM images of thin (7 to 20 Å) CuI films reveal, in addition to atomic modulation, a superstructure with a period of 90 to 100 Å consisting of double stripes. A structure model of the copper iodide surface allowing for CuI lattice contraction and formation of double stripe domain walls is proposed.     

Characterization of periodic domain structures in lithium niobate crystals by scanning electron microscopy and X-ray diffraction analysis

Congruent lithium niobate crystals with periodic domain structures formed by the method of thermoelectric postgrowth treatment have been investigated. Periodic domain structures in the samples of polar ZY-, YZ-, and YX-cuts were characterized by scanning electron microscopy, high resolution x-ray diffractometry, and topography. The evaluation and comparison of the secondary electron (SE) signal cycling, electron emission coefficient, and charge value to equalize the differences in the SE signal have allowed us to specify the features of ZY-and YZ-cut structures. A correlation between the crystal lattice distortions near domain boundaries and the type of domain walls has been found. The domain walls separating the areas with the tail-to-tail P _s vectors not only cause stronger crystal lattice distortions near them, but also are charged less quickly under electron irradiation.