Stable spiral domains in ferrite-garnet films

Stable spiral domain structures—spiral domains stabilized by a bubble lattice and lattices of spiral domains—in epitaxial ferrite-garnet films have been experimentally investigated. The thermodynamic approach based on the concept of magnetostatic pressure is applied to explain the behavior of a spiral domain structure with a change in temperature or magnetic field. It is shown that phase transitions in spiral domains are related to phase transitions in the bubble lattice.     

Three-dimensional diagrams of the regions occupied by induced Anger states in a multidomain magnetic medium

The regions of existence of Anger states in an iron garnet film are plotted and investigated for the first time in a space of three controlling parameters: the frequency and amplitude of the alternating field and the magnitude of the static bias field. It is shown that the dynamic spiral domains differ significantly in their configuration and properties in different regions. A quantitative parameter involving the velocity of the domain walls and the “looseness” (sparseness) of the dynamic array of magnetic domains is introduce to characterize the favorability of conditions for the formation of spiral domains. Fiz. Tverd. Tela (St. Petersburg) 40, 1865–1870 (October 1998)     

Lévy噪声与周期力共同驱动下的螺旋波动力学

研究加性Lévy噪声与周期外力对FitzHugh-Nagumo可激系统中螺旋波动力学行为的影响.螺旋波波头的运动随外力周期在一定范围内呈规则变化,该规则变化可用相应的傅立叶谱理解,维持该规则变化的是锁频行为.Lévy噪声序列中包含着小概率的大尺度噪声,螺旋波波头运动改变主要来自于它们的影响,本文指出Lévy噪声对波头运动的影响也依赖于外力周期的取值.在适当的参数取值下,Lévy噪声的存在也能导致螺旋波的消失,这为螺旋波的控制、消除提供了一种方法.分析了系统周期与外力周期的锁定行为,给出了不同噪声强度下的Arnold舌,指出随机共振行为的存在.     

Structural self-organization, domain structure, and magnetic characteristics of manganites of the La-Sr-Mn-Ti-Ni-O system

The structure and phase composition of La_0.65Sr_0.35Mn_{1 ? x ? y} Ni_xTi_yO₃ manganites (0 ≤ x ≤ 0.3; 0 ≤ x + y ≤ 0.3) obtained by solid-state reactions in air are studied by x-ray diffraction. All the samples are single-phase materials and have an orthorhombic or pseudocubic structure, depending on the chemical composition and the sintering temperature. The saturation magnetization and the temperature dependences of the magnetic susceptibility are measured. The magnetic structure of manganites is characterized by the existence of inhomogeneities in the form of ring domains or drops whose size is in the range 30–100 μm. A study of the surface by scanning tunneling microscopy reveals the occurrence of ring, spiral, or lamellar structures with a characteristic size of the order of 10 nm. A possible approach to the interpretation of the obtained experimental results based on the concept of self-organization processes leading to the segregation of ion pairs and the formation of elastic concentration domains is discussed.     

Temperature and magnetic field dependencies of Condon domain phase in Lifschitz-Kosevich-Shoenberg approximation

The temperature and magnetic field behavior of non-uniform diamagnetic phase of strongly correlated electron gas at the conditions of the dHvA effect is analyzed. It is shown that in the framework of Lifschitz-Kosevich-Shoenberg approximation the magnetic induction splitting, as well as the range of existence of Condon domains, are characterized by strong dependencies on temperature, magnetic field and impurities of the sample.     

热敏神经元网络中螺旋波死亡和破裂的数值模拟

实验发现大脑皮层内出现螺旋波且螺旋波对神经元电信号传递有积极作用.利用细胞网络方法从对大脑皮层观察到的螺旋波进行数值模拟.以包含温度因子的热敏神经元模型在二维空间构造规则网络,研究了神经元膜片温度参数对神经元网络中螺旋波演化影响;定义了一类统计同步因子来刻画温度因子引起螺旋波相变(破裂和死亡)的临界条件.发现在规则网络下,当温度超过一定值后螺旋波会死亡和消失而导致整个网络达到均匀同步;在考虑了弱通道噪声情况下,螺旋波温度超越一定临界值则引起螺旋波的破裂.进一步分析了暂时性发烧昏迷的可能机制在于神经系统某些功能区螺旋波传播电信号的中断.     

Bias-field-induced spiral dynamic domains induced in iron garnet films

The induction of an Anger state (AS) in a multidomain magnetic medium by a static bias magnetic field is observed. The properties of the spiral dynamic domains in the induced AS are substantially different from those of previously studied spiral domains. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 60–64 (10 January 1997)     

Spiral structures in helicoidal magnets

The structure and properties of two-dimensional spiral textures in helical ferromagnets are studied. The existence of novel types of periodic structures, namely, of spiral lattices, is predicted for these magnetic systems.     

Possibility of observing spiral scattering of relativistic particles in a bent crystal

The peak position, impact-parameter range, and optimal conditions for observing spiral scattering of relativistic particles in a uniformly bent crystal are estimated. The existence of spiral scattering with a square-root singularity is pointed out. In this case, the secondary process of volume capture to the channeling mode is absent and the conditions for observing this effect are most favorable.     

Bubble-domain lattices in the vicinity of the compensation point

The behavior of a hexagonal lattice of bubble domains in thin uniaxial films of garnet ferrites is studied in the temperature range from the compensation point to the Néel temperature. Two types of first-order phase transitions (preserving and not preserving the total number of domains in the bubble-domain lattice) occurring with variation of the temperature were studied. It is shown that the type of a phase transition is determined by the temperature dependence of the characteristic length of the film. The existence of two types of phase transitions is explained in terms of magnetostatic pressure existing in a bubble-domain lattice.     

Dynamic stability of a spiral domain in an ac magnetic field

The dependence of the stability of a magnetic spiral domain in a film on the parameters of the film and its domain structure and on an external magnetic field is considered within a phenomenological model. The model allows one to explain a number of experimentally observed properties of dynamic spiral domains resulting from the process of self-organization of domains and domain walls in an iron-garnet film placed in an external ac magnetic field.     

Spiral patterns in magnets

An autowave model of the spiral pattern formation in magnets located in a rotating magnetic field is proposed. The model is based on the overdamped double sine-Gordon equation. Nucleation of spiral domains is associated with the averaged motion of domain walls in the rotating field. A vortex-type defect (Bloch line) is the core of the spiral.     

Universal hysteresis scaling for incommensurate magnetic order in dysprosium

We study the scaling properties of magnetic minor hysteresis loops in a polycrystalline dysprosium metal, varying temperature and magnetic-field amplitudes. We observe irreversibility-related hysteresis loss in the helical antiferromagnetic phase, which is related with remanent flux density as a power law with the same scaling exponent of 1.25±0.05 as that in ferromagnetic materials. In contrast to hysteresis scalings in ferromagnets associated with 180° Bloch walls, the observed law is governed by spiral walls which separate helical domains with oppositely rotating spins.     

Observation of domains in the helical phase of MnP

Chirality domains differentiated by the sense of rotation of the spiral are observed by polarized neutron diffraction topography in single crystals of MnP. They mainly occur as stripes perpendicular to the helix axis about 150 μm in width and several mm in length.The domain patterns and senses of spiral are repeatable on thermal cycling though polishing or application of a uniaxial stress modifies the domain structure and relative volumes. Grossly unbalanced domain populations have been observed.     

Selection of spiral waves in excitable media with a phase wave at the wave back

Universal relationships between the medium excitability and the angular velocity and the core radius of rigidly rotating spiral waves in excitable media are derived for situations where the wave front is a trigger wave and the wave back is a phase wave. Two universal limits restricting the region of existence of spiral waves in the parameter space are demonstrated. The predictions of the free-boundary approach are in good quantitative agreement with results from numerical reaction-diffusion simulations performed on the Kessler-Levine model.     

复Ginzburg-Landau方程中模螺旋波的稳定性研究

研究了复Ginzburg-Landau方程系统中模螺旋波与其他斑图在同一平面内的竞争行为,发现演化结果在系统参数平面内可分为四个主要区域:在I区和III区中,模螺旋波与相螺旋波相比稳定性较差,模螺旋波的空间被相螺旋波所入侵.在II区中,模螺旋波具有较强的稳定性,相螺旋波的空间被模螺旋波所入侵.在IV区内,由于时空混沌所导致的频率不稳定性,演化的结果较为复杂.我们通过对模螺旋波、相螺旋波以及时空混沌的频率分析,发现当模螺旋波的系统参数为α1=-1.34,β1=0.35时,较高频率的模螺旋波具有较好的稳定性,高频模螺旋波可以入侵低频斑图空间.竞争结果主要受系统变量实部的频率影响,频率分析所得到的理论结果与数值实验结果符合得非常好.     

The kinetics of the anomalous state formation and the growth of helium crystals at high supersaturation

The kinetics of the formation of an anomalous state of a helium crystal with a fast-growing surface are investigated. It is demonstrated experimentally that it is external supersaturation that is the determining factor of formation of an anomalous state. The dependence of the time of formation of anomalous state on temperature and initial supersaturation is measured. The problem of crystal growth with the excitation of the first-sound wave in the container is solved. This solution is used to determine the dependence of the kinetic coefficient of growth of anomalous facets on temperature and initial supersaturation. It is demonstrated that the kinetic coefficient of facet growth decreases on approaching the boundaries of the region of the existence of an anomalous state. The kinetic coefficient of growth of atomically rough surfaces in an anomalous state is determined by the damping of pressure oscillation. It is found that the value of the latter coefficient is three-four times that of the respective value for the facets but is considerably, by an order of magnitude, less than the value of the coefficient of growth of such surfaces in the normal state. Phenomena are treated which accompany the spiral growth of facets, namely, the excitation of oscillations of a screw dislocation during spiral rotation and the emergence of vortex rings in superfluid helium. The effect of these phenomena on the kinetics of facet growth and on the formation of an anomalous state is discussed.     

钠离子和钾离子通道噪声扰动对神经网络 时空模式的影响

研究了钠离子和钾离子通道噪声扰动对Hodgkin-Huxley神经网络放电时空模式的影响. 发现无论钠离子通道噪声还是钾离子通道噪声扰动, 当取定一组温度、噪声强度, 随着耦合强度的增大, 神经网络放电时空斑图总能演化出螺旋波, 而且存在形成螺旋波所需的临界耦合强度. 分析发现钠离子通道噪声有利于神经网络螺旋波的形成, 而钾离子通道噪声不利于螺旋波形成. 结果还表明较低的温度能够使神经网络对噪声更加敏感. 最后, 讨论了特定参数下螺旋波与靶波之间的转化现象.     

Bosons as the origin for giant magnetic properties of organic monolayers

Recently, unusual giant magnetic properties were found experimentally in some organized organic monolayers adsorbed on solid substrates. A model is presented which explains the observed phenomenon. The model is based on the special properties of electrons transferred from the substrate to the layer as a result of the adsorption process. Triplet pairing of those electrons is forced by the special 2D properties of the organic layer. Such pairs are confined within domains in the organic layer and their quantum statistics provide a model that explains the unique magnetization as well as all other features of the experimental observations. The model suggests the possible existence of Bose-Einstein condensation at room temperature on the scale of the domains.     

The influence of hexagonal anisotropy on the magnetic structure of antiferromagnetic rare earth metals and alloys

The effect of hexagonal base plane anisotropy on the antiferroniagnetic (AF) spiral spin structure of rare earth metals and alloys has been investigated. Minimization of the anisotropy energy is considered either by a reorientation of rigid coherent sections of the spiral or by a continuous modulation of the spiral periodicity. Both results indicate that when the spiral periodicity is commensurate with the hexagonal anisotropy the energy is greater than for spirals which are non-commensurate. The calculations predict that when the temperature is varied in the AF phase, the magnetic structure becomes unstable as a commensurate turn angle is approached, which may provide a qualitative explanation for the abrupt changes in the c-axis thermal expansion and temperature dependence of the elastic constant C₄₄ that have been observed experimentally in dysprosium and terbium-50% holmium. Further consequences of the spiral perturbation by hexagonal anisotropy are considered.