Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Disordered domain structures and domain boundaries in thin magnetic films

The behavior of a disordered domain structure and domain boundaries with a change in temperature and magnetic field has been studied. The concept of magnetostatic pressure is used to explain the experimental results. It is shown that “the memory effect” and stability interval of a domain structure with a change in temperature or magnetic field depends on the degree of domain structure disorder and the domain boundary energy.     

Properties of ferroelectric thin films with a first-order phase transitions

A Green's function technique is used to investigate the properties of ferroelectric thin films with a first-order phase transitions on the basis of the transverse Ising model. Taking into account the four-spin interactions beside the two-spin interactions the dependence of the polarization on film thickness and temperature and the thickness dependence of the Curie temperature become more complicated.     

Hydrogen embrittlement under load as a first-order phase transition

The stress-strain equation of state for a material with dissolved hydrogen is derived within a two-component continuum model. It is demonstrated that, for the parameters corresponding to conventional metals, it has the shape of a loop similar to the van der Waals loop. On this basis, it is concluded that hydrogen embrittlement of materials under load is a typical first-order phase transition and, therefore, occurs through nucleation of a new phase. Thermodynamic and kinetic theories of this transition are developed, and time dependences of all the main characteristics of this process are calculated.     

Kinetics of the initial stage in a first-order phase transformation in thin films

The nucleation and growth of islands of a new phase on the surface of solids has been studied both experimentally and theoretically for the particular case of the transition from the pyrochlore to perovskite phase in a thin film of a lead zirconate-titanate ferroelectric. This transformation was chosen because the new-phase islands have a stable circular shape in this case, a relatively large size (10⁻⁵–10⁻⁴ m) permitting their observation with an optical microscope, and a low growth rate (10⁻⁸–10⁻⁹ m/s). A theoretical analysis of the process, based on the kinetic theory of first-order phase transitions proposed earlier, has been carried out and the behavior in time of all main characteristics of a phase transformation, namely, nucleation rate, concentration of the new-phase islands, their size distribution, and relative overheating, has been calculated. The same characteristics have been measured experimentally, thus permitting one for the first time to make a thorough comparison of the theoretical with experimental data on the kinetics of first-order phase transitions. They have been found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 121–126 (January 1997)     

Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase. Original Russian Text ? E.M. Artem’ev, M.E. Artem’ev, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 838–840.     

Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase.     

Ball solitons in kinetics of the first-order magnetic phase transition

The theory of magnetic ball solitons (BS) arising as a result of the energy fluctuations, at the spin–flop transition induced by a magnetic field in antiferromagnets with uniaxial anisotropy, is presented. Such solitons are possible in a wide range of amplitudes and energies, including the negative energy relative to an initial condition. When such antiferromagnet is in a metastable condition, ball solitons are born with the greatest probability if the energy of solitons is close to zero. Evolution of these solitons, at which they develop into macroscopic domains of a new magnetic phase, is analyzed, thus, carrying out full phase reorganization.     

Interaction of domain walls in thin magnetic films

Interactions of pairs of parallel Néel walls are investigated by means of Ritz's method calculations. In the case of the symmetric, thin-film mode of Néel walls, unwinding walls show an attractive and winding walls a repulsive interaction for all distances. The functional dependence of the interaction on the distance of the walls is strongly correlated with the wall profiles of the isolated walls with their characteristic extended tails. For the asymmetric Néel wall mode which occurs in thicker films, an additional repulsive interaction of the magnetization vortices in the wall core is found. This leads for unwinding walls to stable configurations of double walls with separations between two and three times the film thickness.     

一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.     

Model of magnetization in thin magnetic films with one domain and two-domain basic structures

The magnetization processes in thin magnetic films are described by a model analysing the behaviour of one domain and two-domain basic structures (BS) in the applied magnetic field. These structures include the film areas with nearly constant crystal and magnetic parameters. The minimum of BS free energy including the energy in the internal magnetic field, the energy of the induced anisotropy and the domain-wall energy are taken into account. The initial and hysteresis curves of the sample depend on the function of distribution for the BSs are calculated. A good qualitative agreement with the results of other authors is observed if the films consists of one-domain or two-domain BS only. Our experimental data give also some support of the model.     

First-order magnetic phase transitions in thin films

Using the simple Landau model, we discuss near-surface magnetic effects for thin films corresponding to first-order phase transitions. The size effects observed in ultrathin ferroelectric films are in agreement with this theoretical interpretation. The presence of a weak external field is also formulated. The text was submitted by the authors in English.     

Effect of synthesis conditions on the metal-semiconductor phase transition in vanadium dioxide thin films

The effect of the conditions of synthesis and of the substrate material on the metal-semiconductor phase transition in thin vanadium dioxide films prepared using laser ablation has been studied. The broadening of the hysteresis loop is shown to be due to a decrease in the size of the crystal grains making up the film. Conjectures are put forward to explain the formation of asymmetric hysteresis loops.     

The effects of uniaxial stress distribution on the ferroelectric properties of thin films with first-order phase transition

Based on the Landau theory, the first-order phase transition properties of ferroelectric thin films have been studied by taking into account uniaxial stress distribution effects. The stress is supposed to decrease from interface to surface exponentially according to the experimental results. It is shown that tensile stress decreases the polarization and the Curie temperature while compressive stress increases the polarization and the Curie temperature. A stress-driven phase transition is found at the critical stress. Our prediction is compared with the available experiment results.     

Third-order phase transition and superconductivity in thin films

We have found a new mean field solution in the BCS theory of superconductivity. This unconventional solution indicates the existence of superconducting phase transitions of third order in thin films, or in bulk matter with a layered structure. The critical temperature increases with decreasing thickness of the layer, and does not exhibit the isotope effect. The electronic specific heat is a continuous function of temperature with a discontinuity in its derivative.     

Epitaxy surface effect on the first-order phase transition properties in a ferroelectric thin film

By modifying the interchange interactions and the transverse fields on the epitaxy surface layer,this paper studies the phase transition properties of an n-layer ferroelectric thin film by the Fermi-type Green’s function technique based on the transverse Ising model with a four-spin interaction.The special attention is given to the effect of the epitaxy surface layer on the first-order phase transition properties in the parameter space constructed by the ratios of the bulk transverse field and the bulk four-spin interaction to the bulk two-spin interaction with the framework of the higher-order decoupling approximation to the Fermi-type Green’s function.The results show that the first-order phase transition properties will be changed significantly due to the modification of interchange interaction and transverse field parameters on the epitaxy surface layer.The dependence of the first-order phase transition properties on the thickness of ferroelectric thin films is also discussed.     

Growth of clusters in a first-order phase transition

The results of computer simulations of phase separation kinetics in a binary alloy quenched from a high temperature are analyzed in detail, using the ideas of Lifshitz and Slyozov. The alloy was modeled by a three-dimensional Ising model with Kawasaki dynamics. The temperature after quenching was 0.59T _c, whereT _c is the critical temperature, and the concentration of minority atoms was=0.075, which is about five times their largest possible single-phase equilibrium concentration at that temperature. The time interval covered by our analysis goes from about 1000 to 6000 attempted interchanges per site. The size distribution of small clusters of minority atoms is fitted approximately byc ₁(1-)³ w(t),c ₁ (1–)⁴ Q _l w(t)^l(2l10); wherec _l is the concentration of clusters of sizel;Q ₂,...,Q ₁₀ are known constants, the cluster partition functions;t is the time; andw(t)=0.015(1+7.17t ^–1/3). The distribution of large clusters (l20) is fitted approximately by the type of distribution proposed by Lifshitz and Slyozov,c _l ,(t)=–(d/dl) [lnt+p (l/t)], where is a function given by those authors and is defined by(x)=C _o e–x-C ₁ e ^–4x/3-C ₂ e ^–5x/3;C ₀,C ₁,C ₂ are constants determined by considering how the total number of particles in large clusters changes with time.Supported by the U.S. Air Force Office of Scientific Research under Grant No. 78-3522 and by the U.S. Department of Energy under Contract No. EY-76-C-02-3077*000.     

Thermodynamically stable fractal-like domain structures in magnetic films

A fractal-like structure of the domain boundaries was revealed in “overcritical” uniaxial Permalloy magnetic films. The fractal dimension of domain boundaries at the film surfaces was determined as a function of the film thickness. It is shown that the phase transition between the two possible types of fractal-like structures is accompanied by a jump in fractal dimension.

     

Phase transitions and domain structures in thin ferroelectric films

Recent advances in technology made available high quality thin ferroelectric films and ferroelectric–paraelectric multilayers. But understanding of the properties of these systems is far from being complete. In particular, it is not clear why various anomalies observed at phase transitions here are different from those in high quality bulk systems. The aim of the paper is to discuss some specific features of ferroelectric phase transitions in thin films and multilayers which are taken into account only partially by the theory. The discussion is limited to revealing the character of ferroelectric state forming just after the transition, i.e. if it is single-domain or multi-domain, if it is single-phase or two-phase. First, thin films with electrodes and on a substrate are discussed. The role of non-ideality of electrodes in realization of one of the first pairs of possibilities is emphasized. A more recent idea is that even for ideal electrodes a domain formation is possible when some conditions on the electrode–ferroelectric interface and the material constants of the material are met. This seems to be quite possible for the considered systems. Clamping by the substrate may lead to formation of a two-phase state which is practically unexplored for very thin films on substrates. Second, ferroelectric–paraelectric multilayers are considered which are more challenging for theoretical study than the thin films. Indeed, despite periodicity in the composition the domain structures are almost never periodic along the multilayer. The non-ideality of electrodes seems to lead to practical impossibility of single-domain ferroelectric phase transition in the multilayers of the considered type.