Ising Films Research Articles

Investigation of the thermal and ferroelectric properties of a Spin-1 Ising thin film: Insight from path integral Monte Carlo

In this work, we provide an analysis of the thermal and ferroelectric properties of an Ising thin film in a transverse field extended for a higher spin (σ=1) within the Quantum Monte Carlo method. For this purpose, several underlying physical quantities were examined in detail including the longitudinal polarization, pyroelectric coefficient, internal energy, and entropy change. This work unravels also a particular contribution of the transverse field Ω/J and surface coupling Js/J on the aforementioned physical quantities, which have significant implication on the phase transition temperature Tc. The polarization values are kept almost unchanged at a low temperature. Furthermore, the peaks corresponding to the entropy change ΔS are broad, and usually occur in a narrow temperature at their corresponding Tc, and result an enlargement of their magnitude upon increasing the external electric field and ultimately their maxima. Meanwhile, a change regarding the interfacial coupling Jint does not significantly affect ΔS and thus Tc. A typical hysteresis behavior is observed to confirm the ferroelectric–paraelectric phase transition.

Computer simulation of three-layer systems based on ferromagnetic nanofilms

Computer modeling of magnetization behavior of thin ferromagnetic Ising films separated by antiferromagnetic film was carried out. Metropolis’s algorithm was used for the simulation. Finite-dimensional scaling theory has been used. Phase transition temperatures were determined by Binder cummulants. Systems with different number of layers in ferromagnetic and antiferromagnetic film have been investigated. Temperatures of phase transitions in ferromagnetic and antiferromagnetic films are determined. A phase diagram of the system has been constructed. There are four phases on the phase diagram. Phase for implementation the spintronic devices is defined. The distribution of magnetization and chess magnetization across the layers of the system in different phases has been investigated.

Probing predictions due to the nonlocal interface Hamiltonian: Monte Carlo simulations of interfacial fluctuations in Ising films.

Extensive Monte Carlo simulations have been performed on an Ising ferromagnet under conditions that would lead to complete wetting in a semi-infinite system. We studied an L×L×D slab geometry with oppositely directed surface fields so that a single interface is formed and can undergo a localization-delocalization transition. Under the chosen conditions the interface position is, on average, in the middle of the slab, and its fluctuations allow a sensitive test of predictions that the effective interactions between the interface and the confining surfaces are nonlocal. The decay of distance dependent correlation functions are measured within the surface, in the middle of the slab, and between middle and the surface for slabs of varying thickness D. From Fourier transforms of these correlation functions a nonlinear correlation length is extracted, and its behavior is found to confirm theoretical predictions for D>6 lattice spacings.

Phase diagrams and magnetic properties of the mixed spin-1 and spin-3/2 Ising ferromagnetic thin film: Monte Carlo treatment

Phase diagrams and magnetic properties of the mixed spin-1 and spin-3/2 Ising film with different single-ion anisotropies are investigated, by the use of Monte Carlo simulation based on heat bath algorithms. The effects of the crystal-fields and the surface coupling on the phase diagrams are investigated in detail and the obtained phase diagrams are presented. Depending on the Hamiltonian parameters, the system exhibits both second- and first-order phase transitions besides tricritical point, triple point, and isolated critical end point.

Study of the Curie temperature in a transverse Ising film with the next nearest-neighbor interactions

The mean-field approximation is used to study the Curie temperature in a transverse Ising film with the next nearest-neighbor interactions. The main emphasis of our research is focused on the dependence of Curie temperature on the decaying exponent δ. In the meanwhile, how the effect of film thickness N on the character of phase transition is also calculated.

Static and dynamic critical behavior of thin magnetic Ising films

This work presents a study of the effect of film thickness on the static and dynamic critical behavior of thin magnetic Ising films. Monte Carlo simulations using the Wolff algorithm were performed to determine the static and dynamic critical exponents of the films. A dimensionality crossover from 2D to 3D (due to the finiteness of the films) in the static and dynamic critical behavior was observed as the film thickness increases. In addition, a slight increase in the effective dimension deff and a considerable increase in the critical temperature Tc(∞) were found. Small values for the dynamic critical exponents were obtained, indicating that the Wolff algorithm is a very efficient method for these magnetic systems.

Unique phase diagrams in ultra-thin Ising films with dilutions

The phase diagrams of two nanoscaled thin films with site (or bond) dilution, described by the spin-1 (or spin-1/2) Ising model, are investigated by the use of the effective field theory with correlations. They are consisted of two layers. Some unique features have been found in the phase diagrams, while they show many similar behaviors for both the spin-1/2 films and the spin-1 films with a positive single-ion anisotropy. It is clarified that the unique behavior of critical concentration in the spin-1 system with site dilution and a positive single-ion anisotropy at which the transition temperature reduces to zero is completely equivalent to that in the spin-1/2 system with site dilution.

The possibility of re-entrant phenomena induced by a transverse field in ultra-thin transverse Ising films

The phase diagrams and temperature dependences of magnetizations in ultra-thin transverse Ising thin films are studied by the use of both the effective-field theory with correlations (EFT) and the mean-field theory (MFA). Novel features, such as the possibility of re-entrant phenomena, are obtained for the magnetic properties in such systems with a zero transverse field at the surfaces, when the EFT is applied to them, although such features could not be found from the use of the MFA. When the transverse field at the surfaces takes a finite value, however, the re-entrant phenomena could not be found from the both formulations of the EFT and the MFA. Similar phenomena are then obtained in the phase diagrams by using the MFA and the EFT.

Unconventional phase diagrams in an ultra-thin spin-1 Ising film with site (or bond) dilution at the surfaces

The phase diagrams of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the spin-1 Ising model with single-ion anisotropy (D), are investigated by the use of the effective field theory with correlations. They are consisted of two layers. A number of characteristic phenomena have been found in the behaviors of transition temperature and tricritical point, which are heavily dependent on the ratios (r=J1/J and d=D/J) and a concentration (p) of bond or site dilution at the surfaces, where J is the exchange interaction in inner layer and J1 is the exchange interaction between the surface.

Equilibrium and nonequilibrium properties of synthetic metamagnetic films: A Monte Carlo study

Synthetic antiferromagnets with strong perpendicular anisotropy can be modeled by layered Ising antiferromagnets. Accounting for the fact that in the experimental systems the ferromagnetic layers, coupled antiferromagnetically via spacers, are multilayers, we propose a description through Ising films where ferromagnetic stacks composed of multiple layers are coupled antiferromagnetically. We study the equilibrium and nonequilibrium properties of these systems where we vary the number of layers in each stack. Using numerical simulations, we construct equilibrium-temperature-magnetic-field phase diagrams for a variety of cases. We find the same dominant features (three stable phases, where one phase boundary ends in a critical end point, whereas the other phase boundary shows a tricritical point at which the transition changes from first to second order) for all studied cases. Using time-dependent quantities, we also study the ordering processes that take place after a temperature quench. The nature of long-lived metastable states are discussed for thin films, whereas for thick films we compute the surface autocorrelation exponent.

Unexpected magnetic properties in an ultra-thin transverse Ising film with bond or site dilution at surfaces

The phase diagrams and magnetizations in a nanoscaled thin film with bond and site dilutions at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations. We find a number of unexpected novel phenomena in them, when the ratio between the transverse field at the surfaces and the transverse field in the inner layer takes a large value; such as the increase of transition temperature, even when the dilution with nonmagnetic atoms at the surfaces is increased.

Characteristic behaviors in an ultrathin Ising film with site- (or bond-) dilution at the surfaces

The phase diagram, magnetization and Arrott plot of an ultrathin film with site or bond dilution at the surfaces are investigated by the use of the effective field theory with correlations (or the mean field theory for comparison). The system is consisted from the two Ising layers. A lot of characteristic phenomena can be found in them. In particular, the behaviors of them are completely different, depending on whether the site or bond dilution is performed at the surfaces. A lot of unexpected phenomena, such as a new type of frustration, have been obtained in the phase diagram and magnetization, when the strength of interlayer coupling between the two layers is changed.

Phase diagrams in an ultra-thin spin-1 transverse Ising film with bond or site dilution at surfaces

The phase diagrams of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the spin-1 transverse Ising model, are investigated by the use of an effective field theory with correlations. A number of characteristic phenomena have been found in them, which are heavily dependent on the ratios (r = J1/J and p = ΩS/Ω, where J is the exchange interaction in the inner layer, J1 is the exchange interaction between the surface and the next inner layer, ΩS is the transverse field at the surfaces and Ω is the transverse field in the inner layer). Some of them have exhibited very similar behaviors found in the two spin-1/2 nanoscaled thin films with bond and site dilutions at the surfaces.

Thickness dependence in nano-scaled transverse Ising films with bond or site dilution at surfaces

Thickness (L) dependence for phase diagrams in nanoscaled thin films with bond and site dilutions at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations, in order to clarify whether some characteristic phenomena found in the previous works for the most thin films (L=3) can be found by increasing the thickness in thin films. We find that the approaches to bulk values, when increasing L, are rather different between the two systems with site and bond dilutions at the surfaces, when the ratio p (p=ΩS/Ω, where ΩS and Ω are the transverse fields at the surfaces and in the inner layer) is given by p=1.0. When p=6.0, many characteristic phenomena can be obtained in the phase diagrams. Unconventional phenomena of phase diagram have also been obtained, when the value of p is changed in the thin films with a very low site or bond concentration q (q=0.1) at the surfaces.

Magnetic properties of a ferromagnetic thin film with four spin interaction: A Monte Carlo simulation study

Monte Carlo simulation has been used to study the critical behaviors and the magnetic properties of a ferromagnetic thin Ising film with a plaquette four spin interaction. The effects of the ratio rs=Js/J of the surface exchange interaction to the bulk one and the four spin interaction on phase diagrams are investigated. A number of characteristic behaviors have been found, which include the first- and second-order phase transitions, thus also the tricritical points, triple point and isolated critical point.

Ferrimagnetism in a ultra-thin decorated Ising film

The phase diagram and total magnetization of a ultra-thin decorated Ising film are examined by the use of the effective field theory with correlations. The system, consisting of spin-1/2 atoms and decorating spin-S (S>1/2) atoms with a crystal–field interaction on the bonds, show ferrimagnetism, compensation point and reentrant behavior, depending on the value of S.

Phase diagrams in a ultra-thin transverse Ising film with bond or site dilution atsurfaces

The phase diagrams and magnetizations of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations. Some characteristic phenomena have been found in them; The transition temperature (TC) of bond dilution is always larger than the corresponding one of site dilution. There exists some region in the phase diagram where the longitudinal magnetization mT for the site dilution is always given by mT=0.0 in the whole temperature region, while the mT for the bond dilution takes a finite value below its Tc. Even in a uniformly applied transverse field, the appearance of a broad maximum in a system with site dilution can be also found, when the variation of TC is plotted as a function of r (r=J1/J, where J is the exchange interaction in inner layer and J1 is the exchange interaction between the surface and the next inner layer). But, such a phenomenon cannot be obtained in the system with bond dilution.

Critical Casimir forces for Ising films with variable boundary fields

Monte Carlo simulations based on an integration scheme for free energy differences is used to compute critical Casimir forces for three-dimensional Ising films with various boundary fields. We study the scaling behavior of the critical Casimir force, including the scaling variable related to the boundary fields. Finite size corrections to scaling are taken into account. We pay special attention to that range of surface field strengths within which the force changes from repulsive to attractive upon increasing the temperature. Our data are compared with other results available in the literature.

Phase transition in Ising, XY and Heisenberg magnetic films

The phase transition and magnetic properties of a ferromagnet spin-S, a disordered diluted thin and semi-infinite film with a face-centered cubic lattice are investigated using the high-temperature series expansions technique extrapolated with Padé approximants method for Heisenberg, XY and Ising models. The reduced critical temperature of the system τc is studied as function of the thickness of the thin film and the exchange interactions in the bulk, and within the surfaces Jb, Js and J⊥, respectively. It is found that τc increases with the exchange interactions of surface. The magnetic phase diagrams (τc versus the dilution x) and the percolation threshold are obtained. The shifts of the critical temperatures Tc(l) from the bulk value (Tc(∞)/Tc(l)−1) can be described by a power law l−λ, where λ=1/υ is the inverse of the correlation length exponent.

First- and second-order wetting transitions in confined Ising films in the presence of nonmagnetic impurities: A Monte Carlo simulation study

In this work, we present the results of a systematic exploration of the effect caused by the introduction of nonmagnetic impurities (or defects) on the stabilization of the interface between two magnetic domains of opposite magnetic orientation. Those defects are simulated as spin vacancies along the center of confined two-dimensional Ising films, which have competing magnetic fields acting on the confinement walls. The calculations are performed for different L×M film sizes and by using the standard Metropolis dynamics. In the absence of defects, the film is characterized by an interface running along the M direction, which is induced by the competing surface fields. That interface undergoes a localization-delocalization transition that is the precursor of a true wetting transition taking place in the thermodynamic limit. When the density of defects is relatively low, our results show that the wetting phase transition is of second order, as in the absence of defects. On the other hand, when the density of nonmagnetic impurities is relatively high, a pinning effect of the interface gives rise to a first-order wetting phase transition. The observed transitions are characterized by measuring relevant properties, such as magnetization profiles, cumulants, magnetization fluctuations, etc., as a function of the density of defects. So, our main finding is that the presence of nonmagnetic impurities introduces a rich physical scenery, such as a line of second-order wetting transitions (observed for low density of defects) that merges into a first-order one just at a tricritical point. Precisely, these two latter findings are the major contributions of our study.