Surface Exchange Interaction Research Articles

Thermally stimulated desorption from the surface of black aluminum layers prepared by PVD methods

Black aluminum is characterized by a highly granular structure and porous surface - i.e. the real surface area is several times larger than its geometric projection to the substrate. In addition, it is a metal with good surface gas sorption capacity and structural stability up to temperatures T ∼500 °C.The surface exchange interactions between the ambient environment and thin layers of black aluminum prepared by different methods are described. These properties are convenient for materials that are suitable for the fabrication of active layers of chemiresistors. The layers were prepared by magnetron sputtering at room temperature and liquid nitrogen temperature, and evaporation with different compositions and gas flow rates. In this work, the process of thermally stimulated desorption was measured using mass spectrometry. The calculation of the activation energy Eact for the release of individual desorbing atoms, or radicals, from the surface of black aluminum was performed. Knowing of the temperature and the number of particles released by desorption allows us to get an idea of the number of defects and the functionalized structure of the surface layer.

Phase diagrams of semi-infinite systems by renormalization group theory and Monte Carlo simulation

In this work, we used the Migdal–Kadanoff renormalization group method to investigate two three-dimensional semi-infinite Ising systems. In the first one, the surface and bulk sites are occupied by spin-3/2 and spin-1/2 respectively, whereas those of the second one are occupied by spin-1/2 on the surface and spin-3/2 in the bulk. Based on the ratio of bulk and surface exchange interactions, we explored different topologies of phase diagrams showing various second order phase transitions, namely ordinary, extraordinary, surface and special transitions. We also found that both systems exhibited first order phase transitions, multicritical points and critical end-points. The existence of a first order phase transition at low temperatures was confirmed by plotting the derivative of the free energy. The Monte Carlo simulation was also used to verify and compare the results obtained by the renormalization group for the two semi-infinite systems.

Electrocaloric properties and critical behavior of a ferroelectric thin film

In this manuscript, the effective field theory (EFT) is used to investigate ferroelectric, thermal, and electrocaloric properties of a ferroelectric thin film. In this regard, the influence of the electric and transverse fields on the aforementioned properties are systematically investigated. Through a detailed analysis, investigations of electrocaloric performance with particular focus on the applied electric field and transverse field are elucidated. As expected, a high electrocaloric effect (ECE) is obtained, however the magnitude corresponding to this effect is enlarged and caused a dielectric breakdown due to the contribution of the transverse field Ω. The peaks corresponding to ECE increase and shift towards higher temperatures after an appropriate increase of the applied electric field. Furthermore, the critical temperature can shift to either lower and higher temperatures compared with the corresponding underlying parameters. On the contrary, the transverse field reduces the peak’s maxima, and therefore the ECE becomes weaker. The influence of the surface exchange interaction and thickness of the film on the entropy change (ΔS/J) and adiabatic temperature change (ΔT/J) were also investigated. This work reveals a large contribution of the applied electric field, surface coupling, and transverse field on the phase transitions. Consequently, the magnitude and ΔT/J can be controlled by appropriately choosing the system’s parameters.

SURFACE PHASE DIAGRAMS OF SEMI-INFINITE FERRIMAGNETIC SYSTEM

The three-dimensional semi-infinite mixed spin-1/2 and spin-3/2 ferrimagnetic Ising system with crystal field is investigated using the mean-field approximation and the Monte Carlo simulation. According to the ratio [Formula: see text] of bulk and surface exchange interactions and the ratio [Formula: see text] of bulk and surface crystal fields, we have classified four qualitative types of phase diagrams characterized by the presence or absence of ordinary, extraordinary, surface and special phase transitions. The critical behavior of the surface and bulk magnetizations has also been highlighted in the vicinity of these different transitions. At low temperatures, two critical end-points appear in the bulk and on the surface in the ordered region limiting two successive first-order phase transitions. Furthermore, we have made a comparison with other works on similar models in pure or mixed versions.

Microwave Giant Magnetoresistance and Ferromagnetic and Spin-Wave Resonances in (CoFe)/Cu Nanostructures

The microwave phenomena that occur in magnetic multilayer (CoFe)/Cu nanostructures, which have a giant magnetoresistance, are studied. The transmission of waves through a nanostructure is used to investigate the microwave giant magnetoresistance effect. The changes in the transmission coefficient at frequencies of 29–38 GHz are found to exceed the relative magnetoresistance, which distinguishes the system under study from the nanostructures studied earlier. Ferromagnetic and spin-wave resonances are used to study the angular dependences of the microwave absorption spectra of a multilayer (CoFe/Cu)n nanostructure. The following parameters are determined: the critical angle that determines the boundaries of the ranges of excitation of uniform and nonuniform spin modes, the type of boundary conditions describing the pinning of spins on the outer nanostructure surfaces, and the surface anisotropy and exchange interaction constants.

Monte Carlo study of thin magnetic Ashkin Teller films at the special point

The magnetic properties and critical behaviours of an Ashkin Teller spin-1/2 model magnetic thin-film systems are studied as part of the Monte Carlo simulation method, based on the Metropolis algorithm, in a simple cubic lattice. The effect of the interaction with the exchange coupling J4 (surface and bulk) is studied. In addition, the phase diagrams present different ratios of the surface exchange interaction at the special point (Rs=J2s/J2b)sp, at which all the films thickness have the same critical temperature, these temperatures can be below, above or equal to the critical system bulk temperature. First and second order transitions appears in the system. Moreover, to the special point descriptions, we found another partially ordered phase at high temperature for certain values of the parameter’s interaction. Finally, the effective surface exponents are computed.

Reduced magnetization curves of Ising nanoparticles with high critical temperature

ABSTRACTThe magnetic properties (phase diagram, the thermal variations of magnetization and the reduced magnetization curve) in graphene-like Ising nanoparticles are examined by the effective-field theory with correlations. The effects of surface exchange interaction JS on the magnetic properties are discussed. When JS is larger than the critical value JSC, a nanoparticle with high critical temperature is possible. The value of JSC depends on the shape and the degree of structural packing of the nanoparticle. It is found that there exist two types of reduced magnetization curves for a nanoparticle with JS > JSC.

Ферромагнитный и спин-волновой резонанс в тонкой пленке (30 nm) сверхрешетки [CoFe/Cu]-=SUB=-N-=/SUB=-

Angular dependencies of the spectrum microwave absorption for the multilayer (CoFe/Cu)N film were studied by ferromagnetic and spin-wave resonance method. The critical angle which indicate the boundaries of ranges where the uniform and non-uniform spin modes are excited, the type of boundary conditions, the values of surface anisotropy constant and exchange interaction were defined in this work. It was shown that the precision of identification of every mode in SWR spectrum is key factor to analyze the measured curves

On the possibility of magnetic ordering (TC(N)) induced by a surface exchange interaction in an Ising nanoparticle with TC(N) > TC(B), where TC(B) is a transition temperature in the corresponding bulk system

The phase transitions of two nanoparticles and their corresponding bulk systems are examined by the effective-field theory with correlations. It is clarified that the transition temperatures of such small nanoparticles may become larger than those of its corresponding bulk systems, when the surface exchange interaction is selected as a large value.

Magnetic and dielectric properties of pure and ion doped RCrO3 nanoparticles

Using a microscopic model and the Green’s function technique the magnetic and dielectric properties of YCrO3 and GdCrO3 nanoparticles (NPs) are studied and compared with the bulk properties. Due to different surface exchange interaction constants the magnetization in YCrO3 increases whereas in GdCrO3 decreases with decreasing particle size. The size dependence of the coercive field in GdCrO3 NPs is also discussed. The bulk dielectric constant ϵ′ in YCrO3 is smaller than that in NPs, whereas in GdCrO3 is observed the opposite behavior. The magnetic field influences ϵ′ in YCrO3 NPs which is an indirect evidence for a strong magnetoelectric coupling. The effects of ion doping on ϵ′ are also studied.

Phase Transition in Cylindrical Ising Nanowires and Nanotubes: Approximation of the Molecular Field Theory

Within the approximation of the molecular field theory, the critical behavior of cylindrical Ising nanowires and nanotubes is studied. The model considered in this work consists of ferromagnetic spins Sc situated in the core of the system and ferromagnetic spins Ss situated on the surface wall; they are interconnected by the exchange coupling J1. The transition temperature Tc for such systems has been calculated as a function of exchange interaction parameters. The influence of the surface exchange interaction and surface coupling on the magnetodynamic behavior of the system have been studied. Some characteristic properties obtained on the phase diagram are related to the ratio of characteristic physical parameters on the surface and core of the abovementioned nanostructures.

Effect of surface and bulk exchange interactions on superlattice materials with a mixed spins: A Monte Carlo study

In the present work, the ground states phase diagrams of superlattice materials with two configurations spins-3/2 and 2 have been studied using Monte Carlo simulations. We have also given the total, partial magnetizations and magnetic susceptibilities of superlattice materials for different magnetic parameters. The effect of surface and bulk exchange interactions and crystal field has been investigated. The magnetic hysteresis cycles of superlattice materials have been found with different values of size, exchange interactions and temperatures.

Фазовый переход в цилиндрических изинговых нанопроволоках и нанотрубках: приближение теории молекулярного поля

AbstractWithin the approximation of the molecular field theory, the critical behavior of cylindrical Ising nanowires and nanotubes is studied. The model considered in this work consists of ferromagnetic spins S _c situated in the core of the system and ferromagnetic spins S _s situated on the surface wall; they are interconnected by the exchange coupling J _1. The transition temperature T _c for such systems has been calculated as a function of exchange interaction parameters. The influence of the surface exchange interaction and surface coupling on the magnetodynamic behavior of the system have been studied. Some characteristic properties obtained on the phase diagram are related to the ratio of characteristic physical parameters on the surface and core of the abovementioned nanostructures.

Unique phase diagrams in a graphene-like transverse Ising nanoparticle

The phase diagrams and the magnetizations in a graphene-like two-dimensional nanoparticle described by the transverse Ising model are investigated by the use of the effective-field theory with correlations. We have found some characteristic behaviors especially for the system with a small transverse field and a small surface exchange interaction, such as the reentrant phenomena.

Phase Diagrams in Graphene-like Ising Nanoparticles

The phase diagrams of four graphene-like nanoparticles described by the Ising model are investigated by the use of the effective-field theory with correlations. The particle is represented by a two-dimensional array of spin-1/2 atoms with two types of exchange interactions, J and JS. The effects of surface exchange interaction JS on the phase diagrams are examined. We have found some characteristic behaviours.

Spin-waves in thin films with Dzyaloshinskii-Moriya interaction

Using the Green’s function method, we calculate the spin-wave (SW) spectrum in a thin film with quantum Heisenberg spins interacting with each other via an exchange interaction J and a Dzyaloshinskii-Moriya interaction of magnitude D. Due to the competition between J and D, the ground state is non collinear. We show that for large D, the first mode in the SW spectrum is proportional to the in plane wave-vector k at the limit k tending to zero. For small D, it is proportional to k2. We show that the surface modes may occur depending on the surface exchange interaction. We calculate the layer magnetizations at temperature T and the transition temperature as a function of the film thickness.

Rapid and convenient removal of fluoride by magnetic magnesium–aluminum–lanthanum composite: Synthesis, performance and mechanism

AbstractBy an easy co‐precipitation method, the magnetic magnesium–aluminum–lanthanum composite was successfully synthesized. The effects of the synthetic conditions such as Mg/Al/La molar ratio and calcination temperature on fluoride removal by the adsorbent were studied. The adsorbents were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), Fourier transform infrared (FT‐IR) and energy‐dispersive X‐ray spectroscopy (EDS). By means of changing various conditions, such as contact time, solution pH, initial fluoride concentration, temperature and coexisting anions, batch adsorption studies were worked out. Results indicated that with a Mg/Al/La molar ratio of 16:1:1, at 500 °C, near neutral pH and room temperature, the adsorbent obtained the maximum capacity of 65.75 mg/g for fluoride adsorption. The adsorption process was fitted well with the Langmuir isotherm model and the pseudo‐first order kinetic model. Adsorption mechanism involved electrostatic interaction on surface and ion exchange interaction. All results indicated that the magnetic magnesium–aluminum–lanthanum composite can be a very promising adsorbent, has potential application in fluoride removal in water treatment and has positive effects on human health. © 2017 Curtin University of Technology and John Wiley & Sons, Ltd.

Structure and magnetism of the(2×2)-FeO(111) surface

Based on ab initio calculations we determine the features and relative stability of different models proposed to describe the $(2 \times 2)$-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values.

The phase diagrams of a ferromagnetic thin film in a random magnetic field

In this paper, the magnetic properties and the phase diagrams of a ferromagnetic thin film with a thickness N in a random magnetic field (RMF) are investigated by using the Monte Carlo simulation technique based on the Metropolis algorithm. The effects of the RMF and the surface exchange interaction on the critical behavior are studied. A variety of multicritical points such as tricritical points, isolated critical points, and triple points are obtained. It is also found that the double reentrant phenomenon can appear for appropriate values of the system parameters.

A Monte Carlo study of the Blume–Capel thin film in the presence of a random crystal field

A Monte Carlo simulation with heat bath algorithm is used to study the effect of random crystal field and surface exchange interactions on the critical behavior and the magnetic properties of a spin-1 Ising ferromagnetic thin film having the simple cubic symmetry. The phase diagram exhibits a rich variety of behaviors such as the double reentrant phenomena and the existence of tricritical points. Thermal magnetization behavior and phase diagrams have been discussed in detail.