Application of Local Information Entropy in Cluster Monte Carlo Algorithms

The chapter refers to a modification of the so-called adding probability used in cluster Monte Carlo algorithms. The modification is based on the fact that in real systems, different properties can influence its clusterization. Finally, an additional factor related to property disorder was introduced into the adding probability, which leads to more effective free energy minimization during MC iteration. As a measure of the disorder, we proposed to use a local information entropy. The proposed approach was tested and compared with the classical methods, showing its high efficiency in simulations of multiphase magnetic systems where magnetic anisotropy was used as the property influencing the system clusterization

Impact of antisymmetric anisotropy on magnetisation of ultra-hard nanocrystalline alloys

This paper describes an application of two-level Stoner–Wohlfarth model including antisymmetric anisotropy and two-level energetic model for simulations of magnetization processes in nanocrystalline, ultra-hard magnetic alloys. The simulation results have been compared with the experimental data obtained for the (Fe80Nb6B14)0:88Tb0:12 alloy. Moreover, the correctness of the proposed model was confirmed by the quantitative agreement with the direct Monte Carlo simulations of similar magnetic systems

Nanoscale analysis of superparamagnetic systems

The paper refers to a kind of nanoscale characterization of superparamagnetic materials above and below the so-called blocking temperature. It is propose to apply the Stoner–Wohlfarth model of nanoparticles magnetization supplemented by the two-level kinetic model, determining behavior in T > 0. This approach allows determination of distribution of magnetic moments and energetic barriers of magnetic objects based on magnetization curves. In many cases, the determined distribution of magnetic moments can be recalculated into average size of the magnetic objects giving finally a nano/microscaled picture of the material. The proposed method was successfully used in characterization of diluted magnetics, nanocomposites, powders and even for human hemoglobin. In the paper the basic theory and its application to the nanoscale characterization is discussed in detail

Magnetization processes of nanoparticles embedded into ferromagnetic matrix

The paper refers to computer simulations of interactions between magnetically hard particle embedded into ultra-hard matrix. We used simulated annealing and Monte Carlo simulations in a frame of the 3D Heisenberg model. The performed simulations reveal that the particles show additional exchange anisotropy dependent on frozen spin direction in the matrix and the angle between matrix and particle easy magnetization axes. The particle matrix coupling are responsible for multi-phase magnetic hysteresis shape and exchange-bias like eff ect

Magnetization processes of irregular dendrite structures - a Monte Carlo study

The paper refers to micromagnetic simulations of magnetization processes of dendrite-like object. The objects were generated by the DLA fractal algorithm that allows obtaining fractals with different ratio of the spins attributed to the surface to volume. The simulations were carried out using the cluster Monte Carlo algorithm designed for spin continuous and multiphase magnetic systems. The presented researches include different magnetic anisotropy of the surface and volume reflected magnetically soft, hard and ultra-high coercive phases. As it was shown, the influence of microstructure on the coercivity mechanism is a complex phenomenon. In the case of the fractals with magnetically soft volume the increasing surface contribution causes either increse or decrease of the coercive field for relatively high or low magnetic anisotropy of the surface, respectively. For the fractals with ultra-high coercive volume the occurrence of the surface anisotropy leads to the significant deterioration of their hard magnetic properties. The obtained spin configurations show that this effect is related to non-colinear directions of the surface anisoropy and strong enough exchange coupling between the surface and volume

The crystal and magnetic properties of some Fe-Nb-B-Ni alloys

The crystal and magnetic properties of (Fe80Nb6B14)1-xNix (x = 0:1, 0.2 and 0.4) bulk alloys prepared by making use of mould casting technique have been studied by X-ray di raction, magnetostatic and Mössbauer e ect methods. Structural and magnetic properties of investigated bulk alloys have been compared with polycrystalline Fe1-xNix (x = 0:1, 0.2 and 0.4) alloys. The measurements have shown that the crystal and magnetic nonhomogeneity for bulk alloys are higher than in polycrystalline compounds, which con rms many magnetic wide magnetic hyper ne eld distribution. The mean diameters of crystallites for (Fe80Nb6B14)1-xNix alloys was calculated from X-ray line broadening and were bigger than 10 nm. The mean magnetic moment and average magnetic hyper ne elds decrease with increase of nickel concentration in investigated compounds

Influence of cooling rate on structural and magnetic properties of (Fe78Nb8B14)1-xTbx alloys

In the presented work we are focused on the influence of cooling rate on structural and magnetic properties of (Fe78Nb8B14)1-xTbx (x = 0.08, 0.1, 0.12) nanocrystalline bulk alloys. The samples were fabricated using the vacuum suction technique with different cooling rates controlled by different sample diameters (from 0.5 to 1.5 mm). The increased Nb content leads to the formation of specific microstructure and allows obtaining ultra-high coercive alloys just after casting without any additional treatment. The coercivity exceeds 8.6 T at the room temperature in case of optimal chemical and preparation conditions (x = 0.12, d = 0.5 mm) and 5.6 T for x = 0.1. The impact of Tb content as well as the cooling rate on magnetic and structural (XRD, SEM, MFM) properties is widely discussed in the context of reduction of rare earths in the RE-based permanent magnets

Magnetism in disordered materials

Purpose: The paper is a review of some problems concerning micromagnetism and magnetism in disordered system.Magnetism of disordered systems is an important problem in analysis of many magnetic materials. As we understand, the term disorder is associated with the both structural (topological and/or chemical) and magnetic (interactions, anisotropy) failures. Typical materials, where phenomena are influenced by the disorder are amorphous and nanocrystalline alloys, nanostructures of magnetic objects, nanoconposites, diluted magnetic materials and intermetallic compounds of rare earth and transition metals. Moreover, in polycrystalline samples can be observed some anomalies related to the area between the grains, which inherently carries some attributes of the disorder. Thus, knowledge of the subject presented here is essential for the proper analysis of magnetics with elements of disorder. In the paper the following problems are discussed: i)magnetization processes in nanosized objects including the famous Stoner-Wohlfarth model, ii) superparamagnetism and magnetic viscosity (time dependent effects), iii) random field Ising model, random bond model and random anisotropy model. Applications of the theories for selected materials (magnetically soft and hard, thin layers, diluted magnetics, and powder systems) are also shown. Design/methodology/approach: Magnetism in disordered materials is a complex problem that, until now, has not exact solutions. There the two approaches. One of them requires some approximation of the problem in order to obtain exact analytical results. The second approach consists in numerical analysis of exact problem that leads to approximated solutions. In the both cases it is important in which stage of a model the disorder is introduced. In the paper the two approaches are widely discussed. Findings: The main conclusion of the paper is that some unusual magnetic properties can be attributed to magnetic and structural disorder. Practical implications: Application of the presented in the paper models indicate that in many magnetic materials the contribution of magnetic disorder plays an important role and should be taken onto account in order to perform correct analysis. Originality/value: The presented collection of different theoretical models including some elements of micromagnetism and magnetism in disordered system as well as applications of the theories to modern magnetric materials is an original idea. The paper is addressed to scientists and researchers that deal with magnetism and related subjects

Phase stability of (Fe80Nb6B14)(0.9)Tb-0.1 bulk nanocrystalline magnet

The paper refers to phase stability of the (Fe80Nb6B14)0:9Tb0:1 bulk nanocrystalline alloys prepared using the vacuum suction casting technique. The samples were in the form of rods with diameters d = 2, 1.5, 1 and 0.5 mm. Heating up to 900 K reveals structural changes that occur at temperatures above 680 K (DSC and M(T) measurements). The phase analysis, using Mössbauer spectra, indicates the decrease of Tb2Fe14B and increase of Fe content in the samples after the heat treatment. The most stable is the alloy with d = 1 mm, where the formation of -Fe phase was not observed. The decrease of d causes signi cant hardening i.e. coercive eld increases from 0.57 T to 2.66 T for d = 2 mm and d = 0:5 mm, respectively

Non-destructive method of determination of elastic properties and adhesion coefficient of different coating materials

Purpose: The paper presents a non-destructive method of determination of Young’s modulus and adhesion coefficient of different coating materials (metallic coatings, polymer, composite etc.). Some of the results obtained by applying this method are discussed in detail. Design/methodology/approach: The presented method consists in measuring the dynamic response of the examined material in the form of a flat rectangular bar subjected to external periodic mechanical stress i.e. the so called vibrating reed technique. General equations describing elastic properties of the sample consisting of a substrate and a deposited coating are derived and discussed in detail. Findings: It was shown that the application of the proposed approach to the metallic, polymeric and composite coatings allowed to obtain a quantitative data concerning the change of both the elastic properties and the adhesion coefficient with a change of: coating thickness, measurement temperature, chemical composition of coating, surface preparation or in the case of epoxy resin coatings with a change of curing time or curing temperature. Research limitations/implications: The proposed method can be applied in many scientific problems in the field of coating materials (e.g. elastic properties of porous coating, crystallization of amorphous coating, adhesion of different polymeric coatings). Practical implications: It was shown that the described method can be successfully used in optimisation of some technological processes of deposition of different coatings on metallic substrate. Originality/value: The paper presents methodology of a non-destructive approach to determination of elastic properties and adhesion coefficient of coating materials with an overview of some applications already publish and also the new ones. Especially interesting are the results concerning the influence of surface preparation on adhesion coefficient which are published for the first time. Keywords: Coating materials; Elastic properties; Young’