Ferroelastic composites are smart materials with unique properties including large magnetodeformational effects, strong field enhancement of the elastic modulus and magnetic shape memory. On the basis of mechanical tests, direct microscopy observations and magnetic measurements we conclude that all these effects are caused by reversible motion of the magnetic particles inside the polymeric matrix in response to an applied field. The basic points of a model accounting for particle structuring in a magnetoactive elastomer under an external field are presented.

Magnetic elastomers (MEs) based on a silicone matrix and magnetically hard NdFeB particles have been synthesized and their magnetic and viscoelastic properties have been studied depending on the size and concentration of magnetic particles and the magnetizing field. It has been shown that magnetic particles can rotate in soft polymer matrix under applied magnetic field, this fact leading to some features in both magnetic and viscoelastic properties. In the maximum magnetic field used magnetization of MEs with smaller particles is larger while the coercivity is smaller due to higher mobility of the particles within the polymer matrix. Viscoelastic behavior is characterized by long relaxation times due to restructuring of the magnetic filler under the influence of an applied mechanical force and magnetic interactions. The storage and loss moduli of magnetically hard elastomers grow significantly with magnetizing field. The magnetic response of the magnetized samples depends on the mutual orientation of the external magnetic field and the internal sample magnetization. Due to the particle rotation within the polymer matrix, the loss factor increases abruptly when the magnetic field is turned on in the opposite direction to the sample magnetization, further decreasing with time. Moduli versus field dependences have minimum at non-zero field and are characterized by a high asymmetry with respect to the field direction.

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Hexagonal orthoferrites of rare earths RFeO3 (R = Lu, Er-Tb) were grown epitaxially on (111)ZrO2(Y2O3) substrates using metal-organic chemical vapour deposition. Temperature and field dependences of magnetization were measured and analyzed for all samples and revealed weak ferromagnetic behavior below T = 120–140 K. The difference in electronic structure along with a distinct similarity in the crystal structure of hexagonal manganites RMnO3 and hexagonal orthoferrites RFeO3 are brought into focus in order to explain the results. Hexagonal orthoferrites are regarded as a promising family of multiferroics.

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The novel layered structures comprising piezoelectric polymer and magnetoactive elastomer (MAE) were developed and investigated. The influence of iron particles content in the elastomeric layer, its thickness and Young’s modulus of silicone on the multiferroic properties of the structures were analyzed. The investigation included the experimental and numerical characterization of the magnetoelectric effect. The giant values of bending deformations of MAEs in the external gradient magnetic field led to giant values of induced voltage (up to nearly 650 mV) in the composite. The displacement of ferromagnetic particles inside the elastomeric matrix under gradient magnetic field became the main basis for numerical modelling. The molecular dynamic method, ‘virtual springs’ method and Verlet algorithm were used to obtain the results of the numerical experiment. The energy transformation and magnetic field response in the novel composite allow it to be used in sensors and energy-harvesting devices.

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Résumé -La caractérisation par la méthode magnétique des catalyseurs Fischer-TropschCet article passe en revue les développements récents dans le domaine de la caractérisation des catalyseurs Fischer-Tropsch à base de cobalt, de fer et de nickel par la méthode magnétique. La caractérisation magnétique fournit des informations précieuses sur la réduction du catalyseur, la taille des nanoparticules ferromagnétiques, la chimisorption, ainsi que sur les réactions topochimiques qui se produisent avec les catalyseurs au cours de la genèse de la phase active et dans des conditions réactionnelles. Les possibilités et les limites de la méthode magnétique sont examinées. Abstract -Magnetic Characterization of Fischer-Tropsch Catalysts -This paper reviews recent developments in the application of magnetic methods for investigation of Fischer