Modifying the temperature dependence of magnetic garnet film coercivity by etching

The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnet films was modified in a defined temperature range by removing the surface layer of the films. Outside the given temperature range the coercivity versus temperature curve did not change. The result supports a model of coercivity according to which different sets of material imperfections are responsible for pinning the domain walls in different temperature regions. Appropriate processing of the samples enables some of the pinning sets to be modified independently of each other

Radiation generated by accelerating and rotating charged black holes in (anti-)de Sitter space

Asymptotic behaviour of gravitational and electromagnetic fields of exact type D solutions from the large Plebanski-Demianski family of black hole spacetimes is analyzed. The amplitude and directional structure of radiation is evaluated in cases when the cosmological constant is non-vanishing, so that the conformal infinities have either de Sitter-like or anti-de Sitter-like character. In particular, explicit relations between the parameters that characterize the sources (that is their mass, electric and magnetic charges, NUT parameter, rotational parameter, and acceleration) and properties of the radiation generated by them are presented. The results further elucidate the physical interpretation of these solutions and may help to understand radiative characteristics of more general spacetimes than those that are asymptotically flat.Comment: 24 pages, 18 figures. To appear in Classical and Quantum Gravit

Gyratons on Melvin spacetime

We present and analyze new exact gyraton solutions of algebraic type II on a background which is static, cylindrically symmetric Melvin universe of type D. For a vanishing electromagnetic field it reduces to previously studied gyratons on Minkowski background. We demonstrate that the solutions are member of a more general family of the Kundt spacetimes. We show that the Einstein equations reduce to a set of mostly linear equations on a transverse 2-space and we discuss the properties of polynomial scalar curvature invariants which are generally non-constant but unaffected by the presence of gyratons.Comment: 15 pages, no figures, journal version extended by appendices B and

Higher-dimensional Kundt waves and gyratons

We present and analyze exact solutions of the Einstein-Maxwell equations in higher dimensions which form a large subclass of the Kundt family of spacetimes. We assume that the cosmological constant may be nonvanishing, and the matter consists of a background aligned electromagnetic field and an additional pure radiation (gyratonic) field with a spin. We show that the field equations reduce to a set of linear equations on the transverse space which can be solved exactly and expressed in terms of the Green functions. We thus find explicit exact gyratonic gravitational and electromagnetic fields created by a radiation beam of null matter with arbitrary profiles of energy density and angular momenta. In the absence of the gyratonic matter we obtain pure nonexpanding higher-dimensional gravitational waves. In particular, we investigate gyratons and waves propagating on backgrounds which are a direct-product of 2-spaces of constant curvature. Such type D or 0 background spacetimes generalize 4-dimensional Nariai, anti-Nariai and Plebanski-Hacyan universes, and conformally flat Bertotti-Robinson and Minkowski spaces. These spacetimes belong to a wider class of spaces which admit the Kahler structure related to the background magnetic field. The obtained wave and gyraton solutions are also members of the recently discussed class of spacetimes with constant scalar invariants (CSI) of the curvature tensor.Comment: 18 pages, no figure

Characterization of microstructural defects from temperature dependence of domain wall coercive field

The steep temperature dependence of the domain wall coercive field. HCW(T), of highly anisotropic rare earth magnetic garnet films, recently described as a piece-wise exponential curve, was analysed. The quantitative analysis revealed the HCW(T) shape to be caused by the exponential temperature dependence of the anisotropy field, modified by functions describing the efficiency of the wall-defects interaction, at least for two different sets of defects. Characteristic periods of the two sets of defects were found and the defects were identified in one case with point defects caused by local fluctuation of the anisotropy constant at sites of individual rare earth ions in the crystal lattice and in the other case with fluctuation of local stresses due to local fluctuation of the material composition

Temperature changes of coercivity in garnets

The temperature dependence of domain wall coercive field, HCW, of an epitaxial magnetic rare earth garnet film was modified by covering the garnet layer with sputtered SiO2. The result supports a recent model of domain wall coercivity. According to this model the typical shape of the HCW(T) curves originate in the temperature dependence of the anisotropy field, modified by several efficiency functions of the wall-defect interaction. The efficiency functions reflect predominance of different sets of sample defects within different temperature regions. Introduction of some extra stress, due to the sputtered SiO2, changed the wall-defect interaction in a certain range of temperatures and altered there the shape of the HCW(T) curve. Characteristics of the original system of defects and estimates of those for the modified sample are presented

Syntéza hvězdicovitých polymerů pro polyurethanové nátery pomocí iontové polymerace

Star polymers were prepared by ionic polymerization. As the star core polyols served e.g.: trimethylol propane (TMP), pentaerythrythol (PETP) and dipentaerythrythol (DPETP) and the star arms were built up from ε-caprolactame, lactide in different ratios of both components. Coatings based on TMP have often anti-slip properties high gloss but lower hardness. On the contrary, coatings based on PETP and DPETP have high hardness, but no anti-slip properties.Hviezdicovité polyméry boli pripravené ionovou polymerizáciou. Ako jadro hviezdy, bol použitý napr. trimetylol propán (TMP), pentaerytritol (PETP), dipentaerytritol (DPETP) a ramená boli syntetizivané z ε-kaprolaktámu a laktidu v roznych pomeroch. Nátery založené na TMP majú proti šmykové vlastnosti, vysoký lesk ale nižšiu tvrdosť. Pre porovnanie, nátery založene na PETP a DPETP majú vysokú tvrdosť, ale žiadne protišmykové vlastnosti