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Peekala M.,University of Warsaw | Wolff-Fabris F.,Helmholtz Center Dresden | Wolff-Fabris F.,Los Alamos National Laboratory | Fagnard J.-F.,University of Liege | And 5 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2013

An orthorhombic DyMnO3 single crystal has been studied in magnetic fields up to 14 T and between 3 K and room temperature. The field dependent ordering temperature of Dy moments is deduced. The paramagnetic Curie Weiss behavior is related mainly to the Dy3+sublattice whereas the Mn sublattice contribution plays a secondary role. DC magnetization measurements show marked anisotropic features, related to the anisotropic structure of a cubic system stretched along a body diagonal, with a magnetic easy axis parallel to the crystallographic b axis. A temperature and field dependent spin flop transition is observed below 9 K, when relatively weak magnetocrystalline anisotropy is overcome by magnetic fields up to 1.6 T. © 2013 Elsevier B.V.


Koutzarova T.,Bulgarian Academy of Science | Kolev S.,Bulgarian Academy of Science | Grigorov K.,Bulgarian Academy of Science | Ghelev C.,Bulgarian Academy of Science | And 6 more authors.
Solid State Phenomena | Year: 2010

Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 × 105 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 × 106 A/m and 2.37 × 105 J/m3, respectively. © (2010) Trans Tech Publications.


Pompeo N.,Third University of Rome | Rogai R.,Third University of Rome | Ausloos M.,SUPRATECS | Cloots R.,University of Liege | And 3 more authors.
Physica C: Superconductivity and its Applications | Year: 2011

We report on microwave measurements on DyBa2Cu3O 7-δ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance R s(H) in the a-b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K-Tc with a static magnetic field μ0H < 0.8 T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behavior of the sample under study. The magnetic field dependence of Rs(H) is ascribed to the dissipation caused by vortex motion. The analysis of Xs(H) points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa2Cu 3O7-δ with nanometric BaZrO3 inclusions. © 2011 Elsevier B.V. All rights reserved.


Laurent P.,SUPRATECS | Fagnard J.-F.,Royal Military Academy of Belgium | Babu N.H.,Brunel University | Cardwell D.A.,University of Cambridge | And 2 more authors.
Superconductor Science and Technology | Year: 2010

In this work we study, both experimentally and numerically, the self-heating of a bulk, large YBCO pellet of aspect ratio (thickness/diameter) ∼0.4 subjected to a large AC magnetic field. To ensure accurate temperature measurements, the sample was placed in an experimental vacuum chamber to achieve a small and reproducible heat transfer coefficient between the superconductor and the cryogenic fluid. The temperature was measured at several locations on the sample surface during the self-heating process. The experimentally determined temperature gradients are found to be very small in this arrangement (<0.2 K across the radius of the superconductor). The time-dependence of the average temperature T (t) is found to agree well with a theoretical prediction based on the one-dimensional (1D) Bean model, assuming a uniform temperature in the sample. A 2D magneto-thermal model was also used to determine the space and time-dependent temperature distribution T (r, z, t) during the application of the AC field. The losses in the bulk pellet were determined using an algorithm based on the numerical method of Brandt, which was combined with a heat diffusion algorithm implemented using a finite-difference method. The model is shown to be able to reproduce the main trends of the observed temperature evolution of the bulk sample during a self-heating process. Finally, the 2D model is used to study the effect of a non-uniform distribution of critical current density Jc(r, z) on the losses within the bulk superconductor. © 2010 IOP Publishing Ltd.


Vanderbemden P.,SUPRATECS | Laurent P.,SUPRATECS | Fagnard J.-F.,Royal Military Academy of Belgium | Ausloos M.,SUPRATECS | And 2 more authors.
Superconductor Science and Technology | Year: 2015

In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a 'forbidden' temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine analytical expressions for two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors. © 2010 IOP Publishing Ltd 1 Printed in the UK & the USA.

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