Papers by Author: M. Tropeano

Abstract: The crystal and magnetic structures of (Pr0.55Ca0.45)(Mn1-yCry)O3 (y = 0.00, 0.03, 0.06) have been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. An orthorhombic-to-monoclinic phase transition occurs on cooling in (Pr0.55Ca0.45)MnO3; at lower temperature charge ordering in the Mn sub-lattice, coupled with orbital ordering, induces an anti-ferromagnetic (AFM) spin ordering within the monoclinic phase. Cr substitution at the Mn site hinders the phase transition, although for the y = 0.03 sample small amounts of the monoclinic phase co-exist at low temperature with the main phase characterized by an orthorhombic structure. In addition Cr favours ferromagnetism (FM) participating to the double exchange mechanism. Cr substitution prevents charge ordering, favouring double exchange and hence FM spin ordering within the Mn sub-lattice. The Curie temperature of the Cr substituted samples is independent on the Cr content.

Abstract: In spite of the relatively short time dedicated to the development of MgB2 conductors since its discovery in 2001, a remarkable progress has been already achieved in terms of critical current density, homogeneity of the conductors and reproducibility of the results over long lengths. Unlike other HTS and LTS materials, MgB2 conductor processing is more open to a number of improvements and modifications that help in making it more attractive for several DC and AC applications. Up to now our work has been focused on the improvement of the production capacity in order to allow for the assembly of large prototype magnets to test the capability of MgB2 to maintain the present level of performance over very long lengths. Long MgB2 tapes of about 1600 m with superconducting and mechanical properties suitable for making low field magnets are already available. Presently we are focusing on the enhancement of the irreversibility field. This would open the possibility of using MgB2 also for high field applications.