The BM_5Se_9 phases: studies of the behaviour of superconductors and ferromagnets

摘要

Syntheses performed in the A_3B/Mse_2 systems lead to the formation of (B/B')(M/A)_5Se_9 phases (ferromagnets or superconductors) {A=M= (Nb, V, Ta); B=B'= (Al, Ge, Ga, Sn, Sb)}. The magnetic studies on GaV_5Se_9, GaV_(0.15)Ta_(4.85)Se_9 and Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9, compared to that of SnNb_5Se_9, GaNb_5Se_9 show that the presence of V modifies the superconducting exchanges and allows ferromagnetic couplings. Through Point-Contact Spectroscopy on SnNb_5Se_9, it is shown that nanoclusters of the pristine NbSe_2 and Nb_3Sn exist, so the properties result from a synergy between them and the matrix, and it could be a mesoscopic superconductor. The superconductors' s T_c values vary from 17.5(2) K for SnNb_5Se_9to 6.0(2) K for SbNb_5Se_9, and the ferromagnetic state transition from 22K for GaV_5Se_9, to 35 K for Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9. The ~(119)Sn and ~(121)Sb Moessbauer studies of Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9 and SbNb_5Se_9 are compared to that obtained for SnNb_5Se_9 itself; for this last one, 3 types of sites corresponding to 2 species of Sn were observed with a reversible dynamic process between the 2 SnII Sn-Se sites correlated to a modification of the Sn-Nb site(l). SbNb_5Se_9, at 5 K, shows just a singlet, attributed to Sb surrounded by Se; from 298 K to 5 K, Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9, presents one asymmetrical Sn-Se site without any transferred field on Sn and an increased asymmetry when T decreases. Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9 is a weak and complicated ferromagnet. In both cases, GaV_5Se_9, and Sn_(0.94)Ga_(0.05)V_(0.30)Nb_(4.70)Se_9, the ESR spectrum observed at 300 K can be fitted with 2 lorentzian distributions: for the first case, with axial symmetries, for the second only one remains with an axial symmetry, belonging to the V sites, its vanadium sites are thus less symmetrical than in the first case.