Cb. Nunes et al., CRITICAL-CURRENT ANISOTROPY IN CONVENTIONAL AND ARTIFICIAL PINNING CENTER ROUND WIRE NB-TI SUPERCONDUCTORS, IEEE transactions on applied superconductivity, 7(2), 1997, pp. 1138-1141
Critical current density anisotropy was detected in conventional and a
rtificial pinning center (APC) Nb-Ti round wires by means of a relevan
t increase of the magnetization M as the wire aspect ratio A(w) (lengt
h L/filament diameter d(f)) was increased. However, contrary to what i
s observed in APC wires, the critical current density J(parallel to) c
alculated from the SQUID measured saturated-magnetization in conventio
nal wires using an anisotropic Bean Model (BM), is significantly diffe
rent from the transport critical current density J(ct). We suggest tha
t the mismatch between SQUID measured J(parallel to) and J(ct) is due
a combination of the discontinuity of the ribbons along the wire lengt
h in conventional Nb-Ti wires and also to the low electric fields gene
rated in SQUID measurements. The low electric fields allows the magnet
ization-induced currents to redistribute inside the wire due to the lo
cal inhomogeneities associated with the presence of alpha-Ti ribbons.
In this case anisotropic BM cannot be used to extract J(c) from M beca
use it will be function of the intrinsic lengths of these ribbons.