Identification of the A15 Contours in Multifilamentary Nb${3}$Sn Wires by Means of a Magnetic Method

A model for evaluating thecontours of the A15 layer in
$\rm Nb_{3}Sn$
wires is proposed. These borders, crucial for the correct assessment of the layer current density
$ J_c$
and the volume layer pinning force
$ F_p (B)$
are in fact non trivial to assess in some cases from scanning electron microscopy (SEM) polished surface images, eventually leading to non-negligible discrepancies between independent measurements reporting the same
$ m-H$
or
$ I_c$
values. A self-reliant method from magnetometry based (Bean) models is proposed, allowing the outer (
$ r_o$
) and inner (
$ r_i$
) contours of the A15 phase to be assessed. The magnetic moment of short samples of a prototype powder in tube (PIT) wire with artificial pinning centres (APC) was measured between 6.5 T (
$ H_0$
) and 6.95 T at 4.2 K in a superconducting quantum interference device (SQUID) magnetometer, allowing the calculation of
$r_{o}$
from the initial slope of
$ dm/dH$
. Then, the derived function for the remanent magnetic moment
$\rm m_{rem}$
is fit to the experimental data in order to derive
$ r_i$
, under the condition that the saturation moment exactly matches the experimental value.