Anomalous increments of AC losses due to flux jumps

AC losses are measured applying magnetic fields of frequencies from 8.6 to 75 Hz and of 3 kG peak values to wires of Nb-60.2Ti-2.8Zr alloy mounted in slots of a bakelite bobbin. The temperature is varied between 1.9 K and the superconducting transition temperature T
c
= 8.9 K using supercritical He as coolant for the higher temperature region. Anomalous increments of AC losses caused by flux jumps are observed at certain field values H
fj
. H
fj
is almost constant at temperatures T = 4.2 to 6 K and decreases if the temperature is reduced below 4.2 K. The temperature dependence of H
fj
can be explained qualitatively by the temperature depencence of both the specific heat and the pinning force of the sample. The quantitative difference as well as a very weak frequency dependence of H
fj
and a reduction of H
fj
by embedding the sample in grease may be due to the influence of the special heat transfer conditions.