  Nb$_{3}$Sn is a superconductor of great relevance for perspective RF
applications. We present for the first time surface impedance $Z_s$
measurements at 15 GHz and low RF field amplitude on Nb$_{3}$Sn in high
magnetic fields up to 12 T, with the aim of increasing the knowledge of
Nb$_{3}$Sn behavior in such conditions. $Z_s$ is a fundamental material
parameter that directly gives useful information about the dissipative and
reactive phenomena when the superconductor is subjected to high-frequency
excitations. Therefore, we present an analysis of the measured $Z_s$ with the
aim of extracting interesting data about pinning in Nb$_{3}$Sn at high
frequencies. From $Z_s$ we extract the vortex motion complex resistivity to
obtain the $r$-parameter and the depinning frequency $\nu_p$ in high magnetic
fields. The comparison of the results with the literature shows that the
measured $\nu_p$ on bulk Nb$_{3}$Sn is several times greater than that of pure
Nb. This demonstrates how Nb$_{3}$Sn can be a good candidate for RF
technological applications, also in high magnetic fields.

Superconductivity