  Polyhydrides are a novel class of superconducting materials with extremely
high critical parameters, which is very promising for applications. On the
other hand, complete experimental study of the magnetic phase diagram for the
best so far known superconductor, lanthanum decahydride LaH$_{10}$, encounters
a serious complication because of the large upper critical magnetic field
$\textit{H}$$_{C2}$(0), exceeding 120-160 T. Partial replacement of La atoms by
magnetic Nd atoms results in a decrease of the upper critical field, which
makes it attainable for existing pulse magnets. We found that addition of
neodymium leads to significant suppression of superconductivity in LaH$_{10}$:
each atomic % of Nd causes decrease in $\textit{T}$$_{C}$ by 10-11 K. Using
strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase
diagram of the ternary (La,Nd)H$_{10}$ superhydride, which appears to be
surprisingly linear with $\textit{H}$$_{C2}$ $\propto$ |$\textit{T}$ -
$\textit{T}$$_C$|. The pronounced suppression of superconductivity in
LaH$_{10}$ by magnetic Nd atoms and the robustness of $\textit{T}$$_C$ with
respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem
indicate the isotropic ($\textit{s}$-wave) character of conventional
electron-phonon pairing in the synthesized superhydrides.
