  Ferromagnets crystallizing in structures described by chiral cubic space
groups, including compounds with the B20 or $\beta$-Mn structures, are known to
host long-period chiral spin textures such as skyrmion lattices. These spin
textures are stabilized by a competition between ferromagnetic exchange and
antisymmetric Dyzaloshinskii-Moriya (DM) exchange, which is enhanced by the
spin-orbit coupling associated with high-atomic-number elements. For real-world
application, it is desirable to find materials that can host compact skyrmion
lattices at readily accessible temperatures. Here, we report on the crystal
chemistry and magnetic phase diagrams of a family of compounds with the filled
$\beta$-Mn structure, FePd$_{1-x}$Pt$_x$Mo$_3$N with $T_C$ ranging from 175 K
to 240 K. DC and AC magnetization measurements reveal magnetic phase diagrams
consistent with the formation of a skyrmion pocket just below $T_C$. The
magnitudes of ferromagnetic and DM exchanges are determined from the phase
diagrams, demonstrating that the introduction of increasing amounts of Pt can
be used to increase spin-orbit coupling in order to control the expected
skyrmion lattice parameter between 140 nm and 65 nm while simultaneously
increasing $T_C$.
