Unusual phase boundary and altered Fermi surface in CeOs$_4$Sb$_{12}$ at high magnetic fields

The filled skutterudite compounds CeOs$_4$Sb$_{12}$ is a 1K antiferromagnetic (AFM) semimetal and candidate topological insulator. Using magnetization ($M$), MHz-conductivity and electrical resistivity ($\rho$) data recorded at magnetic fields of up to $\mu_0 H = 60$ T and temperature $T$ down to 0.4 K, we map out the $(H, T)$ phase diagram. At low $T$ and low $H$ (L phase), the Ce $4f$ electron is delocalized, yielding heavy quasiparticles with a small Fermi surface, while at high $T$ and high $H$ (H phase) the $4f$ electron is quasi-localized, leaving a single, almost spherical Fermi surface of light-mass holes. The behavior of $\rho$ and $dM/dH$ on crossing the L-H boundary, plus comparisons with bandstructure calculations, suggest that the L-H phase transition in CeOs$_4$Sb$_{12}$ is similar in origin to the $\alpha - \gamma$ transition in Ce and its alloys. However, interplay between the free-energy contributions of the AFM and L phases results in a very unusual curvature of the phase boundary at low $T$.