Probing low moment magnetism in a 5d1 material with polarized neutron powder diffraction

5d based transition metal oxides are undergoing broad current interest due to the unique balance of spin-orbit coupling, crystal field splitting, bandwidth and Coulomb interactions that results in emergent quantum properties. While there has been much focus on 5d5 iridates that can host a spin-orbit entangled Jeff=1/2 ground state, there are less investigations of other 5d states. In particular, a 5d1 state may offer a route to enhanced quantum behavior with S=1/2 magnetism. However, two factors present hurdles: (i) 5d1 based materials are rare and (ii) for those that exist the octahedral OsO6 environment and large spin orbit coupling creates a Jeff=3/2 state. To overcome both of these hurdles we present an investigation of an osmium material with an "apatite" structure, Ba5(OsO5)3Cl. The Os environment is a tetragonal pyramid with OsO5 resulting in an altered electronic ground state and a reduced role of spin orbit coupling that allows the formation of a S=1/2 state. Long range magnetic order occurs below 5 K and unusually for an insulator the ordering is predicted to be ferromagnetic. Experimentally accessing a small ferromagnetic moment is challenging with traditional neutron powder diffraction. Instead we utilize half-polarized neutron powder diffraction. This technique offers the extraction of weak ferromagnetic signals (< 0.1µB) and the full matrix tensor for the local site susceptibility in powders and offers a straightforward method to determine crystal field and anisotropy of an ion.