Magnetic Structure and Crystal Field Potential of PrOs$_4$As$_{12}$*

Neutron powder diffraction and elastic neutron scattering have been used to determine the magnetic structure of the Filled Skutterudite compound PrOs$_4$As$_{12}$. The system becomes antiferromagnetically ordered with a Neel temperature ($T_N$) at 2.28K, which has A-type magnetic structure with spins lying along the doubled axis of the magnetic unit cell. The crystal field potential of PrOs$_4$As$_{12}$ has been studied by inelastic neutron scattering (INS). The ground state in the $T_h$ point group symmetry is determined to be a $\Gamma_5$ triplet. This is confirmed by Zeeman effect exhibited at low temperatures under high magnetic fields.