Novel Mn-based Magnetic Material with Strong Spin-Orbit Coupling*

MnPt5M (M=P and As) compounds were designed and synthesized. The crystal and magnetic structure characterizations by both X-ray and neutron scattering show MnPt5M crystalizes in the tetragonal anti-CeCoIn5 layered structure with van der Waals magnetic behaviors. MnPt5P displays an antiferromagnetic ordering ~ 188 K and a spin flop transition at high magnetic field while MnPt5As is a ferromagnet with the T_c ~287 K. The hybridization of the Mn-3d and Pt-5d orbitals plays an important role in the structural stability as well as in their magnetic properties according to the electronic calculations. In addition, the presence of the 4p character of As in the HOMO of the MnPt5As material is critical to the observed ferromagnetic ordering. The lack of strong p character in P in the HOMO orbital of MnPt5P has led to a minimal orbital overlap, thus dominating the interlayer magnetic exchange interactions to give AFM ordering in MnPt5P. The discovery of MnPt5As and MnPt5P opened a tunable system to study the chemical bonding on the magnetic behavior.