  Subtle changes in chemical bonds may result in dramatic revolutions in
magnetic properties in solid state materials. MnPt5P, a new derivative of the
rare-earth-free ferromagnetic MnPt5As, was discovered and is presented in this
work. MnPt5P was synthesized and its crystal structure and chemical composition
were characterized by X-ray diffraction as well as energy-dispersive X-ray
spectroscopy. Accordingly, MnPt5P crystallizes in the layered tetragonal
structure with the space group P4/mmm (No. 123), in which the face-shared
Mn@Pt12 polyhedral layers are separated by P layers. In contrast to the
ferromagnetism observed in MnPt5As, the magnetic properties measurements on
MnPt5P show antiferromagnetic ordering occurs at ~188 K with a strong magnetic
anisotropy in and out of the ab-plane. Moreover, a spin-flop transition appears
when a high magnetic field is applied. An A-type antiferromagnetic structure
was obtained from the analysis of powder neutron diffraction (PND) patterns
collected at 150 K and 9 K. Calculated electronic structures imply that
hybridization of Mn-3d and Pt-5d orbitals are critical for both the structural
stability and observed magnetic properties. Semi-empirical molecular orbitals
calculations on both MnPt5P and MnPt5As indicate that the lack of 4p character
on the P atoms at the highest occupied molecular orbital (HOMO) in MnPt5P may
cause the different magnetic behavior in MnPt5P compared to MnPt5As. The
discovery of MnPt5P, along with our previously reported MnPt5As, parametrizes
the end points of a tunable system to study the chemical bonding which tunes
the magnetic ordering from ferromagnetism to antiferromagnetism with strong
spin-orbit coupling (SOC) effect.
