Mn(Pt1-xPdx)5 P: Isovalent tuning of Mn-sublattice magnetic order

We report the growth and characterization of MnPd5P, a rare-earth-free ferromagnet, with TC≈295K and planar anisotropy, and conduct a substitutional study with its antiferromagnetic analog MnPt5P. All compounds in the family adopt the layered anti-CeCoIn5-type structure with the space group P4/mmm, and EDS and x-ray diffraction results indicate that MnPt5P and MnPd5P form a complete solid solution. Based on measurements of the temperature- and field-dependent magnetization and resistance, we construct a temperature-composition (T-x) phase diagram for Mn(Pt1-xPdx)5P and demonstrate that the essentially antiferromagnetic order found in MnPt5P is extraordinarily sensitive to Pd substitution. At low Pd fractions (x<0.010), the single antiferromagnetic-like transition in pure MnPt5P splits into a higher-temperature ferromagnetic transition followed first, upon cooling, by a lower temperature ferromagnetic to antiferromagnetic transition and then by a re-entrant antiferromagnetic to ferromagnetic transition at even lower temperatures. The antiferromagnetic region makes up a bubble phase that persists up to x≈0.008-0.009 for T≈150K, with all samples 0≤x<0.008 recovering their initial ferromagnetic state upon further cooling to base temperature. Once x>0.010, Mn(Pt1-xPdx)5P undergoes a only single transition into the ferromagnetic phase. The Curie temperature initially increases rapidly with x, rising from TC≈197K at x = 0.013 to a maximum of TC≈312K for x≈0.62, and then falling back to TC≈295K for pure MnPd5P (x=1.00). Given that Pt and Pd are isoelectronic, this work raises questions as to the origin of the extreme sensitivity of the magnetic ground state and the nature of the re-entrant ferromagnetism at dilute Pd levels.