Mn(Pt_1-xPd_x)₅P: Isovalent Tuning of Mn Sublattice Magnetic Order

We report the discovery of MnPd₅P, a rare-earth-free ferromagnet with T_C = 295 K and conduct a substitutional study with its antiferromagnetic analogue MnPt₅P. Based on measurements of the temperature and field dependent resistance and magnetization, we construct a temperature–composition phase diagram for Mn(Pt_1-xPd_x)₅P. The AFM state in pure MnPt₅P is extraordinarily sensitive to Pd substitution, and as little as x = 0.02 stabilizes purely FM order, with T_C maximized at 320 K for x = 0.62. At dilute Pd levels, x < 0.01, both AFM and FM states are observed. Here, the single AFM transition in MnPt₅P splits into a higher temperature PM to FM transition followed on cooling by a lower temperature FM to AFM transition and then by re-entry into the FM state with further cooling. We propose the fantastic sensitivity of the AFM phase in Mn(Pt_1-xPd_x)₅P (x < 0.01) may be associated with change in the Fermi-surface topology, and show electronic band structure calculations are qualitatively in-line with this picture.