Observation of two ferromagnetic phases in Fe 3 Mo 3 N

We report alloying-induced and field-induced ferromagnetism in the $\ensuremath{\eta}$-carbide-type compound ${\mathrm{Fe}}_{3}{\mathrm{Mo}}_{3}\mathrm{N}$, which shows a non-Fermi-liquid behavior in the vicinity of a ferromagnetic quantum critical point. Co substitution induces ferromagnetism in ${(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})$${}_{3}{\mathrm{Mo}}_{3}\mathrm{N}$ in the composition range $0.05\ensuremath{\le}x\ensuremath{\le}0.60$. With increasing $x$, the magnetism varies from the Curie-Weiss-type paramagnetism with a maximum in the temperature dependence of the magnetic susceptibility to another paramagnetism without the maximum via a weak ferromagnetism. An itinerant electron metamagnetic transition is observed for low $x$ at a magnetic field of $\ensuremath{\sim}$14 T. The magnetic phase diagram, in which the alloying-induced and the field-induced ferromagnetic phases are separated, is different from conventional phase diagrams for weak ferromagnets. Taking account of the fact that a magnetic field induces a Fermi-liquid behavior, the quantum criticality for pure ${\mathrm{Fe}}_{3}{\mathrm{Mo}}_{3}\mathrm{N}$ appears to be dominated by the dispersive spin fluctuations observed in the alloying-induced ferromagnetic phase.