Pressure Variation of the Curie Temperature and Spontaneous Magnetization in Fe[sub 2]P and Fe[sub 2]P[sub 0.9]As[sub 0.1]

The transition‐metal pnictides $({\mathrm{M}}_{\text{1−}\mathrm{y}}{\mathrm{M}}_{\mathrm{y}}\mathrm{\prime )}{\mathrm{P}}_{\text{1−}\mathrm{x}}{\mathrm{As}}_{\mathrm{x}}$ exhibit structural relationships and magnetic properties that indicate the presence of filled valence bands, empty conduction bands, and partially filled 3d bands active in metal‐metal bonding. In many cases they support spontaneous magnetism, thereby offering the opportunity to study itinerant‐electron magnetism as a function of 3d bandwidth and occupancy. In particular, the hexagonal system ${\mathrm{Fe}}_2{\mathrm{P}}_{\text{1−}\mathrm{x}}{\mathrm{As}}_{\mathrm{x}}$ is ferromagnetic, but for $\mathrm{x}\text{\hspace{0.17em}<\hspace{0.17em}\hspace{0.17em}0.33}$ its spontaneous moment at $\mathrm{T}\text{\hspace{0.17em}=\hspace{0.17em}0}\mathrm{K}$ is reduced from the ${\mu }_{\mathrm{o}}{\text{\hspace{0.17em}=\hspace{0.17em}3.0μ}}_{\mathrm{B}}/\mathrm{molecule}$ predicted for itinerant, spin‐only ferromagnetism. We investigated the pressure dependence of T_c and μ_o to 11 kbar. In ${\mathrm{Fe}}_2\mathrm{P}$ , the relation between ΔT_c [° C] and P [kbar] is: $\mathrm{P}\text{\hspace{0.17em}=\hspace{0.17em}−0.252(Δ}{\mathrm{T}}_{\mathrm{c}}\text{)\hspace{0.17em}−0.0012(Δ}{\mathrm{T}}_{\mathrm{c}}{)}^2$ . In ${\mathrm{Fe}}_2{\mathrm{P}}_{\text{0.9}}{\mathrm{As}}_{\text{0.1}}$ , it is: $\mathrm{P}\text{\hspace{0.17em}=\hspace{0.17em}−0.71(Δ}{\mathrm{T}}_{\mathrm{c}}\text{)\hspace{0.17em}−0.0017(Δ}{\mathrm{T}}_{\mathrm{c}}{)}^2$ . Pressure did not change significantly the value of μ at 58K, but it promoted a remarkably exchange‐enhanced susceptibility above T_c. We interpret these results to mean that the reduced moment in ${\mathrm{Fe}}_2\mathrm{P}$ is not due to conduction‐band overlap of the Fermi energy, but to a 3d bandwidth that is just narrow enough to support spontaneous ferromagnetism. A critical pressure ${\mathrm{P}}_{\mathrm{c}}\text{\hspace{0.17em}≈\hspace{0.17em}13\hspace{0.17em}}\mathrm{kbar}$ is estimated for a ferromagnetic‐to‐ metamagnetic transition.