Neutron-diffraction study of PuAs and PuSb

The first neutron scattering experiments on PuAs and PuSb single crystals are reported. PuAs is found to be a ferromagnet below $T_c=123\mathrm{}\pm 1$ K. PuSb develops below $T_N=85\mathrm{}\pm 1$ K an incommensurate ordering and undergoes, at $T_{\mathrm{IC}}=67\mathrm{}$ K, a first-order transition to a commensurate phase, ferromagnetic in nature. In both compounds, moments are perpendicular to the ferromagnetic (001) planes, their low-temperature value is $\mu =(0.75\mathrm{}\pm 0.1){\mu }_B$, indicating a ${\Gamma }_8$-type ground state within the Russell-Saunders level scheme. The magnetic phase diagram of PuSb has been determined. The incommensurate phase is suppressed by a small field of 8.5 kOe.