Magnetic properties of NpPd3and PuPd3intermetallic compounds

Abstract

Magnetization, electrical-resistivity, Mössbauer-effect, and neutron-diffraction measurements have been made between 1.5 and 300 °K for two phases of Np${\mathrm{Pd}}_3$ and for Pu${\mathrm{Pd}}_3$. The compounds Pu${\mathrm{Pd}}_3$ and cubic Np${\mathrm{Pd}}_3$ have the Au${\mathrm{Cu}}_3$-type crystal structure. Hexagonal Np${\mathrm{Pd}}_3$ has the Ti${\mathrm{Ni}}_3$-type structure and is the room-temperature equilibrium phase. Cubic Np${\mathrm{Pd}}_3$ can be retained by rapid quenching, and the specimen used in this investigation has a long-range order parameter of $S=0.92\mathrm{}$. The cubic Np${\mathrm{Pd}}_3$ compound is antiferromagnetic with a Néel temperature $T_N=(55\mathrm{}\pm 1)$ °K, and the magnetic transition is apparently first order, since the sublattice magnetization is constant up to 45 °K. A large energy gap $\Delta$ in the spin-wave dispersion relation is observed below 40 °K in the electrical resistivity with $\Delta \simeq 3k{T}_N\simeq k\Theta \simeq k\left(150\mathrm{} °\mathrm{K}\right)$, where $\Theta$ is the Debye temperature obtained from the resistivity data. Cubic Pu${\mathrm{Pd}}_3$ is antiferromagnetic below 24 °K. Cubic Np${\mathrm{Pd}}_3$ and Pu${\mathrm{Pd}}_3$ have a magnetic structure that consists of ferromagnetic (111) planes coupled antiferromagnetically, and they have ordered moments of (2.0±0.1) ${\mathrm{\mu }}_{\mathit{B}}$/Np atom and (0.8±0.1) ${\mathrm{\mu }}_{\mathit{B}}$/Pu atom, respectively. Hexagonal Np${\mathrm{Pd}}_3$ does not possess long-range magnetic order, but does appear to have an abrupt transition to short-range magnetic order distributed throughout the bulk below ∼32 °K, as indicated by an apparent Curie temperature in the magnetization, a spin-disorder resistivity anomaly, and a broadening of the paramagnetic Mössbauer line as the temperature is lowered from 30 to 4.2 °K. Both Np${\mathrm{Pd}}_3$ phases have the same effective paramagnetic moment of 2.74 ${\mathrm{\mu }}_{\mathit{B}}$/Np atom, consistent with the ${\mathrm{Np}}^{3+}$ ionic configuration, the same volume per formula unit, and essentially the same first-and second-nearest-neighbor neptunium environment. The difference in magnetic behavior between the cubic and hexagonal phases of Np${\mathrm{Pd}}_3$ is at present not understood.