Magnetic properties of the neptunium monopnictides

The magnetic properties of the neptunium monopnictides, NpN, NpP, NpAs, and NpSb, have been examined by a variety of experimental techniques. All compounds have the NaCl structure. The present paper reports the results of magnetization, Mössbauer, electrical-resistivity, neutron-diffraction, and lattice-parameter (x-ray) measurements between 1.5 and 300 °K on polycrystalline samples prepared in this laboratory. Particular emphasis is placed on both the complementary information obtained by the different techniques, and how this helps to elucidate the microscopic magnetic behavior. Neptunium nitride is ferromagnetic with $T_C=87\mathrm{}$ °K. Neptunium phosphide orders antiferromagnetically at 130 °K with an incommensurate magnetic structure. At 74 °K the magnetic structure becomes commensurate with the lattice (repeat distance 3.0 unit cells) and at lower temperatures tends to form a 3+, 3- sequence. A complete solution for the magnetic structure at low temperature is obtained by combining the neutron and Mössbauer results. Neptunium arsenide becomes antiferromagnetic at $T_N=175\mathrm{}$ °K. At ∼ 160 °K, the high-temperature 4+, 4- structure transforms to the type-I arrangement (simple +,- sequence). The crystal lattice of NpAs becomes tetragonal at $T_N$, exhibits a first-order transition to cubic symmetry at 142 °K, and remains cubic at lower temperatures. The electrical resistivity increases by an order of magnitude on cooling from 142 to 141 °K. Neptunium antimonide is a type-I antiferromagnet with $T_N=207\mathrm{}$ °K. The variation of the ordered moments ($\frac{1.4{\mu }_B}{\mathrm{Np}}$ in NpN to $\frac{2.5{\mu }_B}{\mathrm{Np}}$ in NpSb), paramagnetic effective moment ($\sim \frac{2.5{\mu }_B}{\mathrm{Np}}$), and the Mössbauer parameters (hyperfine field, quadrupole interaction, and isomer shift) all suggest a trend toward ${\mathrm{Np}}^{3+}$, $5f^4$ free-ion behavior in progressing from the nitride to the antimonide.