Magnetic and Electronic Properties of Npco2: Evidence for Long-Range Magnetic Order

The magnetic and electronic properties of the cubic Laves phase intermetallic compound NpCo${}_{2}$ have been investigated by magnetization, specific heat, and electrical resistivity measurements. Magnetization data confirm previous results, i.e., an antiferromagneticlike order at 12.5 K and a metamagnetic transition at ${B}_{m}$ \ensuremath{\sim} 4.3 T at 3 K with a saturation moment of \ensuremath{\sim}0.6 ${\ensuremath{\mu}}_{\mathrm{B}}$. Although extensive neutron diffraction studies failed to establish the nature of the antiferromagnetic order, its occurrence is confirmed by specific heat measurements that also highlight the presence of magnetic fluctuations suppressed by the application of an external field larger than ${B}_{m}$. The observed high value of the electronic specific heat, ${\ensuremath{\gamma}}_{0}$ \ensuremath{\sim} 330 mJ/mol K${}^{2}$ as $T$ \ensuremath{\rightarrow} 0, and the minute magnetic entropy, 0.1$R$ln2, classify NpCo${}_{2}$ as an itinerant, moderately-heavy-fermion antiferromagnet. The electrical resistivity data agree with the presence of an antiferromagnetic order below ${B}_{m}$ and suggest a non-Fermi liquid behavior at low temperature. The electronic structure of NpCo${}_{2}$ was examined theoretically and compared to experimental data. Local spin density approximation (LSDA) calculations show that this material is close to a magnetic instability and that ferromagnetic alignment of the Np moments is the most stable. LSDA+$U$ calculations in the fully localized limit with $U$ $=$ 0 give reasonable agreement with the experimental Np moment value and their orbital and spin contributions.