The 1-K phase transition in ${\mathrm{UCu}}_5$, showing up in specific-heat data within the antiferromagnetic state below ${\mathit{T}}_{\mathit{N}}$≃15 K, was investigated. Neither the average internal fields seen by the ${\mathrm{\mu }}^{+}$ nor the magnetic and nuclear Bragg reflections in the neutron-diffraction data reflect the phase transition while the muon relaxation rates increase drastically below 1.2 K. These results are interpreted in terms of some additional small-moment magnetic order or spin-density-wave phenomenon, implying the coexistence of two rather independent electronic subsystems, one involving ‘‘heavy’’ electrons, associated with the weak magnetism, and the other associated with the ‘‘conventional’’ antiferromagnetic order.

• Received 9 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2454