Combined ac- and dc-susceptibility, neutron-diffraction, and Mössbauer studies of ${\mathrm{UFe}}_4$${\mathrm{Al}}_8$, ${\mathrm{NpFe}}_4$${\mathrm{Al}}_8$, and ${\mathrm{ThFe}}_4$${\mathrm{Al}}_8$ argue for the establishment of a spin-glass (SG) state below ${\mathit{T}}_{\mathrm{SG}}$(B→0)≊130, 120, and 110 K, respectively. The SG temperature ${\mathit{T}}_{\mathrm{SG}}$(B) decreases with application of external magnetic field B. The ac susceptibility (${\mathrm{\chi }}_{\mathrm{ac}}$) shows a sharp cusp at ${\mathit{T}}_{\mathrm{SG}}$(B≊0). The isothermal and thermoremanent magnetization differ markedly and show temperature-dependent irreversibilities below ${\mathit{T}}_{\mathrm{SG}}$(B). The Mössbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below ${\mathit{T}}_{\mathrm{SG}}$(0). The neutron-diffraction studies of ${\mathrm{UFe}}_4$${\mathrm{Al}}_8$ and ${\mathrm{NpFe}}_4$${\mathrm{Al}}_8$ show that after switching off the external magnetic field greater than 3 T the U and Np momenta (2a site) are frozen parallel to the direction of the external field. We label the freezing temperature under high external fields ${\mathit{T}}_{\mathrm{OG}}$, and show that it serves as the upper limit of ${\mathit{T}}_{\mathrm{SG}}$(B) with ${\mathit{T}}_{\mathrm{SG}}$(B→0)→${\mathit{T}}_{\mathrm{OG}}$. For concentrated SG systems, ${\mathit{T}}_{\mathrm{SG}}$(0) is a reproducible value, independent of the observation time window, which indicates a true thermodynamic phase transition; the cusp in ${\mathrm{\chi }}_{\mathrm{ac}}$, however, is not necessarily a confirmation for the establishment of a SG state.

• Received 22 May 1990

DOI:https://doi.org/10.1103/PhysRevB.42.8507