Spin-glass behavior of AFe4Al8 (A=Th,U,Np) intermetallics.

Combined ac- and dc-susceptibility, neutron-diffraction, and M\"ossbauer studies of ${\mathrm{UFe}}_{4}$${\mathrm{Al}}_{8}$, ${\mathrm{NpFe}}_{4}$${\mathrm{Al}}_{8}$, and ${\mathrm{ThFe}}_{4}$${\mathrm{Al}}_{8}$ argue for the establishment of a spin-glass (SG) state below ${\mathit{T}}_{\mathrm{SG}}$(B\ensuremath{\rightarrow}0)\ensuremath{\approxeq}130, 120, and 110 K, respectively. The SG temperature ${\mathit{T}}_{\mathrm{SG}}$(B) decreases with application of external magnetic field B. The ac susceptibility (${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$) shows a sharp cusp at ${\mathit{T}}_{\mathrm{SG}}$(B\ensuremath{\approxeq}0). The isothermal and thermoremanent magnetization differ markedly and show temperature-dependent irreversibilities below ${\mathit{T}}_{\mathrm{SG}}$(B). The M\"ossbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below ${\mathit{T}}_{\mathrm{SG}}$(0). The neutron-diffraction studies of ${\mathrm{UFe}}_{4}$${\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\ensuremath{\rightarrow}0)\ensuremath{\rightarrow}${\mathit{T}}_{\mathrm{OG}}$. 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 ${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$, however, is not necessarily a confirmation for the establishment of a SG state.