SPIN-GLASS BEHAVIOR OF AFE4AL8 (A = TH,U,NP) INTERMETALLICS

Combined ac- and dc-susceptibility, neutron-diffraction, and Mössbauer studies of UFe4Al8, NpFe4Al8, and ThFe4Al8 argue for the establishment of a spin-glass (SG) state below TSG(B'0)130, 120, and 110 K, respectively. The SG temperature TSG(B) decreases with application of external magnetic field B. The ac susceptibility (ac) shows a sharp cusp at TSG(B0). The isothermal and thermoremanent magnetization differ markedly and show temperature-dependent irreversibilities below TSG(B). The Mössbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below TSG(0). The neutron-diffraction studies of UFe4Al8 and NpFe4Al8 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 TOG, and show that it serves as the upper limit of TSG(B) with TSG(B'0)'TOG. For concentrated SG systems, TSG(0) is a reproducible value, independent of the observation time window, which indicates a true thermodynamic phase transition; the cusp in ac, however, is not necessarily a confirmation for the establishment of a SG state. © 1990 The American Physical Society.