mu SR evidence for the spin-liquid-to-spin-glass transition in beta-Mn1-xAlx

Positive muon spin relaxation (mu SR) has been used to investigate the evolution of spin dynamics in beta-phase Mn1-xAlx alloys over the concentration range 0 less than or equal to x less than or equal to 0.2. A comparison of measured and calculated nuclear dipolar linewidths indicates that the muon is localized at the (3/8,3/8,3/8) interstitial lattice site in the center of a distorted octahedron of "magnetic" site II Mn atoms. For Al concentrations at and below x=0.09 the contribution to the muon relaxation from the dynamic atomic dipolar fields takes the form of a simple exponential e(-lambda t). For pure beta-Mn the associated muon relaxation rate lambda is very small (0.02 mu s(-1)) and temperature independent, while for Al concentrations in the range 0.03 less than or equal to x less than or equal to 0.09 the muon senses a critical slowing down of Mn spin Fluctuations and lambda diverges as temperature decreases towards an apparent critical temperature in the range 3-5 K. For concentrations beyond x=0.09, the mu SR spectra are of a stretched exponential form, i.e., e(-(lambda t)beta), characteristic of those obtained from concentrated spin-glass systems, with beta decreasing from unity to approximately 1/3 as the magnetic transition is approached. At these higher concentrations lambda maintains the same critical form observed for x less than or equal to 0.09, although the associated magnetic transition temperatures are now substantially higher, reaching 38 K at x=0.2. It is suggested that these features are evidence of a quantum spin-liquid-to-spin-glass crossover in the beta-Mn1-xAlx system in which Al substitution leads, on the one hand, to a slowing down of spin fluctuations and partial localization of the Mn moments as the beta-Mn lattice expands, and on the other hand to a lifting of the spin configurational degeneracy associated with topological frustration in the triangular symmetry of the sire II Mn sublattice. [S0163-1829(99)03906-5].