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 evol ution 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 concentrat ions at and below x=0.09 the contribution to the muon relaxation from the d ynamic atomic dipolar fields takes the form of a simple exponential e(-lamb da 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 concentrati ons in the range 0.03 less than or equal to x less than or equal to 0.09 th e muon senses a critical slowing down of Mn spin Fluctuations and lambda di verges 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 o f 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. A t these higher concentrations lambda maintains the same critical form obser ved for x less than or equal to 0.09, although the associated magnetic tran sition temperatures are now substantially higher, reaching 38 K at x=0.2. I t 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 o ther hand to a lifting of the spin configurational degeneracy associated wi th topological frustration in the triangular symmetry of the sire II Mn sub lattice. [S0163-1829(99)03906-5].