Spin-fracton effects in dilute amorphous alloys

Deviations from the Bloch T32 law, observed in dilute amorphous magnets, are shown to result from the crossover from magnon to fracton density of states. Data from a series of amorphous (CopNi1-p)75P16B6Al3 alloys were fitted to an approximate density-of-states function that matches the two regimes at a frequency c(p). Both the magnon stiffness constant and c decrease as powers of p-pc, where pc is the percolation concentration. The exponents are consistent with a value of =1.5 used in the fraction density of states and in agreement with percolation theory.