Critical Fluctuations in a Spin Glass: AN Ultrasonic Study of Copper-Manganese

The critical exponent alpha' for a spin glass was determined. Sound velocity measurements were made on a CuMn single crystal which exhibited the characteristic spin glass cusp in the low-field, ac susceptibility. A sound velocity anomaly oberved in the vicinity of the susceptibility peak temperature was attributed to critical fluctuations. A thermodynamic argument is presented which relates the critical part of the specific heat to that of the second velocity. By using the form delta C sub v = P - Q' t to the - alpha' power for the critical part of the specific heat, the velocity data was fit with alpha' as the only adjustable parameter. The data are well described by this power law for values of t as large as 0.8. This fact is unusual but consistent with current theories which suggest that critical fluctuation phenomena are very large at all temperatures below the spin glass temperature. Alpha' = - 1.9 plus or minus 0.2.