Zero-field magnetic viscosity measurements at low temperatures were made on a series of Pd40Ni40−xFexP20 (x=10–20) samples in order to determine the apparent magnetic-moment-weighted energy barrier distributions for these spin-glass alloys. The distributions observed are best described by a stretched exponential in the form of a truncated Lévy flight distribution. This form suggests a hierarchical landscape of apparent energy barriers arising from interactions between randomly oriented magnetic clusters within the material. The degree of stretching of the exponential form of the energy barrier distributions is found to increase with decreasing iron concentration. © 2004 American Institute of Physics.