Dynamics of a concentrated spin glass: a‐FeB2

Electron spin resonance (ESR) measurements on a‐FeB2 over wide ranges of frequency and temperature are reported. The resonance fields and intensities show anomalies at temperatures that are an order of magnitude higher than the spin‐glass transition temperature. The results cannot be described in terms of existing theories and suggest that new macroscopic variables are needed for adequate description of ESR in spin glasses.