CROSSOVER FROM TRANSIENT SPIN STRUCTURES TO THE FIELD-INDUCED GRIFFITHS PHASE OF FEBR2

Ln the presence of an applied axial magnetic field H-a the uniaxial antiferromagnets FeCl2 and FeBr2 show fluctuating domain-like antiferromagnetic correlations above the phase boundary T-c(H-a). They are detected by SQUID measurements of the low frequency out-of-phase susceptibility chi '' and indicate a field-induced Griffiths phase at temperatures T-c(H-a)< T< T-N. In contrast to FeCl2, important additional frustration-induced intraplanar non-critical contributions to chi '' vs. T are found in FeBr2. For external fields above the T-c(H-a) line, H-a > 2.6 MA/m, they are shown to superimpose linearly on the Griffiths contributions. These dominate at H-a = 2.67 MA/m and are unequivocally modeled within the Landau theory of fluctuations near phase transitions by introducing a Lorentzian T-c distribution.