Crossover from transient spin structures ot the field-induced Griffiths phase of FeBr2

In the presence of an applied axial magnetic field H a the uniaxial antiferromagnets FeC12 and FeBr 2 show fluctuating domain-like antiferromagnetic correlations above the phase boundary To(Ha). They are detected by SQUID measurements of the low frequency out-of-phase susceptibility X" and indicate a field-induced Griffiths phase at temperatures Tc(H a) < T < T N. In contrast to FeC12, important additional frustration-induced intraplanar non-critical contributions to X" vs. T are found in FeBr:. For external fields above the Tc(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 M A / m and are unequivocally modeled within the Landau theory of fluctuations near phase transitions by introducing a Lorentzian T c distribution. The Griffiths phase conjecture was theoretically introduced for diluted Ising-ferromagnets [1]. It is based on the idea of 'local phase transitions' in a diluted system due to the finite probability of arbitrarily large pure and differently diluted clusters. However, despite a possible dynamical signature of the Griffiths phase in inelastic neutron scattering data of KMn0.3Nio.7F 3 [2] and recent Monte Carlo simulations [3], its clear experimental verification is still lacking. A more favorable situation is encountered in an analogous experimental realisation of a Griffiths phaselike phenomenon. It was recently detected on the uniaxial antiferromagnet FeCI 2 in an applied axial magnetic field [4]. Domain-like antiferromagnetic correlations are created by fluctuating demagnetizing fields and, hence, transition temperatures due to the unambiguous relationship T~ = T~(Ha). Within the temperature regime T~(Ha) rs , with t ~ = T T ¢ , t N = T I N , A o and D the Landau expansion coefficients of the quadratic and gradient term of the Gibbs free energy density, E = b / T the temperature-dependent width of the T~ distribution, V the sample volume and q the wave-vector of the order parameter fluctuations. While the field-induced Griffiths phase is driven by inhomogeneous demagnetizing fields, FeBr 2 shows an important additional intrinsic loss mechanism. As outlined in Ref. [7], non-critical fluctuations are attributed to intraplanar frustration. It gives rise to transient non-uniform spin structures, which carry excess magnetisation. Their location in the Ha-T phase diagram is shown in Fig. 1. Below the Tc(H a) line they appear only on the 0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)01360-8 1556 Ch. Binek et aL /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1555-1556