A Mossbauer effect study of the magnetic phase diagram and spin wave excitations in the antiferromagnet Cs2FeCl5.H2O

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Abstract
Mossbauer spectra of Cs2FeCl5.H2O, an antiferromagnet with TN=6.75K, in applied magnetic fields have enabled the magnetic phase diagram to be determined. At the spin-flop field of BSF approximately=1.3 T the spins are observed to re-orient from the a to the c axis suddenly, as expected for a first-order transition, although no hysteresis was detected. By contrast the transitions to the paramagnetic state are continuous, being completed at Bc0 approximately=13 T at 0K with the field required perpendicular to the a axis being slightly larger than for fields applied along the a axis. The saturation value for the magnetic hyperfine field BHF is (52.0+or-0.5)T. At 4.2K, BHF varied with applied magnetic field showing minimum values at BSF and Bc. This arises because this temperature is sufficiently close to TN for the sublattice magnetisation to be affected by the applied field, and may be accounted for by spin wave theory. For small fields along the magnetic (a) axis spin waves are excited and BHF decreases, while for B perpendicular to the a axis fluctuations are suppressed and BHF increases. The fluctuations are largest at the transition fields BSF and Bc which explains the observed minima in BHF. Fairly good agreement is found with spin wave theory with values BE=6.5 T and BA=0.2 T for the exchange and anisotropy fields respectively. The quadrupole splitting is very small at 290K increasing to a constant value of 0.30 mm s-1 between 77K and TN, suggesting a change in crystal structure.