Hysteresis at the spin-flop transition in the antiferromagnetsK2Fe(Cl1

Magnetic hysteresis is found at the first-order spin-flop transition in ${\mathrm{K}}_{2}$Fe(${\mathrm{Cl}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Br}}_{\mathit{x}}$${)}_{5}$\ensuremath{\cdot}${\mathrm{H}}_{2}$O (x=0.1, 0.2, and 0.3). From measurements of the ac susceptibility \ensuremath{\chi} and from a study of the dependence of \ensuremath{\chi} on the modulation field h at the transition, the hysteresis field \ensuremath{\Delta}${\mathit{H}}^{\mathrm{*}}$ is determined. It is found that \ensuremath{\Delta}${\mathit{H}}^{\mathrm{*}}$ decreases with increasing temperature along the first-order line. No hysteresis was observed in pure (x=0) samples.