Criticalbehaviour of the planar helical to or from paramagnetic phase transition in dysprosium and holmium

The planar helical to or from paramagnetic phase transition for Dy is inferred to be of second-order nature from the continuous behaviour of neutron scattering as well as ultrasonic attenuation and velocity measurements. This result is corroborated by the absence of thermal hysteresis at the critical point. Integrated neutron scattering measurements yield a value of the critical exponent beta , which describes the spontaneous sublattice magnetisation, of beta =0.39+or-0.01. Finally, thermal hysteresis was observed for Ho in the ordered region.