NEUTRON-DIFFRACTION AND ULTRASONIC STUDIES OF THE HELICAL[--]PARAMAGNETIC PHASE-TRANSITION IN DYSPROSIUM AND HOLMIUM

Neutron diffraction measurements on high-purity Dy and Ho crystals ind icate no discontinuity in the order parameter at T-N, and no hysteresi s in the value of T-N within 0.05 K between consecutive cooling and he ating runs. This finding is also reflected by the results of ultrasoni c velocity and attenuation measurements, thus corrobrating the continu ous nature of the phase transition. Integrated neutron diffraction mea surements near T-N yield beta = 0.38 +/- 0.02 for Dy and beta = 0.39 /- 0.02 for Ho in agreement with renormalization group predictions (be ta = 0.39) of Mukamel, Krinsky and Bak. Neutron intensities in the par amagnetic region do not adhere to a conventional t(+)(-y) dependence o ver the complete temperature regime. A possible description for the re gion near T-N with a 2D-planar spin model is indicated. The critical v elocity change Delta nu(33) of Ho near T-N is discussed.