Persistence of a non-antiphase modulated magnetic structure to T -> 0 in the heavy electron compound YbPtAl

Measurements by Yb-170 Mossbauer spectroscopy down to 0.025 K and by neutro n diffraction down to 1.7 K have been performed in the heavy electron compo und YbPtAl (T-N similar or equal to 5.8 EC). At 0.025 K, the Mossbauer spec trum evidences a non-square modulation of the magnetic moment, incommensura te with the lattice spacings. This is the first clear example of a Kramers local moment Yb-based system where the Kondo singlet ground state allows a modulated magnetic structure to persist to T --> OK. The 3rd- and 5th-order harmonics of the modulation could be measured, and their thermal variation was followed from 0.025 K to 5 K. An abrupt decrease of both harmonics at 3 K is concomitant with a peak in the specific heat, suggesting a first-ord er phase transition where the modulation transforms into a pure sine wave.