Dresden 2003 – wissenschaftliches Programm

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DY: Dynamik und Statistische Physik

DY 46: Poster

DY 46.91: Poster

Donnerstag, 27. März 2003, 15:30–18:00, P1

Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS — •P. Böni¹, B. Roessli², D. Görlitz³, and J. Kötzler³ — ¹Physik-Department E21, TU München, D-85748 Garching — ²Institut Laue-Langevin, F-38042 Grenoble, France — ³Institut f. Angewandte Physik, D-20355 Hamburg

By inelastic scattering of polarized neutrons near the (200)-Bragg reflection, the susceptibilities and linewidths of the spin waves and the longitudinal spin fluctuations, δ S_sw(q) and δ S_z(q) ∥ M_sp, respectively, were determined separately. For δ S_sw ⊥ q ⊥ M_sp, we explored the statics and dynamics of these modes with transverse polarizations with respect to q. In the dipolar critical regime, where the inverse correlation length κ_z(T) and q are smaller than the dipolar wavenumber q_d, we observe: (i) the static susceptibility of δS_sw^t(q) displays the Goldstone divergence while for δS_z^t(q) the Ornstein-Zernicke shape fits the data. At the smallest q-values we find indications for the predicted 1/q divergence of the longitudinal susceptibility; (ii) the spin wave dispersion as predicted by the Holstein-Primakoff theory revealing q_d=0.23(1) Å⁻¹ in good agreement with previous work above and below T_C; (iii) the (Lorentzian) linewidths of both modes turn out to be identical with respect to the q²-variation, the temperature independence and the absolute magnitude. Due to the linear dispersion of the spin waves they remain underdamped for q<q_d. These central results differ significantly from the well known exchange dominated critical dynamics, but are quantitatively explained in terms of dynamical scaling and existing data for T≥ T_C.