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Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS

P. Boeni·B. Roessli·D. Goerlitz·J. Koetzler
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Abstract

By inelastic scattering of polarized neutrons near the (200)-Bragg reflection, the susceptibilities and linewidths of the spin waves and the longitudinal spin fluctuations were determined separately. By aligning the momentum transfers q perpendicular to both δS_sw and the spontaneous magnetization M_s, 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 kappa_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 with a possible indication of a thermal(mass-)renormalization at the smallest q-values, i.e. 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 in the paramagnetic and ferromagnetic regime of EuS; (iii) within experimental error, the (Lorentzian) linewidths of both modes turn out to be identical with respect to the q^2-variation, the temperature independence and the absolute magnitude. Due to the linear dispersion of the spin waves they remain underdamped for q =T_C. The available mode-mode coupling theory, which takes the dipolar interactions fully into account, describes the gross features of the linewidths but not all details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.Gb

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P. Boeni, B. Roessli, D. Goerlitz, J. Koetzler. 2001-10-18. Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS. https://doi.org/10.1103/physrevb.65.144434

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