SPIN WAVES IN PERIODIC STRUCTURES - Rare Earth Magnetism: Structures and Excitations - page 308

Fig. 6.4. Magnetic excitations propagating in the c -direction in
the one-spin-slip structure of Ho at 20 K, after Patterson et al. (1990;
and to be published). The full curve is the calculated dispersion rela-
tion and the points are the experimental results. The energy gap at
q = 11 (2 π/c ), due to the eleven-layer period, is resolved in these mea-
surements. The linewidth of the scattering peaks behaves anomalously
around q = 11 (2 π/c ) suggesting a gap of the order of 0.3 meV at this
wave-vector. The calculated energy of the long-wavelength modes in the
basal plane is indicated by the line on the left. The discontinuity at q = 0
is due to the dipolar coupling, and the transition to the cone structure is
accompanied by a softening of this lowest-energy mode.
one layer to the next. This alternation doubles the periodicity in the
rotating coordinate system, and thereby halves the Brillouin zone in the
c -direction. The predicted gap is somewhat smaller than the experi-
mental energy-resolution, and is therefore not observed in these mea-
surements. The equivalent gap has however been measured in the one-
spin-slip structure of Fig. 2.5 by Patterson et al. (1990), whose results
are shown in Fig. 6.4. In this case, the 11-layer structure causes an
eleven-fold reduction in the Brillouin zone, but only the first-order gap
at 5 / 11 times 2 π/c is calculated to be readily observable. This gap, on
the other hand, is amplified by about a factor two, as compared to that
in the structure without spin slips. As the number of spin slips increases,
the calculated excitation spectra (Jensen 1988a) become more complex,