Magnon spectrum of the amorphous ferromagnet Co4P from atomistic spin dynamics

The gapped local minimum in the magnon dispersion, located at a finite wave number and frequency, has been observed in the amorphous ferromagnet

Co

4

P

. The feature is called a “rotonlike” excitation and has eluded explanation for decades. We overcome the limitations of previous theories by combining the reverse Monte Carlo method, to determine the atomic structure, with large-scale atomistic spin simulations. This method enables us to include atomic order and spin correlations on an equal footing. We find the rotonlike feature is actually gapless, in contrast to the gapped structure found in previous studies. The gapless feature is attributed to amorphous umklapp scattering caused by residual structural order.