Phonon-assisted relaxation in a frustrated antiferromagnet

A thermally activated magnetic relaxation is observed using neutron spin-echo in the pyrochlore slab ( kagomé bilayer) compound SrCr 9 x Ga 12 - 9 x O 19 ( x = 0.95 ) in a restricted temperature range, 4 K < T < 7 K , above a cross-over to a low temperature relaxation regime with a weaker temperature dependence. The activation energy of the thermally activated relaxation, of the order of 7 meV, coincides with the energy of a phonon mode observed with neutron and Raman spectroscopy, indicating a phonon-assisted regime. The experimental observation of phonon-assisted process gives additional insight to the importance of spin–phonon coupling in frustrated magnets with regard to the models mostly based on purely magnetic interactions.