Fast muon spin relaxation in ferromagnetism of potassium clusters in zeolite A

Potassium clusters arrayed in zeolite A are known to show ferromagnetic properties at low temperatures. A remarkable decrease in the initial asymmetry was observed previously below the Curie temperature in zero-field μ SR by using pulsed muons. In the present work by using dc muons, we clearly observed the fast decay component which was assigned to the decrease in the initial asymmetry in the previous pulsed muon experiment. The relaxation rate amounts to ≃ 20 μ s^{- 1} which corresponds to ≃ 200 Oe in the random static field. This value cannot be explained by the ordinary dipole field, but by the Fermi-contact interaction with electrons in K clusters. The fast decay is easily quenched by applying a weak longitudinal field of ≃ 10 Oe. The low field decoupling of the fast decay component can be explained by the magnetization by the spin-canting mechanism of antiferromagnet under the assumption that some muons stop near the oxygen in 8-membered ring of zeolite framework.