ABSTRACT
A high-resolution neutron spectroscopic technique is used to measure momentum-resolved magnon lifetimes in the prototypical two- and three-dimensional antiferromagnets Rb(2)MnF(4) and MnF(2), over the full Brillouin zone and a wide range of temperatures. We rederived theories of the lifetime resulting from magnon-magnon scattering, thereby broadening their applicability beyond asymptotically small regions of wave vector and temperature. Corresponding computations, combined with a small contribution reflecting collisions with domain boundaries, yield excellent quantitative agreement with the data. Comprehensive understanding of magnon lifetimes in simple antiferromagnets provides a solid foundation for current research on more complex magnets.
ABSTRACT
We used a neutron spin-echo method with microelectron-volt resolution to determine the lifetimes of spin waves in the prototypical antiferromagnet MnF2 over the entire Brillouin zone. A theory based on the interaction of spin waves (magnons) with longitudinal spin fluctuations provides an excellent, parameter-free description of the data, except at the lowest momenta and temperatures. This is surprising, given the prominence of alternative theories based on magnon-magnon interactions in the literature. The results and technique open up a new avenue for the investigation of fundamental concepts in magnetism. The technique also allows measurement of the lifetimes of other elementary excitations, such as lattice vibrations, throughout the Brillouin zone.
ABSTRACT
NaxCoO2, the parent compound of the recently synthesized superconductor Na(x)CoO(2):yH(2)O, exhibits bulk antiferromagnetic order below approximately 20 K for 0.75=x=0.9. We have performed neutron scattering experiments in which we observed Bragg reflections corresponding to A-type antiferromagnetic order in a Na0.82CoO2 single crystal and characterized the corresponding spin-wave dispersions. The spin waves exhibit a strongly energy-dependent linewidth. The in-plane and out-of-plane exchange constants resulting from a fit to a nearest-neighbor Heisenberg model are similar in magnitude, which is unexpected in view of the layered crystal structure of NaxCoO2. Possible implications of these observations are discussed.