1 Department of Materials Science and Engineering, Kyoto University 606-8501, Japan
2 Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
E-mail: nakamura@nmr.mtl.kyoto-u.ac.jp
Received 25 May 2000, in final form 11 October 2000
Abstract
We approach the helimagnetic structure of the itinerant-electron magnet YMn2, in which the Mn sublattice forms a geometrically frustrated corner-sharing tetrahedral lattice, by means of both low-energy neutron diffraction and precise nuclear magnetic resonance (NMR) experiments, by modifying the magnetic state by Tb substitution (the introduction of lattice inhomogeneity), and by considering the origin of the tetragonal lattice distortion detected below TN. The spin reorientation induced by the Tb substitution from the helical state with the propagation vector Q = [Ï„01] (Ï„ = 0.018) and with spins lying in the {100} plane to that with Q = [Ï„ Ï„1] (Ï„ = 0.015) and {111}-plane rotation is interpreted as symmetry lowering from a coherent proper screw state with a high magnetic symmetry to an asymmetric but locally preferable configuration with the easy-plane anisotropy perpendicular to the local symmetry axis. Treating the magnetic state as a weakly coupled assembly of one-dimensional antiferromagnetic spin chains, which is realized as a result of the partial release of the frustration, and taking into consideration the lattice symmetry, the possibility of double-axial helical structures arises and is discussed. In this study, all the inconsistency among previously reported NMR spectra is removed, and the strong frequency dependence of the spin-echo decay time T2 found unexpectedly is explained in terms of a phenomenological model taking into consideration the dynamic motion of electron spins in the locally anisotropic magnetic system.
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