Helimagnetic structure ofYMn2 observed by means of nuclear magnetic resonance and neutrondiffraction

Abstract

We approach the helimagnetic structure of the itinerant-electronmagnet YMn2, in which the Mn sublattice forms a geometricallyfrustrated corner-sharing tetrahedral lattice, by means of bothlow-energy neutron diffraction and precise nuclear magneticresonance (NMR) experiments, by modifying the magnetic state by Tbsubstitution (the introduction of lattice inhomogeneity), and byconsidering the origin of the tetragonal lattice distortion detectedbelow TN. The spin reorientation induced by the Tbsubstitution from the helical state with the propagation vectorQ⃗ = [τ01] (τ = 0.018) and with spins lying in the{100} plane to that with Q⃗ = [ττ1] (τ = 0.015) and {111}-plane rotation is interpreted as symmetrylowering from a coherent proper screw state with a high magneticsymmetry to an asymmetric but locally preferable configuration withthe easy-plane anisotropy perpendicular to the local symmetry axis.Treating the magnetic state as a weakly coupled assembly ofone-dimensional antiferromagnetic spin chains, which is realized asa result of the partial release of the frustration, and taking intoconsideration the lattice symmetry, the possibility of double-axialhelical structures arises and is discussed. In this study, all theinconsistency among previously reported NMR spectra is removed, andthe strong frequency dependence of the spin-echo decay time T2found unexpectedly is explained in terms of a phenomenological modeltaking into consideration the dynamic motion of electron spins inthe locally anisotropic magnetic system.