MAGNETIC ORDER AND MOMENT STABILITY IN YMN2

High-resolution neutron powder diffraction and transverse field-mu-R measurements have been used to examine the nature of the magnetic and structural transition at 100 K in YMn2. The neutron measurements indicate a first-order hysteretic transition occurring at 92.5 K on cooling and 108 K on warming. The low-temperature phase is tetragonal (c/a = 0.995) and antiferromagnetic with localized Mn moments of 2.8-mu-B ordering in a long-wavelength helix characterized by an antiferromagnetic translation vector (tau, tau', 1) where tau = 0.01 8 and tau' = 0.003. The high-temperature phase is cubic and Pauli paramagnetic. The collapse of the Mn moment at the transition is accompanied by a 5% decrease in the cell volume. In the paramagnetic state Of YMn2 measurement of the muon transverse relaxation rate reveals an underlying second-order transition which we associate with the critical slowing of longitudinal spin fluctuations as the transition temperature is approached.