Transition to inhomogeneous ferromagnetic state in - at low temperatures

Institute J. Stefan - Ljubljana, Slovenia

Received: 23 May 2001
Accepted: 22 October 2001

Abstract

NMR spectra of a 40% -enriched well-annealed - powder sample show a huge increase in the NMR linewidth, accompanied by an increase in spin-lattice relaxation time T₁, on cooling below T'=10 . On the other hand, the methyl proton NMR spectra of a well-annealed - single crystal at 6.34 show, in the ferromagnetic phase between and T'=10 , the presence of two broad inhomogeneous lines with their centers shifted by 2.5 and 0.5, respectively, from the proton Larmor frequency. The intensity of the strongly shifted proton NMR line, which dominates the spectra between and T' in the ferromagnetic phase, continuously decreases with decreasing temperature and becomes suddenly very small below T'. This indicates a dramatic decrease of the unpaired spin density at one half of the proton sites and could result from the near disappearance of one of the two alternating ferromagnetic orientations below T' as suggested by the structural model of Narymbetov et al. involving increased orientational disorder at low temperatures.