THE C A RATIO OF HELICAL MAGNETIC-STRUCTURES IN RARE-EARTH MATERIALS/

We have measured the temperature dependence of the c/a ratio of Gd66Y3 4 by neutron diffraction. As the temperature decreases and the helimag netic turn angle tends to zero, the cia ratio of the hexagonal crystal line structure tends to a limiting value of 1.591 +/- 0.001. This is i n keeping with a proposal that the period of the helix is determined b y the c/a ratio and both are linked to a 'ribbon' singularity of the F ermi surface. (C) 1998 Elsevier Science B.V. All rights reserved.