Some Magnetic Structure Properties of Terbium and of Terbium-Yttrium Alloys

Abstract

Neutron diffraction studies have been made on single‐crystal and polycrystalline specimens of terbium. Earlier magnetic and thermal measurements have indicated a transformation to an ordered magnetic state at approximately 230°K, and a subsequent order‐order transformation at approximately 220°K. The neutron measurements show that in the narrow antiferromagnetic region, the magnetic structure of terbium is a helical structure. The interlayer turn angle varies from 20.5° per layer at the Néel point to 18.5° per layer at the lower transition. At this lower temperature the structure transforms, in the absence of any external applied field, to a classical ferromagnetic structure in which the moments are in, or nearly in, the planes perpendicular to the hexagonal axis. At very low temperatures the magnetic moment per atom is very nearly 9.0 Bohr magnetons, the value expected for the ordered tripositive ion. Neutron diffraction measurements have also been made on a series of alloys of yttrium and terbium in order to study the influence of magnetic dilution on the magnetic properties of the rare‐earth metals. Introduction of yttrium into terbium reduces the Néel temperature and broadens the range of existence of the helical phase. At 30‐at. % yttrium and above, the spontaneous transformation to the ferromagnetic state is not observed, for zero applied field, even for temperatures as low as 4.2°K. As in pure terbium the interlayer turn angle ω of the helical structure in the alloys varies with temperature, but the slope of the ω vs temperature curve becomes smaller with increasing yttrium concentration.