Neutron Diffraction Study of Magnetic Ordering in Thulium

Neutron diffraction measurements have been made on a single-crystal specimen of metallic thulium at temperatures ranging from 293° to 4.2°K. In the temperature range between 38° and 56°K the magnetic structure of thulium is similar to that observed for the high temperature form of erbium; namely, a simple oscillating z-component type antiferromagnetic structure. Below 38°K additional satellite reflections are observed in the diffraction patterns from which it is inferred that the sinusoidal modulation is modified below this temperature. At 4.2°K the normal lattice reflections show an increase in intensity in zero field which is consistent with a mean magnetic moment per thulium atom of approximately one Bohr magneton directed parallel to the c axis. The low temperature magnetic structure which is proposed for thulium is one in which the moments are parallel to the c axis of the crystal but change their orientations according to the sequence 4, 3, 4, 3 …. The fundamental period of the modulation remains constant over the entire range of temperature at a value corresponding to 3.5 a3 periods.