ASPECTS OF THE MAGNETIC-STRUCTURE OF TMCU2

An investigation has been made, using neutron diffraction, of the temperature dependence of the complex magnetic behaviour of TmCu2 in its ordered states. The samples were mainly powdered, but some aspects have also been tested by single-crystal measurements. TmCu2 shows at least three different antiferromagnetic structures in its ordered state. Within these phases, the magnetic structures show a strong ferromagnetic coupling of the moments of next-neighbour Tm atoms in the b direction. The low-temperature structure (AFI) is composed by the first two odd harmonics of a propagation vector (2pi/8)a*, where the strongest magnetic intensity is caused by the third harmonic. The first phase transition occurs dose to 3.1 K. The phase that develops here is a mixture of two structures (AFII). At a temperature dose to 4.7 K a third phase (AFIII) appears. The propagation vectors of the structures AFII and AFIII are at least two dimensional. All these structures may be described as sinusoidally modulated with magnetic moments lying in the direction of the easy magnetic axis (b), except for one of the components of AFII, where it is necessary to invoke a component perpendicular to the b axis.