NEUTRON-DIFFRACTION AND X-RAY RESONANT EXCHANGE-SCATTERING STUDIES OFTHE ZERO-FIELD MAGNETIC-STRUCTURES OF TBNI2GE2

The zero-field magnetic structures of the rare-earth intermetallic com pound TbNi2Ge2 have been determined using conventional magnetic neutro n diffraction and high-resolution x-ray resonant exchange scattering t echniques. There are two distinct magnetic phase transitions in this m aterial: one is from the high-temperature paramagnetic state to a long period antiferromagnetic phase at the Neel temperature (T-N=16.8 K), and the other is at a lower temperature (T-t= 9.3 K), where the system locks into a commensurate phase. The structure above T-t is described by a longitudinal, amplitude modulated, sinusoidal wave with propagat ion vector tau(1) approximate to(000.758+/-0.002) in reciprocal-lattic e units (r.l.u.). As the temperature is lowered below T-t, tau(1) lock s at (0 0 3/4) and additional magnetic Bragg reflections corresponding to tau(2) =(1/2 1/2 0) and tau(3) = (1/2 1/2 1/2) develop. A weak mod ulation tau(1)' = (0 0 1/4), related to tau(1) also appears indicating a squaring-up of the low-temperature structure. In this equal moment structure all Tb moments have the saturation value of mu(s) = 9.0 +/- 0.2 mu(B). Both the phases are uniaxial with Tb moments parallel to th e (c) over cap axis of the tetragonal unit cell.