The Cu spins in TlBa2PrCu2O7 order at T-N approximate to 370 K with a spin structure that is collinear and is characterized by the {1/21/21} wave vector, where the nearest neighbor spins are aligned antiparallel along all three crystallographic directions. Lf 50% of the Ba atoms are randomly replaced by the smaller Sr atoms to form Tl(BaSr)PrCu2O7, the T-N of the Cu spins reduces to 350 K but the magnetic structure that forms below T-N is the same. However, at T approximate to 20 K the Cu spins undergo a change in structure, and the spin arrangement is then characterized by the {1/21/21/2} wave vector below T approximate to 12 K. The ground state spin structure of the Cu ions in Tl(BaSr)PrCu2O7 is hence noncollinear, where the spin directions of the nearest neighbor Cu ions in the nb plane remain collinear and antiparallel while along the c axis they are orthogonal. These results demonstrate that the atoms in the BaO layers are also actively participating in the coupling between the Cu ions. (C) 1996 American Institute of Physics.
