COMPLEX MAGNETIC-STRUCTURES OF THE RARE-EARTH CUPRATES R2CU2O5 (R=Y,HO,ER,YB,TM)

The magnetic structures at T = 1.5 K of several members of the R2Cu2O5-type compounds (R = Y,Ho,Er,Yb,Tm) have been determined by means of neutron diffraction. All of them appear to be commensurate with the crystal lattice having propagation vectors k = (0,0,0) for Y, Er, and Tm, k = (0,1/2,0) for Ho, and k = (0,1/2,1/2) for Yb. In all cases, except for Yb, we found that both copper and rare-earth sublattices order simultaneously. The easy axis of the magnetic moments for the two copper sites is along [010] for Y, Er, and Yb, whereas it is along [100] for Ho and Tm, conserving in all the cases the same type of arrangement [A(1) + A(2) = (+--+)(1) + (+--+)(2)]. The arrangement of magnetic moments in the rare-earth sublattices strongly differs between the different compounds indicating the existence of a strong competition between crystal-field and exchange interactions. From the determined magnetic structures, we propose models concerning the metamagneticlike transitions observed in the magnetization curves.