Complex magnetic structures of the rare-earth cuprates ${\mathit{R}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_5$ (R=Y,Ho,Er,Yb,Tm)

The magnetic structures at T=1.5 K of several members of the ${\mathit{R}}_2$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_5$-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, conserv- ing in all the cases the same type of arrangement [${\mathit{A}}^{\mathrm{}\left(1\right)\mathrm{}}$+${\mathit{A}}^{\mathrm{}\left(2\right)\mathrm{}}$=(+--+${)}^{\mathrm{}\left(1\right)\mathrm{}}$+(+--+${)}^{\mathrm{}\left(2\right)\mathrm{}}$]. 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.