Spin-reorientation transition at isoelectronic substitution in double-layer manganites (La$_{1-z}$Pr$_z)_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

The transport, magnetic, and thermal properties of single crystal double-layer manganites of the Ruddlesden-Popper series (La$_{1-z}$Pr$_z)_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ ($z=0.1$ or $0.4$) were studied. The compounds exhibit the colossal negative magnetoresistance effect in the region of a transition into a ferromagnetic state. Upon the isoelectronic substitution of Pr$^{3+}$ for La$^{3+}$, the Curie temperature decreases, while the easy magnetization axis rotates from the $ab$ plane to the c axis. The observed effect is related to a change in the occupancy of $dx^2-y^2$ and $d3z^2-r^2$ orbitals as a result of stretching of the MnO$_6$ octahedra.