Complex interplay of $3d$ and $4f$ magnetism in ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$

We report on structural, magnetic, electrical, and thermodynamic properties of Gd-doped $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_3$ single crystals for Gd doping levels $0⩽x⩽1$. At room temperature, for all doping levels the orthorhombic O’ phase is indicative of a strong Jahn-Teller distortion. All compositions are insulating. The magnetism of ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$ is dominated by the relatively strong $\mathrm{Mn}\mathrm{O}\mathrm{Mn}$ superexchange. For increasing Gd doping, the weakening of the nearest-neighbor exchange interactions due to the significant decrease of the $\mathrm{Mn}\mathrm{O}\mathrm{Mn}$ bond angles leads to the continuous suppression of the magnetic phase-transition temperature into the A-type antiferromagnetic low-temperature phase. The temperature dependence of the magnetization can only be explained assuming canting of the manganese spins. The magnetic moments of Gd are weakly antiferromagnetically coupled within the sublattice and are antiferromagnetically coupled to the Mn moments. For intermediate concentrations compensation points are found, below which the spontaneous magnetization becomes negative. In pure $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$ the Mn spins undergo a transition into a complex, probably incommensurate magnetic structure at $41.5\mathrm{K}$, followed by a further ordering transition at $18–20\mathrm{K}$ revealing weak ferromagnetism due to canting and finally by the onset of magnetic order in the Gd sublattice at $6.5\mathrm{K}$. At the lowest temperatures and low external fields, both magnetic sublattices reveal a canted structure with antiparallel ferromagnetic components.