  We report on structural, magnetic, electrical, and thermodynamic properties
of Gd-doped LaMnO3 single crystals for Gd doping levels 0 <= x <= 1. At room
temperature, for all doping levels the orthorhombic O' phase is indicative for
a strong Jahn-Teller distortion. All compositions are insulating. The magnetism
of La$_{1-x}$Gd$_x$MnO$_3$ is dominated by the relatively strong Mn-Mn
exchange. The weakening of the antiferromagnetic interactions due to the
significant decrease f the Mn-O-Mn bond angles leads to the continuous
suppression of the magnetic phase-transition temperatures into the
antiferromagnetic low-temperature phase for increasing Gd doping. 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 GdMnO3 the Mn spins undergo
a transition into a complex, probably incommensurate magnetic structure at 41.5
K, followed by a further ordering transition at 16-20 K revealing weak
ferromagnetism due to canting and finally by the onset of magnetic order in the
Gd sublattice at 6.5 K. At the lowest temperatures and low external fields both
magnetic sublattices reveal a canted structure with antiparallel ferromagnetic
components.
