  The Ca$_2$MnReO$_6$ double perovskite is a spin-orbit-assisted Mott insulator
with exotic magnetic properties, including a largely non-collinear Mn$^{2+}$
spin arrangement and nearly orthogonal coupling between such spins and the much
smaller Re $5d$ magnetic moments. Here, the electron-doped compound
Ca$_{1.7}$Y$_{0.3}$MnReO$_6$ is investigated. Neutron and X-ray powder
diffraction confirm that nearly full chemical order is maintained at the Mn and
Re sites under the Y substitution at the Ca site. X-ray absorption measurements
and an analysis of the Mn-O/Re-O bond distances show that the Mn oxidation
state remains stable at +2 whereas Re is reduced upon doping. The electron
doping increases the magnetic ordering temperature from $T_c = 121$ to $150$ K
and also enhances significantly the ferromagnetic component of the Mn spins at
the expense of the antiferromagnetic component at the base temperature ($T=3$
K). The lattice parameter anomalies at $T_c$ observed in the parent compound
are suppressed by the electron doping. The possible reasons for the enhanced
magnetism and the suppressed magnetoelastic coupling in
Ca$_{1.7}$Y$_{0.3}$MnReO$_6$ are discussed.
