Interplay of superexchange and orbital degeneracy in Cr-doped ${\mathrm{LaMnO}}_3$

We report on structural, magnetic, and electron spin resonance (ESR) investigations in the manganite system ${\mathrm{LaMn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ $(x<~0.5).$ Upon Cr-doping we observe a reduction of the Jahn-Teller distortion yielding less distorted orthorhombic structures. A transition from the Jahn-Teller distorted ${\mathrm{O}}^{\prime }$ to the pseudocubic O phase occurs between $0.3<x<\mathrm{}\mathrm{0.4.}\mathrm{}$ A clear connection between this transition and the doping dependence of the magnetic and ESR properties has been observed. The effective moments determined by ESR seem reduced with respect to the spin-only value of both ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ ions.