Comment on ``Strain effect and the phase diagram of La₁₋ₓBaₓMnO₃ thin films''

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Recent experiments in La₁₋ₓBaₓMnO₃ thin films by Zhang et al. [Phys. Rev. B 64, 184404 (2001)] showed that the ferromagnetic Curie temperature Tc is enhanced (reduced) by tensile (compressive) strain. The results are regarded to be anomalous because it is generally understood by the authors that the tensile strain should lead to a reduction of the electron hopping t (and thus also Tc) due to an elongation of the (in-plane) Mn-O bond length. In this comment such a general understanding of the strain effects is disapproved. It is argued that the tensile strain leads primarily to a larger Mn-O-Mn bond angle (with the Mn-O bond length nearly unchanged), thus the pure geometric consideration on t is already in the correct direction towards the experimental findings by Zhang et al. At the same time, an indirect strain effect under consideration of the dynamic electron-phonon coupling is proposed, which may improve the quantitative explanation.