A comparative study of ${\mathrm{La}}_x{\mathrm{Bi}}_{1-x}\mathrm{Mn}{\mathrm{O}}_3$ thin films grown on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates is reported. It is shown that these films grow epitaxially in a narrow pressure-temperature range. A detailed structural and compositional characterization of the films is performed within the growth window. The structure and the magnetization of this system are investigated. We find a clear correlation between the magnetization and the unit-cell volume that we ascribe to Bi deficiency and the resultant introduction of a mixed valence on the Mn ions. On these grounds, we show that the reduced magnetization of ${\mathrm{La}}_x{\mathrm{Bi}}_{1-x}\mathrm{Mn}{\mathrm{O}}_3$ thin films compared to the bulk can be explained quantitatively by a simple model, taking into account the deviation from nominal composition and the Goodenough-Kanamori-Anderson rules of magnetic interactions.

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