Strain relaxation studies in epitaxial magnetite,

{Fe}_{3} O_{4}

, thin films grown on

{MgAl}_{2} O_{4}

(100) substrates are reported. The study shows that the films were relaxed in line with the theoretical model prediction with a critical thickness,

t_{c} = 5 nm

. Antiphase boundaries (APBs) are not expected to form in

{Fe}_{3} O_{4}

films grown on

{MgAl}_{2} O_{4}

substrates because both film and substrate have the same crystal symmetry. In contrast, our study reveals the formation of APBs within the

{Fe}_{3} O_{4}

films. Our analysis shows that the APBs in a

{Fe}_{3} O_{4} / {MgAl}_{2} O_{4}

heteroepitaxial system are formed by partial dislocations, which accommodate the misfit. This formation mechanism of APBs is fundamentally different from the one found in the

{Fe}_{3} O_{4} / MgO

system, where APBs are formed as a consequence of equivalent nucleation sites on the MgO substrate separated by nontranslational vectors of the

{Fe}_{3} O_{4}

lattice. The mechanism for the formation of antiphase boundaries through partial dislocations should be applicable to a wide range of epitaxial systems having identical symmetries of the film and the substrate and significant lattice mismatch.

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