We report spin-polarized transport experiments across antiphase domain boundaries which act as atomically sharp magnetic interfaces. The antiphase boundaries are prepared by growing Fe(3)O(4) epitaxially on MgO, the magnetic coupling over a large fraction of these boundaries being antiferromagnetic. Magnetoresistance measurements yield linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane, respectively. This behavior can be explained by a hopping model in which spin-polarized electrons traverse an antiferromagnetic interface between two ferromagnetic chains.

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The antiphase domain structure in epitaxial Fe 3 O 4 films determines its physical properties such as superparamagnetism, resistivity, and magnetoresistance. A good knowledge and control of the domain sizes in these films is therefore of utmost importance. We report on the finding that the antiphase domain boundaries anneal out via a diffusive mechanism at relatively low temperatures. This has been demonstrated by postannealing the films at 250°C, 300°C and 350°C. The boundary migration process is a thermally activated process with an activation energy of 26 kJ/mol ͑250 meV͒. We have further studied the domain size in epitaxial Fe 3 O 4 films as a function of growth parameters. A linear relationship has been obtained for the logarithm of the domain size versus the inverse of the growth temperature ͑in the range of 125°C to 300°C), which supports the diffusional mechanism. The domain size is not influenced by the iron flux, but does depend on the oxygen flux. This suggests that the critical nuclei are pairs of iron and oxygen atoms and that iron is more mobile than oxygen.

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Magnetic field induced step-like changes in magnetization and resistivity of Sm1−xSrxMnO3 manganites were studied. A strong dependence of these features on the cooling rate was observed. Magnetostriction, however, does not show the presence of large strain in our samples. From all these features we can rule out the conventional explanation of magnetization jumps as a consequence of martensitic transition. We propose instead that quenched by fast cooling disorder leads to the formation of an inhomogeneous metastable state and to subsequent magnetization jumps.