We study the magnetic behavior of Fe 3 O 4 ͑111͒ thin films with thicknesses between 5 nm and 50 nm. The films are epitaxially grown on ␣-Al 2 O 3 ͑0001͒ single crystals by atomic-oxygen-assisted molecular beam epitaxy. The Fe 3 O 4 ͑111͒ thin films exhibit high structural order with sharp interfaces and low roughness and exhibit a Verwey transition for thicknesses above 8 nm. However, the samples have magnetic properties that deviate from the bulk ones. The magnetic moment varies between 2.4 B for 5-nm-thick film and 3.2 B for 50-nm-thick film in a field of 1.2 T, which is lower than that of bulk samples (4.1 B /Fe 3 O 4 formula). Still the magnetic saturation is never reached, even in fields as large as 2 T. The thinner the film, the slower the approach to saturation. Structural analysis, performed using high-resolution transmission electron microscopy, reveals the presence of antiphase boundaries (APB's), the density of which decreases when the thickness increases. Using a model of ferromagnetic domains separated by antiferromagnetically sharp interfaces, we show that the slow approach to saturation observed in the films as a function of thickness is driven by the APB density.

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1ϫ10 15 Ga ions/cm 2 , but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5ϫ10 12 ions/cm 2 , the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5ϫ10 12 and 1ϫ10 13 Ga ions/cm 2 , a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic to paramagnetic transition at fluences around 1ϫ10 15 Ga ions/cm 2 . A brief comparison with the effect of irradiating with He ions is given.

The resistive effect of domain walls in FePd films with perpendicular anisotropy was studied experimentally as a function of field and temperature. The films were grown directly on MgO substrates, which induces an unusual virgin magnetic configuration composed of 60 nm wide parallel stripe domains. This allowed us to carry out the first measurements of the anisotropy of domain wall resistivity in the two configurations of current perpendicular and parallel to the walls. At 18 K, we find 8.2% and 1.3% for the domain wall magnetoresistance normalized to the wall width (8 nm) in these two respective configurations. These values are consistent with the predictions of Levy and Zhang.