Spin reorientation has been observed in CoFe 2 O 4 thin single crystalline films epitaxially grown on ͑100͒ MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned. The importance of magnetic anisotropy is well recognized in many technical applications such as magnetic and magneto-optic recording. The large interest for high anisotropies is motivated by technological demands such as increasing the magnetic recording density. With large anisotropy, the superparamagnetic limit can be pushed down, and a stable magnetization can be promoted in ultrasmall nanosized magnetic structures, which are needed in advanced media for ultrahigh density recording. Besides the intrinsic anisotropy of the bulk, other sources of anisotropy may be enhanced in artificial structures and contribute to their magnetic properties. Depending on their relative orientations and magnitudes, the involved anisotropies may compete between each other, leading to spin-reorientation phenomena in the system. For example, the broken symmetry at the interfaces in ultrathin films generates a perpendicular anisotropy, which overcomes the shape anisotropy.1 However, increasing the layer thickness reduces the ratio between the surface and the volume atoms, leading to an in-plane alignment of the easy axis.2 In obliquely sputtered metallic thin films, we established the existence of an in-plane reorientation of magnetic anisotropy.3 Depending on the film thickness and due to the shadow effect during the growth, the layer can develop columns or nuclei able to confine the anisotropy parallel or perpendicular to the longitudinal direction ͑projection of the incident beam in the film plane͒.Ferrites cover a large family of oxides, including soft as well as hard magnetic materials. Hard ferrites such as the hexagonal ͑BaFe 12 O 19 ͒ and the spinel ͑CoFe 2 O 4 ͒ are particularly attractive for magnetic and magneto-optic recording applications due to their large magnetocrystalline anisotropy and high chemical stability. Recent studies demonstrated that integrating cobalt ferrite as a pinning layer in the spin valve architecture can strongly enhance the magnetoresistance effect of the sandwiched structure. 4 In epitaxial hexaferrite thin films, the uniaxial magnetocrystalline anisotropy is strong enough to dominate all the other sources of anisotropy and to keep the spin alignment constant regardless the film thickness and the preparation conditions. 5 However, cobalt ferrite rep...

Magnetic properties of CoFe2O4 thin films epitaxially grown on (100) MgO by pulsed layer deposition are controlled by substrate temperature and film thickness. At 300 °C a strong dependence of magnetic anisotropy on thickness is observed indicating that stress is dominant. However at 800 °C magnetic properties are less sensitive to film thickness suggesting that magnetocrystalline is the only prevailing form of anisotropy. We also show that magnetic domains exhibit a cluster-like structure rather than stripe.

Magnetic domains in obliquely sputtered Co films on a polymer substrate have been investigated with magnetic force microscopy. The growth angle has been found to strongly affect the domain structure as well as the magnetic properties, due to changes in the microstructure. At large angles finite-size elongated domains oriented in the longitudinal direction ͑projection of the growth direction on the film plane͒ were energetically favorable. However, at intermediate angles a transition to stripe domains occurs due to an increase in exchange coupling. These domains exhibit a width of 400 nm and lie along the longitudinal direction. In the remanent state ͑after saturation͒, circular as well as elliptical magnetic bubbles coexist, but coalesce in the dc demagnetized state to form stripe domains.

The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments, and Errata. Communications are limited to three and one halfjournal pages, and Notes, Comments, and Errata are limited to one and three-fourths journal pages as described in the Announcement in the I July 1991 issue.

Reorientation in the magnetic anisotropy as a function of film thickness has been observed in Co-Ni and Co thin films, obliquely sputtered on a polyethylene terephthalate substrate at a large incidence angle ͑70°͒. This effect is a consequence of the low magnetocrystalline anisotropy of the films ͑fcc structure of Co͒ and changes in microstructure from nuclei to columns according to the thickness. The critical thickness for this transition was estimated to be 30 nm for Co-Ni.