We report on the properties of ultrathin ͑2, 4, 6, and 8 nm͒ epitaxial films of magnetite, Fe 3 O 4 , grown on MgO ͑100͒. Atomic force microscopy image and V-I curves suggest that the films at this thickness are still continuous. The resistivity versus temperature results imply that the conductivity mechanism in all these films is similar. The resistivity of 4 nm thick film is much greater than that of 6 and 8 nm films. The films show ferrimagnetic instead of reported superparamagnetic behavior. The dead layer formed by Mg diffusion between MgO substrate and magnetite films and also the dead layer on the top uncapped film could be the possible reasons for the anomalous resistivity and magnetic properties of the ultrathin films. The effect of the "dead layer" in the thinner film is relatively greater than the one in the thicker film and should lead to a lower magnetoresistance.
Abstract.Giant Planar Hall effect (GPHE) has been observed in epitaxial magnetite (100) films grown on MgO substrates. The effect is manifested as jumps in the transverse resistivity when the film is subjected to a swept, in-plane magnetic field. The jumps are two orders of magnitude higher than previously observed in metallic ferromagnets.Recently, the same effect has been reported for other materials, but unlike our results, they present GPHE at low temperature only. The magnitude of the GPHE observed at room temperature has potential applications such as magnetic sensors and nonvolatile memory elements.
Step-induced anisotropy of electron transport in ultrathin Fe film was investigated. The Fe films ͑2 and 10 nm͒ were deposited on vicinal MgO ͑100͒ substrate using molecular-beam epitaxy. It is found that the films with a thickness of 10 and 2 nm are continuous and discontinuous, respectively, which was determined from their resistivity values, the temperature dependency of the resistivity and the V-I curve. The enhanced magnetoresistance in the continuous and the discontinuous films was observed when the current flows parallel and perpendicular to the miscut direction, respectively. We suggest that the atomic steps in the continuous films nucleate additional domain walls acting as scattering centers and the extra scattering was introduced for the current perpendicular to the step edges. The mechanism of the influence of the atomic steps on the electron-transport properties is different in the continuous and discontinuous films. We further suggest that in the discontinuous films, the atomic steps result in the anisotropic growth of the islands and the anisotropy in the transport behavior as a consequence.
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