Abstract
The relationship between the microstructure and the magnetic properties of evaporated iron thin films of various thicknesses has been investigated to reveal the reason why iron films more than 100 nm thick evaporated at an incidence angle greater than 75° have high coercive force (Hc) and high squareness ratio (Mr⁄Ms). Iron films were deposited on polyimide films or on glass substrates at an oblique incidence angle of 80° in a vacuum of 1.1×10−3 Pa. To evaluate Hc and Mr⁄Ms, M−H curves were measured using a vibrating sample magnetometer. The morphological structure of the films was observed by examining in a transmission electron microscope cross-sectional specimens prepared by the ultramicrotome method. The crystallographic structure was analyzed by electron diffraction. The structure of films etched chemically after evaporation was also investigated. The results are summarized as follows:
(1) Iron thin films 25 nm thick evaporated at oblique incidence of large angles have relatively low Hc and Mr⁄Ms, whereas the films more than 100 nm thick have much greater Hc\varparallel and (Mr⁄Ms)\varparallel in the direction parallel to the incidence plane.
(2) Iron films evaporated at oblique incidence of large angles consist of two layers. The lower layer, less than 40 nm thick, shows a narrow columnar structure composed of fine crystallites and has chemical activity. The upper layer consists of wider columns, with interstices, composed of greater crystallites. The films evaporated on polyimide substrates have smaller crystallites in the lower than the films on glass substrates.
(3) Improvement in Hc\varparallel and (Mr⁄Ms)\varparallel is attributable to the columnar structure formed in the upper layer. The magnetic properties do not depend on the texture axis and its orientation.
(4) Chemical etching results in the formation of iron films less than 100 nm thick with a high Hc\varparallel and a good (Mr⁄Ms)\varparallel.
