1. Field of the Invention
This invention relates to the field of magnetic, microwave materials and more particularly to the field of epitaxial, thin film, magnetic, microwave materials.
2. Prior art
Lithium ferrite is recognized as a highly desirable microwave magnetic material chiefly because of its low linewidth and corresponding low loss at microwave frequencies. In addition, lithium ferrite has a high Curie temperature, high saturation magnetization and a substantially square hysteresis loop. Each of these characteristics contribute to the desirability and vast usefulness of lithium ferrite as a microwave material.
Prior art lithium ferrites have been available as bulk monocrystalline and polycrystalline material and as thin films formed by chemical vapor deposition (CVD) techniques. Flux grown bulk monocrystalline lithium ferrite often includes crystalline voids resulting from flux inclusions within the crystal. In addition, there is often a variation in the composition of the film from the inner or first grown portion of the crystal material and the outer or last grown portion of the crystal material as a result of segregation effects within the melt. Thus, such single crystalline bulk materials do not have uniform characteristics. Further, there are cutting and polishing problems which are well known in the art with respect to the obtaining of very thin films from bulk single crystals. Thicker films obtained from bulk crystals have the problem of void and the composition variation. With respect to the polycrystalline material, this material does not have the narrow ferromagnetic resonance linewidth of the single crystalline material and therefore is not as beneficial as single crystalline material in those applications which require narrow linewidths. CVD lithium ferrite films have been disposed on only a few substrate materials. Such films have been under significant strain due to lattice constant mismatch between the films and substrate. Unfortunately, since lithium ferrite is a magnetic material, strain in a film of lithium ferrite acts through the magnetostriction constant of the material to influence the properties of the strained sample and has the undesired effect of broadening the ferromagnetic resonance linewidth of the material. The broadening of the linewidth which results from extreme film tension partially negates the advantages of lithium ferrite as a magnetic microwave material.
Magnesium oxide has satisfactory characteristics for use as a microwave substrate, in that it does not significantly adversely affect the propagation of microwaves. Unfortunately, the relative lattice constants of lithium ferrite and magnesium oxide are such that lithium ferrite films grown by prior art techniques on a magnesium oxide substrate are under such tension that they crack. The usefulness of such lithium ferrite films for microwave applications is drastically reduced by the resulting cracks in the film. These cracks result in a non-uniform material having microwave characteristics which are significantly inferior to what they would be if the film were uncracked. Such inferiority results from the strain in the individual portions of the film which are bounded by cracks and by the discontinuities in the film which are created by the cracks.
In order to obtain the full benefits potentially available from use of lithium ferrite as a microwave material, unstrained epitaxial films disposed on non-magnetic substrates are needed.