Atomic scale design and control of cation distribution in hexagonal ferrites for passive and tunable microwave magnetic device applications.

摘要

A vast body of knowledge on the structure and properties of hexagonal ferrites has been accumulated in the last sixty years driven in part by the technological significance of these materials in diverse applications, such as permanent magnets, microwave devices, and magnetic recording media. In this work, the Alternating Target Laser Ablation Deposition (ATLAD) technique is applied in the growth of epitaxial hexagonal ferrite films. As a result, unique magnetic properties, including 50 degrees increase in the Neel temperature and 20% increase in the saturation magnetization compared to conventionally prepared materials, are realized by controlling the cation distribution at the atomic scale.;Lowest energy distributions resulting from the localization of Mn cations in the spinel block of the hexagonal M-type unit cell were theoretically determined by ab-initio calculations. ATLAD deposition routine was designed to deposit epitaxial thin films with the cation distribution identified by ab-initio calculations. The films were fully characterized in terms of composition, crystal structure, surface morphology, static and dynamic magnetic properties, and cation distribution. Enhanced magnetic moment (+20%) and Neel temperature (+50 K) were measured in the films. These improved magnetic properties were correlated with the occupation and valence of specific interstitial sites by Mn cations, in good agreement with theoretical predictions. The localization of Mn cations in 4fIV and 12k sublattices has fundamentally modified superexchange interactions in the unit cell, as confirmed by spinwave resonance measurements.;A novel approach to the design of tunable microwave devices based on hexagonal and cubic ferrites by taking advantage of the magnetoelectric effect is presented. The proposed planar and compact devices, including phase shifters and filters, were designed in microstrip geometry with low magnetic bias field requirements. The devices were designed and simulated using commercial finite element software (Ansoft HFSS) for cubic ferrites and specially developed numerical method based on Galerkin's approach in spectral domain for highly anisotropic hexagonal ferrites. Prototypes were fabricated using standard photolithographic techniques. The active tuning of the devices was realized using voltage controlled magnetic fringe fields emanating from stress coupled magnetoelectric heterostructures. The proposed approach has the potential benefits of drastically reduced power consumption, improved response time, as well as reduced size, cost, and weight.