Fabrication and properties of microwave lithium ferrites

Lithium ferrites are low-cost materials which are attractive for microwave device applications. For a number of years these materials have been prominent in the computer core industry because of excellent temperature performance and the squareness of their hysteresis loops. Previous attempts at exploiting lithium ferrites for microwave applications have met with limited success. Properties which required further improvements were the dielectric and magnetic loss tangents, anisotropy, coercive force, and density. This paper reports primarily the development of microwave S - and C -band lithium ferrites with substantially improved properties, comparable to those of the more expensive ferrimagnetic garnets currently in use. Extensive applications are also expected for higher frequencies up to and including K u band. Because a number of factors can affect the performance of microwave devices, the lithium ferrites reported herein are heavily doped to optimize the numerous properties of interest. The following additives were employed: 1) Ti-to tailor saturation magnetization for various frequencies; 2) Zn-for its beneficial effects on coercive force, magnetic loss, and densification; 3) Mn-to decrease the dielectric loss, improve the remanence, and reduce the stress sensitivity of the remanence; 4) Co-to provide a range of spinwave linewidths ; 5) Ni-for increased remanence ; and 6) Bi (the most important additive)-to increase the densification at low firing temperatures and thereby provide low dielectric loss and low coercive force. The microwave lithium ferrites offer a high degree of latitude for tailoring various parameters for special device requirements. However, the optimization of selected properties is sometimes accomplished at the expense of other properties. Although this investigation was directed mainly toward applications for latching devices, lithium ferrites have been altered for use in a variety of devices such as limiters and circulators. The requirements for ferrite materials in latching phasers are reviewed, the compositional factors which result in useful properties are discussed, and performance in circulators and latching phasers is reported.