Microwave Lithium Ferrites
Among the various magnetic oxide materials used in devices operating at microwave frequency ranges lithium ferrite is presently occupying a prominent position. Although its potential as rectangular loop material was being utilised for long in memory devic
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1. INTRODUCTION Among the various magnetic oxide materials used in devices operating at microwave frequency ranges lithium ferrite is presently occupying a prominent position. Although its potential as rectangular loop material was being utilised for long in memory devices the exploitation of its attractive feature as a microwave material was much delayed due to excessive losses observed in early samples. Many researchers working in the field put up a good deal of effort in the early 70s to obtain lithium ferrites with improved dielectric properties. Preparation of low loss lithium ferrite was first reported in 1971 by Collins and Brown [1]. Considerable interest has persisted in lithium ferrites since then. With the establishment of reproducible procedures for the preparation of lithium ferrites, there has been no looking back and at this juncture lithium ferrites are among the most wanted materials for the microwave frequency ranges. The family of lithium ferrites has expanded to such an extent that it now covers a very wide spectrum of properties for utilisation in microwave devices. The frequency range of its use extends even to millimeter wave bands. Braun [2] was first to show that lithium ferrite with chemical formula Lio.sFe2.S04 is an inverse spinel. The divalent ion in this case being a composite ion (Li ~~s Fe~:S). The compositional variations made possible by incorporation of a variety of other metal ions in its formula unit have given rise to lithium ferrite family as a class of material with many special features [3, 4]. These features are: 1. Unsubstituted lithium ferrite (Lio.sF~.s04) has the highest Curie temperature (Tc • 640 0c) among the ferrimagnetic oxide materials. High Tc signifies temperature stability of its magnetic properties. The substituted compositions of lithium ferrites also inherit this property from the parent material. A case of materials belonging to different ferrite families and a comparison of Tc values for a typical value of magnetisation (4nMs = 1000 Gauss) is illustrated in Fig. 1. 2. The lithium ferrites possess excellent rectangular loop characteristics. From its earlier use in memory core industry the present application in latching microwave devices is a natural extension. V. R. K. Murthy et al. (eds.), Microwave Materials © Springer-Verlag Berlin Heidelberg 1994
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FIg. 1. Saturation magnetisation (4nMs) vs Curie temperature (Tc) for different microwave ferrites.
3. The saturation magnetisation (4nMs) of unsubstituted lithium ferrite at room temperature is - 3750 Gauss. It has become possible to vary this parameter to cover values upto 5000 Gauss by. suitable compositional adjustments. Before this NiZn ferrite system was the only one available for such high values of 4nMs for microwave applications. Other properties of technical importance such as rema
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