The microwave properties of aligned hexagonal ferrite fibers

Abstract

Among the new class of planar hexagonal ferrites discovered between 1952 and 1956 by Philips [1, 2] were Y ferrite (Ba2M2Fe12O22), W ferrite (BaM2 Fe16O27) and Z ferrite (Ba3M2Fe24O41). These are all constructed from basic units of hexagonal barium M ferrite and cubic spinel ferrites in various combinations, and they retain a hexagonal structure, usually with the direction of magnetization parallel to the c-axis. However, if the metal M ˆ cobalt(II), then Co2Y, Co2W and Co2Z are formed, known as ferroxplana ferrites, so called because their preferred direction of magnetization is at an angle to the caxis [2]. As a consequence of this, at room temperature these materials are magnetically soft because, although a large amount of energy is needed to move out of this plane or cone, the magnetic vector can easily rotate within the preferred plane or cone [3]. Therefore, these materials are of little use as permanent magnets despite their spontaneous magnetization and high thermal stability, but they have a much higher permeability and ferromagnetic resonance in the GHz region compared to the 300 MHz ceiling encountered with the spinel ferrites [4]. Because of this, the ferroxplana ferrites are commercially important materials which are exploited for their unique magnetic properties, especially in the microwave frequency regions. These materials are used for inductor cores, u.h.f. communications and other high frequency applications [3]. The crystals are naturally isotropic within the basal plane, and their permeability can be raised even further by the alignment of individual crystals in the preferred direction, without a lowering of the limiting frequency or the loss maximum [5]. In previous publications, both random [6] and aligned hexagonal ferrite ®bers [7] have been reported, which were produced from an aqueous sol gel route and show improvements in microstructure over conventional polycrystalline materials ®red at equivalent temperatures. The many advantages of sol-gel processes for manufacturing ®brous materials compared to bulk polycrystalline powders and the potential enhancement of their magnetic properties have been discussed in these articles. The hexagonal ferrites are commonly manufactured as either a powder or single crystal, but the development of a ®brous form of these ferrites and their subsequent incorporation into a composite matrix will result in a generation of novel magnetic materials with many potentially useful applications. We now report the microwave permeability spectra of these novel ®brous hexagonal ferrites, and compare them with literature data on the same phases in bulk powder form, where it is available. The precursor sols for the Co2Z, Co2Y and Co2W ferrites were produced as previously reported [8], and blow spun as gel ®bers using a proprietary spinning process [9]. Both low porosity and a submicrometer grain size are desirable properties in polycrystalline microwave ferrites, but the commercial methods used to obtain them [10] such as hot pressing, cannot be used in the production of a ®brous material [11]. Therefore, a 0.67% calcium oxide-doped Co2Z ferrite was also produced in an attempt to encourage either formation of the Z phase at a lower temperature, or an improved microstructure at equivalent temperatures. The ®bers were characterized upon subsequent heat treatments, and all appeared to form single phase samples of the relevant Co2Z, Co2Y and Co2W ferrites from their X-ray diffraction (XRD) data [6±8]. The complex permeability (i ) spectra of the ceramic ferrite ®bers were determined using the following technique. The ®bers were milled to a ®ne powder and dispersed in paraf®n wax to a volume loading of 30 % ferrite. The wax=ferrite mixture was die-pressed to form a toroidal specimen which was inserted in a coaxial cell. Measurements of transmission and re ection were made between 0.5 and 18 GHz using a Hewlet Packard HP8510C vector network analyzer coupled to an HP8515A Sparameter test-set. A schematic of the measurement system is shown in Fig. 1. The values of i for the ferrite=wax composite were calculated from the measured S-parameters using the transmission line method of Nicholson and Ross [12]. The intrinsic ferrite properties were isolated using the Lichtenecker effective medium expressions: