Behaviors of Additives in Ferrites

The influence of various cations ranging from alkali metals to transition metals according to the periodic table were discussed. The ferrites used were Ni ferrite, Mg ferrite and Zn ferrite having different cation distributions in the spinel lattice. Cations having appropriate ionic radii and valencies are soluble in ferrite. Several additives formed other phase in addition to the spinel phase. There are many additives by which magnetic and electric properties of ferrites are affected. 1 . Introduction. It is well known that additives play an important role in improving the magnetic characteristics of ferrites. For instance, the role of small amounts of Ca and Si in high permeability of Mn-Zn ferrite [I] and the role of a small amount of Bi in improving the residual magnetization and the coercive force of Ba ferrite [2] were investigated. Moreover magnetic properties of various oxides with spinel structures were investigated [3]. It is possible to expect many effects from various additives in ferrites. However, the behaviors of additives have not been clarified enough. In this report the influence of various cations ranging from alkali metals to transition metals according to the periodic table were studied. 2. Experimental. Three kinds of ferrite having different cation distributions in a spinel lattice were selected. They were Ni ferrite as an inverse spinel, Mg ferrite as an intermediate spinel, and Zn ferrite as a normal spinel. They were synthesized by usual methods. Ni ferrite or Zn ferrite having a stoichiometric composition could be obtained always. Mg ferrite was also stoichiometric a t the initial stage of the experiment, but in subsequent procedure after adding lIIb group metals the composition of the used ferrite deviated slightly to the magnesium excess side. As additives 1-6 at % of cations were added to each ferrite respectively, in the form of carbonate in the case of alkali metals and alkaline earth metals except Be and Mg, or in the form of oxide in the case of other metals including Be and Mg. After satisfactory mixing, each ferrite powder with additives was pressed into toroids and disks. Firing was carried out at 1 300 OC for 3 hours in air. To identify the formed phases and to determine the lattice constants of the ferrites after adding additives, X-ray diffraction pattern measurements were made with a diffractometer with manganese filtered iron radiation. The formed phases were identified by the diffraction patterns at 2 8 < 80°, and lattice constants were determined from the (533), (731) and (800) spinel reflections. Curie temperatures were measured with a magnetic balance using small pieces taken from a fixed position of the disk specimens. Magnetic permeabilities at 10 kHz were measured on toroidal specimens with an impedance bridge and electric resistances were measured on disk specimens with a vibrating reed volt ammeter. Observations with an electron microscope or analysis with an electron probe micro analyser were carried out when necessary. 3. Result. 3.1 ALKALI METALS. The lattice constant, Curie temperature and the electric resistance showed the replacement of divalent cations in each ferrite by Li or Na within a limited amount. Li is not so effective for Ni ferrite as for nickel oxide. The behavior of Li in Mg ferrite is particular. The electric resistance decreased remarkably by adding up to 4 at 0/,, and the magnetic permeability increased correspondingly (Fig. 1). The behavior of Na in Ni ferrite is similar to Li in Mg ferrite (Fig. 2). K has a too large ionic radius to form a solid solution with each ferrite. The possibility of formation of a solid solution was suggested in a slight increase of the lattice constant by adding K to Zn ferrite as shown in figure 3. Rb and Cs formed neither solid solutions nor other phases with any ferrite. However, it is observed that the existence of Rb affects the micro structure of sintered ferrites. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1977134 C1-176 J. TASAKI AND T. IZUSHI . \ ~ n o t ~ n t o f . \ d t l i t i v c s at'A , \ m o u n t o f A d d i t i v e s a t " , FIG. I . Magnetic permeabilities of Mg ferrite with several FIG. 3. Lattice constants of Zn ferrite with alkali metals. additives. FIG. 2. Magnetic permeabilities of Ni ferrite with several additives. 3.2 ALKALINE ARTH METALS. Except Ca, alkaline earth metals had little effect on the lattice constant of Ni ferrite or Zn ferrite. The replacement of Ni or Zn by Mg is confirmed from the drop of Curie temperature of Ni ferrite and the slight decrease of the lattice constant of Zn ferrite. In Mg ferrite these cations except Be are effective for the lattice constant (Fig. 4) and Curie temperature. The behavior of Ca in each ferrite is particular. The variation of the lattice constant or Curie temperature is remarkable. The magnetic permeability increases in Ni ferrite Amount o f Additives a t % FIG. 4. Lattice constants of Mg ferrite with alkaline earth metals. (Fig. 2) and especially in Mg ferrite (Fig. 1). As the amount of Ca increased above 4 at % another phase Ca2Fe20, was confirmed with Ni ferrite and Zn ferrite by X-ray diffraction. Another phase, SrFe,O,, was also found by adding Sr above 2 at % to Zn ferrite. 3 . 3 IIIb GROUP METALS. The solubility of Al, Ga, and In among this group in Ni ferrite and Zn ferrite is shown clearly by the lattice constant or Curie temperature. The lattice constant varied according to the ionic radii of Al, Ga and In (Fig. 5 ) and Curie temperature decreased with the amount of these additives (Fig. 6). In Mg ferrite the variation of the lattice constant or Curie temperature was somewhat complicated because of the excess magnesium content of the starting Mg ferrite. Ga and BEHAVIORS OF ADDITIVES IN FERRITES C1-177