Observations of the influence of ion-bombardment on the microstructure of synthetic and natural ferrite crystals

Abstract

It has been widely reported that spinels, prepared for TEM study by ion-beam thinning, exhibit a fine "mottled" texture on a scale of 10nm [1-6]. The observations of Frost and Gai [7] on spinels also appear to show a similar background contrast. The microstructure which produces the mottled contrast is almost certainly developed during the thinning process, because spinels exhibit no mottled contrast when they are prepared for TEM observation by crushing, yet show it strongly when prepared by ionbombardment. This mottled texture, however, is not characteristic of all ion-bombarded spinels; in particular, no strong mottled contrast appears to occur in ulvospinel, Fe2TiO4 [4-6], or in the spinel polymorph of Mg2 SiO4 [6, 8]. It should be noted that the nonoccurrence of mottling in ulvospinel remains debatable [9]. This communication concerns some new and novel observations of ion-bombardment induced microstructures in a synthetic ferrite and the natural spinelstructured ferrite minerals, jacobsite and franklinite. The synthetic material came from a manganese-zinc ferrite ([Mn-Zn]Fe204) single-crystal boule supplied by the Fuji Electrochemical Company. The same material has been the subject of an electron microscopy study of its oxidation behaviour [10]. The composition of the boule was determined by wet chemical analysis to be 55.10mo1% Fe203 (assuming all of the iron to be in the form Fd+) , 27.94mo1% MnO and 16.96mo1% ZnO. The lattice parameter of the asgrown crystal was found by means of X-ray diffraction to be 0.8487 nm [11]. Synthetic Mn-Zn ferrite samples, cut from the boule, were heat-treated at different oxygen partial pressures (Po2) to achieve different oxygen stoichiometries. The prevalent nonstoichiometry in Mn-Zn ferrite occurs through the incorporation of excess oxygen into the structure, which is reflected in the oxidation of Fe 2+ and Mn 2+ and the creation of associated cation vacancies [12, 13]. The Fe 2+ concentration was measured using a coulometric titration technique [14]. Table I gives the different types of heat-treated Mn Zn ferrite materials studied, and their estimated cation vacancy concentration, determined by means of a weight and charge balance calculation [11]. Material B was annealed at 1100 ° C in an evacuated sealed fused-silica tube for 30 days, to equilibrate the cation vacancy concentration. Materials C and D were cooled under conditions of varying Po2, in order to keep the samples on isocompositional lines (using T A B L E I The cation vacancy content of the synthetic Mn-Zn ferrite materials, heat-treated as shown