SUBSOLIDUS EQUILIBRIA RELATIONS IN CoQ-MgO-Fe203 SYSTEM IN AIR AND MAGNETIC BEHAVIOUR OF ITS FERRIMAGNETIC SPINEL PHASE

Abstract

Phase equilibria relations are determined for the system CoO - MgO - Fe^O^ at variable temperatures in air by conventional quenching technique. Three isothermal planes at 800°, 1000° and 1150°C are taken into consideration. It is noted that the isothermal section at 1150°C exhibits two single-phase (one phase with protoxide structure and the other with spinel structure) and two biphase zones (spinel+protoxide, and spinel+hernatite). The protoxides are solid solutions of MgO and CoO containing a little amount of Fio0r(. The spinels are solid solutions of magnesiof err i.te (^i+x^e2+x^4+^ anc* cobalt-bearing spinel (Co, Fe^, 0. ). One of the biphase zones is in effect a cross-section of a ternary solvus where different compositions split up into protoxide and spinel phases. The isothermal section -it 1000°C shows the same phases as observed at 1150°C but here the biphase regions are expanded, thereby restricting the fields of single phases. At 800°C, the phase diagram becomes complicated with the appearance of another type of spinel. This spinel is primarily iron-cobaltite with a minor amount 2+ of Mg going into its structure. Tie-lines are drawn within the solvus region

connecting the compositions of coexisting phases. The proportion of phases are calculated from the relative heights of principal reflections and then Lever rule is applied to determine the orientation of tie-lines. In few cases EPMA data are also used. Lattice para meters of the phases are calculated from the positions of the X-ray diffraction peaks. Magnetic moment measurements at variable temperatures (from 77°K to the neighbourhood of their respective Neel temperatures) under a constant field of 5 kGauss are carried out on the single-phase compounds prepared at 1150 C in air. All the spinel mixtures show typical characteristics of ferrimagnetic compound when specific magnetisation (magnetisation per gram) versus temperature variations are plotted. It is also noted that at a given temperature, with the increase of cobalt content the magnetic moment rises for all the samples. To establish the crystal chemistry and solid solution behaviour of the spinel series, five mixtures lying along the join (MgOjggfFegOu)-- and (Co°)25(Fe2°3^75 in the 1150°C isothermal section are taken into consideration. Various studies made on these samples and the results obtained therefrom are outlined below. Mdssbauer spectra of the samples are taken at room temperature in absence of any external field.

Spectra of all the patterns show two superimposed six-finger Zeeman patterns, one due to the tetrahedral and the other due to the octahedral iron ions. The values of Mossbauer parametero suggost that all the 3+ iron is in Fe state. The difference in isomer shift between the two sites, dA-dB,is negative in every case. The data on relative intensity confirm that the distribution of iron between the tetrahedral and octahedral sites are 42 and buy., respectively for all the mixtures. There is no variation of quadrupole splitting with respect to composition and their values are small (can be assumed to be zero within the limit of experimental error). The observed variation in the internal hyperfine magnetic field against composition is explained in the light of relative distribution of cations between the two sites and the superexchange interactions resulted therefrom. Magnetic moment data al room temperature are combined with the Mossbauer d.ita to obtain the distribution of cobalt ions between the two sites. 2+ It is seen that as Co ions are added to magnesioferrite, migration of these ions takes place in both the sites, although primarily to the octahedral site. Thus an increase in magnetic moment per grain of the samples takes place with progressive addition of cobalt in magnesioferrite.

Lattice parameters and intensity ratios -7-7-^ are obtained from the X-ray dlffractometer data. These data are plotted against composition and an attempt has been made to explain the cation distribution between the two sites. These characteristics also agree with the magnetic moment and Mossbauer data. Calculations of thermodynamic coefficients (a and p) in the neighbourhood of Neel points are done with varying field strengths. Neel temperatures are then calculated precisely from the values of a and p. The variation of Neel temperature with composition shows that there is a progressive nonlinear increase in its value due to addition of cobalt in the magnesioferrites.