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CUBIC-TETRAGONAL TRANSFORMATION AND
MAGNETIC PROPERTIES IN COPPER FERRITES
WITH EXCESS Fe2O3
M. Sugimoto
To cite this version:
JOURNAL DE PHYSlQUE Colloque C1, supplement au no 4, Totne 38, Auril 1977, page (21-117
CUBIC-TETRAGONAL TRANSFORMATION AND MAGNETIC PROPERTIES
IN COPPER FERRITES WITH EXCESS F e 2 0 3
M. SUGIMOTO
Department of Electronic Engineering, Saitama University, Urawa, Japan
Rhumb.
-
On a mesure les temperatures de transition cubique-tktragonale et les proprietes magnetiques d'un ensemble d'tchantillons du systkme binaire CuO-CuFezO4, contenant plus de Fez03 que la stoechiometrie. Un durcissement magnetique marque a Cte observe sur un Cchantillon de 40 Cu0-60 FezO, ; on a verifie par analyse aux rayons X et par spectrometric Mossbauer que ce phenomene provenait de la precipitation de la phase z-Fe20, durant le recuit.Abstract. - Measurements have been performed of cubic-tetragonal transition temperatures and magnetic properties on a set of those samples of the binary system CuO-Fez03 which contain Fez03 more than stoichiometric CuFezO4. A marked magnetic hardening was observed on a sample of 40 Cu0.60 Fez03, which has been verified, through the X-ray analyses and measurements of Mossbauer spectra, to result from the precipitation of a-Fe203 taking place during annealing.
1. Introduction. - Many reports were presented concerning the cubic-tetragonal transformation in CuFe204. By way of example, Snoek [I] reported that CuFe204 to be cubic above 7600C and tetragonal below this temperature. Whereas, Takei [2], Stiers- tadt [3], and Inoue and Iida [4] reported that the transition temperature was 3600C. In connection with this, Ohnishi and Teranishi 153 pointed out that the transition a s quoted above depends upon the temperature at which the quenching of CuFe204 commences, as was acknowledged in the results of the Mossbauer study and so forth [6].
Besides, Bertaut [7] and Finch el al. [8] examined the relationship between the cation ordering on the octahe- dral sites and the axial ratio as a function of tempera- ture, and attributed the transition to an ordering of transition metal ions on the octahedral sites. Dunitz and Orgel [9] ascribed the tetragonal deformation of CuFe204 to the lattice distortion due to the Jahn- Teller effect. As depicted above, a lot of reports have been presented on the cubic-tetragonal transfor- mation in the stoichiometric copper ferrite. However, no report has been published on the study devoted to those samples containing the excess Fe203.
The present report is a study in a series of researches concerning the magnetic hardening of ferrites, and covers the cubic-tetragonal transition temperatures as determined by means of the X-ray analysis on those samples of the binary system CuO-Fe203 which contain Fe203 in excess, as well as the changes of magnetic properties, in particular, the magnetic har- dening associated with the transition.
2. Experimental procedures. - A family of samples of the binary system CuO-Fe20, with eight different compositions was prepared from the pure CuO and Fe203 through a conventional ceramic process. The contents of Fe20, of those samples ranged from 40 to 90 mol.
%.
The transition temperatures were measured by high temperature X-ray apparatus on powdered samples, which were sintered at 900 OC for five hours and subsequently cooled in the electric furnace a t an extremely low rate.In addition, on samples which were sintered in the air a t 900 OC to 1 050 OC and then quenched, and on these samples as annealed at 300 OC to 700 OC after quenching, measurements have been carried out of magnetic properties, X-ray analyses and observation of microstructures in conjunction with Mossbauer absorption spectra. The saturation magnetization was measured on the powdered sample by means of a vibrating sample magnetometer, while the demagneti- zation curve was measured on a rod sample measuring 10 mm in diameter and 25 mm in length by the same method as that of the previous paper [lo]. Mossbauer spectra were also measured on the powdered sample using 5 5 C o ( C ~ ) as the source.
3. Experimental results. - 2.1 TRANSITION TEM- PERATURES.
-
Figure 1 shows the transition tempera- tures from tetragonal to cubic for those samples which were heated at a rate of temperature rise of about 0.1 OC/min. For CuFe20, with the stoichiome- tric composition, the transition temperature is measur- ed to be 360 OC. As the Fe203 content increases overC1-118 M. SUGIMOTO
I I I I
CURIE TEMPERATURE ( AS QUENCHED FROM 1.000' C )
-
U-
w CUBIC a 2 4 0 0 - A NO 4 a Cu 0 W a 3 + 0 TETRAGONAL1
TETRAGONIL TETRAGONAL A N 0 3 0 0 1 AND C u 0 Fe z 0 3 1 40 5 0 6 0 7 0 F e Z 0 3 CONTENT ( MOL .% )FIG. 1. - Transition temperatures and Curie temperatures as measured on the copper ferrites containing Fe203 in excess.
the stoichiometry, the transition temperature shows a nearly linear increase, reaching eventually a plateau temperature of 415 OC for Fe203 contents of 60 mol.
%
or more.
It was found that the transition temperature when heating is about 10 OC higher than for the descent, especially on those samples containing Fe203 in excess. This difference may be ascribed to the difference of activation energy for the migration of transition metal ions between the 8a site and the 16d site.
Also illustrated is in figure 1 a part of the pseudo- phase diagram which was established from the results of X-ray analysis. As evident from the diagram, the sample containing Fe203 of 60 mol.
% is located at a
position in the two-phases region.2.2 MAGNETIC PROPERTIES. - Those samples containing Fe203 in excess of the stoichiometry, which were quenched from temperatures ranging from 1 000 OC to 1 050 OC and subsequently annealed at somewhere between 400 OC and 600 OC, have been found to show a marked increase of the coercive force. This phenomenon of magnetic hardening was most significant when the sample was sintered at 1 000 OC, subsequently quenched, and further annealed a t 500 OC for 10 hours. As evident from figure 2, the annealing caused the remanent magnetization and the coercive force in a sample of 40 Cu0.60 Fe,O, to be enhanced respectively to about 3 and about 10 times of those which was quenched. During the annealing, however, the saturation magnetizations at room tempe- rature undergo a marked reduction as shown in figure 3.
Figure 1 includes the measurements of Curie temperature. As the content of Fe203 increases over the stoichiometry, the Curie temperature climbs to the
--x-x- Hc -0 - - -0- AS ANNEALED -0- - -0- Hc AS QUENCHED 0 4 0 0
-
Z 0-
C N F w (I, T C Z y 2 0 0 4 W (L n 4 0 5 0 6 0 7 0 8 0 90FeZOQ CONTENT ( M0L:I. )
FIG. 2. - Dependence of the remanent magnetization and the coercive force upon the heat processing in the copper ferrites
containing the excess Fez03.
-
(I,-.
3 30 z W-
z 0 C 4 N r 2 0 W ZS
Z 0 C 4 = 10 3 2 m 0 I 4 0 5 0 6 0 7 0 8 0 F e 2 O 3 CONTENT ( MOL .'fa )RG. 3.
-
Influence of heat processing on the saturation magne- tization at room temperature.CUBIC-TETRAGONAL TRANSFORMATION AND MAGNETIC PROPERTIES IN COPPER FERRITES C1-119
was confirmed that, when quenched from 1000 OC, samples containing 50 mol.
%
to 60 mol.%
Fe,03 had a single phase.4. Discussion.
-
All the samples used in the present work might not have reached the complete equilibrium state because of a rather short heat-treatment time. However, the examination of X-ray analyses indicated that all samples were identically close to the equili- brium.It has been clarified by a number of studies that the cubic-tetragonal transition temperature in CuFe204 depends only upon the ordering of copper ions on the 8a site, and that the transition temperature rises linearly with decreasing the ordering of copper ions on the 8a site. The variation of transition temperature in the present study is supposed to come from the same origin mentioned above, that is, the temperature rise in the samples containing the excess Fe,03 might be attributed to the decrease of the ordering of copper ions on the 8a site due to an essential deficient in copper ions accompanying the increase of the Fe203 content. Besides, a small amount of Fe2+ and/or Cuf ions which might have been formed by the firing at higher temperature [ l I] may have been a cause for this transition.
Further experiments were conducted to find the cause of the marked magnetic hardening. From the facts that this phenomenon was significant in the samples containing the excess F e 2 0 3 and that it heavily depends upon the annealing temperature and time, the magnetic hardening has been inferred not to stem from the crystalline distortion due to the Jahn-Teller effect, but to stem from the precipitation of Fe203 as solved into the spinel. An attempt was made to verify the precipitation of Fe,03 by the observation of the microstructure, and it was found that the crystal grains are not great enough to distinguish the phases and the precipitates. By means of the X-ray analyses, however, it has been confirmed that those samples containing larger amount of Fe203 permit the F e 2 0 3 phase to be created after the annealing at 5000C. Besides, the X-ray diffraction exhibited an appearance of rather diffuse side bands on the low angle which were dominant at the earlier stage of annealing. The presence of the side bands to the main diffraction lines is interpreted as resulting from the spinodal decomposition [12].
Figure 4 shows the Mossbauer absorption spectra as measured on a sample of 40 Cu0.60 Fe,03 which was quenched from 1 000 OC, and that on the same sample as annealed for 15 hours after quenching. In evidence from the figure, the average internal magnetic field increases with annealing, and the annealed sample yields two spectra for the precipitated Fe,O,. The intensity of the absorption spectra due to the precipi- tated Fe203 became stronger with increasing anneal- ing time. This experimental finding also indicates that
. .;
A:.&
* [ %. AS ANNEALED AT 5009: 9% ''':'
I
- .
. .
...
AFTER QUENCHING.
.
I I I I I Fe,O, REFFRENCEFIG. 4. - Mossbauer absorption spectra for a sample of 40 Cu0.60 Fez03 quenchcd from 1 000 "C and for the same
sample further annealed at 500 OC for 15 hours.
the magnetic hardening results from the precipitation of Fe203.
5. Conclusion.
-
Measurements were carried out of the cubic-tetragonal transition temperatures and the magnetic properties on those samples of the binary system CuO-Fe,O, which contain between 40 mol.%
to 90 mol.% of Fe203, from which following conclu-
sion have been drawn.a) The sample of stoichiometric CuFe204 a s
sintered at 900 OC in the air and subsequently cooled very slowly has a transition temperature of 360OC. As the F e 2 0 3 content is increased over the stoichiometry, the transition temperature climbs nearly linearly, settling to a fixed temperature of 415 @C for F e 2 0 3 contents over 60 mol.
%.
b) A marked magnetic hardening was observed on a
sample of 40 Cu0.60 Fe,O, as annealed at 500 OC for 10 hours after quenching from 1 000 0C. This phenomenon has been confirmed to come from the precipitation of Fe203 through the X-ray and the Mossbauer spectra analyses.
C1-120 M . SUGIMOTO
References
[l] SNOEK, J. L., Rev. Techn. Philips 8 (1946) 359. [2] TAKEI, T., J. Inst. Elect. Eng. Japan 59 (1939) 274. [3] STIERSTADT, K., 2. Phys. 146 (1956) 169.
[4] INOUE, T., IIDA, S., J. Phys. SOC. Japan 13 (1958) 657. [5] OHNISHI, H., TERANISHI, T., J. Phys. Soc. Japan 16 (1961)
35.
[6] YAMADAYA, T., MITSUI, T., OKADA, T., J. Phys. Soc. Japan
17 (1962) 1897.
[7] BERTAUT, E. F., J. Physique Radi~lm 12 (1951) 252.
181 FINCH, G . I., S I N ~ A , A. P. B., SINHA, K. P., Proc. R. Soc
Ser. A 242 (1957) 28.
[9] DUNITZ, J. D., ORGEL, L. E., J. Phys. Chem. Solids 3 (1957)
20.
[lo] S~JGIMOTO, M., IEEE Trans. Mag-11 (1975) 1309.
[I 11 GADALLA, A. M. M., WHITE, J., Trans. Brit. Ceram. Soc. 85 (1966) 1:
