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EFFECT OF MAGNETIC ORDER ON THE
CONDUCTIVITY OF Ni-Zn FERRITES
A. Ghani Awad, M. Ahmed
To cite this version:
JOURNAL DE PHYSIQUE Colloque C1, supplhment au no 4, Tome 38, Avril 1977, page Cl-237
EFFECT OF MAGNETIC ORDER ON
THE
CONDUCTIVITY OF
Ni-Zn
FERRITES
A. A. GHANI AWADPhysics Department, Faculty of Science, Ain Shams University, Cairo Egypt M. A. AHMED
Physics Department, Faculty of Science, Tanta University, Tanta, Egypt
R h r n b .
-
Des mesures prkises de la variation thermique de la rQistivit6 Clectrique et de la puissance thermo-blectrique ont ktk effectuh sur une skrie d'khantillons du systhme Nil-,Zn,FetO4 avec x = 0,2 ; 0,4 et 0,6. Les modifications observhs au voisinage de la tempkra- ture de Curie confirment des rksultats publi6s auparavant pour le ferrite de Cu-Cd et sont expliquks par une assez forte interaction d'echange B-B dans ces ferrites.Abstract.
-
Precise measurements of the temperature dependence of the electrical resistivity and thermo-power were performed for a series of samples of the system Ni1-~Zn~FezO4 (x = 0.2,0.4 and 0.6). Observed changes around the Curie temperature confirm previously reported data for Cu-Cd ferrite and were explained as being due to the fairly strong B-B exchange interaction in these ferrites.
1. Introduction.
-
In a ferrite the transition from the ordered ferrimagnetic state to the paramagnetic state is accompanied by changes in various physical properties. The activation energy for electrical conduc- tivity has been found to be smaller in the ferrimagnetic region than in the paramagnetic region [I, 2, 31. For ferrites containing non magnetic ions such as ZnZ+ or Cd2" contradictory results have appeared in the literature. In some works [ I , 21 a pronouncedchange of the activation energy for the conductivity,
E,, was observed a t the transition temperature while
in another [3] no change of E, was reported a t this tem- perature.
In a previous paper we attempted to explain this discrepancy for Cu-Cd ferrites [2]. The aim of the
present work is to study the behaviour of the conducti- vity and the thermo-power for Ni-Zn ferrites during the transition to the paramagnetic state.
for ferrites and magnetic oxides [7, 81. On measuring
the temperature dependence of the thermo-power, 0,
our results also confirmed the conclusion of Simsa [5] and Grifiths [6] that reproducible data can be obtain- ed without introducing any additional contact material between the highly smooth and well ground surfaces of the sample and the platinium electrodes. Resistance and thermopower were measured using a potentio- metric arrangement with high-input-impedence null indicators. The accuracy of measurement ranged from f 1
%
to +_ 2 "/,. For stability of the measure- ments it was found necessary to maintain a vacuum up to 10-2-10-3 mm Hg in the measuring chamber.3. Results. - Figure 1 shows the temperature dependance of p and 0 for compositions x = 0.2 and x = 0.4 (samples 1 and 2). An increase in the activa-
tion energy of the resistivity was found on passing
2. Experimental.
-
A series of samples of the1
system Ni,-,Zn,Fe,O, with compositions x = 0.2,
0.4 and 0.6 were prepared by the coprecipitation
method. This method has been proved to give more ol
homogeneous and stoichoimetric final products than
2
- the usual ceramic technique [4]. All previous data onsuch materials were obtained on samples prepared by the ceramic method. In order to allow comparison
between the results obtained on samples prepared by 1 5 2;O
both methods, an additional sample with the composi- 2;5
0,
tion x = 0.6 was also prepared. 2 - - - - -- ---.
I . --For measurement of the electrical resistivity p, the
@F
-
- -
='
'samples were rubbed by an In-Ga alloy until a uniform
"
Ilayer was formed On the ground surface. Flo. 1.
-
Temperature dependence of resistivity p and (thermo-Such a contact material has proved to be most suitable power 8 for samples 1 and 2 (see table I).
C1-238 A. A. GHANI AWAD AND M. A. AHMED
through the Curie point T,. The thermopower express- ed in units of 2.3 kle = 198 pV/degree varies only slightly with the temperature in the whole range without any observable discontinuity in the region around the transition temperature.
Figure 2 shows a comparison between the behaviour of p and 8 for the samples with x = 0.6 which were obtained either by the coprecipitation or by the cera- mic technique (samples 3 and 3s).
FIG. 2. - Temperature dependence of resistivity p and thermo- power 0 for sample No. 3 (see table I). P-Sample prepared by the coprecipitation method. S-Sample prepared by the ceramic
technique.
In table I we give the values of the activation energy for all samples in the ferrimagnetic region E,, in the paramagnetic region E,, the change of this activation energy AE, values of the Cuire temperatures Tc and the exchange energy
Ei
= kTc(k
being the Boltzmann constant).4. Discussion and Conclusions. - For compositions 1,2 and 3 the activation energy for conduction increases in the paramagnetic region by amounts between 37 and 49 percent compared to its value in the ferri- magnetic region (see Table I). This relatively large effect of magnetic order on conduction process in these ferrites may be explained as follows :
Zn2+ ions occupy A sites in the spinel lattice and,
being non magnetic, diminish the strenght of the A-A and A-B exchange interactions. Mossbauer studies
of Ni-Zn ferrites made by Satya Murthy [9] and later by Petitt [lo] showed that the B-B exchange interaction on the contrary increases with the increase of Zn content. Therefore ferrites containing appreciable amounts of Zn are characierized by a fairly strong B-B exchange interaction. Electronic conductivity also occurs through the B-sublattice due to exchange of electrons between divalent and trivalent iron ions. This exchange also mainly occurs through the separating oxygen ions. Existence of the strong magnetic exchange between ions on the B-sites of such ferrites will greatly affect the activation energy needed for the transfer of the electron between Fe2+ and Fe3+ ions. In such a case this results in a considerable decrease of the activation energy for conduction in the magnetically ordered phase.
The thermo-power for these samples weakly depends on temperature without observable discontinuities in the region around the transition temperature. This behaviour of 0 implies that the concentration of charge
carriers remains constant in the whole temperature region and consequently the activation energy for conduction is totally consumed in the hopping process of these carriers between energetically equivalent sites [I 11. These results therefore give a further evi- dence which favours a localized rather than a band picture for conductivity in ferrites.
For sample 3 s prepared by the ceramic method the thermo-power shows an increase with temperature in the region just above Tc and remains constant for other temperatures. This change in thermo-power is about 10
"/:
of the value observed below Tc. A similar decrease of 8 was previously reported [2] in the case of Cu-Cd ferrite containing about 0.05 Fe2+ ion per molecule. Another corresponding change was found by Griffiths [ 6 ] for magnetite which had only divalent and trivalent iron ions in B-sublattice. This change was about+
13%
of the observed value a t T,. We may consider therefore that the change A8 near the transi- tion point may be connected with the presence of a considerable quantity of F e z + ions in the composition of the ferrite. Therefore we may attribute the diffe- rence between the behaviour of 0 for sample 3 s andthat for sample 3 to a small increase of Fe2' ions in the sample prepared by the ceramic method. The
Comparison between the changes AE itt activation energy for conductivity during transition to the paramagnetic state and values of the exchange energy for some composition of the system. Ni, -,Zn,Fe204
EF
E~
AE Ei=kT,Sample x AE
-
AEe. v. e. v. e. v. Tc k e. v. El
-
EFx
EFFECT OF MAGNETIC ORDER ON THE CONDUCTIVITY OF Ni-Zn FERRITES C1-239
origin of the excess of Fez' for the sample prepared by We conclude that the behaviour of the conductivity the ceramic method may be due to : and the thermopower during the transition to the
(i) small amounts of iron eroded from the ball mills [12] during the n~illing process (Samples prepared by the coprecipitation method are not subject to this process) ;
(ii) sintering of this sample at 1 350 OC, which is
100 OC higher than the sintering temperature for the sample prepared by the coprecipitation method.
paramagnetic state depends on the concentration of Fe2+ ions which might be present in the ferrite. A relatively large increase of the activation energy for the conduction process (AE = 0.11-0.14 eV) was
found on some samples of Ni-Zn ferrites and was attributed to the fairly strong B-B exchange interaction in these ferrites.
References [I] S u e ~ o v , A. E., Fiziko Metallov I Me!falovedene7 (1959) 317. [2] GHANI, A. A. and MIRIJASOV, N. Z., Sov. Phys. Sol. Stare
(USA) 13 (1972) 2627.
[3] MLESCU, M. N., C. R. Hebd. Sian. Acad. Sci. 263 (1969) 136.
[4] CYSINA, 2. M. et al., Physical and Physico-chernical proper- ties of ferrites (1966) 42.
[5] SIMSA, Z., Chzech. J. Phys. B 16 (1966) 919.
[6] GRIPFTHS, B. G., ELWLLL, D. and PARKER, R., Phil. Mag.
22 (1970) 163.
[7] KRAUSSE, J., HADEREY, F. and DULLENKOPF, P., Z. Angew. Phys. 20 (1 966) 367.
[8] TAKATA, M., TSUBONE, D. and YANAGIDA, H., Phys. Absfr.
78 (1975) 6805.
19) SATYA MLIRTHY, B. S., NOTERA, A. G. and YOSSEF, S. I . ,
Phys. Rev. 181 (1969) 696.
[lo] PETITT, G. A., SolidState Cotrrnlwz. 13 (1973) 1611. [ l l ] AUSTIN, I. G. and Mom, N . F., Adv. Phys. 18 (1969) 41. [I21 BROCKMAN, F. G . and MATTESON, K. E., J . Amer. Cerumic
