HIGH-FREQUENCY PROPERTIES OF Ni-Zn-Co FERRITES IN RELATION TO IRON CONTENT AND MICROSTRUCTURE

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HIGH-FREQUENCY PROPERTIES OF Ni-Zn-Co

FERRITES IN RELATION TO IRON CONTENT AND

MICROSTRUCTURE

J. de Lau, A. Broese van Groenou

To cite this version:

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JOURNAL DE PHYSIQUE Collogue CI, supplement au n° 4, Tome 38, Avril 1977, page Cl-17

HIGH-FREQUENCY PROPERTIES OF Ni-Zn-Co FERRITES

IN RELATION TO IRON CONTENT AND MICROSTRUCTURE

J. G. M. DE LAU (*) and A. BROESE VAN GROENOU Philips Research Laboratories Eindhoven, The Netherlands

Résumé. — La Co-substitution d'ions Co3+ et des ions Co2+ dans des ferrites de Ni-Zn ainsi

que la réduction de la taille des grains conduisent toutes deux à une importante amélioration des propriétés magnétiques dans la gamme de fréquence des MHz. Cette amélioration est attribuée à la stabilisation des parois. La teneur en Co3+ et avec elle les propriétés à haute fréquence sont

déterminées par rapport au manque d'ions Fe.

A partir de mesures de la relaxation au-dessus de la température ambiante dans la gamme des kHz, on peut conclure que la stabilisation des parois est due à l'ordonnancement des ions Co2+

qui sont fortement anisotropes. Les ions Co3+, et probablement les vacances en cations qui leur sont

associées, sont à l'origine de la migration des ions Co2+ à travers le réseau cristallin.

Abstract. — Both the substitution of Co3+ ions in addition to Co2+ in Ni-Zn ferrites and the

reduction of grain size lead to a great improvement of magnetic properties at frequencies in the MHz range. This improvement is attributed to domain-wall stabilization. The Co3+content and

with it the high-frequency properties are determined by the extent of iron deficiency.

From relaxation measurements above room temperature in the kHz range it can be concluded that the domain-wall stabilization is caused by ordering of the strongly anisotropic Co2+ ions.

The Co3+ ions and presumably cation vacancies in association with Co3+ provide the means

whe-reby Co2+ is transported through the lattice.

1. Introduction. — Low magnetic losses at high frequencies in Ni-Zn ferrites are obtained by stabiliz-ing domain walls [1]. Domain-wall stabilization can be brought about in two ways : firstly, by induced magnetic anisotropics which can be obtained by sub-stitution and ordering of small numbers of highly anisotropic C o2 + ions [2, 3, 4, 5] and, secondly, by

means of a fine-grain microstructure [6, 7].

The presence of an iron excess leading to cation vacancies [2, 3], or of an iron deficiency compensated by Co3 + ions [4, 8] is necessary for the ordering process

of the C o2 + ions.

The present paper deals mainly with iron-deficient Ni-Zn-Co ferrites, which are the most important ferrites for practical applications.

The most important chemical parameters investigat-ed are the iron content and the concentrations of Co3 +

ions and cation vacancies.

Special preparation techniques were used to vary the iron content within a very small range. Co3 +

concen-trations were measured very accurately by careful chemical analyses. The main micro structural para-meter investigated was grain size. Very fine-grain materials were made by means of hot pressing. Per-meabilities and magnetic losses were measured as functions of frequency and temperature. In this paper we shall mainly deal with the results of the magnetic measurements.

Details of preparation and measurement techniques (*) Present address : Elcoma Ceramic Laboratories, N. V. Phi-lips Gloeilampenfabrieken, Eindhoven, The Netherlands.

as well as of chemical aspects can be found in refe-rence [1].

2. Influence of chemical composition. — The pre-sence of C o3 + ions in Ni-Zn-Co ferrites is related to

an iron deficiency. This is demonstrated in figure 1 which gives the amount of C o3 + as a function of the

parameter x for a series of sintered ferrites of composi-tion : Nl0.772Zn0.193^-00,035Fe2 + xO4+y . nCo3' k 0.015 - \ 0.010 - \ . • \ 0.005 - \ -0.0A -0.02 0 0.02 • • x

FIG. 1. — The number of Co3+ ions per molecular formula unit

("co3+) a s a function of the compositional parameter x for

ferrites of composition : Nio.772Zno.i93Coo.o35Fe2+3;04+v. The

samples were sintered at 1 180 °C for 2 hours in oxygen and are characterized by closed porosity. The Co3+ content was

deter-mined by means of an active oxygen analysis [1]. The continuous line represents the theoretical amounts for a stoichiometric

composition, i. e. y = 4/3 x.

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C1-18 J. G. M. DE LAU AND A. BROESE VAN GROENOU

FIG. 2. -The real and imaginary parts of the permeability,

p' and p", measured a t 100 MHz as a function of the compositional parameter x for the series of ferrite samples of figure 1.

The parameter x expresses the excess of iron, x Ordering of Co2+ ions also takes place in iron- being negative for iron deficiency. excess materials with cation vacancies. This is shown For small negative values of x (0

>

x

>

-

0.02) in figure 3 which gives p' and the loss factor tan the amount of Co3 + as analysed [I] corresponds to (6)/,u = ,u"/(,u')~ as functions of x for another series of

the theoretical amount for a stoichiometric composition (the continuous curve). This means that the cation- 50

anion ratio remains 314, or y = 413 x. For large negative values of x (x < - 0.02) the amount of Co3' reaches a saturation value and remains constant.

t

In this composition range a second phase can be observed microscopically. ₃₀

Results of magnetic measurements at 100 MHz

on the same samples are shown inifigure 2, where the tons loL _-. 20

real part of the permeability (p') and the imaginary part (p") are plotted as a function of x. The curves of

,u' and ,uN show a sharp increase between x =

-

0.02 and x = 0, i. e. the region where the amount of Co3+ decreases.

A practically constant level of ,u' and y" is attained in the region where the Co3+ content is constant (x < - 0.02). The lowering of y' and ,u" with increasing amount of Co3 + is attributed to increasing domain-

wall stabilization. This domain-wall stabilization is caused by an induced anisotropy which in turn is due to ordering of CoZf ions. One of the aims of this investigation is to know more about the role of Co3 +

-

ions in the ordering process.

I

0 I

I

-

I

f = 100 MHz

_I

Fm. 3. - Permeability p' and loss factor tan (6)/p measured at

50 MHz as a function of the x parameter. The continuous curves correspond to samples slowly cooled from the Curie temperature to room temperature. The results after field disturbance at room temperature are given by the broken curves. The compositions are about the same as those of the samples of the figures 1 and 2. The samples were sintered at 1 200 OC in oxygen

and they are characterized by a high and open porosity.

IJ"

T

60 10 LO 12 ₂₀ -010 0 0.10 020 030 0.40 050 0 6 0

ferrites with larger differences in the iron content. The materials were prepared and sintered in such a way that the iron-excess ferrites could be oxidized at low temperatures so that the Fez+ content is very low [I]. The continuous curves correspond to samples slowly cooled from the Curie temperature and show a maximum at x = 0. Apparently domain walls are not stabilized at this composition and they can freely move, resulting in a high permeability and high losses at 50 MHz. Both with excess and deficiency of iron p1 and tan (6)ly decrease as a result of domain-wall stabilization caused by Co2+ ordering. In the iron- excess region this ordering takes place by diffusion via cation vacancies which are present in large concentrations in oxidized materials [3].

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Ni-Zn-Co FERRITES IN RELATION TO IRON CONTENT AND MICROSTRUCTURE C1-19

and that after field disturbance for the iron-deficient materials.

The improvement of the magnetic properties as a result of the substitution of Co2+ and Co3+ ions in Ni-Zn ferrites is demonstrated in figure 4. Here p'

F I ~ . 4.

-

p' (continuous curves) and ,u" (broken curves) as a function of frequency for a Co-substituted material (I), with composition : Ni0.6gZn0.~9C00.04Fe1.9904 and a Mn-substituted

material (2) with composition : Nio. 7~Zno. 3oMno. 0lFe1.9904. The materials were sintered at 1 160 OC for 2 hours in oxygen. There is a slight difference in average grain size : for material (1)

(1) 5 = 4 I.lm and for material (2)

5

= 2 pm.

and p" have been plotted as a function of frequency for two different materials (compositions are given in the figure caption). Both materials contain a deficiency of iron and are characterized by the same Ni-Zn ratio. Material 1 contains small amounts of Co2+ and Co3+, while a small amount of Mn3+ has been substituted in material 2 in order to obtain high electrical resistivity. At frequencies higher than 150 MHz the magnetic spectra are rather similar. The behaviour of p' and p" in this region is determined by ferromagnetic resonance phenomena.

Below 150 MHz the spectra show great differences. That between the constant levels of the p' - f curves (continuous curves) at low frequencies is attributed to the domain-wall stabilization in the Co-substituted material. The p" - f curves (the broken curves) also show great differences. In the Mn-substituted material (2) losses increase sharply around 10 MHz, whereas in the Co-substituted material (1) this only occurs around 100 MHz. If p' at low frequencies corresponds to the rotational permeability (p,,,), the resonance frequency

(A,,)

can be calculated with the aid of Snoek's formula [9] :

where y is the gyromagnetic ratio and M, the saturation magnetization.

For the Co-substituted material we calculate a resonance frequency of 260 MHz, which accords very well with the observed maximum of p" in the p"

-

f

curve, in agreement with the assumption that the permeability is purely rotational. On the other hand a marked deviation is found for the Mn-substituted material : the calculated value off,,, is 117 MHz whereas the maximum in the p" - f curve is found at 55 MHz. This means that moving domain-walls make a considerable contribution to the permeability.

Information on the mechanism of the Co2 + -Co3 +

ordering can be obtained by measuring the relaxation of the induced anisotropy, which in this case can be measured in the kHz frequency range above room temperature. Magnetic relaxations manifest them- selves as a decrease of p' with increasing frequency, while p" has a maximum near the relaxation frequency (Ae1). The relaxation time (z) is given by :

Activation energies can be derived from log z- 1/T

plots. Relaxation measurements were done with Ni (-Zn) -Co ferrites having different Co2+ and Co3+ contents [I, 81.

In figure 5 values of 1/t measured a t 150 O C have

been plotted as a function of the Co3+ concentration for three series of materials. Data on composition and

1031

I O - ~

-

I O - ~ lo-'

coJf per formulo unlf Fig. 5.

-

Relaxation time z measured at 150 OC as a function of the Co3+ content for ferrites with composition :

Nio.g8C0o.oz-zFez+~O4+~ and x varying between - 0.02 and

-

0.08 (the curves 1 and 2) and ferrites of composition : N ~ O . ~ ~ Z ~ O . I ~ C O ~ . O ~ F ~ ~ + ~ O ~ + ~ and x varying between 0 and

- 0.02 (curve 3). The sintering temperature was 1 260 OC in all cases and the oxygen pressure 1 atm (curves 2 and 3) or 50 atm

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C1-20 J. G. M. DE LAU AND A. BROESE VAN GROENOU

sintering conditions are given in the figure caption. For each of the series a straight line with slope

+

1 is found to describe the results fairly well ; in other words, l/z is proportional to the Co3+ content. This means that the presence of Co3+ is necessary for the relaxation process to occur. Curve 1 differs markedly from curve 2, though both are for series of materials of the same composition, sintered under different oxygen pressures. Apparently some pheno- menon related to the manner of sintering is involved. We shall discuss this further in section 3. Activation energies of the Co2

'

-Co3

'

relaxation were found [I, 81 to be 0.7 eV for Ni-Co ferrites and 0.9 eV for Ni-Zn-Co ferrites.

It should be noted that activation energies for the Co2+ cation-vacancy relaxation process vary between 1 and 2 eV [1,8]. Another important quantity which can be derived from the relaxation spectra is the strength of the relaxation. The decrease of p1 and the maximum value of p" are measures for the relaxation strength. The latter is found to be proportional to the Co2+ content [8]. No systematic variation with the Co3 + content is found. From the result of the relaxation measurements we can conclude that it is the Co2' ion whose migration changes the anisotropy and thus the permeability and that the Co3+ ion is involved in the process as a means of transport for the Co2+ ions.

3. Influence of microstructure. - A small grain size is favourable for the high-frequency properties. This is demonstrated in figure 6 which contains the magnetic spectra of three materials with the same composition but with different grain size. The materials do not

FIG. 6 . - Frequency dependence of p' and p" for samples of composition : Nio. 8Zno. 2Mno.olFe1.9904 and different average grain size

5.

The samples were sintered or hot-pressed in oxygen at different temperatures : 1 240 OC

(5

= 12 pm), 1 160 OC

(D = 2 pm) and 950 OC

(5

= 0.3 pm).

contain Co. The effect of grain size reduction can be compared with that of Co2 + -Co3' substitution as

shown in figure 4. The low-frequency level of p1 decreases and at the same time p" is decreased over a broad frequency range. Apparently the domain walls are effectively pinned by the presence of many grain boundaries.

In the case of ferrites whose domain walls have already been stabilized by Co2+-Co3+ substitution the effect of grain size reduction is less pronounced [I]. The degree of improvement depends on the Ni-Zn ratio which is shown in figure 7. Here room tempera-

'on 6 -. P

I

10 - I .

FIG. 7. - The loss factors tan (6)/p as a function of frequency for two compositions : Nio.99Coo.ozFe1.9904 (the continuous curves) and Ni0.579zn0.386Co0.04sFel.9904 (the broken curves).

The samples were sintered or hot-pressed in oxygen at the temperatures given under figure 6.

ture values of tan (8)/p have been plotted as a function of frequency for six different materials. The solid curves and the broken curves correspond to two different compositions, namely a Ni-Co ferrite and a Ni-Zn-Co ferrite composition, respectively. The effect of the grain size is greatest in the Ni-Co ferrite.

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Ni-Zn-Co FERRITES IN RELATION TO IRON CONTENT AND MICROSTRUCTURE C1-21

(2.6 eV) is comparable with that of the curve present- ing the cation-vacancy concentration as a function of the reciprocal temperature (2.2 eV). It seems plausible that cation vacancies in very small concentrations play a role in the ordering mechanism of the Co2+ ions. It must then be assumed that the cation-vacancy concentration is determined only by the sintering temperature ; in other words the cation vacancy concentration does not change during the post- sintering cooling period. The latter is acceptable because we are only dealing with materials with closed porosity and with relatively small cation-vacancy concentrations (the compositions are practically stoichiometric, see figure 1). Both factors are unfavou- rable for oxidation during cooling and for increase of cation vacancy concentration according to the reac- tion :

FIG. 8. - Relaxation time z measured at 150 OC as a function of the average grain size

5

for a number of iron-deficient Ni-Zn-Co ferrites of about the same composition : Ni-Zn ratio 4 and Co content between 0.03 and 0.035. The samples were sintered or hot-pressed in oxygen at temperatures varying between 925

and 1 260 OC.

1 200 OC and 1 atm oxygen pressure the concentration is very low, namely of the order of whereas at 600 OC it is already higher than If the relaxation times from figure 8 are replotted as a function of the reciprocal sintering temperature

(T,)

a straight line as shown in figure 9 is found. The slope of this curve

FIG. 9.

-

Relaxation time z measured at 150 OC but now plotted as a function of the reciprocal of the sintering or hot-pressing temperature Ts. The samples were the same as those in figure 8.

The cation-vacancy concentration established during sintering depends on the oxygen pressure. The cation- vacancy concentration would be expected to increase and the relaxation time consequently decrease with higher oxygen pressure. This has indeed been found [I, 81 and explains the shift of curve 1 with respect to curve 2 in figure 5.

We conclude that Co3 + ions are certainly and cation vacancies most probably involved in the Co2+ ordering process. From the literature [12] it is known that diffusion of divalent ions by way of cation vacancies can be facilitated if the divalent ion temporarily give off an electron and becomes trivalent. In the case of Co2+ diffusion it is quite conceivable that the electron transfer from a Co2+ to a Co3+ ion is followed by an association with a cation vacancy. The diffusion of the Co3+ ion can now take place with an activation energy between 0.7 and 0.9 eV. After the Co3 + ion has

found a new site the vacancy-Co3 + pair dissociates and the Co3+ ion can take up an electron. The result is a displacement of the Co2+ ion.

4. Conclusions. - Low magnetic losses at high frequencies of iron-deficient Ni-Zn-Co ferrites are attributed to domain-wall stabilization which is caused by ordering of Co2+ ions in the spinel lattice. The time needed for the CoZf ordering depends on the Co3 + concentration, which is primarily determined by the iron deficiency. The rate of ordering is also determined by the temperature and the oxygen pressure at which the material has been sintered.

In the Co2+ ordering process not only Co3+ ions but very probably also cation vacancies in small concentrations are involved.

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C1-22 J. G. M. DE LAU AND A. BROESE VAN GROENOU

results in properties which are comparable with those cient Ni-Zn-Co ferrites, though in materials with a high of ferrites with Co2 + -Co3+ substitutions. The effect of Ni-Zn ratio considerable improvements can be obtain-

grain-size reduction is less pronounced in iron-defi- ed by reducing the grain size to below 1 ym.

References

[I] DE LAU, J. G. M., Philips Res. Repts. Suppl. No. 6 (1975). [8] BROESE VAN GROENOU, A., CREYGHTON, J. H. N., and

121 HECK, C. and VACCARI, G., Z Angew. Phys. 17 (1964) 92. DE LAU, J. G. M., J. Phys. Chem. Solids 35 (1974) 1081. [3] MIZUSHIMA, M., Japan J. Appl. Phys. 3 (1964) 82. _{[9] SMIT,}_J._{and WIJN, H. P.}_J.,_{Ferrites, Philips Technical} [4] SIXTUS, K. J., Solid State Phys. Electr. Telecomm. 3 (1960) _{Library, Eindhoven (1959).}

91.

[5] MARAIS, A., MERCERON, Th. and PORTE, M., Phys. Stat. [lo] REIJNEN, P. J., Philips Res. Repts. 23 (1968) 151.