THE SIZE DEPENDENCE OF THE THERMALLY AND MAGNETICALLY INDUCED WALL PINNING IN A Ni-Zn FERRITE

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THE SIZE DEPENDENCE OF THE THERMALLY

AND MAGNETICALLY INDUCED WALL PINNING

IN A Ni-Zn FERRITE

T. Postupolski, A. Wiśniewska

To cite this version:

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JOURNAL DE PHYSIQUE Colloque CI, supplement au n° 4, Tome 38, Avril 1977, page Cl-31

THE SIZE DEPENDENCE OF THE THERMALLY

AND MAGNETICALLY INDUCED WALL PINNING IN A Ni-Zn FERRITE

T. POSTUPOLSKI and A. WISNIEWSKA

Unitra-Elpod-Polfer, Ul. Dzielna 60, 01-029 Warszawa, Poland

Résumé. — Les différences des états magnétiquement neutres (DNS) d'un ferrite de Ni-Zn soumis à des désaimantations thermiques et magnétiques parallèles et perpendiculaires apparaissent dépendre distinctement de la taille moyenne des grains, ce qui est discuté.

Abstract. — The differences in the magnetic states, DNS, of a Ni-Zn ferrite, subjected to thermal and to magnetic parallel and perpendicular neutralizations, appeared to depend distinctly on the mean grain diameter what is discussed.

1. Introduction. — Since a long time [1] it has been known that in the after-effect-free magnetics their neutral state f/) after thermal neutralization, TN, may be different from that after magnetic neutraliza-tion, MN. In such cases the initial permeability of a sample may, in the same external conditions, have different values after TN than after MN, /iT # /%. These differences in the neutral states, DNS, expressed as

DNS = fctr - nM)/fiM (1)

may even reach 30 % and more. Then they are of importance not only from physical point of view but also for the practical applications.

In this report some characteristic results obtained on a polycrystalline Ni-Zn ferrite are presented. All the samples under study had the same fundamental properties (a, 6C) and were differentiated by the

micro structural ones (grain diameter, density, poro-sity).

2. Samples and experimental procedure. — Samples of chemical composition Zn0.275Ni00,245(Fe203)o.48 were obtained by usual ceramic technique with a presintered ferrite mass. Presintering was done to assure the same specific saturation magnetization for all the samples under study. Microstructural parameters were controlled by peak temperature and sintering time.

Initial permeability, /i, taken as a measure of the magnetic state, was measured at low frequency. The thermal neutralization, TN, was performed by cooling, at a given rate, the samples from a temperature

exceed-C1) Neutral state : a state of magnetic material or body in which the resultant flux density and the field strength are zero over any region having dimensions large compared with the domain size.

Neutralize : to bring a magnetic material to a neutral state [2].

ing by 20-30 °C their Curie point, 9C, to the measuring

temperature. During the whole TN period the samples were protected against any disturbance. Magnetic neutralization, MN, was performed isothermically by applying to the sample an alternating magnetic field of a strength monotonically decreasing to zero. This field was generated by a transformer single-wire secondary winding (/max < 700 A) or by an electro-magnet in its air gap (uniform flux density dropped from 3.5 kGs in the absence of the sample). The effects of all the above kinds of neutralization were fully reproducible.

3. General description. — The ring-shaped samples were subjected to TN and to two kinds of MN : parallel, MN||, and perpendicular, MNX, to the direc-tion of the measuring field. The mutual posidirec-tions of the permeabilities and thus the corresponding magnetic states resulting from these neutralizations appeared to depend on the mean grain diameter D of a given sample, as it is shown in figure 1. All these states

,,JUL TN

mm\ ffj" ^ ^ MN

X B B a a MN ^ M N , TN1 E3S3 m^ D —

FIG. 1. — Positions of permeabilities after given kinds of neutralization for different grain diameters D.

proved to be insensible to vibrations and mechanical shocks (unlike in the case of nickel reported in [1]), and to temperature cycling within + 25 — + 170 °C range. After temperature cycling had increased over

+ 170 °C, the MN states were obviously transformed

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C1-32 T. POSTUPOLSKI AND A. WISNIEWSKA

into TN state. It was also found that the rate of temperature variations both during cooling from 0, and during cycling had not noticeable influence on any of these states. These and other results as frequency spectra of p, dependence of p on the magnetic field strength etc. have proved unambiguously that the phenomenon under study is of a purely domain origin. 4. Results and discussion. - The results described

were obtained on ring cores. To avoid a suspicion that anisotropy of domain arrangement along the direction of minimum reluctance (the so called toroid symmetry or the crystallographic axis continuity effect [3, 41) is involved in the results measured, the cubes of 15.007 f 0.005 mm in the edge were cut out of large ferrite blocks having different grain diameters. Each of these cubes was found to be magnetically isotropic. First the cubes were subjected to TN and, subsequently, to MN along three cube axes. After the given neutralization had been made, the cube apparent permeability, pap,, was measured along three cube axes by means of a suitably matched winding. The material permeability p was next calculated from papp knowing a demagnetization factor for the cube- winding combination used. The values of p,,

yll

and pL SO obtained are shown in figure 2 as a function of

4 2 3 4 D m--

i*

FIG. 2.

-

Permeability ,ti (density correction taken into account) as a function of mean grain diameter 5 after thermal (T),

parallel

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and perpendicular (I) neutralizations.

mean grain diameter

5.

As it can be seen, for

3 <

1.2 pm p,

<

pT

<

pll ; for

3

E 1.2 pm

p, < pT = pll ; for D > 1.2 pm pL < pll < p,. Using a diagram shown in figure 3 the following interpretation is proposed. First, the situation concerns the ferrite whose larger grains have defects that pin the domain walls. This is evidenced by the deviation of curves p(D) from the straight line of Globus' p - D proportionality law [5, 61. When the grains are free of defects (small grains in our case), DNS results only from anisotropy of the magnetically demagnetized state (anisotropy of demagnetization

-

AD). AD consists in the orientation of a statistically greater

rot.

I/-1

1

;:5-

FIG. 3. - Interpretation diagram.

number of walls along the direction of magnetic neutralization [7, 8, 91. One can reasonably expect that in an internally isotropic cube the AD

-

effect will be a symmetric one in relation to the average value

pav = (pII

+

p1)/2, and that in the defect-free magnetics, having isotropic form in considered directions (cube, sphere, disc, etc.), pa, = p,. As our experiment showed, the domain orientation of the TN-state is perfectly at random (practically identical p, value along three cube axes). Figure 3 shows that for D < ~ l pav=p~ ; > P T > ~ L and DNSII '(PT-P~~)/ p

<

0. When

D l

< D

<

D3 : pT > pa,, pT still fulfils the Globus' law but pll, pav and pI does not continue to do it. We suppose that within this D-range the internal grain defectiveness is moderate. In the TN- state the walls are pinned at the grain perimeters preferring rather local continuity of domains in successive grains than being positioned in the deepest potential wells. The MN puts the walls down into these wells located at grain boundaries as well as inside the grain. The domain continuity decreases, less after MNII, more after MN,. Consequently, the deviation from p

-

5

proportionality begins, in scale, first for p, and then for pa,, pll and, finally, for pT. There is a characteristic point D, tor which pll =pT. In this point the parallel AD-effect compensates the pa, decrease, DNSII = 0. D, is the point of apparent insensitivity of domain configuration to MNII. Above D3 p, deviates distinctly from Globus' law ; this gives evidence of the appearance of large intragranular defects (pores) which pin the walls by themselves [5, 61.

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THERMAL AND MAGNETIC WALL PINNING IN Ni-Zn FERRITE (21-33

increased. This implies a general question : if the decrease of the wall diameter is accompanied by an increasing number of domains and walls inside one grain, could it result in a decrease of permeability, as is shown by the experiment ? It is believed that formula (2), being an extended Globus concept [ 5 ] over a sin-

FIG. 4. - Factor of formula (2) depending on number of 180°

domains, n, or number of walls, i = n - 1, within the single grain (this grain is shown in lower right corner).

gle multidomain grain, figure 4, can qualitatively answer this question :

where M, : saturation magnetization, o : wall energy per surface unit, : mean grain diameter, n : number of domains and i = n - 1 : number of walls within the grain.

Although this model is very simplified, quite reaso- nable conclusions may be drawn from it. The factor in eq. (2), depending on wall number, increases quickly with i as is shown in figure 4.

If one supposes I to be a half of D, and grain to

have i = 2 instead of one wall

-

the permeability will be 21

%

smaller than for I = D and i = 1 ; but if for the same I = 0.5 D one takes i = 3

-

permeability will increase by 11

%.

However, for I = 013 and i = 3

-

permeability will decrease by 23

%.

Thus it can be concluded that in a grain containing defects able to pin the walls, these defects do not generate a considerable number of domains and, consequently, of walls. (Assumption that p decrease is caused by the occupation of some grain volume by

created closure domains with hardly movable walls does not change this conclusion.)

Now we intend to draw attention to the meanning of ha,. It is a measure of an imaginary isotropic state after the magnetic undirectional (i. e. imaginary) neutralization. p, is a measure of the also isotropic state but resulting from real TN. Thus Ap = p,

-

paV is a measure of a pure difference in isotropic TN and MN states ; it is due only to the grain defectiveness and excIudes unavoidable AD effects (these should also be expected in a monodomain grains). Thus Ap, if attai- nable, may be used in qualitative detecting of grain defectiveness already in the range of Globus' law, i. e. where it is not yet expected. Thus the troublesome microscope procedure may be avoided and the result is obtained by magnetic way.

The experimental values of DNS for MNII, MNT and MN,, states are shown in figure 5.

FIG. 5. - Differences in the neutral states, DNS, of the cubes examined as a function of mean grain diameter 5.

(DNS,, = (1.1,

-

pav)/pav = A,u/pav is the relative value of Ap). For the

D

< 1 pm DNS = 0 what corres- ponds to D-range where only AD-effects occur ; for

-

D

>

1 ym, DNS,, > 0, the macrostructural defects are expected to be present within the grains. For

-

D > 2 pm the grains are strongly defected. As it can be seen from figure 5 all these results, expressed as the DNS(D) dependences, are very distinct.

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C1-34 T. POSTUPOLSKI AND A. WISNIEWSKA

DNS,, does not reach the zero value for the lowest D ing Co ferrite reveal that the situation for p, > p,,

values what may be caused by the aforementioned may occur which is now being studied. This indicates additional domain continuity anisotropy. that further and more extensive investigations in this

The latest results obtained by us for Ni-Zn contain- field are desirable.

References

[I] SNOEK, J. L., FAST, J. F., Nature 4101 (1948) 887. LOWSKA, M., Proc. of 5th Conf. on Ceramics in Elec-

121 IEC Publication 50 (901), 1st edition (1973), Section 901- tronics, 22-24. April 1974, Liblice, CNRS.

02. [7] BARBIER, J. C., FERLIN-GUION, B., J. Appl. Phys. Suppl. 33

[3] GLOBUS, A., DUPLEX, P., Phys. Stat. Sol 31 (1969) 765. (1962) 1226.

[41 K I ~ E L , Ch., Rev. Mod. Phys. 21 (1949) 541. [8] VERGNE, R., PORTESEIL, J. L., BLAZEK, Phys. Stat. Sol. (a)

25 (1974) 171.