A NEW FERRITE MATERIAL FOR VIDEO RECORDING HEADS

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Submitted on 1 Jan 1977

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A NEW FERRITE MATERIAL FOR VIDEO

RECORDING HEADS

H. Löbl, P. Neusser, M. Zenger, J. Frey

To cite this version:

H. Löbl, P. Neusser, M. Zenger, J. Frey. A NEW FERRITE MATERIAL FOR VIDEO

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JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 4, Tome 38, Auril 1977, page Cl-345

A NEW FERRITE MATERIAL FOR VIDEO RECORDING HEADS

H. LOBL, P. NEUSSER, M. ZENGER and J. FREY Siemens AG, Miinchen, Unternehmensbereich Baulernente, R. F. A.

Rhumb.

-

Une nouvelle technique modifik de pressage a chaud permet la fabrication de ferrites de Mn-Zn presentant une structure granulaire homogkne, un diametre moyen de grain ajustable entre 0,l et 1 mm et une porosite infkrieure a 0,l %. La mEme composition preparee par la technique ordinaire de pressage chaud fournit un diamktre de grain de 10 A 20 pm seulement. Le materiau B gros grain, qui posskde des joints de grain d'une forme sinueuse particuliere, prksente

une permkabilite aux basses frequences p" plus faible mais montre des pertes moins Clevks et une permkabilite plus grande dans une gamme de frequences atteignant 10 MHz. Les avantages de ce materiau pour les applications h I'enregistrement video sont discutks.

Abstract.

-

A new modified hot-pressing technique enables the fabrication of Mn-Zn ferrite materials with a homogenous grain structure, an adiusted average grain size of 0.1 to 1 mm and a porosity lower than 0.1 percent. The same composition prepared by a normal HP technique shows a grain-size of only 10 to 20 lm. The large grain material, which possesses a peculiar snaky form of grain-boundaries, has a lower permeability p' at low frequencies but shows lower losses and a higher permeability at frequencies in the range up to 10 MHz. The suitability of this material for video-recorder application is discussed.

1. Introduction. - Hot-pressed ferrites are beside l pressure

single-crystals of great interest for the fabrication of - - - -Alumina punch

magnetic recording-heads, according to their good thermocouple -- 1

mechanical properties based on their high density .-- - -;/Sic punch

and their being free of pores [I, 21. / /

Within the last decade no information has been

-- - S I C die

given, that shows a significant superiority of single-

crystals to hot-pressed ferrites, and in this situation the - - - ferrite specimen

decision for the one or the other material can be _-furnace

influenced by the complexity of the fabrication- technique, which is important to the price. The physical main difference between the two types of material is the presence or absence of grain-boundaries, and it was the

aim of our investigations to find hot-pressed bodies - --hydraulic r a m

with very large crystals, in the hope to combine some

useful properties in one material [3]. FIG. 1. - Hot-pressing apparatus. 2. Experiments and results. - The ferrite investi-

gated has a composition of 52,5 rnol

%

Fe203, 25 mol

%

MnO and 22,5 mol

%

ZnO.

This composition lies within the region wherein highest permeabilities can be reached. Depending on the sintering conditions it shows a secondary permeability maximum near room temperature.

With the mentioned composition various experiments concerning the hot-pressing methods have been made. The apparatus used is shown schematically in figure 1. It consists of an electric furnace and an inserted die of Sic. The punches consist of several parts - in the region near the sample they are made of Sic. I n the regions more outside other ceramic materials with lower thermal conductivity based on Al,03 are used for the punch.

Substantial for a good hot-pressing result is the preparation of the ferrite-powder. It is necessary t o choose a high prefiring-temperature, which has the effect, that at the chosen hot-pressing temperature n o sintering takes place without application of pressure at least in comparable times.

The prefired material is ball-milled and isostatically pressed to cylindrical samples, which are inserted into the hot-pressing apparatus.

The essential steps of the hot-pressing process :

Slowly heating in air to 1200

...

1 250 OC without pressure. After a thermal equilibrium is established, pressure is applied growing from 0 to 5 000 N/cm2 within 5 minutes. We used a sintering time of about

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2 hours, although the final density is reached earlier.

Figure 2 shows the density dependent on soaking-time. Cooling is performed in air without pressure.

hot-press parameter 1220°C 500 b a r

.

powder l hour at 900°C pre-fimd S powder l hour at lOOOOC

and l hour at 1200T

.+-

0 l 2 3 L 5 /.- 1Om1n 60

FIG. 2. - Density of hot-pressed ferrites as a function of soaking time.

The material obtained (we call it A) has an average grain-diameter from 20 to 40 pm. Figure 3 shows its permeability versus temperature curves before and after annealing at 1 100 OC and 2 hours The increase of

rc

is due to the decrease of mechanical stresses.

20 000-

after annealing

before annealing

figure 4. Like in the normal process, heating is performed without pressure to a first temperature 6,. Here pressure is applied and a thermal equilibrium is established. After this the temperature is slowly raised to a second value d,, while pressure is still applied. Cooling is done without pressure.

pressureless

i

hot -pressing cooling '

I without pressure

. . 1 . .-P

FIG. 4. - Temperature-time-diagram of hot-pressing-cycle in order to obtain uniform large-grain-growth.

It was found, that by this method grain-sizes up to

1 mm can be reached. Decisive for the grain-size is the temperature interval A6 = 6, - 6,, which is passed through with pressure applied. Figure 5 gives a grain-

L A b I d

-- 50 0 50 1m OC 0 10 20 30 -C

'

3

;

-

O

"

1009:

9 --c A9 --t

FIG. 3. - ~(6)-curves of material A (grain diameter 20

...

30 pm) FIG. 5.

-

Grain-size versus A 6 ; A6 = temperature interval before and after annealing. with pressure applied.

The other important magnetic properties of material A will be discussed in comparison with our new material B. As already mentioned we wanted to get larger grains in a hot-pressed material. One reason was the idea, that grain-boundarier in the contact surface of the magnetic head with the tape can act as parasitic gaps with the writing-in process and in this way lower the signal to noise ratio f4, 51.

Within the investigations concerning the grain-size, it proved to be impossible to increase the grain-size by increasing pressure, time or temperature of the sintering process. The only way to get a strong and uniform grain-growth is shown in the diagramm in

size versus Ad-diagram. Fixed parameters are the end- temperature 6, = 1 350 OC and a time of 3 hours in

which AS is passed through. Figure 6 shows the micro- structure of examples of the materials A and B.

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A NEW FERRITE MATERIAL FOR MDEO RECORDING HEADS 61-347

FIG. 6.

-

Micros~ructures or materials A and B. I I

Ql Q.2 a5 7 2 5 loMHz

f-

Another result is shown in figure 7. The perrneabi- FIG. 8.

-

hrnplex permeability versus frequetlcy of material A lity of the samples made by using the methods above and B.

mentioned decreases with grain-size

-

an effect, which Ikeda er al. suppose to be dependent on special forms

FIG. 7.

-

Permeability versus grain-size for material of type 8

o f boundaries, the so-calIed snaky boundaries 161.

Contrary to material A the p-values cannot be influenc- ed by annealing I. e. their low values are not simply

caused hy mechanical stresses due to the fabrication process. It can be of great practical advantage, if the properties of a head material will not be jnfluenced by

machining or by heating during thc glass-bonding process.

The complex permeability versus frequency curves of material A and B are shown in figure 8. They show in respect to p a rather normal khaviour - the stronger inner,fidd [TI, which causes the decrease in p

yields a shift of the loss maximum or the region of the pilecrease to higher frequencies. This p-decrease combined with a frequency-shift can be a useful compromise in respect to the working-frequency- range of a video-recordi ng-head.

To get an idea how the compIex permeability influences the behaviour of a recording-head, we made some calculations based on the theory of Ieakdge reactance transformers. Figure 9 demonstrates the equivalent circuit on which the estimation is based.

recording b e uivalent c) equivalent

head k~akage[reacto~e) e l e d r ~ c

transformer circuit

FIG. 9.

-

Magnetic recording hcad and i t s equivalent electric

circuil.

The

recording-head (a) with an impressed flux-change

on

one edge of the gap can be seen as a leakage reac- tance transformer with an impressed voltage into a primary winding (b). The third part of the picture

(c) sives the equivalent electric circuit where the magne-

tic leakage is presented by a series inductance, the

magnetic loss by a parallel resistance. Since p' and p" are measured by comparison with series inductances and resistances one

must

calculate the respective values

of a parallel circuit for each frequency. As a result of

these calculations one can expect in the range of 5 MHz

a signal voltage about 50

%

higher

by

using material

B

Physical Properties of Materials A B

-

Initial Permeability

*

pl 3000 Coercive Force Hc 7 Curie Temperature 9, > 130 Flux Density cat H = 3 0 0 0 A j m ) B 390

Resistivity Q 1

Density 5120

Porosity <0.1

Yickers Hardness

HV

50 8000

Average Grain Size 20

* before annealing

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than material A, that means, that material B is better is its grain-structure, and our conjectures mentioned matched to video-head-application. are about to be tested in practical use.

But at the end of this paper, it must be repeated, The last figure 10 presents a table of the essential that the most interesting feature of the new material data of the two materials considered in this paper

References

[ l ] SUGAYA, H., IEEE Ti-ans. Mugn. Mag-4 (1968) 295. [5] WATANABE, H., YAMAGA, I., IEEE Trans. Magn. Mag-8 (1972)

[2] HIROTA, E., MIHARA, T., IKEDA, A., CHIBA, H., IEEE 497.

Trans. Magn. Mag-7 (1971) 337. [6] TKEDA, A., SATOMI, M., CHIRA, H., HIROTA, E., Ferrites :

[3] ITHO, S., Radio Mentor 6 (1972) 279. Proceedings of the International Conference, Japan 337 [4] MONFORTE, F. R., CHEN, R., BABA, P. D., IEEE Trans. (1970).