recording experiments with multi-track thin-film heads

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IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-18, NO. 6, NOVEMBER 1982 RECORDING EXPERIMENTS WITH MULTI-TRACK THIN-FILM HEADS E.L.M. Raemaekers ABSTRACT Since high coercive tapes are used for short wavelength recording, both heat dissipation and saturation of the magnetic circuitmay cause problems in thin-film recording heads because of the required writing current. Experiments carried out to determine the optimum dimen- sions of our 64-channel single-turn heads will be described. It is con- cluded that theseheadsaresuited for recording tapes havingcoer- civities up to 80 kA/m (1000 Oe). INTRODUCTION In a fixed head digital recorder we can apply separate multi-track thin-filmrecordingandreproducing [l] heads.Thispaperdescribes experiments carried out to determine the influence of the Inte- grated Recording Head (IRH) dimensions on recording performance. Since the required recording field in front of the IRH is propor- tional to the tape-coercivity H,, we need high writing cdrrents. However, the writing current in an IRH is restricted owing to the heat dissipa- tion. Saturation of the magnetic circuit will set upper bounds to the recording field. Therefore, the investigation aims at achieving the optimum recording quality of short wavelengths at a minimum writing current. In an early stage we decided to apply a single turn in our IRH since the efficiency of the magnetic circuit of a multi-turn IRH is lower owing to the magnetic circuit length: The writing current decreases less than inversely proportional to the number of turns. Also from a technological point of view it is advantageous t o make a single-turn IRH. The followingparameters will be discussed: (fig. 1) the track width W, the distance between the polished head surface and the turn: GH, the gap length: G, the choice of the substrate, the pole-tip length P and heat dissipation. Fig. 1. Schematic view of thin-film head. 1143 In all the experiments usehasbeenmadeofmodifiedmulti- -trackheadswithinwhichonlythequantity to be investigatedwas varied, so that otherinfluences owingto technolorn and encapsulation remained restricted to a minimum. Unless otherwise stated, tape with a coercivity H, of 50 kA/m (625 Oe) was used and sinusoidal writing currents are applied without high frequency bias currents. No difference in recording performance can be measured when the direction of the tape transport is reversed. 1. TRACK WIDTH Special multi-track heads with varying track widths have been made in order to determine the influence of this quantity on the recording. Table I shows that the track width can be decreased to 10 pm without any deterioration of the recording performance if broadened back cir- cuits (fig. 1, channel 2) are applied for the narrow track widths. TABLE I Normalized (with respect to the track width) output P = 4 pm, G = 1 pm, wavelength ~4.75 pm, track width (pm) decrease (dB) track width* decrease (Pm) (dB) 600 0 200 0 100 50 - 0.3 30 20 50 0 30 20 10 - 0.9 - 2.5 * These tracks have broadened backcircuits. 2. GH We found that GH, the distance between the polishedheadsurface and the turn is an important parameter. It is the only quantity of the head that is not determined by the thin-film technology but by the encapsulation. In order to find the place down to where the head has to be polished the influence of GH has been determined experimen- tally as follows: A 12 mm wide,64-channelheadwasbevelled so that GH differed slightly from channel to channel. Fig. 2 shows the sum of high fre- quency bias current and signal current required for recording a 50 pm wavelength signal at aprox. 100 pWb per mm trackwidth (10 mM/mm) as a function of GH. For everychanneltheoptimum of the high- -frequency bias current was determined at a low signal current level. After that, the signal current was so increased that the 100 pWb/mm signal level was measured at thereproducing head Owing to the importance of low writing currents in connection with heat dissipation and saturation of the magnetic circuit, a GH of 5 pm was chosen in all followi,ng experiments. In order to be cer- tain that wear and tear will not shorten thelife of the head in a serious measure, a continuous test was carried out: 1000 hours at 9.5 cm/s (CrOz)-tapespeed.Froman electrical point of view no decay was observable, while the reduction of the gap height was less than 0.2 pm. Manuscript received July 22, 1982 The author is with Philips Research Laboratories P.O.B. 80.000, 5600 JA Eindhoven, The Netherlands 0018-9464/82/1100-1143$00.75O1982 IEEE

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IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-18, NO. 6 , NOVEMBER 1982

RECORDING EXPERIMENTS WITH MULTI-TRACK THIN-FILM HEADS

E.L.M. Raemaekers

ABSTRACT

Since high coercive tapes are used for short wavelength recording, both heat dissipation and saturation of the magnetic circuit may cause problems in thin-film recording heads because of the required writing current. Experiments carried out to determine the optimum dimen- sions of our 64-channel single-turn heads will be described. It is con- cluded that these heads are suited for recording tapes having coer- civities up to 80 kA/m (1000 Oe).

INTRODUCTION

In a fixed head digital recorder we can apply separate multi-track thin-film recording and reproducing [l] heads. This paper describes experiments carried out to determine the influence of the Inte- grated Recording Head (IRH) dimensions on recording performance.

Since the required recording field in front of the IRH is propor- tional to the tape-coercivity H,, we need high writing cdrrents. However, the writing current in an IRH is restricted owing to the heat dissipa- tion. Saturation of the magnetic circuit will set upper bounds to the recording field.

Therefore, the investigation aims at achieving the optimum recording quality of short wavelengths a t a minimum writing current. In an early stage we decided t o apply a single turn in our IRH since the efficiency of the magnetic circuit of a multi-turn IRH is lower owing to the magnetic circuit length: The writing current decreases less than inversely proportional to the number of turns. Also from a technological point of view it is advantageous t o make a single-turn IRH.

The following parameters will be discussed: (fig. 1) the track width W, the distance between the polished head surface and the turn: GH, the gap length: G, the choice of the substrate, the pole-tip length P and heat dissipation.

Fig. 1. Schematic view of thin-film head.

1143

In all the experiments use has been made of modified multi- -track heads within which only the quantity to be investigated was varied, so that other influences owing to technolorn and encapsulation remained restricted to a minimum. Unless otherwise stated, tape with a coercivity H, of 50 kA/m (625 Oe) was used and sinusoidal writing currents are applied without high frequency bias currents.

No difference in recording performance can be measured when the direction of the tape transport is reversed.

1. TRACK WIDTH

Special multi-track heads with varying track widths have been made in order to determine the influence of this quantity on the recording. Table I shows that the track width can be decreased to 10 pm without any deterioration of the recording performance if broadened back cir- cuits (fig. 1, channel 2) are applied for the narrow track widths.

TABLE I

Normalized (with respect to the track width) output P = 4 pm, G = 1 pm, wavelength ~ 4 . 7 5 pm,

track width (pm)

decrease (dB)

track width* decrease (Pm) (dB)

600 0 200 0 100

50 - 0.3 30 20

50 0 30

20 10 - 0.9

- 2.5

* These tracks have broadened backcircuits.

2. GH

We found that GH, the distance between the polished head surface and the turn is an important parameter. I t is the only quantity of the head that is not determined by the thin-film technology but by the encapsulation. In order to find the place down t o where the head has to be polished the influence of GH has been determined experimen- tally as follows: A 12 mm wide, 64-channel head was bevelled so that GH differed slightly from channel to channel. Fig. 2 shows the sum of high fre- quency bias current and signal current required for recording a 50 pm wavelength signal at aprox. 100 pWb per mm trackwidth (10 mM/mm) as a function of GH. For every channel the optimum of the high- -frequency bias current was determined at a low signal current level. After that, the signal current was so increased that the 100 pWb/mm signal level was measured at the reproducing head

Owing to the importance of low writing currents in connection with heat dissipation and saturation of the magnetic circuit, a GH of 5 pm was chosen in all followi,ng experiments. In order to be cer- tain that wear and tear will not shorten the life of the head in a serious measure, a continuous test was carried out: 1000 hours at 9.5 cm/s (CrOz)-tape speed. From an electrical point of view no decay was observable, while the reduction of the gap height was less than 0.2 pm.

Manuscript received July 22, 1982

The author is with Philips Research Laboratories P.O.B. 80.000, 5600 JA Eindhoven, The Netherlands

0018-9464/82/1100-1143$00.75O1982 IEEE

1144

L

0 L 8 12 16 20

Fig. 2. Writing current as a function of GH. P = 4 pm, G = 1 pm,

3. GAP LENGTH

Within one 64-channel IRH the gap length was varied between 0.3 pm and 2 pm. Table I1 shows for three wavelengths how the reproducing signal and the writing current depend on gap length.

As our experiments show, short wavelengths can best be re- corded with short gap-length heads. Moreover, the writing current is then low. Yet we prefer 0.5 or 0.7 pm gap lengths, because of the deterioration of the long wavelength response when the writing current is adjusted optimally for a 1 pm wavelength.

TABLE I1

Normalized (0 dB : maximum reproducing signal level recorded by a

three wavelengths. P = 4 pm, W = 140 pm.

wavelength (pm)

G = 0.3 pm head) recording performance and optimal writing current for

1 2 5 G

pm I(mAp) V(dB) I(mAp) V(dB) I(mAp) V(dB)

2 210 - 4 280 - 1 350 + 6 1 200 - 1.8 245 - 1 280 + 2 0.7 175 - 1.4 200 - 1 225 f 2 0.5 120 - 0.3 200 0 22 5 f 2 0.3 120 0 180 0 200 0

4. SUBSTRATE AND FIRST MAGNETIC LAYER

The choice of the substrate appeared to be of importance to the recording performance: A head made on silicon (fig. 3a) andconsisting of two 4 pm NiFe layers as a magnetic circuit records a 1.5 pm wave- length signal 3 dB better (at 280 mAp writing current) than a head of which a NiZn-ferrite substrate is the one pole-tip and 4 pm NiFe the other (at 175 mAp writing current, fig. 3b).

Probably saturation occurs in the ferrite pole-tip, leading to widening of the magnetic gap. Therefore we cover the ferrite substrate (4 n M, = 0.4T) with a 0.5 pm NiFe layer (4 H M, = 1 T, fig. 3c). This head indeed combines the good recording properties of the head of fig. 3a with the lower writing current of the head of fig. 3b.

Fig. 3 . Schcmatic view of thin-film hcads on silicon and ferrite substrates.

5. POLE-TIP LENGTH

Table 111 shows to what extent the recording performance of a head having a pole-tip length P of 1.5 pm decreases as compared with that of a head with a P of 4 pm. The required writing current in the case of P = 1.5 pm is 300 mAp for all wavelengths while no optimum in the current occurs for the shortest wavelengths. This can be explained by saturation of the magnetic circuit.

TABLE I11

Decrease in the recording performance of a head having P = 1.5 pm with respect to a head having P = 4 pm. Both heads have W = 140 pm, G = 1 pm.

wavelength (pm) decrease (dB)

2 4 8

16

- 3.0 - 6.0 - 6.9 - 7.3

6. HEAT DISSIPATION

In thin-film heads heat dissipation may offer problems in two ways: 1. When a single channel is used, the recording properties may dete-

riorate owing to the high writing current. In order to investigate this, an off-size head (see sections 3 and 5) with a relatively large gap length (2 pm) and a short NiFe pole-tip length (2 pm) was used. The turn is made of Au and the thickness is approx. 1 pm. In fig. 4 curve A represents the reproducing signal of a 2.4 pm wavelength as function of the writing current. Curve B shows the erasing performance. In both cases a sinusoidal current is applied and a decrease of the head properties occurs at writing currents above 300 mAp: The maximum erasing effect is only - 23 dB. However, using a pulsating erasing current (duty cycle 1 : IO), the erasing effect is improved to - 36 dB, curve C. Ob- viously, heat production is the cause of the deterioration. By measuring the increase in the winding resistance [2] it was possible to calculate that its temperature is approx. 130°C a t a sinusoidal current of 300 mAp (curve D) so that the Curie temperature of NiZn ferrite is reached [ 3 ] and the magnetic gap widens.

2 . In the case of simultaneous application of sinusoidal currents through all channels of an IRH, the heat dissipation may result in an intolerable increase in temperature. This is prevented by sending the pulsated currents (see section 6.1) through the chan- nels one by one [4]. The pulse width of the writing current can be decreased down to 0.5 ps so that 20 channels can be used within a 10 ps bit period, giving a total bit stream of 20x100 kbit/s = 2 Mbitls.

1145

ACKNOWLEDGMENT

The author wants to thank L. Postma, A. Bakens and A. Courage for preparing the special wafers and the WA-Head Workshop for careful encapsulating the recording and reproducing heads. He is indebted to Dr. J. de Wilde and Dr. W.F. Druyvesteyn for numerous stimulating discussions.

REFERENCES

1. W.F. Druyvesteyn et at., IEEE Trans. Mag. MAG 17, 2884 (1981). 2 . Gerard Somers, private communication. 3. Philips Data Handbook, Soft Ferrites CM 4a, 5 1 (1 1-78). 4. 'W.J. van Gestel, L.M.H.E. Driessen, J.C.F. Moeskops, AES pre- - I IrnAp) print 1832 (1-4), presented at the 70th Convention, 1981

0 100 200 300

[:is, 4. I<ccording pcrformancc and tcmperature of the turn, as a October 30 - November 2, New York. function o f thc writing currcnt.

7. HIGH COERCIVE TAPE

Fig. 5 shows the frequency response of some recording heads. The signals were reproduced by a yoke magnetoresistive head [l] having a gap length of 0.3 pm and a track width of 125 pm, narrower than any recorded track in this experiment. The writing currents are adjusted for a maximum 1 pm wavelength response. If we use an IRH as dis- cussed in section 4, fig. 3c, on Metal-tape (H, = 80 kNm, 1000 Oe) instead of CrOz tape (H, = 50 kA/m, 625 Oe), the gain in the signal a t short wavelengths is 4 dB. The same factor is found for the ratio of 60th tape-coercivities and required writing currents indicating the absence of saturation of the magnetic circuit.

- Wavelength iprnl

L O - -- fig 3c,H,=50kA/rn,I=175 mAp. - - - ferrlte head,g =05prn

v=9.5crn/s,erased tape

Af = 300 Hz

''0 1 0 2b 30 LO SO 60 7'0 60 90 Id0 - Frequency l kHz l

Fig. 5 . I<cproducing signal o f a Yoke-Magneto-Resistive Head (Sensing currcnt: 21 mA, rcsistance: 45 1;2) measured at 9.5 cmls .

CONCLUSIONS

Single-turn Integrated Recording Heads are suited for recording short wavelengths (1 pm) on tapes having coercivities up to 80 kAlm (1000 Oe).

Heat dissipation in the multi-track heads is prevented by duty cycling of the writing current, while 0.7 pm gap length, 5 pm gap height and 4 pm pole-tip length are the optimal head dimensions.

The track width can be decreased to at least 10 pm. A 3 dB improvement of the recording performance was made possible by covering the ferrite substrate with a 0.5 pm NiFe layer.