Electrodeposition of Highly Functional Thin Films for Magne

Electrochimica Acta 45 (2000) 3311–3321

Electrodeposition of highly functional thin films formagnetic recording devices of the next century�

Tetsuya Osaka *Department of Applied Chemistry, School of Science and Engineering,

Kagami Memorial Laboratory for Materials Science and Technology, Waseda Uni6ersity, 3-4-1 Okubo, Shinjuku-ku,Tokyo 169-8555, Japan

Received 23 November 1999; received in revised form 15 January 2000

Abstract

A review is presented of the recent progress in research and development of soft magnetic films for magneticrecording heads of the future primarily on the basis of the work performed by the author’s research group. Films ofCoNiFe ternary alloy with high saturation magnetic flux density BS and low coercivity, HC were successfully producedby electrodeposition. A typical film, designed as ‘HB-CoNiFe’, had the composition of Co65Ni12Fe23 (at.%) withBS=2.0–2.1 T and HCB2 Oe. Properties other than BS and HC were also investigated; namely, magnetostriction, lS,corrosion properties, and film resistivity r. The key to the success in obtaining low HC with high BS was to form filmwith very fine crystals. The inclusion of small amount of sulfur was found to be essential for producing such a filmwith the desired magnetic properties. The film has been applied to the construction of a new type of merged-GMRhead, which is considered as a breakthrough for materializing ultra high-density magnetic recording. © 2000 ElsevierScience Ltd. All rights reserved.

Keywords: Electrodeposition; Soft magnetic film; High BS film; High r film; CoNiFe alloy

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1. Introduction

Fig. 1 shows how rapidly the areal density of harddisk drives (HDDs) has been increasing over the past15-year period. Several critical innovations were neces-sary to bring about such a rapid progress in the field ofmagnetic recording. One of the most significant innova-tions from the viewpoints of material improvement isthe electrodeposition of permalloy (Ni80Fe20), whichwas introduced by IBM in 1979 as the core material ofa thin film inductive head to increase the magneticrecording density [1]. After the introduction by IBM in1991 of the magneto-resistive (MR) element as read

head and the electrodeposited permalloy as a writehead [2], the speed of increase in the recording densityof HDD jumped from 10 times per decade to 100 timesper decade. In ultra-high density recording of the fu-ture, issues such as the saturation of the write head coreand the thermal relaxation of recording medium areexpected to become serious. For magnetic recordingmedia some new proposals have been made to alleviatethe problem of instability. The perpendicular magneticmedium is one of the candidates, and presently it offersthe possibility for solving the thermal stability [3–6].

For the write head core material, both saturationmagnetic flux density (BS) and specific resistivity (r)must be increased. Recently, the permalloy with thecomposition of Ni45Fe55 has been in use instead ofNi80Fe20, although corrosion resistance of the lattermaterials is insufficient [7]. However BS and r, values

� Pergamon Medal Award Lecture* Tel.: +81-3-52863202; fax: +81-3-32052074.E-mail address: [email protected] (T. Osaka)

0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0013 -4686 (00 )00407 -2

T. Osaka / Electrochimica Acta 45 (2000) 3311–33213312

Fig. 1. Trend of increase in recording density of hard diskdrives (HDDs) during the past 15 years.

Fig. 3. Diagram showing regions of composition with lowcoercivity (HCB2) for CoNiFe thin films electrodepositedfrom the baths containing various additive; A, saccharin bath;B, thourea bath; C, no-sulfur-containing additive (SCA) bath.

of permalloy films (BS=0.9–1.0 T and r=20–25 mVcm for Ni80Fe20, and BS=1.5–1.6T and r=40–45 mVcm for Ni45Fe55) are still unsatisfactory.

Many different electrodeposited films have been re-ported to possess high BS values; for example, CoFewith BS=1.8–1.9 T [8–11], CoNiFe with BS=1.6–1.8T [12–15]. Sputtered films have also been reported toposses high BS value; e.g. BS=1.4–1.7 T for FeSiB-CuNb and Fe-(Ta,Hf,Nb, etc.)–(N,C,B,O) [16–18],and ‘very high BS’ for FeN with [19–21]. As a processfor fabricating merged MR head, however, an electro-chemical process is preferred.

This presentation reviews primarily the contributionin the area of research and development of electrode-posited soft magnetic film for MR head of next century[22,23].

2. Soft magnetic materials

2.1. Magnetic properties of new CoNiFe film

The composition-BS diagram for CoNiFe bulkternary alloy published by Bozorth [24] is reproduced inFig. 2. Many high BS materials with composition nearFe60Co40 are known, but they do not possess low HC

values. Thus, it was a challenge for one to create thehigh BS (\2 T) CoNiFe soft magnetic films by elec-trodeposition. The diagram of Fig. 3 shows composi-tional regions of low HC of the CoNiFe film which wasprepared by electrodeposition. The region of BS\1.8 Ton electrodeposited film was found to be located inapproximately the same area as that shown in Fig. 2 forthe bulk material. This area is apparently determinedby relative concentrations of the three elements consti-tuting the material. It is seen from Figs. 2 and 3 thatthe B and C regions are located in the Co-rich area withBS\1.8T, whereas the A region is located in the low BS

area.The baths used to generate A and B regions in Fig. 3

contain saccharin [25] and thiourea [26,27], respectively,which caused codeposition of a small amount of sulfurin the films, whereas the bath used to form the films inthe C region does not contain such a sulfur-containingadditive (hereafter designed as SCA). The compositionand operating conditions of the baths used are listed inTable 1. The films deposited in the saccharin andthiourea baths contained sulfur of 0.35 and 0.9 at.%,respectively, whereas the sulfur content of the film from

Fig. 2. Diagram showing region of composition with highsaturation magnetic flux density for bulk CoNiFe ternary alloypublished by Bozorth [24].

T. Osaka / Electrochimica Acta 45 (2000) 3311–3321 3313

Table 1Basic bath composition and operating condition for electrode-posited CoNiFe soft magnetic films

Chemicals Concentration(mol dm−3)

0.03–0.0875CoSO47H2O0.2NiSO46H2O

FeSO47H2O 0.005–0.0450.4H3BO3

NH4Cl 0.280.01Na laulyl sulfate (g dm−3)

Sulfur containing additive (SCA)a 0–10(g dm−3)

2.8Bath pHBath temperature Room temp.Counter electrode Pt wire

3–20Current density (mA cm−2)

a SCA; saccharin (as Na–salt) 0–10 g dm−3; thiourea 0–0.06g dm−3.

magnetic film can be prepared only by electrodeposition[29,30]. The mixed phase lines are clearly dependent onthe trace amount of sulfur inclusion. The film electrode-posited with no-SCA gave the fcc–bcc mixed phase linein the highest BS region.

It is important to note that lS of the film to be usedfor fabricating the magnetic head core should be lowbecause magnetic field does not produce stress in suchfilms whose lS value is nearly equal to zero. In addi-tion, the characteristics of lS of the electrodepositedfilms are generally different from those of the filmsprepared by other thin film techniques such as sputter-ing and quenching. Fig. 6 shows the zero magnetostric-tion lines for the three electrodeposited CoNiFe filmsand the quenched CoNiFe alloy [28]. The trend for thequenched alloy, which is described in Ref. [28] is quitedifferent from that of the electrodeposited films. Thethree electrodeposited films exhibited almost the sametendency as shown in Fig. 6. The zero lS line shifts tothe region of lower Fe contents with an increase in Scontent. Namely, the film with low lS shifts to thehigher BS region with a decrease in S content. Theresults show that the zero lS line of the CoNiFe filmfrom the bath with no-SCA is located near the low HC

and high BS region. Thus, one of the most suitable filmshas a composition of Co65Ni12Fe23 (at.%) with high BS

(2.1 T), low HC (1.2 Oe (95 A m−1)), low lS (1.8×10−6) and usual r (21 mV cm). This film is designatedas ‘HB-CoNiFe’.

It is clearly seen from in Figs. 4 and 6 that theinclusion of a small amount of sulfur in the CoNiFefilm drastically affects the formation of fcc–bcc mixedphase and the location of the zero lS line.

no-SCA bath was less than 0.1 at.%. The sulfur analysiswas initially performed by the TEM–EDX and EDXmethods to make relative comparison between manysamples prepared under widely different conditions.The absolute sulfur contents were determined later bythe conventional combustion method. It was found thatthe values obtained by the TEM–EDX and EDX meth-ods were approximately three times as large as thosefound by the combustion method. Therefore propercorrection was applied to the TEM–EDX values tocompare absolute sulfur contents on the normalizedbasis.

The lines indicating fcc–bcc phase boundary areshown in Fig. 4, in which line D for rapidly quenchedribbon was taken from Ref. [28], representing the caseof no sulfur inclusion. If the mixed fcc–bcc phase isassumed to consist of small crystallites, low HC filmshould form near the region of fcc–bcc phaseboundaries, because the formation of small crystallitesis one of the important factors leading to low HC. Infact, it has already been reported that an electrode-posited CoNiFe film with an especially low HC value of0.06 Oe (4.8 A m−1) contains small crystallites near theregion of fcc–bcc phase boundaries [14]. However,CoNiFe films with HCB2 Oe (159 A m−1) and BS\1.9 T or even any soft magnetic films with HCB2 Oe(159 A m−1) and BS\2.0 T, have not been reportedthus far. Fig. 5 shows bright field images of bcc singlephase, fcc–bcc mixed phase, and fcc single phase of thehigh BS CoNiFe films. With the fcc–bcc mixed phase,the low HC value was obtained. From these images, it isclear that the fcc–bcc mixed phase is formed with finecrystal grains compared with those of bcc or fcc singlephases. It is thus likely that the newly developed soft

Fig. 4. Bcc-fcc phase boundaries of electrodeposited CoNiFethin films deposited from the baths containing various addi-tives; A, saccharin bath; B, thourea bath; C, no-sulfur-contain-ing additive (SCA) bath. Line D for rapidly quenched ribbon[28] is shown comparison.


Fig. 5. TEM bright field images of electrodeposited CoNiFe thin films deposited from the no-sulfur-containing additive (SCA) bathwith various crystalline structures; C1, bcc structure; C2, bcc-fcc mixed structure; C3, fcc structure. THEED patterns are also shown.

2.2. Corrosion resistance of new CoNiFe film

Corrosion resistance of CoNiFe film was investigatedin a 2.5 wt.% NaCl solution [29,31,32]. The corrosionresistance should be considered as an indication ofdurability of the film material. Fig. 7 shows anodicpolarization curves of the HB-CoNiFe film, the elec-trodeposited CoNiFe film prepared in the saccharinbath, and the permalloy film electrodeposited from abath also containing saccharin. The conventional bcc-CoNiFe film had the same composition as the HB-CoNiFe film except for the inclusion of a small amountof sulfur in the former (CoNiFe, 0.3 at.% sulfur;permalloy, 0.1 at.% sulfur; HB-CoNiFe, less than 0.1at.% sulfur). Comparison of the polarization curves forthe two types of CoNiFe ternary alloy films shows thatthe anodic dissolution potential (or rest potential) ofthe HB-CoNiFe film, −870 mV versus SCE, is slightlymore cathodic than that of the permalloy and theCoNiFe films, −820 mV, whereas the pitting corrosionpotential of the HB-CoNiFe film is −65 mV, which ismuch more anodic than that of the bcc-CoNiFe film,−420 mV (pitting corrosion potentials are indicated bydotted lines in Fig. 7). Pitting corrosion potential ismore important in practical application than dissolu-tion potential. The pitting corrosion potential of theHB-CoNiFe film is even more anodic than that of thepermalloy film. The results show that the HB-CoNiFefilm has the most superior anti-corrosion propertyamong the three films tested, and that the pittingcorrosion potential is strongly dependent on the verysmall amount of S inclusion, even although the differ-ence in anti-corrosion property between the twoCoNiFe films with same sample composition results notonly from the difference in sulfur inclusion, but alsofrom the difference in crystal size of the films.

3. Basic research on the effect of sulfur containingadditives

Since the role of the SCA is so important in provid-ing such excellent properties to the CoNiFe film, thebasic investigation was performed to understand themechanism of inclusion of sulfur from SCA additives[33].

The mechanism of inclusion of impurity elements innickel films electrodeposited from the well-known

Fig. 6. Zero magnetostriction lines for electrodepositedCoNiFe thin films deposited from the baths containing variousadditives; A, saccharin bath; B, thourea bath; C, no-sulfur-containing additive (SCA) bath. Line D for rapidly quenchedribbon [28] is shown for comparison.


Fig. 7. Anodic polarization curves for electrodeposited softmagnetic thin films recorded in 2.5 wt.% NaCl solution; A,80-permalloy; B, bcc CoNiFe; C, HB-CoNiFe. The electrolytesolution was degassed by bubbling N2 for 5 min prior to use.Dotted lines show pitting corrosion potentials.

The first attempt to observe adsorbed saccharinmolecules was made by the author [33] by immersing afresh Au(111) electrode prepared by the anneal andquench method in an aqueous 0.1 mM (18 mg dm−3)saccharin solution for 10 min, followed by rinsing theelectrode thoroughly in pure water and transferring itinto the STM cell containing 0.05 M HClO4. AnAu(111) electrode was used because it is most com-monly used for EC-STM observations. In the potentialrange where no faradaic current flowed, only the struc-ture of Au(111) was observed, indicating that adsorbedsaccharin molecules, if any, had been removed from theelectrode surface during the water rinse. Thus, thesecond attempt was made by adding a small amount of0.5 mM (96 mg dm−3) saccharin directly into the STMcell containing 0.05 M HClO4 to make the final saccha-rin concentration equal to approximately 50 mM (9.6mg dm−3). The potential of the gold electrode was heldat 0.75 V versus RHE. Upon injection of the saccharin

Fig. 8. Coercivity and sulfur inclusion of electrodepositedCoNiFe thin films deposited from the sulfur-containing addi-tive (SCA) baths; A, saccharin bath; B, thiourea bath.

Watts bath containing saccharin, thiourea, and otheradditives was studied in detail during the period of thelate 1950s to the 1970s [34–47]. Because nickel is amajor constituent of the CoNiFe alloy, and also be-cause all three components of the alloy belong to thesame group in the Periodic Table of the Elements, theinteraction of the bath additives with CoNiFe can beanticipated to be similar, at least qualitatively, to thatwith pure Ni. In a recent research, in-situ EC-STM experiments were carried out with a singlecrystal Au(111) electrode on the basis of this consider-ation.

Effects of saccharin and thiourea concentrations onthe sulfur content and the coercivity of CoNiFe filmsare shown in Fig. 8A and B, respectively. With saccha-rin as the additive, the S content was essentially invari-ant at 0.390.05 at.% between the saccharinconcentrations of 2 and 10 g dm−3, where the HC valuewas also nearly constant at 4292 Oe (33209160 Am−1). With thiourea, on the other hand, the S contentincreased almost linearly with its concentrationbetween 0.005 and 0.06 g dm−3. The HC valueinitially decreased sharply with increasing thiourea con-centration, reaching a minimum of approximately2 Oe (160 A m−1) at the thiourea concentration of0.015–0.02 g dm−3. Further increase in thioureaconcentration caused a linear increase in HC atleast up to 20 Oe (1600 A m−1) at the thioureaconcentration of 0.06 g dm−3. The films obtained withsaccharin are not useful as a soft magnetic film due tohigher coercivity, whereas, the use of thiourea permitsthe formation of CoNiFe films of practical value[27,28].


Fig. 9. In-situ STM images of saccharin adlayer on Au(111)substrate in 0.05 HClO4 containing 0.05 mM (96 mg dm−3)saccharin. Applied potential, 0.75 V vs. RHE. Frame size: A,100×100 nm; B, 30×30 nm.

(molecules) at the edges of tightly packed domainsmoved, causing clear bright spots to become unclearand vice versa. These observations indicate that saccha-rin molecules adsorb only weakly on the Au(111)surface.

The STM observation of adsorbed thiourea was car-ried out under the conditions identical to those em-ployed in the first experiment with saccharin describedabove. Namely, after the Au(111) substrate was im-mersed in a 0.1 mM (7.6 mg dm−3) thiourea solutionfor 10 min, it was removed from the solution and rinsedthoroughly with pure water prior to transferring intothe STM cell containing 0.05 M HClO4. Unlike saccha-rin, adsorbed thiourea survived the water rinse andyielded the STM images reproduced in Fig. 10A and B,which were recorded at 0.65 V versus RHE. It is seenfrom Fig. 10A that the Au(111) terraces separated by

Fig. 10. In-situ STM images of thiourea adlayer on Au(111)substrate in 0.05 HClO4 containing 0.1 mM (9.6 mg dm−3)thiourea. Applied potential, 0.75 V vs. RHE. Frame size: A,100×100 nm; B, 30×30 nm.

solution, the image of the hexagonal atomic structureof bare Au(111) surface changed dramatically into thatof adsorbed saccharin molecules, which is shown in Fig.9A and B.

Fig. 9A shows that a large number of bright spotsare distributed over the entire terrace. Each bright spotis more clearly seen in the enlarged image of the area of30×30 nm2 in Fig. 9B. From the size of each spot (ca.0.8 nm) and the size of a saccharin molecule (see Fig.11A), it is clear that each bright spot in the STM imagecorresponds to an individual saccharin molecule. It isseen that the saccharin molecules are tightly packed onthe surface, but they do not form any ordered structure.Molecular defects and diffuse surroundings of thebright spots are seen especially clearly in Fig. 9B.During the imaging it was frequently seen that the spots


Fig. 11. Schematic drawings of molecular structures; A, sac-charin; B, thiourea.

4. Research on high r CoNiFe film

From the viewpoint of the rate of data recording, ahigh resistivity (r) of the head core materials is re-quired to decrease eddy-current loss. Electrodepositedsoft magnetic materials with high r have not beenstudied as much as high BS materials, but some at-tempts have been made to prepare soft magnetic thinfilms with high r values such as electrodeposited NiFe[49], NiFeMo [50,51], FeP [52,53], and NiFeP [54].There is no report on soft magnetic materials satisfyingcondition of both high BS and high r for the write-headof the high-density magnetic recording system of thefuture.

To develop a new soft magnetic thin film with bothhigh BS and high r, an attempt was made to increasethe resistivity of the electrodeposited high BS CoNiFethin film. It was recently found that an electroless NiPfilm with an extremely high r value of 5000 mV cm canbe deposited from a bath containing a complexingagent having –NH2 groups [55,56].

This finding led one further to an attempt to elec-trodeposit a the highly resistive soft magnetic film froma bath containing an organic compound agent having–NH2 groups as an additive instead of a complexingagent. With diethylenetriamine (DET) as an additive[49,54], the r value of the Ni80Fe20 permalloy film wasfound to increase from 20 to 60 mV cm. It has beenshown that a small amount of C incorporated in thefilm from the additive of DET increases the resistivityof the film, where the C content was determined with acombustion method.

The electrodeposited CoNiFe film made with noSCA, which exhibited higher performance as a softmagnetic film than permalloy as described in the pre-ceding section, was examined for the improvement ofresistivity with the DET additive in the deposition bath[57]. Fig. 12 shows the dependence of BS and HC on theDET concentration in the plating bath. The metalcomposition of all CoNiFe films was kept atCo56Ni13Fe31 (the deviation was less than 1 at.%). Thevalue of BS was maintained as high as 1.9 T the DETconcentration of up to 8.0 g dm−3, although it wasslightly lower than 2.1 T, which is the value in theabsence of DET. The value of HC was lower 2.5 Oe(180 A m−1) up to 8.0 g dm−3 of DET, but this valuerapidly increased with the addition of more than 8.0 gdm−3 of DET. Fig. 13 shows the dependence of r

value on DET concentration. The value of r graduallyincreased with DET concentration, and the film pre-pared from the bath containing 20 g dm−3 of DET hada high r value of 130 mV cm. These results show thatthe desirable soft magnetic CoNiFe thin films with ahigh r value of 90 mV cm can be deposited from thenew bath containing 8.0 g dm−3 of DET as anadditive.

monatomic steps 0.24 nm in height were covered withmany string-shaped substances. Those short pieces ofstrings are seen more clearly at an expanded scale inFig. 10B. Comparison of the dimensions of each pieceof string with the size of a single thiourea molecule,shown in Fig. 11B, seems to indicate that a single pieceof string represents a polymeric molecule consisting ofseveral, linearly combined molecules of thiourea. Al-though the string-shaped ‘polymer’ molecules areclosely packed, forming a layer, no ordered structure orregularity in molecular orientation was found with re-spect to the structure of the Au(111) surface. Thesefeatures were observed consistently in the potentialrange of 0.4–0.8 V versus RHE, suggesting thatthiourea molecules are strongly adsorbed on Au(111)surface. Furthermore, Fig. 10A and B show the pres-ence of many pits on the surface. Such pits were notfound on clean Au(111) surfaces. The depth of thesepits is almost equal to the height of monoatomic stepsexisting on the Au(111) surface, that is 0.24 nm. Carefulobservation of the inside of the pits revealed that thestring-shaped material existed also at the bottom of thepits, as can be seen in Fig. 10B. In view of the knownhigh stability of soluble Au(I)–thiourea complex,Au(CS(NH2))2

+ [48], the pit formation is likely to be aresult of the dissolution of gold to form this complexspecies.

To compare the size of saccharin and thioureamolecules, the schematic drawings of molecular struc-tures of the two molecules are shown in Fig. 11. Fromthe STM images shown results in Figs. 9 and 10, it itsclear that the additives of saccharin and thiourea actquite differently on the Au(111) electrode surface. It isconcluded that the saccharin adsorbs weakly and onlyphysically on the Au(111) surface, whereas thioureaadsorbs strongly to form chemisorbed polymerizedthiourea molecules.


Fig. 12. Magnetic properties of electrodeposited CoNiFe thinfilms as a function of diethylenetriamine (DET) concentration.

Fig. 14. Carbon content of electrodeposited CoNiFe thin filmsas a function of diethylenetriamine (DET) concentration.

was three times as large as the carbon content of thefilm prepared without DET. The desirable soft mag-netic CoNiFe film with a high r value of 90 mV cmprepared from the bath with 8.0 g dm−3 of DETcontained 0.24 at.% carbon. Therefore, it is consideredthat the increase of r value is attributable to the changein microstructure and the effect of impurity scatteringresulting from the co-deposition of carbon in the film.

Properties of the various electrodeposited CoNiFefilms are listed in Table 2. As a result of the frequencydependence of permeability of CoNiFe films with bothlow and high resistivity, the loss at higher frequencieswas clearly diminished with an increase in r [57].

Thus, the desirable property of the new CoNiFe thinfilm is attributable to the decrease of eddy-current lossresulting from the increased r value.

5. Application of new materials to fabrication of GMRhead

Fig. 15 shows an SEM image and a schematic of aGMR head with the high BS CoNiFe film as thewriting-head core material [58,59]. The core was fabri-cated with a 0.7 mm thick high BS CoNiFe film and thethicker permalloy films of 2.3 mm.

In order to clarify the mechanism of the increase in r

value, the crystal structure of the films prepared fromdifferent baths with or without DET additive was ana-lyzed. The X-ray diffraction profiles of bcc-Fe disap-peared as the DET concentration was increased. It wasthus confirmed that the crystallinity of the film pre-pared with DET was lower than that prepared withoutDET additive.

In addition, the film compositions of the films pre-pared from different baths with or without DET addi-tive were analyzed. Fig. 14 shows the dependence ofcarbon content on the DET additive concentration. TheCoNiFe thin film prepared without DET contained avery small amount of carbon, i.e. less than 0.1 at.%.The carbon content gradually increased as the DETconcentration was increased, and the film prepared with20 g dm−3 of DET contained 0.3 at.% carbon, which

Fig. 13. Resistivity of electrodeposited CoNiFe thin films as afunction of diethylenetriamine (DET) concentration.

Table 2Magnetic properties and resistivity of electrodeposited CoNiFesoft magnetic thin films with high BS and/or high r

CoNiFe CoNiFe+DET

0 8 20DET (g dm−3)1.7BS (T) 2.1 1.9

9025 130r (mV cm)2.31.2 5HC (Oe)


Fig. 15. SEM image and schematic illustration of GMR headstructure.

Table 3Write head dimensions and read head element

Type II Type IIIType I

P2CoNiFe locationa P1 and P2None1.7–2.5 1.5–1.8Write track widthb 1.1–1.2

(mm)0.35Write gap length 0.30 0.20

(mm)AMRc GMRdRead element AMRc

a P1 is for the bottom pole, and P2 is for the top pole.b Read track width is smaller than write track width by

0.2–0.4 mm.c AMR is MR element using anisotropy magnet-resistive

effect.d GMR is GMR element using giant magnet-resistive effect.

25 nm. A write current of 35 mA was enough to writein the media with an HC of 7.0 kOe (560 kA m−1). Thisis the champion data to be recorded in the highestcoercivity medium in the world. Fig. 17 shows themagnetic force microscopy (MFM) image of recordingpatterns in 7000 Oe medium. The magnetic patterns areclearly formed by the new type head using HB CoNiFefilm (Type III).

First the effect of CoNiFe location on overwriteperformance was examined. In this experiment, theflying height was about 35 nm, and the estimatedmagnetic spacing was 58 nm (i.e. spacing between thesurface of magnetic recording layer, not of mediumsurface, and the head surface). The write frequency forthe overwrite measurement was 60 MHz/10 MHz (HF/LF). The corresponding linear density for HF was 200kFCI. The write current was 35 mA0–p for Types I andII, and 50 mA0–p for Type III. The head properties areshown in Table 3.

An overwrite requirement (\26 dB) limits the maxi-mum medium coercivity that one can use. For Type Ihead, it was 3.0 kOe (240 kA m−1). For Type II head,it was 3.5 kOe (280 kA m−1). Furthermore, Type IIIhead wrote well in the media with an HC as high as 5.0kOe (400 kA m−1).

The effect of the flying height is summarized in Fig.16. By reducing the flying height to 25 nm (estimatedmagnetic spacing is 47 nm), the overwrite limit wasincreased to 7.0 kOe (560 kA m−1) for Type III head.The calculated maximum write field of a well head,when the magnetic spacing was 47 nm, was about 10kOe (800 kA m−1).

The write saturation curves of overwrite for Type IIIhead are also shown in Fig. 16. The flying height was

Fig. 16. Overwrite characteristics of writing head with orwithout high BS CoNiFe film; (1) without CoNiFe, flyingheight=35 nm; (2) without CoNiFe, flying height=25 nm,(3): with CoNiFe, flying height=35 nm; (4) with CoNiFe,flying height=25 nm. Recording performance was as follows;gap length, 0.20 mm; track width, 1.1 mm; recording density,110 kBPI, 33 kFRPI signal overwritten with 200 kFRPI signal.Dotted line shows the value of overwrite requirement (\26dB).


Fig. 17. Magnetic force microscopy (MFM) image of record-ing pattern detained by writing head with CoNiFe film. Thevalue of coercivity of the medium was 7000 Oe, and recordingperformance was as follows: gap length, 0.20 mm; track width,1.1 mm; recording density, 110 kBPI.

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6. Conclusion

The recent progress of research and development onnew soft magnetic films was reviewed for the author’saward lecture (1999 Pergamon Gold Medal). New softmagnetic films of CoNiFe for magnetic recording of thenext century were fabricated with an electrodepositiontechnique. The new CoNiFe soft magnetic film withhigh BS (=2.0–2.1 T) was created by controlling theinclusion of a very small amount of sulfur in the film.The most suitable film of ‘HB-CoNiFe’ group possessesa very fine crystalline structure with 10–15 nm indiameter of fcc–bcc mixed phase near the zero lS line.Corrosion properties of the film are nearly identical tothose of the plated 80-permalloy film. The GMR headfabricated with the CoNiFe film demonstrated superiorperformances leading to a breakthrough for materializ-ing ultra high-density magnetic recording.

Acknowledgements

The author would like to thank deeply to his col-leagues and students at Applied Physical ChemistryLaboratory, Waseda University, for their contributions,without which we could not succeed in reaching thetarget described in this review. Especially, Dr MadokaTakai, Mr Akiyoshi Nakamura, Mr KatsuyoshiHayashi and Mr Tokihiko Yokoshima were the keypersons in this successful project. Dr Keishi Ohashi andhis group of NEC Corporation also contributed greatlyto the development of the film and to the transfer of thebasic results of research to the application.

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