IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008 113

High Magnetic Saturation Poles for Advanced Perpendicular WritersMark T. Kief1, Venkat Inturi1, Mourad Benakli2, Ibro Tabakovic1, Ming Sun1, Olle Heinonen1,

Steve Riemer1, and Vladyslav Vas’ko1

Seagate Technology, Minneapolis, MN 55378 USASeagate Technology, Pittsburgh, PA 15222 USA

Advanced perpendicular writers continue to demand maximum write fields, fast rise times at ever vanishing head-media spacing andstrict reliability standards. This means that the asymptotic progression to a 2.4 T pole with nearly ideal magnetic response continues.At the same time we must control critical pole dimensions, fabricate at a reasonable cost while protecting against corrosion and erasurerisks. We will review progress made to meet this challenge in a discussion of high moment materials utilizing electroplating and sputterdeposition for single layer films and laminates. Concerns for corrosion will be assessed and minimized by controls on key contaminants.Micromagnetic modeling will be used to study the phenomenology and expected performance impacts of these various materials andstructures. Results will be used to provide a better insight into the potential materials/design tradeoffs that must be made. In conclusion,performance and reliability will be assessed through electrical testing.

Index Terms—Ferromagnetic materials, magnetic anisotropy, magnetic heads, soft magnetic materials.

I. INTRODUCTION

I T is generally understood that the magnetic properties ofpolycrystalline thin films are strongly dependent upon the

microstructure and grain size. High moment compositions ofFeCo alloys are no exception to this rule. Until the mid-1990sit was not well understood how to produce magnetically softFeCo thin films [1], [2]. FeCo35-40 are particularly importantalloys since they exhibit the highest known saturation magneti-zation at noncryogenic temperatures (with the possible excep-tion of reports [3] of Fe N that are still very controversial).The importance of grain size can qualitatively be explained asarising from the exchange averaging of local anisotropies whenthe grain size is well below the magnetic exchange length. Thisgeneral behavior is described by Hoffman’s ripple model [4]as summarized below and also by Herzer [5], with exchange ,grain diameter , anisotropy , saturation magnetization ,local anisotropy , and number of grains in coupling volume

and deviation of crystallites

where the structure factor (1)

The coercivity is significantly influenced by grain diameter andeffective averaging of local anisotropies.

It is also worth noting that the exchange averaging is most ef-fective for anisotropy variation within the plane. For thin sput-tered films with typical columnar grain structure, any local per-pendicular anisotropy may be reduced but never made zero. Thisis an especially important consideration for FeCo alloys thathave a very high magnetostriction value . ForFeCo35 thicknesses 100 nm (i.e., greater than the stripe do-main critical thickness) combined with a moderate compressive

Digital Object Identifier 10.1109/TMAG.2007.911028

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Fig. 1. Sputter deposited FeCo TEM cross sections and associated B(H) loops.(a) Unseeded FeCo. (b) Cu seeded FeCo. (c) FeCo/AlO laminate. (d) FeCo/Rulaminate.

stress these results in a perpendicular anisotropy component andstripe domains. Therefore, it is essential that that the FeCo stressbe positive with a preferably low value [6] to eliminate strip do-mains and the associated high .

II. MATERIALS REQUIREMENTS

With these considerations in mind, one can appreciate thecritical role of grain size in producing FeCo with soft mag-netics or low coercivity. Fig. 1 illustrates this behavior in com-paring FeCo35 grain size with the films M(H) loop for 200-nm-thick films. Fig. 1(a) shows FeCo35 sputter deposited directlyupon an oxide layer using a common physical vapor deposi-tion (PVD) system. The grain diameter is relatively large,about 100 nm, and the B(H) loop show a high . Oe.Addition of a 1 nm seedlayer such as Cu [Fig. 1(b)] causesa significant decrease in grain diameter to about 20 nm andin to 10–20 Oe. Note that these 200-nm-thick films showonly very weak uniaxial anisotropy. Fig. 1(c) illustrates eight

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114 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008

Fig. 2. Real part of permeability for materials: FeCo, FeCo/AlO laminate,FeCo/Ru laminate.

FeCo laminations with AlO/NiFe. The insertion of the oxidedisrupts the columnar growth and results in smaller grain sizeand coercivity. The FeCo/AlO laminate displays very low co-ercivity with Oe, Oe and a clear uni-axial anisotropy with Oe for nm. Thesesoft magnetic properties are very desirable of main pole appli-cations. In addition, the ferromagnetic/nonmagnetic layer struc-ture can be very helpful to control main pole remanence as pro-posed by Nakamoto et al. [7]. Another potentially interestinglaminate structure is FeCo/Ru. Fig. 1(d) illustrates eight lami-nates of FeCo/Ru. Note that the Ru is less effective in disruptingthe columnar grain structure nm but still reduces thecoercivity Oe while achieving a very lowremanence because of the Ru antiferromagentic interlayer ex-change coupling.

In addition to the quasi-static magnetic properties, it is alsointeresting to understand the dynamic switching properties ofthe main pole material. Naturally, one desires a good perme-ability at high frequencies for the writer yoke and pole. The highfrequency permeability was studied using a high frequency per-meameter with a passband from 100 MHz to 3 GHz. Thisinstrument uses a small drive field to characterize the low fieldresponse of a small, sheet film sample [8]. Fig. 2 plots the per-meance for samples of FeCo, FeCo/AlO laminate, and FeCo/Rulaminate. As expected, the low field permeability scales like theinitial permeability or . The permeability of the FeCoand FeCo/AlO laminate is relatively flat until near the FMR res-onance at 1.9 GHz. Since the FMR frequency , thehigh and high of FeCo alloys provide a high resonance.The FeCo/Ru shows decreasing permeability with frequency. Itis assumed that this is due to the weak driving field that has diffi-culty overcoming the strong exchange coupling in this laminate.It is important to understand that these measurements do not in-clude the effects of the demagnetizing field for a deep submi-cron-scale writer pole. This is best described by micromagneticmodeling. Nevertheless for the yoke or main pole laminate ma-terials with relatively thin magnetic layers, the behavior shouldbe similar if the effective anisotropy is considered [9]. The effec-tive anisotropy of the main pole is will be larger than the sheetfilms due to shape anisotropy and coupling of adjacent laminatelayers.

Although high saturation magnetization with low coercivityand soft magnetics are desirable for the main pole material, theseare not sufficient. The small dimensions of the main pole andthe close proximity of the media SUL can produce conditions

favorable for a vortex magnetization state and perpendicular re-manence (i.e., in the direction out of the air-bearing surface orABS). These conditions are not desirable since they may im-pede with the high speed switching behavior of the pole andcan cause on-track erasure of recorded data. One solution to thisproblem is to control the main pole remanence by introductionof laminations in the main pole [7]. These laminations can ef-fectively use the demagnetization energy to control remanencerather than contribute to its cause. A precise understanding ofthis behavior requires micromagnetic simulation. However, onecan gain insight into this problem through application of thelaminate model described by Slonczewski [10] and that was pre-viously presented by a couple of these authors [11].

It has been shown that a perpendicular remanence is pro-moted by a main pole breakpoint that is long compared to thewidth [12]. For illustration, we consider the limiting case of along breakpoint and media soft underlayer as described by along, narrow prism with laminations of alternating magneticand nonmagnetic layers. The remanent domain state is givenby minimization of the energy contributions from intrinsicanisotropy, closure domains, and wall energies includingedge-curling walls. The edge-curling walls describe the cou-pling between ferromagnetic layers at the edges of the prism,which are more realistic than demag fields. For the desired casewhen the remanent magnetization is in the plane of the ABS,the energy of the edge-curling walls must be sufficiently lowto be favored in comparison to the other energies. We comparethe energies of the easy axis, EA, laminate state assumingedge-curling walls and the hard axis state [10] for an elementof width, , anisotropy, , saturation magnetization, , andthe thickness of the magnetic, , and nonmagnetic layers, :

Energy per unit length of EA laminate state

where (2)

Energy per unit length of HA state

(3)

This indicates that the EA laminate state is favored relative tothe hard axis (HA) state for large widths, low thicknesses, low

, and high . To estimate this further, we assume a rela-tively large total pole thickness with many laminations and a toppole width (TPW) set by the targeted track density. We considercases of and nm in Fig. 3, where we plotthe boundary between perpendicular remanence and in-plane re-manence with laminates. The plot illustrates that the AF lami-nate state can be achieved even in this worst case scenario forthe 200 and 100 nm widths, but only when the nonmagneticspacer is very thin. This condition becomes harder to maintainas TPW is reduced. For the nm width the boundarycrosses the point nm and spacer nm. For

nm, the boundary crosses the point nmand nm. For these estimates, we assume no interlayerexchange coupling. This can be added and will promote the de-sired laminate state.

For a finite pole length, laminations can also produce a anti-ferromagnetic configuration oriented along the pole. This con-figuration will have a lower remenance than the HA case de-scribed above. This can offer further benefits in practice.

KIEF et al.: HIGH MAGNETIC SATURATION POLES FOR ADVANCED PERPENDICULAR WRITERS 115

Fig. 3. Control of Main Pole remanence and on track erasure. (a) Illustratesdesired EA remanent state and undesired HA state. (b) shows boundary betweenEA and HA states for TPW of 0.2, 0.1, and 0.05 �m.

Although useful, lamination is not an ideal solution. This isbecause the targeted density effectively fixes the cross section ofthe writer pole, therefore the introduction of nonmagnetic layersinto the pole has the net effect of diluting the average magneti-zation. A reduction is magnetization will result in a lower writefield than the limiting case of a solid 2.4 T FeCo pole. This isan increasingly undesirable tradeoff as the writability becomesmore stressed with increasing areal density. For a top pole thick-ness of 200 nm having ten FeCo laminations and nine nonmag-netic spacers at 2 nm thick, this translates to a dilution of 9%or an average magnetization of 2.18 .

Before the introduction of perpendicular writers, it was typ-ical for the main pole to be deposited by electroplating. In fact,Seagate was the first to introduce 2.4 FeCo plated materialfor the main pole. Typical advantages of electroplating versussputtering deposition include reduced process content, reducedvariance, and reduced cost. As we described above, there aremultiple reasons to consider laminated main pole materials. Thisis true for plated as well as sputtered.

As in the case of sputtered magnetic films, the coercivity andanisotropy of electroplated films can often be improved by thereduction of the magnetic materials grain size. Given the in-herent properties of FeCo alloys, this can be particularly impor-tant. Composition, grain size, stress, and even crystalline ordermay be optimized for FeCo by the plating bath constituents andprocessing conditions. Grain size reduction can also be achievedby lamination in plated films. As in the sputtered material case,a good nonmagnetic spacer material must be very smooth, con-trollable for subnanometer thicknesses, and provide good mi-crostructural growth of the FeCo. Although many possibilitiesexist, we illustrate this principle with the lamination of FeCoand NiP layers. Plated NiP is known to be amorphous for con-centration of P exceeding 10% wt. Fig. 4 shows TEM imagesof plated FeCo and FeCo/NiP laminate films. We estimate thatthe grain size is reduced from about 60 to 30 nm with lami-nation. Furthermore, the anisotropy and coercivity are suitablyimproved as shown by B(H) loops for FeCo and FeCo/NiP lam-inate in Fig. 4. Typically, the grain size and microstructure de-pend upon the film thickness for magnetic and nonmagneticlayers. This is also the case for FeCo/NiP. The hard axis coer-civity is shown to decrease as the NiP spacer thickness increasesand the number of laminations increases for a fixed total thick-ness. For a NiP thickness of 2 nm, low Oe can bewell achieved for eight or more laminations. This matches wellthe rough targets we proposed earlier using the simple analyticlaminate model. In other words, good control of remanence isachievable for either sputtered or plated FeCo laminations as

Fig. 4. Electroplated FeCo TEM cross sections and associated B(H) loop for(a) FeCo, and (b) FeCo/NiP laminate.

TABLE ICORROSION PROPERTIES OF ELECTROPLATED (EP) AND SPUTTERED (SP)

MAGNETIC ALLOYS

shown here. This is a valuable demonstration since it allowsgood flexibility in our processing approach and also good op-portunity for cost savings.

We have discussed the challenges presented by FeCo alloys toproduce high moment materials with soft magnetic properties.In addition to performance driven requirements, another crit-ical consideration must be reliability. Unfortunately, FeCo al-loys are inherently more susceptible to corrosion that past mag-netic alloys such as NiFe or CoNiFe. A primary function ofthe head overcoat is to prevent corrosion of the transducer in-cluding writer pole at the slider ABS. The overcoat thickness isextremely thin and increasingly stressed by an asymptoticallyvanishing HMS and ever more demanding environmental condi-tions, which decreases its ability to prevent corrosion. We musttherefore consider the potential corrosion risks of FeCo. Table Icompares the corrosion potentials for several sputtered and elec-troplated magnetic alloys at acidic and near neutral pH con-ditions. The more negative the corrosion potential, the greaterthe risk. Table I shows that the risks for FeCo NiFeNiFe and for pH 3.0 pH 5.9. More interesting, the potentialillustrates the common observation that electroplated alloys canoften have greater risk than sputtered materials. The origin ofthis difference has been subject of investigation for many years.It has become a greater concern with perpendicular writers andenvironmental stresses. It is generally understood now that amajor factor in the increased corrosion potential is the pres-ence of catalytic elements in the material especially sulfur in theform of metal sulfides and also other contaminants [13]. One ap-proach to this problem is addition of a third element such as Nito the alloy. This will reduce the corrosion potential but with as-sociated loss in . A better solution to this problem is elimina-tion of these contaminants from the material and environment.We have previously shown that for electroplated FeCo a majorsource of sulfur is the industry standard additive Saccharin [13].Replacing Saccharin with a non-sulfur additive produces dra-matically improved corrosion properties while preserving the

116 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008

Fig. 5. Effects of sulfur upon FeCo corrosion properties. Electrochemical po-tential improves (less negative) with removal of sulfur and process optimization.

other desirable properties of the FeCo material. This is shownin Fig. 5. Elimination of the sulfur containing additives results insignificant reduction in corrosion potential. Additional enhance-ments in the electroplating process to further reduce contami-nants can produce corrosion potentials comparable to electro-plated permalloy in near neutral solution—also shown in Fig. 5.

III. FINITE-ELEMENT MODELING

With the insights of some material options and simple magne-tostatics, we now examine a more accurate micromagnetic sim-ulation of the remanent state and switching behavior. In par-ticular, the effect of the exchange coupling mediated by thelamination was studied using large-scale micromagnetic sim-ulation. For simplicity, the writer structure used consisted onlyof a top pole and a yoke, with no return poles. The top polelength in the downtrack direction was 150 nm, and the widthin the cross-track dimension was 100 nm. The width of thewriter structure was 4 m, and its height 5 m. A first break-point with a total flare angle of 70 was located at 150 nmabove the ABS; a second breakpoint with a flare of 120 wasat 850 nm above the ABS. A yoke of thickness 350 nm was re-cessed 800 nm from the ABS. The writer was excited with athree-turn helical coil recessed 1 m. A soft underlayer (SUL)of dimensions m m and of thickness 120 nm wasincluded in the simulations, with a distance of 40 nm betweenthe ABS and the SUL. The writer top pole consisted of ferro-magnetic material with a saturation magnetization density of1920emu/cm . The thickness of the top pole was divided intofour equal layers that were coupled to each other through lami-nation layers. The strength of the interaction was varied between

erg/cm and erg/cm . The saturation magnetization den-sity of the yoke and SUL were 1440 emu/cm and 955 emu/cm ,respectively. The top pole and yoke had uniaxial anisotropiesin the cross-track direction with anisotropy fields of 10 and5 Oe, respectively, and ferromagnetic exchange couplings of1.5 erg/cm. The SUL consisted of two 60-nm-thick laminatedlayers with a weak interfacial coupling of 0.01 erg/cm betweenthe layers; it had a uniaxial anisotropy of strength 2 Oe in thecross-track direction, and a ferromagnetic exchange couplingof 1.2 erg/cm. The writer was excited using write currents of60 mA and 90 mA (0-p) with a period of 1 ns and a risetime (0to peak value) of 20 ps.

The writer and soft underlayer were placed in a parallelepipedsimulation box and all space in the box meshed using a cubic

Fig. 6. Write field for micromagnetic writer with 90 mA bipolar current. J =�1:0,�0:1,+1:0 erg/cm for triangle, cross, and box, respectively.

mesh of side 10 nm. The Landau–Lifshitz–Gilbert (LLG) equa-tions for the whole system with dimensionless damping equalto 0.05 in the writer structure and 0.01 in the SUL were inte-grated using a Bulirsch–Stoer integrator parallelized and opti-mized for the LLG equations. In the results presented here, wewill show the magnetostatic field due to writer and SUL at thetrailing edge of the writer at a center location in the cross-trackdirection and 15 nm below the ABS.

As we discussed above, lamination that provides an antiferro-magnetic coupling (AFC) between adjacent layers may help tocontrol the remanent state of the writer pole tip. A sufficientlystrong AFC forces the pole tip into a scissor-like remanent state,in which the magnetization in consecutive layers is antiparalleland largely in the ABS plane. Such a state has low magneticcharge density on the ABS and therefore smaller stray fields thatthe vortex state that is common with no or ferromagnetic lamina-tion. However, a concern is that a strong AFC will make it moredifficult to saturate the top pole and the available write field willsuffer, as a consequence. In addition, the field rise time may beaffected by the strength of the coupling. Fig. 6 shows the per-pendicular field at the trailing edge for a 0-p current of 90 mA.The exchange coupling strengths mediated by the laminates are

1.0, 0.1, and 1.0 erg/cm . There are two conclusions fromthis figure. The first one is that for this relatively large current,the writer is well saturated and there is almost no difference inthe magnitude of the field for the different couplings—the fieldfor an AFC of erg/cm is only about 100 Oe. The secondobservation is that the rise time is generally reduced slightly forAFC coupling.

If the write current is reduced, the writer becomes lesssaturated and the effect of AFC laminations becomes moreapparent. In Fig. 7, the current is reduced to 60 mA. Whilethere is no discernible difference in the field between strongferromagnetic and weakly AFM couplings (1.0 erg/cm and

0.1 erg/cm , respectively), the stronger AFC ( 1.0 and5.0 erg/cm ) clearly have reduced field strengths by about

500 and 3000 Oe, respectively.We also performed simulations to study the remanent field

of the writer at short time scales (limited by simulation time).Fig. 8 shows the result of exercising the writer with a 90 mA(0-p) current of a period of 2 ns for a few periods. The currentwas then turned off but the integration continued to calculatethe remanent field for up to 2.5 ns after the current was turned

KIEF et al.: HIGH MAGNETIC SATURATION POLES FOR ADVANCED PERPENDICULAR WRITERS 117

Fig. 7. Write field for micromagnetic writer with 60 mA bipolar current. J =�5:0,�1:0,�0:1,+1:0 erg/cm for box, triangle, cross, and box, respectively.

Fig. 8. Maximum write field for micromagnetic writer with 90 mA peak currentand relaxation. J = �5:0,�1:0,�0:1,+1:0 erg/cm for box, triangle, cross,and box, respectively.

Fig. 9. Average write field for micromagnetic writer with 90 mA bipolar cur-rent and relaxation. J = �5:0, �1:0, �0:1, +1:0 erg/cm for box, triangle,cross, and box, respectively.

off. In this figure is shown the maximum perpendicular compo-nent of the field under the pole tip. There is a clear trend here:the maximum field increases with the strength of AFC, while itis smallest for weakly AFC erg/cm . The average field(averaged under the area of the pole tip) shows the same trend,although now the fields from FM and weakly AFC are indistin-guishable (Fig. 9)

The fact that strong AFC gives “spikes” in the field can be un-derstood by examining the magnetization configuration of thepole tip. Fig. 10 shows the magnetization components at theABS surface of the pole tip for AFC of erg/cm (top panel),and the corresponding field map 15 nm below the ABS. It isclear that the peaks in the perpendicular component (along they-axis) is due to the magnetization at the corners of the pole tip.At those location, the magnetization density has a large curva-ture, which gives rise to a large charge density . Thischarge density is the source of the field spikes. Increasing AFC

Fig. 10. Magnetization and field components of pole tip. Magnetization (top)and Field (bottom) components x, y, z (left, center, right).

Fig. 11. HGA Overwrite (a) and BER results for electrical Write Plus Erase(WPE) in microinch (b) for perpendicular main poles with materials: FeCo,FeCo/AlO laminate, FeCo/Ru laminate.

TABLE IISUMMARY OF ERASURE AFTER WRITE (EAW) TESTING RESULTS FOR VARIED

POLE MATERIALS

coupling between laminations drives increased charge densityat corners of pole and field spikes.

IV. CONCLUSION AND SUMMARY

The industries requirement is for the design, process, and ma-terials to produce a well performing writer. For current perpen-dicular writers this simply requires high write flux, sharp writegradient, and good reliability. We have argued that FeCo alloysare a logical choice given their very high saturation magnetiza-tion. Unfortunately, these alloys often have poor magnetic prop-erties due to the high magnetostriction and anisotropies. We

118 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008

have shown how the use of suitable laminations can result inboth improved magnetics and controlled remanence. Further-more, we have shown that very good magnetics, thin film lami-nations, and good corrosion can be engineered in both sputteredand electroplated high moment alloys. The overall validationof this approach is shown in Fig. 11 where we compare HGAdata for FeCo/NiFe, FeCo/AlO laminates and FeCo/Ru lami-nate main poles. The geometry of these write poles are similarwith a to nm, to nm andBreakpoint nm. Fig. 11(a) shows comparable overwritefor all materials with potentially some weak benefit for FeCo asmight be expected. Fig. 11(b) shows good bit error rate (BER)from 7 to 8 for all materials. These are good results and con-sistent with micromagnetic predictions. Table II shows EAWfor same writers and indicates no remanence and Erasure AfterWrite problems for both AlO and Ru laminates but significanterasure for most FeCo poles. This is consistent with modelingand discussions of perpendicular remanence for these structures.

In summary, we have highlighted the basic materials and de-sign requirements for perpendicular writer poles. We have ar-gued that microstructurally engineered FeCo alloys are an ex-cellent fit to meet current product requirements for both per-formance and reliability. Finally we report electrical data thatlargely confirms our expectations of main pole materials impactupon perpendicular writer field, bit error rate and erasure. In thefuture, these requirements will be stressed further as writabilitydemands become increasingly more challenging.

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[9] E. van de Riet and F. Roozeboom, “Ferromagnetic resonance and eddycurrents in high-permeable thin films,” J. Appl. Phys., vol. 81, no. 1,pp. 350–354, Jan. 1, 1997.

[10] J. C. Slonczewski, B. Petek, and B. E. Argyle, “Micromagnetics oflaminated permalloy films,” IEEE Trans. Magn., vol. 24, no. 3, pp.2045–2054, May 1988.

[11] M. Kief and V. Vas’ko, “Interlayer magnetostatic coupling—a con-tinuum of interaction strengths,” in APS Meeting, Mar. 2000, K24.9.

[12] M. Mochizuki, C. Ishikawa, H. Ide, K. Nakamoto, Y. Nakatani, and N.Hayashi, “Remanent head field study of single pole-type head based onmicromagnetics,” J. Appl. Phys., vol. 93, no. 10, pp. 6748–6750, May2003.

[13] I. Tabakovic, S. Riemer, K. Tabakovic, M. Sun, and M. Kief, “Mecha-nism of saccharin transformation to metal sulfides and effect of inclu-sions on corrosion suceptibility of electroplated CoFe magnetic films,”J. Electrochem. Soc., vol. 153, pp. C586–C590, 2006.

Manuscript received May 18, 2007. Corresponding author: M. T. Kief(e-mail: Mark.T.Kief@Seagate.com).