Journal of Magnetism and Magnetic Materials 264 (2003) 275–283

Overview of magneto-resistive probe heads for nanoscalemagnetic recording applications

Dmitri Litvinov*, Sakhrat Khizroev

Seagate Research, 2403 Sidney Street, Pittsburgh, PA 15203, USA

Received 30 September 2002; received in revised form 12 December 2002

Abstract

This work investigates various types of reader designs as applied to magnetic recording. Reciprocity principle is used

to study playback response. It is shown that both shielded and not shielded differential readers exhibit magnetic

playback properties advantageous for achieving extremely high areal bit densities. Modified design of differential

readers with one MR sensor is proposed to overcome the manufacturability issues associated with a conventional

double-MR sensor differential reader.

r 2003 Elsevier B.V. All rights reserved.

Keywords: Magnetic recording; Magnetoresistive probe heads; Terabit per square inch data storage

1. Introduction

As the areal densities continue to increasesteadily, the requirements for playback headsbecome increasingly strict. A common approachutilized today for improving the performance ofMR playback heads [1–5] is a continuous im-provement of the MR sensor properties while thebasic magnetic configuration of a playback headremains largely unchanged. Today, the mostcommon playback head design in use is shieldedMR head. While such MR heads are the mostexplored and utilized playback heads with estab-lished manufacturing processes, it is important torealize that these playback heads are not fullyoptimized in terms of both playback amplitude

and spatial resolution. It is the intention of thiswork to discuss alternative playback head designsand to compare their recording characteristics tothe ones of conventional shielded MR readers.

The following basic reader configurations willbe discussed (see Fig. 1): (a) unshielded reader [1];(b) shielded reader [2,3]; (c) differential reader[6–9]; (d) shielded differential reader [10,11].Variations of shielded, differential, and shieldeddifferential readers with the emphasis on variousaspects of recording performance and manufactur-ability will be considered as well. Playback from aperpendicular recording medium with a softunderlayer and a single-layer longitudinal record-ing medium will be investigated. For completeness,more exotic configurations such ad longitudinalrecording medium with a soft underlayer (keeperlayer) and perpendicular recording medium with-out a soft underlayer will be considered as well.

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*Corresponding author.

E-mail address: dmitri.litvinov@seagate.com (D. Litvinov).

0304-8853/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0304-8853(03)00107-0

Information Storage: Basic and Applied

The reciprocity principle is used to study play-back performance of various readers [12–14].According to the reciprocity principle, the play-back voltage of a linear playback head is equal tothe convolution of the sensitivity field (function) ofthe reader with the magnetization pattern writteninto the recording layer. The sensitivity field iscalculated as the field generated by the read-head,in which the read sensor is substituted with anequivalent soft magnetic material with a currentcarrying imaginary coil wrapped around [15]. Thereciprocity expression for the playback voltage isgiven by

SB1

I

ZM �H qr; ð5Þ

where M is the magnetization in the recordinglayer and H is the sensitivity field generated by theimaginary current, I :

The reader design parameters used in calcula-tions are similar to the ones suggested by Mallaryet al. [16] for a 1Terabit/in2 perpendicular record-ing system design. Unless specified otherwise, themagnetic thickness of an MR sensor, tMR; isassumed to be 10 nm. The cross-track width of anMR sensor, wMR; is 40 nm. The height of an MR

sensor, hMR; is 40 nm. The spacings between anMR sensor and shield, dMR�shield; and betweenMR sensors in a differential design, dMR; are set to10 nm. The flight-height, FH ; is 5 nm and themedia thickness, tRL; is 10 nm. The shields thick-ness, tshield; is 100 nm, the shield cross-track width,wshield; is 400 nm, and the shield height, hshield; is220 nm. The dimensions mentioned above areshown in a schematic drawing of a shieldeddifferential reader in Fig. 2. The presence of asoft underlayer is modeled with symmetric bound-ary conditions about the top surface of the soft

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MR

Sen

sor

Recording Medium Recording Medium

Recording MediumRecording Medium

MR

Sen

sor

MR

Sen

sor

(a) (b)

(c) (d)

shield shield

shield shield

MR

Sen

sor

MR

Sen

sor

MR

Sen

sor

Fig. 1. Schematics of various reader designs: (a) unshielded reader, (b) shielded reader, (c) differential reader, (d) shielded differential

reader.

ShieldShield

Recording Layer

tMR

dMR

tshield

hMR

tRL

FH

hshieldx

zdMR-shield

Fig. 2. Schematic of a shielded differential reader with the

relevant dimensions outlined.

D. Litvinov, S. Khizroev / Journal of Magnetism and Magnetic Materials 264 (2003) 275–283276

Information Storage: Basic and Applied

underlayer. As shown in Fig. 2, x-axis representsdirection along the track; z-axis represents direc-tion perpendicular to the plane of the medium. x ¼0 represents the middle of an MR sensor for single-element readers and midway between the MRsensors for differential readers. Magnetic fieldmodeling based on boundary element approachwas utilized throughout the paper.1 The presentedfield profiles are evaluated in the middle of arecording layer. The roll-off curves are calculatedassuming perfect step transitions using reciprocityintegral given by Eq. (5).

2. Basic reader designs comparison

The presented calculations of the playback arebased on reciprocity principle. The reciprocityprinciple requires the knowledge of the sensitivityfunctions for the playback heads. Figs. 3a and bshow the z (vertical) and x (horizontal) compo-nents of the sensitivity fields along the track for thefour types of heads, respectively.

Playback off a perpendicular recording mediumwith a soft underlayer and of a single-layerlongitudinal medium versus linear density for fourreader designs is shown in Figs. 4a and b,respectively. While conventional shielded readerprovides improved performance over unshieldedreader, both differential reader configurations

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-200 -100 100 200

-6

-4

-2

0

2

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6H

z (a

.u.)

Distance along the track (nm)

No shield (sul) Shield (sul)Diff (sul)

Shield Diff (sul)

-200 -100 0 100 200

-2

-1

0

1

2

3

4

5

Hx

(a.u

.)

Distance along the track (nm)

(a) (b)

0

No shield (no sul) Shield (no sul)Diff (no sul)

Shield Diff (no sul)

Fig. 3. (a) Vertical component, Hz; and (b) horizontal component,Hx; along the track of the sensitivity field for different reader types.

500 1000 1500 2000 2500

0

5

10

15

20

25

30

35

Play

bac

k (a

.u)

Linear Density (kfci)

No shield (sul)

Shield (sul)

Diff (sul)

Shield Diff (sul)

0 500 1000 1500 2000 2500

0

5

10

15

Play

bac

k (a

.u.)

Linear Density (kfci)

No shield (no sul)

Shield (no sul)

Diff (no sul)

Shield Diff (no sul)

(a) (b)

Fig. 4. Playback off (a) a perpendicular recording medium with a soft underlayer and (b) a single-layer longitudinal recording medium

versus linear density for four reader designs.

1Amperes, the boundary element solver by Integrated

Engineering Software, Winnipeg, Canada.

D. Litvinov, S. Khizroev / Journal of Magnetism and Magnetic Materials 264 (2003) 275–283 277

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offer substantial performance improvement interms of higher playback amplitude and higherresolution. Unshielded differential reader offershigher signal amplitude at lower linear densities.Shielded differential reader offers the highestspatial resolution of all four designs.

3. Parallels between perpendicular and longitudinal

recording

It should be recalled that a conventionalshielded reader as applied to longitudinal record-

ing is equivalent to a differential reader as appliedto perpendicular recording [17]. This is illustratedin Fig. 5a where the sensitivity fields of a (shielded)differential and a shielded reader are compared.The normalized sensitivity fields for the readersabove are shown in Fig. 5b. It can be seen thatnormalized sensitivity functions of a shieldedreader and of a shielded differential reader arealmost identical, while the sensitivity function of anot shielded differential reader has slightly widertails.

For completeness, it is instructive to comparethe performance of the above-mentioned reader

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itivi

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ield

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Shielded Reader (Hx) Diff Reader (Hz) Shield Diff (Hz)

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-1.0

-0.5

0.0

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1.0

Sens

itivi

ty F

ield

(a.

u.)

Distance along the track (nm)

Shielded, Hx Differential, Hz Shielded Diff, Hz

(a) (b)

Fig. 5. (a) Sensitivity fields of a shielded, differential and shielded differential readers. (b) Normalized sensitivity fields for a shielded,

differential, and shielded differential readers.

0 500 1000 1500 2000 2500 30000

5

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35

Play

bac

k (a

.u.)

Linear Density (kfci)

No shield (perpendicular) Shielded (perpendicular) Differential (perpendicular) Shield Diff (perpendicular) No shield (longitudinal) Shielded (longitudinal) Different ial (longitudinal) Shield Diff (longitudinal)

0 1000 2000 2500

0

5

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15

20

Linear Density (kfci)

Shielded (longitudinal) Differential (longitudinal) Shield Diff (longitudinal) Shielded (perpendicular (no sul)) Differential (perpendicular (no sul)) Shield Diff (perpendicular (no sul))

(a) (b)

Play

bac

k (a

.u.)

1500500

Fig. 6. (a) Perpendicular and longitudinal systems playback for four reader designs. (b) Perpendicular without a soft underlayer and

longitudinal systems playback for three reader designs.

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designs as applied to perpendicular and long-itudinal recordings. Fig. 6a compares playbackversus linear density for perpendicular mediumwith a soft underlayer and a longitudinal single-layer medium with equivalent recording layers. Itcan be observed that for all reader designs, theplayback amplitude is higher for perpendicularrecording than the playback amplitude for long-itudinal recording, which is clearly an advanta-geous feature of perpendicular recording. Forcomparison, Fig. 6b shows playback signals of aperpendicular system with a medium without asoft underlayer and of a single-layer longitudinalmedium. Comparing Figs. 6a and b, it can be seenthat the higher playback amplitude in perpendi-cular recording is mostly due to the utilization ofmedia with soft underlayers.

4. Influence of shields

4.1. Number of shields

Playback off a perpendicular recording mediumwith a soft underlayer and a single-layer long-itudinal recording medium versus linear densityfor not shielded, one-side-shielded, and double-shielded single-MR-sensor reader designs areshown in Figs. 7a and b, respectively. As it hasbeen reported previously, the addition of shields

improves the resolution of the reader at higherlinear densities for both perpendicular and long-itudinal recordings.

4.2. Shield thickness

Figs. 8a and b show sensitivity functions androll-off curves, respectively, for double-sidedshielded readers for the cases of a single-layerlongitudinal medium and a perpendicular mediumwith a soft underlayer for 100 and 10 nm thickshields. Only a very weak dependence on the shieldthicknesses can be observed for the cases pre-sented.

5. Soft underlayer versus no soft underlayer

Figs. 9a and b show roll-off curves forperpendicular and longitudinal systems, respec-tively, for three reader designs for the cases ofmedia with and without a soft underlayer. It canbe observed that while in perpendicular system theaddition of a soft underlayer substantially in-creases the playback amplitude, in longitudinalsystem the addition of a soft underlayer (keeperlayer) leads to a substatial drop in the playbacksignal. The physical explanation of the phenomenais illustrated in Fig. 10 where imaging properties ofa soft underlayer film are outlined for the cases of

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0

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30

Play

bac

k (a

.u.)

Linear Density (kfci)

No shield z (sul) One shield z (sul) Two shield z (sul)

0 500 1000 1500 2000 2500

0

5

10

15

(a) (b)

Play

bac

k (a

.u.)

Linear Density (kfci)

No shield x (no sul) One shield x (no sul) Two shield x (no sul)

Fig. 7. Playback off (a) a perpendicular recording medium with a soft underlayer and (b) a single-layer longitudinal medium versus

linear density for not shielded, one-side-shielded, and double-shielded single-MR-sensor reader designs.

D. Litvinov, S. Khizroev / Journal of Magnetism and Magnetic Materials 264 (2003) 275–283 279

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Play

bac

k (a

.u.)

Linear Density (kfci)(a) (b)

Thick shield, Hx (nosul) Thick Shield, Hz (sul) Thin Shield, Hx (no sul) Thin Shield, Hz (sul)

0

Fig. 8. (a) Sensitivity functions and (b) playback versus linear density for perpendicular and longitudinal shielded readers of different

shield thicknesses: thick shield—100nm; thin shield—10nm.

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5

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20

25

30

35

Play

bac

k (a

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Shield (sul) Shield (no sul) Diff (sul) Diff (no sul) Shield Diff (sul) Shield Diff (no sul)

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0

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Play

bac

k (a

.u.)

Linear Density (kfci)

Shield (no sul) Diff (no sul) Shield Diff (no sul) Shield (sul) Diff (sul) Shield Diff (sul)

(a) (b)

Fig. 9. Comparison of playbacks of three reader designs (shielded, differential, and shielded differential) for the cases of (a)

perpendicular media with and without a soft underlayer and (b) longitudinal media with and without a soft underlayer (keeper layer).

SUL

SUL

perpendicular

longitudinal

Fig. 10. Illustration of the imaging properties of soft underlayer for the cases of perpendicular and longitudinal recording schemes.

D. Litvinov, S. Khizroev / Journal of Magnetism and Magnetic Materials 264 (2003) 275–283280

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perpendicular and longitudinal recordings. Inperpendicular recording, the addition of a softunderlayer effectively doubles the recording layerthickness, thus increasing the amplitude of thestray fields. In longitudinal recording, the additionof a soft underlayer film effectively creates a layerunderneath the recording layer with the magneti-zations oriented opposite to the magnetizationwritten into the recording layer. As a result, the netstray field decreases.

6. Differential reader optimization and a single-

MR-sensor differential readers

The performance of differential readers can befurther optimized by adding a soft magneticmaterial bridge that magnetically couples the twosensors as shown in Figs. 11a and b. Theadditional configurations of a differential readerthat are worth considering are shown in Figs. 11cand d, where one of the MR sensors is substitutedwith an equivalent soft magnetic materials—yoke.The latter is simpler to manufacture as building

differential readers represents technological chal-lenges associated with manufacturing of double-MR elements with the outputs connected to form adifferential circuit. To avoid electrical shortage ofthe MR element in Figs. 11c and d designs, theyoke can be made of an insulating ferrite-basedmagnetic material.

Figs. 12a and b show z and x components ofthe sensitivity functions for three configurationsof not shielded differential reader, respectively.It can be seen that addition of the bridgeconnecting the two MR elements substantiallyincreases the magnitude of the sensitivity func-tion. Single-MR-element-based readers haveasymmetric profiles along the track. Similarly,Figs. 13a and b show z and x components of thesensitivity functions for three configurations ofshielded differential reader, respectively. Again, itcan be seen that addition of the bridge connectingthe two MR elements substantially increases themagnitude of the sensitivity function. As in thecase of a not shielded reader, single-MR-element-based shielded readers have asymmetric profilesalong the track.

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MR

Sen

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MR

Sens

or

(c) (d)

shield shield

Recording MediumRecording Medium

MR

Sen

sor

MR

Sen

sor

(a) (b)

shield shield

MR

Sen

sor

MR

Sen

sor

Fig. 11. Schematics of bridged differential readers: (a) not shielded, (b) shielded, (c) not shielded with one MR element, and (d)

shielded with one MR element.

D. Litvinov, S. Khizroev / Journal of Magnetism and Magnetic Materials 264 (2003) 275–283 281

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Figs. 14a and b compare roll-off curves fordouble-MR-element differential readers and mod-ified single-MR-element differential readers shownin Fig. 11 for the cases of perpendicular andlongitudinal recordings, respectively. It can be seenthat in both perpendicular and longitudinalrecordings, single-MR-sensor differential readersgive better performance at lower linear densitiesthan double-MR-sensor differential readers. Theperformance at higher linear densities is approxi-mately the same for all types of readers.

7. Summary

Different reader designs applied to perpendicu-lar and longitudinal recording systems have beeninvestigated. It is demonstrated that differentialreader configurations possess advantageous play-back characteristics in terms of both the playbackamplitude and the spatial resolution as comparedto conventional shielded readers. Single-MR-element differential reader, a simplified versionof a double-MR-element differential reader, is

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-200 -100 0 100 200

-10

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0

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10

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(a.u

.)

Distance along the track (nm)

Bridged (sul) Diff (sul) Half Diff (sul)

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0

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4

6

8

10

Hx

(a.u

.)

Distance along the track (nm)

Bridged (no sul) Not bridged (no sul) Half Diff (no sul)

(a) (b)

Fig. 12. (a) Vertical component and (b) horizontal component of the sensitivity function for three types of differential readers: diff:

conventional differential reader; bridged: differential reader with two MR elements connected by a soft magnetic bridge; and half diff:

bridged with one of the MR elements substituted with soft magnetic material.

-200 -100 100 200-10

-5

0

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Hz

(a.u

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Distance along the track (nm)

Bridged (sul) Diff (sul) Half Diff (sul)

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0

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4

6

8

Hx

(a.u

.)

Distance along the track (nm)

Bridged (no sul) Diff (no sul) Half Diff (no sul)

(a) (b)

0

Fig. 13. (a) Vertical component and (b) horizontal component of the sensitivity function for three modifications of shielded differential

readers: diff: conventional differential reader; bridged: differential reader with two MR elements connected by a soft magnetic bridge;

half diff: bridged with one of the MR elements substituted with soft magnetic material.

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proposed as a relatively easy to build playbackhead with improved signal resolution.

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Diff (sul)

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Half Diff (sul)

Shield Half Diff (sul)

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20

Linear Density (kfci)

Diff (no sul) Shield Diff (no sul) Half Diff (no sul) Half Shield Diff (no sul)

(a) (b)

Play

bac

k (a

.u.)

Fig. 14. Playback off (a) a perpendicular recording medium with a soft underlayer and (b) a single-layer longitudinal medium versus

linear density for double-MR-sensor and single-MR-sensor differential readers.

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