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can be adjusted by the energy level and pulse dura-

tion of the laser pulse, resulting in a well-controlled

surface roughness of the landing zone (Baumgart

et al. 1995). During contact of the slider on this

landing zone, the formation of meniscus areas is re-

stricted to the areas around the ‘‘bumps.’’ Thus, the

stiction force is controlled by the number and shape

of the laser bumps. Because of this, stiction of laser-

textured disks is much less sensitive to changes in

the environmental conditions or the number of CSS

cycles. An array of laser bumps with a pitch distance

of approximately 40 mm between rows is shown in

Fig. 5.

4.3 Load/Unload

Laser zone texturing becomes ineffective if the height

of the laser bumps is required to decrease to less than

10 nm. To avoid high stiction of the head/disk inter-

face in the latter case, ramp load/unload has been

introduced as an alternative to laser texturing. In the

case of ramp load/unload, the slider is loaded onto

the disk during startup of the drive using the ramp

mechanism shown in Fig. 6. During shutdown of the

drive, the slider is lifted from the disk surface using

the ramp mechanism. The use of a ramp load mech-

anism avoids stiction completely during start/stop.

Ramp load/unload is difﬁcult to implement, how-

ever, because of severe tolerance requirements during

assembly. Ramp load/unload mechanisms are used

mainly in laptop computers, but have begun to

appear also in 95 mm hard disk drives.

5. Wear

Wear of the head/disk interface occurs on both the

slider and the disk as the result of head/disk inter-

actions during CSS or steady-state operation. Wear

of sliders and rigid disks can occur as adhesive wear,

abrasive wear, impact wear, or tribochemical wear.

Intermittent slider/disk impacts during steady-state

operation lead to impact wear. In addition, tribo-

chemical wear occurs if oxygen from the atmosphere

absorbs chemically on the surface of the carbon layer,

leading to the formation of surface oxides. Thermal

desorption of these species leads to the formation of

CO and CO

, which depletes the surface of carbon.

For lubricated disks, the lubricant ﬁlm tends to act as

a chemical barrier, thereby reducing the occurrence

of tribochemical wear in favor of other types of wear

(Talke 1995).

Disks and sliders are typically coated with a thin

layer of carbon to protect the magnetic materials

from wear and corrosion. Carbon overcoats used for

rigid disks and sliders are typically diamond-like car-

bon (DLC), amorphous DLC (a-C), hydrogenated

DLC (a-C:H), and nitrogenated DLC (a:CN

) (Grill

1997). Hydrogenated carbon ﬁlms (a-C:H) show gen-

erally improved wear performance over a-C ﬁlms.

The hardness and the elastic modulus of DLC in-

crease with hydrogen concentration in the DLC be-

cause hydrogen tends to increase sp

bonds in DLC

(Lee et al. 1992). Incorporation of nitrogen in DLC

also improves the wear resistance of DLC ﬁlms.

Nitrogenated ﬁlms have gained wide application as

thin-ﬁlm disk and slider overcoats.

6. Lubricants and Additives

In general, perﬂuorinated polyethers (PFPE) are used

for disk lubrication since they have good load carry-

ing capability and exhibit a very low vapor pressure.

Sliding contact between a slider and a disk tends to

remove the lubricant in the sliding track. To prevent

lubricant depletion, a disk lubricant should have good

mobility so that it can recoat areas of the disk de-

pleted from lubricant. However, since high mobility

Figure 5

Array of laser bumps with bump spacing between rows

of 40 m m.

Figure 6

Ramp load/unload mechanism.

670

Magnetic Reco rding: Rigid Media, Tribology

increases lubricant spin-off, a compromise must be

made between mobility and bonding. A number of

commonly used disk lubricants is shown in Table 1

(Homola 1996).

Most disk lubricants exhibit thermal degradation

problems when exposed to elevated temperatures.

Furthermore, PFPE are susceptible to catalytic de-

composition when in contact with Al

, a common

slider material component. To reduce the degrada-

tion of disk lubricants caused by Al

it is custom-

ary to eliminate the exposure of disk lubricants to

by coating the slider surface with a thin carbon

ﬁlm. Another option is to use additives in the lubri-

cant such as X-1P (Perettie et al. 1999). However,

difﬁculties with phase separation have been observed

with phosphazine additives at high temperature and

humidity owing to the fact that these additives are

insoluble in PFPE-based oils (Kang et al. 1999).

7. Future Trends in Tribology of Rigid Media

To increase the recording density to 100 Gbits in

2

(15.5 Gbits cm

2

), it is anticipated that the ﬂying

height of the slider must be of the order of 5 nm, that

the carbon overcoat on the slider and the disk should

be reduced to less than 2–3 nm, and that the root

mean square surface roughness be approximately

0.2 nm.

Owing to the complexity of these requirements, the

question arises as to whether it would not be better to

implement contact recording, in which case the air

bearing between the slider and the disk would be

eliminated completely. Although the latter situation

is highly desirable, contact recording would require

an extremely lightly loaded slider/disk interface in

order to keep wear of the slider and the disk within

acceptable limits (Talke 1995). Furthermore, a lightly

loaded interface is prone to stick/slip and friction-

induced vibration, making this approach all but tech-

nically unfeasible. However, the trend towards re-

duced ﬂying heights will continue and lower and

lower ﬂying heights will be implemented in future

disk-drive products. Thus, even more difﬁcult tribo-

logy problems will be encountered.

See also: Magnetic Recording: Rigid Media, Pre-

paration; Magnetic Recording Devices: Head/Medi-

um Interface; Magnetic Recording Rigid Media,

Recording Properties; Magnetic Recording Technol-

ogies: Overview

Bibliography

Baumgart P, Krajnovich D J, Nguyen T A, Tam A C 1995 A

new laser texturing technique for high performance magnetic

disk devices. IEEE Trans. Magn. 31, 2946–51

Grill A 1997 Tribology of diamondlike carbon and related

materials: an updated review. Wear 94–95, 507–13

Gui J, Marchon B 1995 A stiction model for a head–disk

interface of a rigid disc drive. J Appl. Phys. 78, 4206–17

Homola A M 1996 Lubrication issue in magnetic disk storage

devices. IEEE Trans. Magn. 32, 1812–8

Hu Y, Bogy D B 1996 Dynamic stability and spacing modu-

lation of sub-25 nm ﬂy height sliders. ASME J. Trib. 119,

646–52

Kang H J, Perettie D J, Talke F E 1999 A study of phase

separation characteristics of perﬂuoropolyethers/phospha-

zene (x-1p) lubricant mixtures on hard disk surfaces. IEEE

Trans. Magn. 35, 52385–7

Lee H J, Lee J K, Zubeck R, Smallen M, Hollars D 1992

Properties of sputtered-deposited hydrogenated carbon ﬁlms

as a tribological overcoat used in rigid magnetic disks. Surf.

Coat. Technol. 54–55, 552–6

Perettie D J, Morgan T A, Zhao Q, Kang H J, Talke F E 1999

The use of phosphazene additives to enhance the perform-

ance of PFPAE lubricants. J. MMM 193, 318–21

Talke F E 1995 On tribological problems in magnetic disk re-

cording technology. Wear 190, 232–8

Wahl M H, Lee P, Talke F E 1996 An efﬁcient ﬁnite

element-based air bearing simulator for pivoted slider

using bi-conjugate gradient algorithms. Trib. Trans. 39,

130–8

Zhao Q, Talke F E 1999a Effect of environmental conditions on

the stiction behavior of laser textured hard disk media. Trib.

Int. 33, 281–7

Zhao Q, Talke F E 1999b Stiction and deformation analysis of

laser textured media with crater-shaped laser bumps. Trib.

Trans. 43 (1), 1–8

F. E. Talke

University of California, San Diego, La Jolla

California, USA

Table 1

Molecular structures of typical commercial disk lubricants.

Tradename Molecular structure End group

Fomblin Y CF

O[CF(CF

)CF

–(CF

–CF

Krytox AD F–[CF(CF

)CF

–CF

–F,–CF

Fomblin Z CF

O(CF

–(CF

–CF

Demnum F–(CF

–CF

–F,–CF

Fomblin Z-DOL HO–CH

O–(CF

–(CF

–CF

–OH –OH

Fomblin AM2001 P–CH

OCH

–(CF

– (CF

–CF

OCH

–P –CH

–pheQ(O)

QCH

671

Magnetic Recording: Rigid Media, Tribology

Magnetic Recording: VHS Tapes

The home video market is massive and is the domi-

nant form of magnetic media sold in the world mar-

ket. Estimated world sales for VHS (Video Home

System) media for 1988 were equivalent to 2686 mil-

lion E180 cassettes shipped at a total factory gate

price of $3335 million. Figure 1 shows a comparison

of VHS sales to other media. Originally used prima-

rily for recording of TV programs for later viewing,

use has now extended to home viewing of pre-

recorded ﬁlm, camcorder applications, and is making

inroads into multimedia PC programs.

Although VHS is the most popular video system,

its technology has been overtaken by other formats.

However, because of the large number of video

machines in homes and the considerable investment

in prerecorded ﬁlm for sale and rental, it is likely to

remain the dominant system until replaced by digital

technology. There have been some additions to the

format that allow stereo sound to be recorded, a

long-play facility with slower tape speed, a compact

cassette version for use in camcorders which can

then be played in a standard video cassette recorder

(VCR) using an adapter, and a super VHS (S-VHS)

with improved performance quality. S-VHS requires

a special machine and tapes to take advantage of the

improved quality but is backward compatible with

standard VHS tapes.

VHS tape sales are very competitive and prices have

fallen dramatically over time. This has reduced profit

margins so that many of the large media producers

have withdrawn from the business to concentrate on

more profitable ‘‘high-tech’’ media applications or

have transferred production to low-cost regions of the

world such as China and the East Asian Pacific Rim

countries.

1. The VHS Format

VHS is a helical scan system with the record/replay

heads mounted on a drum that rotates at an angle to

the tape transport direction. This produces recorded

data tracks at an angle to the tape direction. This

angle is dependent on the TV format and allows a

relatively high head speed across the tape surface at a

slow tape speed producing an efﬁcient use of the re-

cording area for the serially recorded signals (for the

625 line–50 ﬁeld TV system, the angle of scan is

51 57

50.3

at a head speed of 3.92 ms

1

and a tape

speed of 23.39 mms

1

(EN 60774 1994)). For a de-

tailed description of helical scan recording see Mee

and Daniel (1996).

With all removable media systems it is important

that exact standards are applied to the media to allow

writing and replay in different machines and main-

tenance of replay quality under a range of chang-

ing conditions. Standards were originally established

by JVC, the manufacturer who developed VHS, but

detailed speciﬁcations are now incorporated into

international standards (e.g., European Standard

EN60774-1). Many of the details are mechanical,

such as the dimensions of the cassette (104 mm 

188 mm 25 mm), the reel dimensions (62 mm diam-

eter), and the tape winding path and its control. The

tape has a speciﬁed width of 12.65 mm70.01 mm

(0.5 in) and a maximum thickness of 20.0 mm includ-

ing the magnetic coating. Playing times are speciﬁed

as 30 min, 60 min, 90 min, 120 min, and 180 min,

which are determined by the length of tape in the

cassette. Modern VCRs usually have a long-play

facility that doubles the playing time by reducing the

tape speed. This does produce some loss of quality

but modern head technology minimizes noise levels.

The magnetic coating has to be of a high resolu-

tion, longitudinally oriented, and with a coercivity

of approximately 50 10

1

(628 Oe). Invaria-

bly the magnetic coating consists of acicular oxide

Figure 1

Chart of total sales of VHS media compared to other

major magnetic storage media for 1988. Because some

sales are in the form of prerecorded tape, estimates of

VHS sales have been converted into the form of E180

cassettes (estimates by the Magnetic Media Information

Service). (a) Total sales for 1998 as a percentage of total

value ($13.833 billion.) (b) Total sales for 1998 as a

percentage of total units (10.627 billion units.)

672

Magnetic Reco rding: VHS Tapes

particles, usually cobalt-modiﬁed iron oxide (g-Fe

)

although chrome dioxide (CrO

) can be used. The

latter is reputed to have a better high-frequency res-

ponse, but the cost is generally higher than for iron

oxide. Most CrO

tapes on the market now contain a

mixture of CrO

and cobalt g-Fe

to limit price.

2. Magnetic Par ticles

The properties of magnetic particles determine the ﬁ-

nal magnetic properties of the manufactured tape.

Magnetostatic interactions between particles play

an important role in determining properties such as

coercivity, and the measured values for individual

particles, pigments, and tapes can be very different

(McConochie et al. 1996). Magnetic properties of

particles are described in detail in Magnetic Recording

Materials: Tape Particles, Magnetic Properties. Ideal-

ly all particles should be single domain with identical

magnetic properties. In practice this is not possible,

but particles are manufactured so that parameters

such as volume and aspect ratio have as narrow

a distribution as possible. Typically particles have a

length of about 600 nm and an aspect ratio of 6:1.

Ferric oxide particles are the most commonly used,

but basic g-Fe

has a coercivity controlled primarily

by shape anisotropy and can only be used to make

tapes with coercivities up to about 25 10

1

(B300 Oe). To increase this further, particles are

surface doped with cobalt. CrO

has a higher intrinsic

coercivity partly associated with crystal anisotropy

and is suitable for VHS applications in its basic form.

3. Media Production

The magnetic tape coating consists of small mag-

netic particles dispersed in a nonmagnetic polymer

matrix with additional nonmagnetic particles for other

important functions. A wet dispersion of all the com-

ponents is produced which is then coated onto plastic

ﬁlm and dried. The tape formulation is proprietary

and each tape manufacturer will have its own choice of

components. Manufacturers of precursors often have

test formulations that are freely available to customers

and produce an acceptable ﬁnal product. All disper-

sions have essentially the same components that serve

the same specific general functions as follows:

(i) Magnetic particles—see above.

(ii) Wetting agents—usually a surfactant whose

function is to coat the magnetic particles and prevent

agglomeration after dispersion. Some part at least

can be replaced by ‘‘wetting’’ polymer resins, which

have functional groups effective in wetting. This re-

duces the ﬁnal wetting agent content of the tape to

the advantage of wear and friction. Although orig-

inally developed for advanced particulate media

manufacture, they are in common use for commod-

ity products such as VHS.

(iii) Polymer resins—to produce the ﬁnal solid

matrix and generally comprising a mixture of seve-

ral types which can be classiﬁed as soft, such as

polyurethane, and hard, such as vinyl. The correct

combination determines the ﬁnal mechanical pro-

perties of the tape, e.g., stiffness and temperature

resistance characteristics, which are important for

controlling properties such as magnetostriction asso-

ciated with changing magnetic properties, as particles

are stressed and the matrix is strained during tape

ﬂexure.

(iv) Cross-linking agents—added just before ﬁnal

coating of the tape to strengthen the polymer matrix

and to ensure that particles are strongly bound.

(v) Solvents—to dissolve the resins and the wetting

agents.

(vi) Carbon black—to reduce static electricity in

the tape.

(vii) Lubricants—additional surfactants and other

lubricants which are incorporated into the matrix.

These leach out during the life of the tape to lubricate

the surface and ensure smooth transit across the

heads and guides during record and replay.

(viii) Abrasives—such as aluminum oxide parti-

cles, which help to clean debris and deposits from

heads and other parts in contact with the tape during

use.

Magnetic particles are supplied by pigment man-

ufacturers in a clustered form to aid handling of

what, individually, would be a very ﬁne and hazard-

ous powder of particles with the dimensions of smoke

grains. The main function of the wet dispersion pro-

cess is to separate these particles without damaging

them to produce a slurry of monodispersed magnetic

particles which can be coated and oriented. This dis-

persion is very important in determining the ﬁnal

quality of the tape. Not only does it affect the ability

of particles to be oriented by application of a mag-

netic ﬁeld during production, but poorly dispersed

particles will increase the roughness of the tape sur-

face and affect noise characteristics (Clarke et al.

1991). A detailed discussion of wet dispersions can

be found in Magnetic Recording Media: Particulate

Dispersions but the essential processes of tape manu-

facture are as follows:

(i) Premixing—some of the components including

the magnetic particles, wetting agents, and a portion

of the solvents are mixed at high shear to wet the

particle surfaces and exclude any water adsorbed on

the surfaces.

(ii) Milling—the components are mixed in a bead

mill or similar device to generate shear and separate

particles. Milling times and rates are important as

‘‘over milling’’ generates broken particles whereas

‘‘under milling’’ leaves agglomerates in the system.

Broken particles will have a low switching ﬁeld or

may be superparamagnetic and are detrimental to

the recording properties of the tape resulting in

‘‘print through’’ and a lower saturation remanence.

673

Magnetic Recording: VHS Tapes

Agglomerates will cause an increase in the tape noise

and poor orientation.

(iii) Let down—additional solvents are added to

control the ﬁnal viscosity of the slurry ready for

coating. This stage has to be completed carefully to

prevent solvent shock, which can cause ﬂocculation

of the particles.

(iv) Filtering—any remaining agglomerates or

oversized particles are removed by passing through

a ﬁne mesh ﬁlter.

(v) Coating—the ﬁnal wet dispersion is applied to

a plastic substrate using a knife coater or gravure

roller. This is part of the mass-production process.

Coating is at high speed onto wide plastic webs.

(vi) Orientation—immediately after coating and

before the ﬁlm has dried, the web is passed through

an arrangement of magnets that increases particle

alignment along the tape axis.

(vii) Drying—the web is then passed through an

oven where the solvents are driven off and collected

for recycling. The dried web is then wound onto rolls

and stored to allow further curing of the polymers.

(viii) Calendaring—the web is passed between

heated rollers that have specially prepared paper-

coated surfaces. High pressure is applied so that the

combination of heat and pressure produces a smooth

tape surface. This is important in maintaining a good

head/tape contact in the record and replay processes

to minimize asphericity and surface roughness noise.

(ix) Back coating—a nonmagnetic layer contain-

ing carbon black is coated onto the back surface of

the web. This prevents slippage during use and also

reduces buildup of electrostatic charge.

The large rolls of completed media, referred to as

jumbos, are then slit to the ﬁnal tape width, slitting

debris is removed, and the tape is wound onto indi-

vidual reels, called pancakes. In a separate process,

tape is spliced to plastic leader tapes already attached

to reels in cassettes and the appropriate length of tape

is loaded ready for ﬁnal labeling and packaging of the

cassettes.

4. Media Properties

The performance of a tape is deﬁned in terms of its

recording and replay properties, and characteristics

such as signal to noise ratio. These are speciﬁed in the

international standards and comparison is made to

a reference tape VRT-2, which can be provided by

JVC. In practice, having established particle supply

and the tape production process, quality can be

controlled by static magnetic measurements using a

vibrating sample magnetometer. The only static

magnetic property identiﬁed in the standards is the

approximate coercivity (50 10

1

). However, in

general all tapes have very similar major hysteresis

loop shape and characteristics. Figure 2 shows the

major hysteresis loop for a typical proprietary VHS

tape, measured in the tape direction and transverse to

it. The effect of particle orientation can be clearly

seen by comparison of the shape of the two loops.

Listed below are some parameters that have been

devised to characterize recording media and values

measured for this sample.

(i) Coercivity, H

(52 kAm

1

(648 Oe))

(ii) Saturation magnetization, M

, usually expressed

as a moment per unit area (490 Am

3

(49 memucm

2

))

(iii) Remanent magnetization, M

(370 Am

3

(37 memucm

2

))

(iv) Squareness, M

(0.76)

(v) Orientation ratio, M

r[in tape direction]

r[transverse]

(1.67)

(vi) Switching ﬁeld distribution (SFD), obtained

by differentiating the hysteresis loop. This produces a

peak around the coercivity. The SFD is deﬁned as the

peak width at half height (27 kAm

1

(340 Oe))

(vii) Coercivity squareness, S*, a measure of the

steepness of the loop at the crossing point (0.68). It is

deﬁned by (Ko

ster 1984):



M¼0

ð1  S



5. Camcorder Applications

A major use of video tape is for camcorder recording.

The standard VHS tape is far too cumbersome for this

application and so a compact VHS cassette has been

developed. This uses the standard 12.65 mm tape but

in a cassette measuring 59 mm 92 mm 22.5 mm,

which is about a quarter of the volume of a standard

cassette. The compact cassette can be inserted into an

Figure 2

Hysteresis loops for a cobalt g-Fe

VHS tape

measured along the tape axis direction and transverse to

it. The differential of the hysteresis is also shown and

indicates the switching ﬁeld distribution measured at

half height.

674

Magnetic Reco rding: VHS Tapes

adapter, which then allows it to be replayed or edited

in a standard VHS VCR. There are rivals for ana-

logue camcorders, the most notable being the 8 mm

system, which like the later development of Hi8 use

8 mm wide tape in a much more compact cassette and

an increased coercivity. The 8 mm tapes are based on

metal particles, usually passivated iron, and the Hi8

uses either metal evaporated tape or advanced metal

particles. The 8 mm systems have an improved re-

sponse over VHS. However, in spite of taking a large

part of the camcorder market, 8 mm VCR machines

have failed to make much impact on the television-

related use for time shifting and replay of prerecorded

‘‘videos.’’ Digital camcorders are now available and

these will replace the older VHS and 8 mm analogue

systems very rapidly.

6. Prerecorded Tape

Much of the tape that is produced is sold with pre-

recorded ﬁlms and other entertainment material. Mak-

ing copies used to involve playing a master tape on

one VCR that was re-recorded on a number of copy

machines. This process is very slow, equipment inten-

sive, and is not cost effective. A process of anhysteretic

duplication has been developed that produces a mag-

netic image of a master tape in a high-speed process.

The principles of the technique are based on an-

hysteretic remanence. This process involves the appli-

cation of a small d.c. bias magnetic ﬁeld to a magnetic

sample subjected to a saturating a.c. ﬁeld which is then

slowly removed. The d.c. bias ﬁeld is then removed

and a remanent magnetization is left. Figure 3 shows

the anhysteretic remanent magnetization (ARM) as a

function of d.c. bias ﬁeld for a typical VHS tape.

For comparison, the isothermal remanence (IRM)

and the d.c. demagnetization remanence (DCD) are

also shown. The IRM curve is a plot of the reman-

ence produced by the application of a d.c. magnetic

ﬁeld to an initially a.c. demagnetized specimen while

the DCD curve is a plot of the remanence produced

in an initially negatively saturated sample. The IRM

and DCD curves are used extensively as an indication

of the nature of interactions in recording media

(Spratt et al. 1988, Kelly et al . 1989). The steep slope

of the initial anhysteretic curve is evident in Fig. 3.

In anhysteretic duplication, a mirror image of the

required video tape is produced on a master tape.

When placed in contact with a blank VHS tape

(magnetic surfaces together) and subjected to an a.c.

ﬁeld, the stray ﬁelds from the master tape magnetic

recording are sufﬁcient to produce an anhysteretic

magnetic image imprinted on the VHS tape. It is clear

that if the master tape is to withstand demagnetiza-

tion in the process, the a.c. ﬁeld must be sufﬁcient to

saturate the VHS tape but small enough so as not

demagnetize the master tape, which must have a high

coercivity (typically 160 10

1

(2 kOe)).

7. Signal Loss and Archival Problems

Tape replay is a dynamic process involving a magnetic

interaction between the tape and the replay head. This

produces a change in the magnetization of the tape that

has to be reversible if the recorded signal is to remain

unchanged. Magnetic changes do not affect the basic

structure of the tape, unlike mechanical deterioration,

which can result from rep lay in poor ly maintained

VCRs. Magnetic deterioration is not evident from vis-

ual inspection and re-recording restores the good qual-

ity video signal. First generation tapes did exhibit a

signal loss with replay that was identiﬁed as being

related to magnetostriction effects (Flanders 1976,

Flanders et al. 1979). However, changes in formula-

tion to reduce particle stresses improved tape perform-

ance. This effect was not evident in later generations of

particles and this type of signal loss is no longer re-

garded as a problem in commercial tapes. However,

some problems do still exist. All tapes are subject to

dropouts associated with localized physical or magne-

tic defects where the recorded signal is defective or

unstable. With replay, these dropouts increase in

number resulting in a ‘‘snowy’’ TV image. Magnetic

debris trapped on guides and heads can also cause sig-

nal loss, which is evident as missing lines on the TV

image. With continual replay these can merge into

bands and are referred to as magnetic scratching

(S-Ararghi et al. 1992). Replay can also lead to a

general increase in noise levels (McCann et al. 1999).

This is believed to be a result of magnetostric tion as

identiﬁed in the signal loss of ﬁrst generation tapes but

at a much lower level so that it is only observed in the

noise signal.

Figure 3

ARM for a cobalt g-Fe

VHS tape. Isothermal

remanent magnetization and d.c. demagnetization

curves are included for comparison.

675

Magnetic Recording: VHS Tapes

8. Conclusions

The VHS system has to be regarded as a commercial

success story. Having been adopted as the preferred

home video system, production of VCRs and video

tape has produced one of the major volume storage

media products. Although the technology has been

surpassed by other systems, there is a tremendous

inertia to change generated by the high domestic in-

vestment in machines and availability of prerecorded

tapes. The VHS system is likely to remain the pre-

ferred medium for some time and will only be re-

placed with the move to digital recording.

See also: Magnetic Recording Materials: Tape Par-

ticles, Magnetic Properties; Magnetic Recording

Media: Particulate Dispersions; Magnetic Recording

Devices: Inductive Heads, Properties; Magnetic Re-

cording Technologies: Overview

Bibliography

Clarke M D, Bissell P R, Chantrell R W, Gilson R G 1991

Experimental studies of particulate recording media noise.

J. Magn. Magn. Mater. 95, 17–26

EN 60774-1 Part 1 1994 VHS and compact VHS video cassette

system. European Standard. European Committee For Elect-

rotechnical Standardization

Flanders P J 1976 Changes in recording tape magnetization

produced by stress. IEEE Trans. Magn. MAG-12, 348–55

Flanders P J, Kaganowicz G, Takei Y 1979 Magnetostriction

and stress-induced playback loss in magnetic tapes. IEEE

Trans. Magn. MAG-15, 1065–8

Kelly P E, O’Grady K, Mayo P I, Chantrell R W 1989 Switch-

ing mechanisms in cobalt–phosphorus thin ﬁlms. IEEE

Trans. Magn. 25, 3880–3

ster E 1984 Recommendation of a simple and universally ap-

plicable method for measuring the switching ﬁeld distribution

of magnetic recording media. IEEE Trans. Magn. 20, 81–3

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Magnetic Refrigeration at Room

Temperature

Magnetic refrigeration, based on the magneto-

caloric effect (MCE), has recently received increased

attention as an alternative to the well-established

compression–evaporation cycle for room-tempera-

ture applications. In the magnetic refrigeration cycle,

depicted in Fig. 1, initially randomly oriented mag-

netic moments are aligned by a magnetic ﬁeld, result-

ing in heating of the magnetic material. This heat is

removed from the material to the ambient by heat

transfer. On removing the ﬁeld, the magnetic moments

randomize, which leads to cooling of the material be-

low ambient temperature. Heat from the system to be

cooled can then be extracted using a heat-transfer

medium. Depending on the operating temperature, the

heat-transfer medium may be water (with antifreeze)

or air, and for very low temperatures helium. There-

fore, magnetic refrigeration is an environmentally

friendly cooling technology. It does not use ozone de-

pleting chemicals (CFCs), hazardous chemicals (NH

or greenhouse gases (HCFCs and HFCs). Another

key difference between vapor-cycle refrigerators and

magnetic refrigerators is the amount of energy loss

incurred during the refrigeration cycle. The cooling

efﬁciency in magnetic refrigerators working with

gadolinium has been shown (Zimm et al.1998)to

reach 60% of the theoretical limit, compared to only

B40% in the best gas-compression refrigerators. This

higher energy efﬁciency will also result in a reduced

release. However, with most magnetic materials,

this high efﬁciency is only realized in high magnetic

ﬁelds, 5 T. Therefore current research aims at new

magnetic materials displaying larger magnetocaloric

effects, which then can be operated in lower ﬁelds,

B2 T, that can be generated by permanent magnets.

The heating and cooling described above is pro-

portional to the size of the magnetic moments and the

Figure 1

Schematic representation of a magnetic-refrigeration

cycle, that transports heat from the heat load to the

ambient. The lighter- and darker-shaded square

containers depict material in low and high magnetic

ﬁeld, respectively.

676

Magnetic Refrigeration at Room Temperature

applied magnetic ﬁeld. This is the reason that, now-

adays, research in magnetic refrigeration is almost

exclusively conducted on super-paramagnetic mate-

rials and on rare-earth compounds (Tishin 1999). For

room-temperature applications like refrigerators and

air-conditioners, compounds containing manganese

should be a good alternative. Manganese is a tran-

sition metal with high abundance. Also, there exist in

contrast to rare-earth compounds, an almost unlim-

ited number of manganese compounds with critical

temperatures near room temperature. However, the

magnetic moment of manganese generally is only

about half the size of heavy rare-earth elements. En-

hancement of the caloric effects associated with mag-

netic moment alignment may be achieved through the

induction of a ﬁrst order phase-transition or better, a

very rapid change of magnetization at the critical

temperature, which will bring along a much higher

efﬁciency of the magnetic refrigerator. In combina-

tion with currently available permanent magnets, this

will open the path to the development of small-scale

magnetic refrigerators, which no more rely on rather

costly and service-intensive superconducting mag-

nets. Another prominent advantage of magnetoca-

loric refrigerators is that the cooling power can be

varied by scaling from milliwatt to a few hundred

watts.

Following the discovery of a sub-room-temperature

giant-MCE in the ternary compound Gd

(Ge

1x

)

(0.2pxp0.5)(Pecharsky and Gschneidner 1997), there

is a strongly increased interest from both fundamental

and practical points of view to study the MCE in Gd-

based materials (Choe et al.2000,Morellonet al.

2000). The common feature of these compounds is

that they undergo a ﬁrst-order structural and magnetic

phase transition, which leads to a giant magnetic ﬁeld-

induced entropy change, across their ordering temper-

ature. Many other materials have been studied with

respect to their (magneto-caloric) properties, here we

will mention only a few of them. The lanthanum–

manganese–perovskite oxides La

1x

MnO

show

considerable MCE values (Zhang et al.1996,Hueso

et al. 2002). Transition metal pnictides such as

MnP

1x

(Krokozinsky et al. 1982) and chalcogen-

ites such as Cr

(Hashimoto 1981) have been stud-

ied. The nearly equiatomic alloy FeRh undergoes a

magnetic and structural order–order transformation

and exhibits giant MCE between 296 K and 316 K but

its MCE is irreversible and disappears after the ﬁrst

application of the magnetic ﬁeld (Annaorazov et al.

1992, 2002). The Heusler alloys Ni–Mn–Ga show

large magnetic-entropy changes that are due to an

abrupt change of the magnetization near the marten-

site–to–austenite structural transition (Hu et al. 2000,

2001a). The compound La(Fe,Co)

11.83

1.17

with a

very low rare-earth content, also shows a consider-

able magnetocaloric effect (Hu et al. 2001b). The

MCEs in most transition-metal-based alloys are lower

than those in lanthanide-based alloys for the same

temperature range, especially if one takes into account

the fact that DT also depends on the specific heat of

the compound (Pecharsky et al.2001).Thismakes

them unlikely candidates for use as magnetic refriger-

ant materials below room temperature. However,

above room temperature there exist only a very lim-

ited number of rare-earth compounds exhibiting mag-

netic ordering, a prerequisite for showing large

magnetocaloric effects. Fortunately, there exist many

manganese compounds, such as MnAs (Pytlik and

Zieba 1985), which display a variety of structural and

magnetic phase transitions in the relevant temperature

range. For a working material in a magnetic refriger-

ator it is however essential that the phase transition is

also reversible.

1. Theoretical Considerations

When a material is magnetized in an applied mag-

netic ﬁeld, the entropy associated with the magnetic

degrees of freedom, the so-called magnetic entropy

, is changed as the ﬁeld changes the magnetic order

of the material. Under adiabatic conditions, DS

must be compensated by an equal but opposite

change of the entropy associated with the lattice, re-

sulting in a change in temperature of the material.

This temperature change, DT

, is usually called the

magnetocaloric effect. It is related to the magnetic

properties of the material through the thermodynam-

ic Maxwell relation



ð1Þ

For magnetization measurements made at discrete

temperature intervals, DS

can be calculated by

means of

ðT; BÞ¼

iþ1

ðT

iþ1

; BÞM

ðT

; BÞ

iþ1

 T

DB ð2Þ

where M

iþ1

ðT

iþ1

; BÞ and M

ðT

; BÞ represent the val-

ues of the magnetization in a magnetic ﬁeld B at the

temperatures T

iþ1

and T

, respectively. On the other

hand, the magnetic entropy change can be obtained

more directly from a calorimetric measurement of the

ﬁeld dependence of the heat capacity by integration:

ðT; BÞ¼

CðT

; BÞCðT

; 0Þ

ð3Þ

where C ðT; BÞ and CðT; 0Þ are the values of the heat

capacity measured in a ﬁeld B and in zero ﬁeld, re-

spectively. It has been conﬁrmed that the values of

ðT; BÞ derived from the magnetization measure-

ment coincide with the values from calorimetric

measurement (Gschneidner et al. 1999, Tegus et al.

2002b).

677

Magnetic Refrigeration at Room Temperature

The adiabatic temperature change can be inte-

grated numerically using the experimentally meas-

ured or theoretically predicted magnetization and

heat capacity:

ðT; BÞ¼

CðT; B



ð4Þ

Obviously, the MCE is large when



is large

and CðT; BÞ is small at the same temperature. Since



peaks at the magnetic ordering temperature, a

large MCE is expected close to this magnetic phase

transition.

Recently we discovered that MnFeP

1x

com-

pounds exhibit a large magnetic entropy change

(Tegus et al. 2002a), of a similar magnitude as the so-

called giant MCE material Gd

, but whose

optimum use temperature is at and above room tem-

perature. This result is of significant practical impor-

tance, because it makes these compounds excellent

working materials for magnetic refrigeration in

household refrigerators or air-conditioning.

In the intermediate composition range (0.15p

xp0.66), compounds in the MnFeP

1x

system

crystallize in the hexagonal Fe

P-type of structure.

They have most interesting magnetic properties in-

cluding a ﬁeld-induced ﬁrst-order magnetic transition

(Zach et al. 1990). Polycrystalline samples can be syn-

thesized starting from the binary Fe

PandFeAs

compounds, Mn chips and P powder mixed in the ap-

propriate proportions by ball milling under a protec-

tive atmosphere. After this mechanical alloying process

one obtains amorphous powder. To obtain dense ma-

terial of the crystalline phase, the powders are pressed

to pellets and sealed in molybdenum tubes under an

argon atmosphere. These are heated at 1273 K for

120 h, followed by a homogenization process at 923 K

for 120 h and ﬁnally by slow cooling to ambient con-

ditions. The powder x-ray diffraction patterns show

that the compound crystallizes in the hexagonal Fe

P-

type structure. In this structure the Mn atoms occupy

the three(g) sites, the Fe atoms occupy the three(f) sites

and the P and the As atoms occupy two(c) and one(b)

sites statistically (Bacmann et al. 1994). From the

broadening of the x-ray diffraction reﬂections, the av-

erage grain size is estimated to be B100 nm.

Figure 2 shows the change in critical temperature

on the application of a magnetic ﬁeld. As pointed out

by Pecharsky et al. (2001), this change in critical

temperature is a good indication for the temperature

change induced by the application of a magnetic ﬁeld.

The inset of Fig. 2 shows the temperature dependence

of the magnetization near room temperature of

MnFeP

0.45

0.55

and Gd (Alfa Aesar3N) as deter-

mined in an applied magnetic ﬁeld of 1 T. It is

obvious that the change in magnetization in (Mn,Fe)

(P,As) compounds at the ordering temperature T

is much larger, despite the fact that the magnetic

Figure 2

Variation of critical temperature with applied magnetic ﬁeld, the line is a least square ﬁt of the data. Inset:

Temperature dependence of the magnetization for MnFeP

0.45

0.55

and Gd measured with increasing (heating) and

decreasing (cooling) temperature in a ﬁeld of 1 T.

678

Magnetic Refrigeration at Room Temperature

moment of Gd is much larger at low temperatures

248 Am

1

compared to 125 Am

1

. Variation

of the P/As ratio between 3/2 and 1/2 makes it pos-

sible to tune T

and the optimal operating temper-

ature between 200 K and 350 K (70 1C and 80 1C),

without losing the large magnetocaloric effect. The

inset shows the temperature dependence of the mag-

netization measured with increasing and decreasing

temperature in an applied ﬁeld of 1 T. The thermal

hysteresis is signature of a ﬁrst-order phase transi-

tion. Because of the small size of the thermal hyster-

esis (o1 K), the magnetization process can be

considered as being reversible in temperature. From

the magnetization curve at 5 K, the saturation mag-

netization was determined as 3.9 m

/f.u. This high

magnetization originates from the parallel alignment

of the Mn and Fe moments, though the moments of

Mn are much larger than those of Fe (Beckmann and

Lundgren 1991). Variation of the Mn/Fe ratio may

also be used to further improve the magnetocaloric

effect.

We calculated the magnetic-entropy changes, DS

from magnetization data by means of the Eqn. (2).

The results are shown in Fig. 3. The calculated

maximum values of the magnetic entropy change is

14.5 J kg

1

for a ﬁeld change from 0 T to 2 T.

The maximum magnetic entropy in 3d materials

depends on the spin moment S. Because there are

two magnetic ions per formula unit, one has S

2R lnð2S þ 1Þ, where R is the universal gas constant.

From the saturation magnetic moment, we estimate

the average S value of the magnetic ions to be equal

to S ¼1 thus S

¼18.3 J mol

1

K ¼117 J kg

1

which is about six times larger than the value

obtained from the magnetization measurements. For

comparison, the magnetic-entropy change of the com-

pound Gd

and Gd is also shown in the Fig. 3.

It is evident that the MCE in MnFeP

1x

com-

pounds is comparable with that of Gd

though

the Gd compound has a larger magnetic moment at

5 K. The origin of the large magnetic-entropy change

should be attributed to the comparatively high three

moments and the rapid change of the magnetization

in the ﬁeld-induced magnetic phase transition. In rare-

earth materials, the magnetic moment fully develops

only at low temperatures and therefore the entropy

change near room temperature is only a fraction of

their potential. In 3d compounds, the strong magneto-

crystalline coupling results in competing intra- and

inter-atomic interactions and leads to a modiﬁcation

of metal–metal distances which may change the iron

and manganese magnetic moment and favors the spin

ordering.

Bearing in mind the use of these materials in mag-

netic refrigerators, next to the magnetocaloric prop-

erties also the electrical and heat conductivity is of

utmost importance. There is hardly any information

on the electrical-transport properties of this system.

The electrical resistance can also be useful for a more

detailed investigation of the magnetic phase transi-

tion because it is very sensitive to changes in the in-

teractions between magnetic ions. The availability of

electrical-resistance data would make it possible to

compare the critical magnetic ﬁelds derived from

magnetic and electrical measurements and to under-

stand the role of the electron–phonon and electron–

magnon interactions in the magnetic phase transi-

tions. The temperature dependence of the electrical

Figure 3

Magnetic-entropy changes of MnFeP

0.45

0.55

for external magnetic ﬁeld changes of 0–2 calculated from the

magnetization data, in comparison with those of Gd (m) and Gd

(.).

679

Magnetic Refrigeration at Room Temperature