Buschow K.H.J. (Ed.) Concise Encyclopedia of Magnetic and Superconducting Materials

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magnetism in these oxides is that the exchange inter-

action between the magnetic ions is mediated by

the intermediate oxygen ion through super exchange

(Anderson 1963, Goodenough 1963). Assuming that

the magnetic ions M

and M

lie on a straight line

through the intermediate oxygen ion O, the magnetic

super exchange interaction can be explained in the

following covalent bonding picture for magnetic ions

with at least a half-ﬁlled shell: one of the electrons of

the oxygen p-orbital directed to the magnetic ions will

be excited or transferred to the ion M

On M

the electron can only enter an empty orbital

with its spin direction antiparallel to the magnetic

moment of M

. The other electron of the p-orbital

pointing along the M

–O–M

bond can only be ex-

cited to the M

ion and lower the total energy of the

system, if the magnetic moment of M

is oriented

antiparallel to M

. This is so because both electrons

involved originate from the same oxygen p-orbital,

and thus their spins must be antiparallel according to

the Pauli exclusion principle. Hence, the magnetic

(spin) moments of both ions should be aligned an-

tiparallel to obtain the lowest energy state and to

fulﬁll the Pauli principle at the same time. More

about the super exchange interaction can be found in

the reviews by Anderson (1963) and Goodenough

(1963). For the super exchange interaction it can be

derived that it is strong and antiferromagnetic along

a 1801 bond, i.e. when M

–O–M

form a straight line,

while it is weak and ferromagnetic along a 901 bond.

This explains that the interaction between the mag-

netic ions on the A- and B-sites in spinel ferrites,

which make an angle of around 1251 (see Fig. 1), is

much stronger than between those on the B-sites

which make an angle of around 901 (Smit and Wijn

1959). Thus in ferrites the dominant magnetic inter-

action is the AB interaction, which is typically 15–20

times stronger than the BB interaction. This causes

the magnetic moments on the A and B sites to be

antiparallel oriented whereas those on the B-sites are

parallel to each other.

The net magnetic moment of spinel ferrites is thus

determined by the sum of the magnetic moments of

the two spins on the B-site minus the moment of the

A-site. This results in typical saturation magnetiza-

tions, M

, of spinel ferrites between 143–560 kAm

1

at 0 K and 120–477 kAm

1

at room temperature

(Smit and Wijn 1959), which are lower than what can

be attained in metallic magnetic materials (where the

dominant magnetic interaction is ferromagnetic).

Most of the spinel ferrites, with the exception of

ZnFe

, have a Curie temperature T

in excess of

room temperature, with typical values ranging from

300 1C for MnFe

to 670 1C for Li

0.5

2.5

(Smit

and Wijn 1959). This T

of ferrites is acceptable for

most applications that require typically an operating

temperature of the ferrite of 80 1C. In most commer-

cial applications either MnZn-ferrites or NiZn-fer-

rites are used. The MnZn-ferrites have the advantage

that high values of the permeability m can be reached,

whereas NiZn-ferrites offer advantageously higher

values of the resistivity rE10

Om versus 1 Om for

MnZn-ferrites. This makes NiZn-ferrites more suited

for high frequency application, in particular for fre-

quencies X1 MHz. For applications which require a

high resistivity and low material costs, MgZn-ferrites

(rE10

Om) are used. For instance, the yoke-rings in

the deﬂection unit of cathode-ray tubes are made of

MgZn-ferrites as this makes it possible to directly

wind the deﬂection coil on the ferrite yoke.

For applications the magnetic softness character-

ized by the permeability m ¼dB/dH is relevant as it

gives the response of a ferrite to an applied ﬁeld H.

Depending on intrinsic properties (composition,

additions) and extrinsic properties (microstructure,

preparation) permeabilities of X15 000 can be ob-

tained. The highest permeabilities are typically

reached in MnZn-ferrites with large grains, which

are free from pores. NiZn-ferrites generally have

lower permeabilities, in commercial applications be-

tween 100 and 1000. This is because compensation of

the magnetocrystalline anisotropy, as can be done in

MnZn-ferrites, is not possible in NiZn-ferrites (Smit

and Wijn 1959).

The only commercial application in which mono-

crystalline spinel ferrites are used on a large scale is

the magnetic recording head production for VCRs.

For most applications ferrites are made by ceramic

processing which results in polycrystalline materials.

Ceramic processing is done by mixing and calcining of

oxide and carbonate powders of the constituent met-

als. Subsequently, the powder is pressed in the desired

shape and the ﬁnal processing step is a sintering proc-

ess following a selected temperature proﬁle under a

carefully controlled atmosphere. Typically sinter tem-

peratures needed to have the required solid state re-

actions proceed are 1100 1C or more. Details can be

found elsewhere (Smit and Wijn 1959, Snelling 1988).

For further information about soft ferrites the

reader is referred to the book by Smit and Wijn

(1959) and the review by Brabers (1995) and also to

Transition Metal Oxides: Magnetism.

1. Applications

Traditionally the main application areas of soft-fer-

rites are transformer cores, inductors, and yoke-rings

in cathode-ray tubes (Snelling 1988). Since the 1990s

new applications have arisen, such as cores for in-

duction lighting and magnetic antennas for pagers.

The trend in all these applications and the driving

force for ferrite development is that the operating

frequency increases to achieve miniaturization. This

trend is illustrated in Fig. 2 which gives the weight

and operating frequency of a 100 W power supply

since the 1960s. A reduction in weight from approx-

imately 8 Kg to around 100 g has been achieved.

180

Ferrites

The reason that miniaturization necessitates higher

operating frequencies can be understood by consid-

ering the throughput power P

thr

of a transformer

core, which can be given as:

thr

¼ I

 V

ð1Þ

with I

the input current and V

the input voltage.

Expressing V

in terms of the change of ﬂux in time

dF/dt gives:

thr

¼ I

 N

¼ I

 N2pfBA ð2Þ

with N the number of turns of the primary winding of

the transformer, f the operating frequency, B the in-

duction level in the core, and A the cross-sectional

area of the core. Since the maximum induction level

within the core is limited by the saturation magnet-

ization of the ferrite, M

, it is obvious from Eqn. (2)

that in order to decrease the size and hence cross-

sectional area of the transformer, without sacriﬁcing

throughput power, the operating frequency f should

be increased. Hence, a key aspect of modern research

and development in soft ferrites is to reduce the dis-

sipation, i.e., losses in ferrites at increasing frequen-

cies, in the range of 100 kHz to 3 MHz.

Traditionally, three contributions to the dissipa-

tion or loss, P

tot

, in soft ferrites are distinguished: d.c.

hysteresis loss, P

; eddy current loss, P

; and residual

loss, P

. The justiﬁcation for this subdivision resides

in part in the differences in frequency response of

each contribution, although it is to a large part phen-

omenological. The d.c. hysteresis loss corresponds to

the dissipation incurred by cycling the d.c. B-H loop

of a soft-magnetic ferrite. Through the number of

times the loop is cycled per unit time, P

depends on

frequency f as P

¼f

BdH. The standard approach

to limit the hysteresis loss in ferrites (Smit and Wijn

1959, Snelling 1988) is to facilitate domain wall

movement by using ferrites with a low coercivity, H

This is done by selecting ferrite compositions which

have a low magneto-crystalline anistropy constant K.

Ideally the ferrite should have its anisotropy com-

pensation point, at which K passes through zero, at

the required operating temperature. Typically this

is 80 1C. In addition, the ferrite composition used

should have a low magnetostriction constant and in-

ternal and external stress should be avoided to limit

adverse magnetostriction effects. Finally, ferrites with

a large grain size are used as H

1

, with D the

grain size (see van der Zaag 1999).

The eddy current contribution is the dissipation

caused by eddy currents in the ferrite. Although, due

to their oxidic character, ferrites have a much higher

resistivity than metallic soft magnetic materials, their

conductivity is not zero and eddy currents can occur.

One can derive that the dissipation P

is (Snelling

1988):

¼ C

=r ð3Þ

with C

a dimensional constant and r the resistivity at

the measurement frequency. Hence, the P

contribu-

tion can be separated by plotting P

tot

/f vs f, as has

been done in Fig. 3. Traditionally, the remainder of

the dissipation, such as the dissipation due to ferro-

magnetic resonance (Snoek 1948) has been treated as

the residual loss contribution, P

. The residual loss is

thus the extra loss under a.c. measurement conditions

not attributable to eddy current loss (i.e., not match-

ing Eqn. (3)). Thus P

has a higher than quadratic

dependence on the frequency f. As Fig. 3 illustrates at

higher frequencies both the eddy current and residual

loss dominate the ferrite dissipation. Consequently,

considerable work was done in the 1990s to deter-

mine and minimize the mechanisms responsible for

these losses (van der Zaag 1999).

To reduce the eddy current loss, the resistivity of

ferrites should be increased. The resistivity of the

ferrites used in commercial applications is determined

by the resistivity of the grain boundary in these po-

lycrystalline materials. Hence, considerable effort has

been devoted to increase the grain boundary resist-

ance by careful control of the processing conditions

and by adding dopants such as Nb

, ZrO

, and

besides the well-known glass-forming addi-

tions CaO and SiO

(Snelling 1988). The glass form-

ers segregate to the grain boundary to form a

secondary phase, which forms an insulating layer

around each grain preventing current ﬂow from one

grain to the next. At higher frequencies also the ca-

pacitive shunting of the grain boundary impedance

becomes an issue. It is thought that the addition of

Figure 2

The miniaturization trend in the weight of transformer

cores since the 1960s. The resulting operating

frequencies are indicated. The inset shows various sizes

of E-cores used for transformers.

181

Ferrites

dopants like Nb

, ZrO

, and Ta

play a role in

reducing this effect.

The residual loss has been shown to be a dom-

inant contribution to the dissipation of ferrites in the

500 kHz to 3 MHz frequency range. Microscopic

mechanisms contributing to the residual loss are: the

ferromagnetic resonance loss (Snoek 1948) and the

intragranular domain wall loss (van der Zaag 1999).

The former is caused by excitation of the uniform

spin precession in the ferrite. The latter is due to

domain wall movement of intragranular domain

walls within the grains of a polycrystalline ferrite.

Both contributions can be eliminated by using a

ﬁne-grained ferrite material. In the case of the ferro-

magnetic resonance loss this causes a reduction in

the permeability of the ferrite and hence an upward

shift of the ferromagnetic resonance (Snoek 1948).

In well-developed ferrites the ferromagnetic reso-

nance should be above the frequency at which the

ferrite is operated. The intragranular domain wall

loss is eliminated by using ﬁne-grained ferrites in

which all the grains are monodomain, i.e., intra-

granular domain walls are absent. Such a situation

occurs in ﬁne-grained ferrites, since below a certain

grain size the monodomain state has a lower mag-

netostatic energy than a domain state in which a

domain wall is present (van der Zaag 1999). For

further reading on the subject of dissipation in fer-

rites the reader is referred to (Snoek 1948, van der

Zaag 1999).

2. New Developments: Ferrite Films

A new development in the ﬁeld of soft-ferrite research

and their application is their deposition and use in

thin ﬁlms (see Brabers 1995). Spurred by the work on

high-T

superconductors, there has been considera-

ble improvement in the deposition of thin ﬁlm oxides,

making it possible to deposit thin ﬁlm ferrites in the

nanometer to micrometer thickness range. The inter-

est in these thin ﬁlm ferrites is driven in part by the

possibility to improve integrated inductors (Suzuki

et al. 1996).

In the IC industry a problem is that present in-

ductors, amongst others for RF-applications, with

typical diameters of 0.2–0.8 mm are huge in compar-

ison to the active elements on ICs. Thus they require

a too large surface area to make integration on chip

economically attractive. A possible solution would be

to apply a magnetic substrate or a thin magnetic ﬁlm

on the coil as this may result in a reduction to a

the area needed for the same inductance while main-

taining the quality factor Q (as is illustrated in Fig. 4

by using a ferrite substrate). In view of their resistiv-

ity, which makes application at higher frequencies

possible, ferrite ﬁlms deposited on the integrated in-

ductors can provide a possible solution. A problem is

the high substrate temperature (X600 1C) needed to

assure the formation of a ferrite ﬁlm with a proper

cation distribution and an adequate grain size to as-

sure that the ferrite ﬁlms have a sufﬁciently high per-

meability. Wet-chemically prepared ﬁlms grown by

ferrite plating at 80 1C provide a useful method as

permeabilities X25 have been obtained in mm thick

ﬁlms (van der Zaag et al. 1999).

Figure 4

The quality factor, Q ¼oL/R with o the angular

frequency, L the inductance and R the resistance of the

inductor, of two planar inductors, one on a glass

substrate and the other on a NiZn-ferrite substrate.

Although both inductors have comparable inductances

(L ¼33 and 28.4 nH) and Q, the inductor using a ferrite

substrate has half the diameter, i.e., needs 1/4 of the

area.

Figure 3

Schematic diagram of the total dissipation, P

tot

, versus

the frequency, f, for soft-magnetic ferrites. The various

contributing terms to the ferrite dissipation: hysteresis

loss P

, eddy current loss P

and residual loss P

are

indicated. All terms have been scaled by the frequency.

182

Ferrites

Another interesting trend is the work on magnetite,

, ﬁlms in view of their intrinsic fully spin-polar-

ized electron transport for magnetoresistive sensors

(Coey et al. 1998, Seneor et al. 1999). Fe

is a half-

metallic ferromagnet (see Half-metallic Magnetism)as

it has a gap in the minority spin band, whereas there is

no gap in the majority spin band (Brabers 1995). Thus

for the majority spins Fe

behaves as a metal with

spin-polarized electron transport along the B-sites by

hopping conduction. This property is of interest as it

could be employed in magnetic tunnel junctions lead-

ing to a magnetic ﬁeld sensor element with, theoret-

ically, an inﬁnite magnetoresistance effect, as electrons

with minority spin cannot pass the Fe

layer. The

degree to which a proper Fe

layer can be grown in

these magnetic tunnel junctions, or needs to be grown

to have a positive effect on the magnetoresistance is

not known precisely. The same holds for the role

played by the interfaces of the Fe

layer or anti-

phase boundaries, causing so-called ‘‘dead’’ magnetic

interface layers. A ﬁrst indication of an increased

magneto-resistance effect due to the incorporation of a

-like layer in a magnetic tunnel junction has

been reported by Seneor et al. (1999).

See also: Ferrite Ceramics at Microwave Frequencies

Bibliography

Anderson P W 1963 Exchange in insulators: superexchange,

direct exchange and double exchange. In: Rado G T, Suhl H

(eds.) Magnetism I. Academic Press, New York, Chap. 2

Brabers V A M 1995 Progress in spinel ferrite research. In:

Buschow K H J (ed.) Handbook of Magnetic Materials. Else-

vier, Amsterdam, Chap. 3

Coey J M D, Berkowitz A E, Balcells L, Putris F F, Parker F T

1998 Magnetoresistance of magnetite. Appl. Phys. Lett. 72,

734–6

Goodenough J B 1963 Magnetism and the Chemical Bond.

Wiley, New York, pp. 165–85

Seneor P, Fert A, Maurice J-L, Montaigne F, Petroff F, Vaure

A 1999 Large magnetoresistance in tunnel junctions with an

iron oxide electrode. Appl. Phys. Lett. 74, 4017–9

Smit J, Wijn H P J 1959 Ferrites. Wiley, New York

Snelling E C 1988 Soft Ferrites: Properties and Application, 2nd

edn. Butterworths, London

Snoek J L 1948 Dispersion and absorption in magnetic ferrites

at frequencies above one Mc/s. Physica 14, 207–17

Suzuki Y, van Dover R B, Gyorgy E M, Philips J M, Korenivski

V, Werder D J, Chen C H, Cava R J, Krajewski J J, Peck W F

Jr., Do K B 1996 Structure and magnetic properties of epi-

taxial spinel ferrite thin ﬁlms. Appl. Phys. Lett. 68,714–6

van der Zaag P J 1999 New views on the dissipation in soft

magnetic ferrites. J. Magn. Magn. Mater. 196–7, 315–9, and

references therein

van der Zaag P J, Lubitz P, Kitamoto Y, Abe M 1999 The per-

meability of plated ferrite ﬁlms. IEEE Trans. Magn. 35, 3436–8

P. J. van der Zaag

Philips Research Laboratories, Eindhoven

The Netherlands

Ferroﬂuids: Applications

The ﬁrst synthesis and application of a magnetic ﬂuid

as a magnetocaloric device was reported by Neuringer

and Rosensweig (1964), although attempts to create

stable ferromagnetic colloids can be traced back to

(Elmore 1938). Papell (1965) at NASA was awarded

the ﬁrst patent for the process of making ferroﬂuid.

The ﬂuid was developed for the purpose of retention

and orientation of rocket fuel with an external mag-

netic ﬁeld under reduced gravity. Although the earlier

applications never materialized, the new discovery

nonetheless laid the foundation of a whole new indus-

trial sector (Berkovski 1996).

For his lifelong contributions to the ﬁeld, Ronald

Rosensweig is considered the father of the technol-

ogy. His NASA-funded research, which began in

1965 at AVCO Corporation, demonstrated that

ferroﬂuids can be prepared in a wide variety of car-

riers and in high magnetization values. His patents

in magnetic ﬂuids, staged rotary seals, magneto-

gravimetric separators, and display devices created

a base for industrial enterprise. Serious marketing

efforts began when Ronald Rosensweig and Ronald

Moskowitz founded Ferroﬂuidics Corporation in

1968. The ﬁrst product based on ferroﬂuid, intro-

duced in 1969, was the dynamic seal capable of

operating under pressure or vacuum. Since then a

number of new applications of the technology have

been discovered and are listed in chronological order

in Table 1. Although the technology may be present

in many consumer products, the ferroﬂuid in the

devices is hidden from view. It is estimated that over

200000 rotary vacuum seals, 350 million loud-

speakers, 500 million computer disk drives, and 15

million DVD–ROM drives have been built with

ferroﬂuids.

Table 1

Chronology of commercial ferroﬂuid applications.

1969 Vacuum rotary seal

1970 Ferroﬂuidic display ‘‘Fluidler’’

1973 Loudspeaker

1975 Sensor

1977 Inertia damper

1978 Computer disk drive

1982 Domain visualization

1985 Ferroﬂuid ﬁlm bearing

1987 Turbine blade

1989 Stepper motor

1991 Emission seal

1992 Material separation

1998 CD/DVD-ROM

1999 Ferroﬂuidic adventure science kit

183

Ferroﬂuids: Applications

1. Ferroﬂuids

In ferroﬂuids, magnetic and liquid states coexist. On

the surface, they are deceptively simple systems: a

homogeneous phase comprised of three constituents,

namely the magnetic particles, surfactant, and the

liquid carrier. However, a complex chemistry and a

balance of inter-particle forces determine the colloi-

dal stability (Rosensweig 1979). The magnetite par-

ticles must be in the range of 10 nm. The surfactant

tails should be long enough to prevent particle ag-

glomeration. Ferroﬂuids synthesized about 30 years

previously are still stable.

Both magnetic and liquid properties inﬂuence the

device performance. The saturation magnetization

values of commercial ferroﬂuids range from 0.0001–

0.1 T. Liquid properties such as viscosity, volatility,

working temperature range, environmental compat-

ibility, thermal degradation, and dielectric behavior

dictate the choice of the carrier in which the particles

are suspended. The most commonly used ferroﬂuids

fall in the viscosity range of 1–2000 cp at 27 1C. There

exist several different chemical families of ferroﬂuids

such as the petroleum oils, synthetic esters and hy-

drocarbons, perﬂuoropolyethers, silicones, water, or-

ganic solvents, etc. Within each family a range of

physical properties may be achieved by altering the

concentration of magnetic solids and the molecular

weight of the carrier. Thus, literally hundreds of dif-

ferent commercial magnetic colloid formulations are

available.

2. Applications

Ferroﬂuids offer design solutions to a myriad of

complex engineering problems. Patents in the ﬁeld

exceed 2800.

Magnetic and liquid properties in a ferroﬂuid, al-

though mostly independent of each other, work to-

gether to create successful products (Raj and Chorney

1998). The rotary shaft seal relies on the magnetic

properties of ferroﬂuid for pressure capacity, but the

liquid carrier determines the service life, operating

range, power requirement, and high vacuum perform-

ance of the product. In a loudspeaker, the magnetism

associated with the colloid simply retains the ﬂuid in

the air gap, but the carrier provides the beneﬁts of

damping and heat transfer. Similarly the viscosity and

lubricity of the base oil are relevant parameters for the

performance of a ferroﬂuid ﬁlm bearing. When ferro-

ﬂuid is used in a domain detection application, the

particles migrate through the carrier and accumulate

at the transition zones of the specimen. The carrier

subsequently evaporates making the domain bound-

aries visible in colorful patterns.

Magnetic ﬂuids exhibit levitation phenomena, a

unique feature not found in ordinary liquids. A per-

manent magnet with density much greater than the

ﬂuid ﬂoats in the ferroﬂuid (Rosensweig 1966a). A

pool of magnetic colloid exposed to a gradient mag-

netic ﬁeld can suspend heavy nonferrous objects

(Rosensweig 1966b). These two principles form the

basis of several industrial devices such as dampers,

sensors, and material separators.

A list of ferroﬂuid applications, the types of ﬂuids

used, and their key properties are presented in Table 2.

Devices such as seals and loudspeakers generally re-

quire ferroﬂuid amounts in microliters, material sep-

arators in liters and power transformers in gallons.

The multistage rotary shaft seal is the oldest and most

recognized ferroﬂuid-based product. This device is

used in many industries such as the semiconductor,

nuclear, aerospace, laser, and lamp manufacturing

industries. The basic construction of the seal is shown

in Fig. 1. A magnetic circuit is formed to create al-

ternate regions of high and low magnetic ﬁelds in an

air gap. The shaft is supported by two high precision

ball bearings to allow for rotation. A tooth or ridge

structure either on the pole pieces or shaft conﬁgures

the magnetic ﬁeld into the desired pattern. A ferro-

ﬂuid ﬁlls the regions of high magnetic ﬁeld forming a

series of liquid O-rings with intervening annular open

spaces. Each ﬂuid O-ring sustains a pressure differ-

ential dictated by the ﬁeld strength and the magnet-

ization of the ferroﬂuid. The total pressure capacity

of the seal is the sum of the capacity of individual

O-rings and generally exceeds two atmospheres. The

use of a low vapor pressure ferroﬂuid maintains a

high vacuum integrity, up to 10

9

torr, in the process

chamber. The seal is leak tight, essentially nonwear-

ing and consumes low power. It is typically used in

gaseous environments but can also withstand occa-

sional liquid splashing.

An example of an application of a ferroﬂuid vacuum

seal is shown in Fig. 2. The seal is mounted on a ster-

ilizer and rotates an impeller at speeds of 1750–

3500 rpm inside a chamber charged with a gaseous

mixture of ethylene oxide and nitrogen at temperatures

close to ambient. The system may be used to sterilize

heart pacemakers and sutures. Due to the toxic nature

of the gas, the process must be carried out under leak-

tight conditions. To ensure personnel safety, dual

magnetic ﬂuid seals are employed and the space be-

tween the two seals is monitored. If the process end

seal fails, escaping gas can be detected while the sec-

ondary seal prevents gas emission into the work area.

Manufacturing of integrated circuits requires that

the environments be free of any particulate contam-

inants. In Fig. 3, a coaxial seal is employed in a ro-

botic arm to transport 200 mm silicon wafers in a

cluster tool. The arm picks up wafers from a carousel

and places them sequentially at different locations.

This seal is composed of two coaxial shafts, which

provide two independent angular drive mechanisms

to the arm.

Thin proﬁle ferroﬂuid seals with a typical thickness

of about 1mm are employed in computer disk drive

184

Ferroﬂuids: Applications

spindles, Fig. 4. The seal pressure capacity is in the

region of 100 mm H

O and operating speed 3600–

7500 rpm. Several million of these seals are produced

each year. The seals prevent the bearing lubricant and

other airborne contaminants from reaching the disk

cavity. The ﬂying height of the read–write head above

the disk surface is only 0.2 mm. The presence of even a

small dust particle (24 mm) can easily cause a head

crash or soft error.

As an electromechanical device, a dynamic loud-

speaker, Fig. 5, has a natural resonance point in its

operating frequency range of 20 Hz to 20 KHz. At

resonance the system is underdamped. Also the coil

easily burns out when the speaker is over powered.

The air surrounding the coil cannot remove heat ef-

ﬁciently. Ferroﬂuid is incorporated into the speaker

gap. Thermal conductivity of ferroﬂuid is about four

times higher than air and consequently the transient

power handling of the speaker is increased. The

damping requirements of the driver can be achieved

with a precise selection of ferroﬂuid viscosity. The

ﬂuid is naturally retained in the gap by the prevailing

magnetic ﬁeld. Ferroﬂuid also aids with voice-coil

centering, reduction in distortion and simpliﬁcation

of cross-over network.

The magnetic permeability of a ferroﬂuid can pro-

duce measurable inductive changes in an electrical

circuit. In a tilt sensor, Fig. 6, ferroﬂuid is used as a

movable magnetic core. An angular displacement of

the tube moves the ﬂuid partly from one side to the

Table 2

Types of ferroﬂuids employed in applications and key properties.

Typical applications Types of ferroﬂuids Main ferroﬂuid property

Physical parameters (room

temperature)

Biomedical/catalysis/

magnetic adhesive/

magnetic shielding

Surfacted magnetite

microspheres

Micromagnetics 10 nm supermagnetic

particles

Sealing Perﬂuropolyethers

Polyphenyl ethers

Hydrocarbons

Silahydrocarbons

Esters

Chemically inert

Radiation resistant

Wide temperature

range Hydrolytic

stability Good lubricant

0.02–0.06 T 200–2500 cp

Damping/heat transfer/

noise control

Hydrocarbons Esters

Perﬂuoropolyethers

Silicones

Low volatility Wide

viscosity Fire resistant

High viscosity index

0.01–0.04 T 200–10000 cp

Sensing Mineral oils

Hydrocarbons Esters

Large liquid range

Thermal stability

Boundary lubricant

0.005–0.004 T 2–100 cp

Domain detection/

material separation/

polishing/ magnetic

ink/display/inspection

Water Environmentally safe 0.0001–0.004 T 1–5 cp

Power transmission Mineral oils Vegetable oils Improved dielectrics

Biodegradable, cooling

0.0001–0.001 T 10–25 cp

Magnetic ink/magnetic

polymer/research

activities

Organic solvents

(heptane, xylene,

toluene, MEK)

High volatility 0.005–0.1 T 1–25 cp

Figure 1

Multistage ferroﬂuid rotary seal.

185

Ferroﬂuids: Applications

other creating an inductive unbalance and an elec-

tronic signal proportional to the tilt angle. These

sensors operate in the region of 7451 and are utilized

in the construction industry to measure the tilt of

underground pipes.

The levitation of a permanent magnet in a pool of

ferroﬂuid is the basis of another type of sensing de-

vice used in the oil well industry. The position of the

freely suspended magnet, inﬂuenced by the inclina-

tion or acceleration of the encasement, can be de-

tected by means of inductance coils. This rugged

sensor accurately measures the orientation of an un-

derground drill bit during oil prospecting.

The apparent density of a ferroﬂuid increases

steadily with increasing magnetic ﬁeld gradient pro-

ducing a levitation force upon immersed nonmagnetic

objects. Thus, nonferrous materials of various density

values ﬂoat to different levels in the ﬂuid. Two

different commercial separators utilize this principle:

one is the static (sink/ﬂoat) apparatus in which ferro-

ﬂuid is stationary and the other is a dynamic system

in which ferroﬂuid is rotating in a vertical duct. These

machines are employed for separation of scrap

metals and minerals from ore deposits. Figure 7 is a

Figure 3

Coaxial seal used in a robot arm.

Figure 4

Computer disk drive with magnetic ﬂuid seals.

Figure 5

Moving coil loudspeaker using ferroﬂuid.

Figure 2

Seal application in a sterilizer.

Figure 6

Ferroﬂuid tilt sensor.

186

Ferroﬂuids: Applications

schematic of a sink/ﬂoat device. A mixture of ﬁne

particles is passed through a pool of ferroﬂuid. The

magnetic ﬁeld gradient is adjusted so that the heaviest

metal sinks to the bottom and the remainder ﬂoats to

the top. The mixed light fraction can be further proc-

essed until all grains are sorted out. These scrap

metals, which are generally dumped in a landﬁll as

waste, now become a valuable commodity through

recycling.

The CD-ROM and DVD-ROM technology, which

has revolutionized the entertainment industry with

crisp sound and sharp images, relies on ferroﬂuid for

high performance. A laser beam retrieves recorded

digital signals from a rotating disk, Fig. 8. A ﬂexible

lens assembly, integral to an optical actuator, directs

the beam at the disk. The lens is supported by two

cross coils: the focusing coil aims the beam precisely

on the disk surface and the tracking coil ensures that

the beam stays on the optical track. If an error oc-

curs, a feedback signal actuates the coils to reposition

the lens and thus the beam. The coils must be damped

to minimize settling time and improve the access

time. Additionally, the ﬂuid noticeably reduces res-

onance and hence reading errors.

The mono domain Fe

particles in ferroﬂuids

are nontoxic and can pass easily through capillaries

of living systems. The particles may be coated with

biocompatible dispersants such as dextran or starch

and suspended in water. Such a magnetic liquid

holds great potential in diagnostic and therapeutic

ﬁelds (Ha

feli et al. 1997). Magnetic microspheres

based on ferroﬂuid particles are commercially avail-

able for in vitro applications such as separation of

cells, enzymes, proteins, antibodies, DNA, and

RNA, and attachment of bacterial and virus cells

for immunoassays. Immunomagnetic beads can trans-

port drugs to a specific organ or tissue. Thus, radio-

nuclides can be targeted to a lesion for radiation

therapy. Also, the magnetic carriers may be heated

in vivo with an external radio frequency source (hype-

rthermia) to a temperature of up to 47 1C, thereby

destroying AIDS viruses and tumors. The particles

have been shown to enhance the magnetic resonance

(MR) imaging, thus making it possible to detect met-

astases. Superparamagnetic iron oxide as a contrast

agent has been approved by the FDA for clinical trials.

3. Future Trends

New applications of ferroﬂuid with significant

growth potential include: NMR contrast agents for

both biomedical and oil prospecting ﬁelds, quiet so-

lenoid valves for the medical industry, environmental

seals for pollution control, cooling and dielectric

beneﬁts in power transformers, replacement of toxic

mercury in switches, displays for education and en-

tertainment markets, a new generation of sensing de-

vices, low noise and efﬁcient steppers, magnetic ink

printing, actuator damping, optical shutters, preci-

sion ﬂuid ﬁlm bearings, critical lubrication, magnet-

ically-responsive adhesives, precision grinding, and

fabrication of high strength composites. The next

generation of ferroﬂuids may combine magnetism,

electrical conductivity, and liquidity into one state,

which will undoubtedly uncover new technological

frontiers. Magnetic liquids, due to their inherent

composition, may also have a role in the development

of nanodevices.

See also: Ferroﬂuids: Introduction; Ferrohydro-

dynamics; Ferroﬂuids: Magnetic Properties; Ferro-

ﬂuids: Preparation and Physical Properties

Figure 7

Recycling of scrap metals with sink/ﬂoat apparatus.

Figure 8

CD/DVD-ROM actuator damping with ferroﬂuid.

187

Ferroﬂuids: Applications

Bibliography

Berkovski B (ed.) 1996 Magnetic Fluids and Applications Hand-

book. Begell House, New York

Elmore W C 1938 Ferromagnetic colloid for studying magnetic

structures. Phys. Rev. 54, 309–10

feli U, Schu

tt W, Teller J, Zborowski (eds.) 1997 Scientific

and Clinical Applications of Magnetic Carriers. Academic

Press, New York

Neuringer J L, Rosensweig R E 1964 Ferrohydrodynamics.

Phys. Fluids 7, 1927–37

Papell SS 1965 Low viscosity magnetic ﬂuid obtained by col-

loidal suspension of magnetic particles. US Patent 3215575

Raj K, Chorney A F 1998 Ferroﬂuid technology—an overview.

Indian J. Eng. Mater. Sci. 5, 372–89

Rosensweig R E 1966a Buoyancy and stable levitation of a

magnetic body immersed in a magnetizable ﬂuid. Nature 210,

613–4

Rosensweig R E 1966b Fluidmagnetic buoyancy. AIAA4. 10,

1751–8

Rosensweig R E 1979 Fluid Dynamics and Science of Magnetic

Fluids. Advances in Electronics and Electron Physics, Vol.

48. Academic Press, New York, pp. 231–54

K. Raj

Ferroﬂuidics Corporation, Nashua

New Hampshire, USA

Ferroﬂuids: Introduction

It is unlikely that any liquid will be found that is

intrinsically ferromagnetic. Nickel, cobalt, and iron,

for example, are only ferromagnetic in the solid state.

It is ﬂuid media composed of solid magnetic particles

of very small size, colloidally dispersed in a liquid

carrier, that are the basis for the highly stable,

strongly magnetizable liquids known as magnetic ﬂu-

ids or ferroﬂuids, see Fig. 1. Ferroﬂuids are extremely

interesting due to the interplay between hydrody-

namic and magnetic phenomena with detailed infor-

mation the subject of several books (Bashtovoy et al.

1988, Berkovsky 1996, Blums et al. 1997, Rosensweig

1985, and others). The typical particle size in a ferro-

ﬂuid is about 10 nm, which is smaller than a magnetic

domain, hence each particle is permanently magnet-

ized. The particles experience appreciable thermal

translation that maintains them in suspension against

the force of gravity, while in the absence of ﬁeld

thermal reorientation of the particles results in zero

remanence or coercivity, i.e., ferroﬂuids are magnet-

ically soft. The ferroﬂuids behave remarkably as

highly homogeneous, magnetizable media that retain

their ﬂuid property in the most intense applied mag-

netic ﬁelds and magnetic ﬁeld gradients.

Following a brief discussion of their origin, this

article outlines methods for the synthesis of ferroﬂuids

and discusses physical requirements for their colloidal

stability. In addition, basic magnetic and physical

properties are described quantitatively.

1. Origin

By remarkable coincidence, ferroﬂuids were invented

in the USA independently at least three times in the

1960s. Of these inventions, the best known is that of

Papell (1965) who employed grinding in the presence

of a carrier liquid and a surfactant to reduce large

grains of magnetite (Fe

) to colloidal size. The

physical chemistry of the process was clariﬁed and

the process further developed by Rosensweig et al.

(1965). O’Connor (1962) synthesized a ferroﬂuid

using chemical precipitation with an ion exchange

technique to purify the product. Gable employed

chemical synthesis, ﬁling a patent application in 1965

that issued only much later (Gable and Kerr 1977).

Dry powders of Gable display remarkable solubility

in water. All these ferroﬂuids use a surface-sorbed

layer of a long-chain surfactant, polymer, or protein

molecule to provide a short-range repulsion prevent-

ing particles from sticking to each other.

Massart (1981) developed ferroﬂuids in aqueous

media that are stabilized by the electric double layer

mechanism.

Elmore (1938a, 1938b) produced ﬁne dispersions of

magnetite (Bitter colloid) used to detect the bound-

aries of magnetic domains in bulk ferromagnetic ma-

terials. But as the particle size was too large (20 nm)

to prevent sedimentation the material does not qual-

ify as a ferroﬂuid.

2. Preparation

Chemical precipitation is the most often employed

process although grinding is useful in some circum-

stances. Chemical decomposition and vapor-liquid

reaction are used to produce certain specialized ferro-

ﬂuids. In all cases, a repulsive layer is provided that

envelopes each particle to prevent them sticking to

each other.

The essential chemical reaction in the precipitation

of ferroﬂuid particles is indicated by the reaction:

2Fe

3þ

þ Fe

2þ

þ 8OH



¼ FeO  Fe

þ 8H

O ð1Þ

where FeO Fe

is magnetite written to emphasize

the ratio of divalent to trivalent iron atoms in it,

III

/Fe

¼2. The iron salts used in the preparation

may be any combination of soluble salts such as the

chlorides or sulfates and the alkali can be ammonium

hydroxide, sodium hydroxide, or another base. Fol-

lowing precipitation in the procedure of Khalafalla

and Reimers (1974) the small particles are extracted

into a mixture of surfactant in an organic solvent.

Massart (1996) discusses procedures for bonding spe-

cies of small ions of a desired electical sign to the

188

Ferroﬂuids: Introduction

surface of oxide particles. In this manner, ferroﬂuids

can be conferred stability over ranges of pH values.

In the grinding process, magnetic powder, most

commonly magnetite of several microns size, is mixed

with 10–20 vol.% of dispersing agent based on sol-

vent volume, and 0.2 kg l

1

of magnetite may be used.

The proportion of dispersant to solid corresponds

closely to a monolayer coating on the particles in the

Figure 1

Attraction of ferroﬂuid against gravity to the poles of a magnet; peaks on the surface result from a competition

between gravitational, surface, and magnetic energies (courtesy of Ferroﬂuidics Corporation).

189

Ferroﬂuids: Introduction