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

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Absent are the complex branched surface domains

that typically occur in misoriented grains of iron-

based material. Only occasionally regular domains

may be replaced by a smoothly varying magnetiza-

tion conﬁguration as actually expected for zero an-

isotropy (Fig. 6(c)). In ﬁne-grained Permalloy

material (Fig. 6(b)), the domain structure resembles

that of Permalloy thin ﬁlms in which the domains

extend over the grains (see Fig. 1, and the interpre-

tation in Sect. 2.5). Uniformly magnetized grains are

also found in coarse-grained NiFe materials after

annealing in magnetic ﬁelds. Such regular magnetic

microstructures cause reversible magnetization pro-

cesses and thus low noise in sensor applications.

Similar domains as in coarse-grained Permalloy

ribbons are observed in soft magnetic spinel ferrites

with a sufﬁciently large grain size (Fig. 6(d)). Ferrites

are prepared by sintering and they are used in high-

frequency applications due to their insulating prop-

erties (see Ferrites). Domain wall processes are

important in the whole application range of ferrites

up to GHz frequencies.

2.4 Amorphous Ribbons

Amorphous magnetic ribbons are alloys with typi-

cally 80% iron, cobalt, or nickel, and 20% ‘‘metal-

loids’’ like silicon or boron. They are quenched from

the melt with a thickness around 20 mm (see Amor-

phous and Nanocrystalline Materials). In application,

they compete with high-permeability NiFe alloys.

While crystal anisotropy is small in NiFe materials, it

cannot exist in metallic glasses by definition. Never-

theless, ‘‘regular’’ domains do exist in such ribbons,

again caused by residual anisotropies.

The main source of anisotropy is internal stress,

frozen in by the rapid quenching process. The mag-

netization is coupled to the stress by the magneto-

striction constant, so that the domain complexity in

as-quenched ribbons depends on magnetostriction as

shown in Fig. 7. Two types of domains are typical:

wide curved domains that follow a varying in-plane

easy axis (caused by dominating tensile stress for

positive magnetostriction constant), and narrow ﬁn-

gerprint domains. The ﬁngerprints are closure do-

mains of underlying perpendicular domains, caused

by planar compressive stress which induces an easy

axis perpendicular to the surface. Internal stress can

not only vary laterally, but also in depth. This leads

to layered domain structures as demonstrated in

Fig. 8. A layered in-plane anisotropy with twisted

easy directions is visible in Fig. 7(a).

Even magnetostriction-free metallic glasses show

regular (but wide) domains in the as-quenched state

(Fig. 7(d)). They are often aligned along the ribbon

axis, caused by magnetization-induced anisotropy

(see Sect. 1). The easy axis follows the magnetization

direction present during quenching, which is frozen in

while cooling through the Curie point and which is

along the ribbon axis for magnetostatic reasons. Such

induced anisotropies are also present in magnetos-

trictive materials where it is difﬁcult to separate them

Figure 7

Typical zero-ﬁeld domain patterns in 20 mm-thick

amorphous ribbons of different magnetostriction

constant l.

Figure 8

Demagnetized state (a) of a magnetostrictive amorphous

ribbon, and the image difference (b) between image

(a) and a state where a small magnetic ﬁeld was applied,

showing domain changes. The 1801-wall displacement

continues in the area of the ﬁngerprint structure (see

arrow in (b)), proving that the ﬁngerprint pattern must

be restricted to the surface zone as sketched. Obviously

there is a layered stress distribution: compressive stress

near the surface and dominating tensile stress

underneath.

850

Magnets, Soft and Hard: Magnetic Domains

from stress-induced contributions. Residual anisotro-

pies may also be caused by the ﬂow pattern during

quenching, by the surface structure of the ribbons

(due, for example, to ‘‘air bubbles’’ on the wheel

side), and by mechanical creep due to different

quenching speed at different ribbon locations.

Controlling anisotropy in metallic glasses is possi-

ble. Frozen-in stress can be relaxed by more than

95% by carefully annealing below the crystallization

temperature. In addition, annealing can be performed

in uniform magnetic ﬁelds below the Curie temper-

ature or under external mechanical load. This leads to

well-deﬁned anisotropies by the same mechanisms as

discussed before. A variety of ordered domain-states

is possible in this way (Fig. 9), allowing tailoring of

magnetization curves for specific applications. For

instance, transverse and perpendicular domains

shown in (a) and (e), respectively, are essentially

magnetized by rotation in ﬁelds along the (vertical)

ribbon axis, causing a linear hysteresis curve with

only minor domain rearrangement processes. Losses

are correspondingly low.

2.5 Nanocrystalline Ribbons

Modern nanocrystalline ribbons with grain sizes in

the 10 nm range compete in high-permeability appli-

cations with classical Permalloy and amorphous rib-

bons (see Amorphous and Nanocrystalline Materials).

The best-known alloy is the FeSiBNbCu system,

which consists of nanocrystalline SiFe grains embed-

ded in an amorphous magnetic matrix. Because of the

small grain size, the exchange interaction averages

over anisotropic properties of the individual grains,

so that anisotropy is smoothed out and the material is

effectively anisotropy-free (the same mechanism ap-

plies to the Permalloy ﬁlms discussed in Sect. 1 and

the ﬁne-grained Permalloy sheets of Sect. 2.3).

Together with a very small magnetostriction, excel-

lent soft magnetic properties can be achieved.

Well-deﬁned domains and walls are found in these

materials (Fig. 10), which cannot be distinguished at

optical resolution from those in metallic glasses. An-

other class of nanocrystalline materials contains some

zirconium and has a higher iron content to obtain a

higher saturation magnetization. The patchy modula-

tion of magnetization (Fig. 10(b)) seems to be caused

by the effects of the irregular polycrystalline nature of

this material. Hard and immobile patch domains are

observed when the crystallites are getting too big or if

the coupling between the crystal grains is too weak.

3. Domains in Hard M agnetic Materials

The magnetic microstructure of hard magnets is

trivial in the permanent magnetic state: the elemen-

tary carriers of the magnetic moment, the more-or-

less independent grains or particles in these materials,

Figure 9

Ordered domain states in metallic glasses after different

annealing treatments. The ribbon axis is along the

vertical direction in all images. Annealing in a ﬁeld

transverse to the strip axis results in wide transverse

domains (a). A strongly reduced domain width is

obtained after annealing in ﬁelds perpendicular to the

ribbon surface, resulting in an oblique (b) or

perpendicular anisotropy (c) in dependence of the

annealing ﬁeld strength. The orientation of the narrow

stripe domains in the case of a perpendicular anisotropy

depends on the magnetostriction constant. If the ribbon

is demagnetized in an alternating ﬁeld along its axis, the

stripes are oriented parallel to the ﬁeld axis for a low-

magnetostriction material (d) and perpendicular in a

high-magnetostriction material (e). This can be

explained by the compatibility of magnetostrictive

deformations: a positive magnetostriction favors stripe

nucleation perpendicular to the ﬁeld (sketch e) as

opposed to parallel stripes (sketch d) in which the

deformations are incompatible. The latter is only

possible in low-magnetostriction materials and is

preferred there because a parallel stripe pattern saves

wall energy. The transverse pattern in (f) is obtained by

a ‘‘creep’’ annealing treatment under mechanical load.

At zero ﬁeld, the magnetization is forced in the

transverse in-plane direction to reduce magnetostatic

energy.

851

Magnets, Soft and Hard: Magnetic Domains

keep saturated along a preferred direction in reman-

ence. Domains are not connected to the permanent

magnet function and should be completely eliminated

to obtain a high coercivity and remanence. They only

play a role during switching. Two main classes of

permanent magnets must be distinguished: Small-par-

ticle magnets and anisotropy-based coarse-grained

magnets (see Hard Magnetic Materials, Basic Princi-

ples of ). Most hard magnetic materials rely on a

strong uniaxial anisotropy with Qb1, as opposed to

soft magnetic materials, which typically have a man-

ifold of easy directions.

3.1 Coarse-grained Magnets

In large-grained sintered magnets, two classes can be

separated: nucleation- and pinning-type magnets. The

difference becomes evident in the initial magnetization

curves. Starting from the thermally demagnetized

state, every grain in a nucleation-type material con-

tains many domain walls that can be displaced easily

(see Fig. 11(a), (b)), leading to a large initial permea-

bility. The permanent magnet properties appear only

when the initial domain walls are driven out in a large

ﬁeld (Fig. 11(c)) so that also residual domains, which

are a natural consequence of particle shape and local-

ized stray ﬁeld concentration, are completely annihi-

lated. This requires magnetic ﬁelds that may well

exceed the saturation ﬁeld M

along the preferred axis,

in particular for isotropic samples or for misoriented

grains in textured material. The magnet can be de-

magnetized after this step only if new domain walls are

nucleated. In contrast, in a pinning-type magnet, the

domain walls are effectively pinned by precipitates,

leading to a low initial permeability.

Examples of nucleation-type magnets are hexa-

ferrite magnets (see Alnicos and Hexaferrites) and

high-quality rare-earth magnets of the SmCo

and

the Nd

B types (see Magnets: Sintered). They

consist of highly anisotropic grains in a textured, po-

lycrystalline compound, which is prepared in such a

way that switching of one grain has little inﬂuence

on its neighbors. Coercivity is thus primarily given

by some mean switching thresholds of the individ-

ual grains. Both the initial and the ﬁnal states of

the switching process are essentially saturated in

Figure 10

(a) Ordered domain structure on a nanocrystalline

FeSiBNbCu-based soft magnetic ring core annealed in a

ﬁeld perpendicular to the tape direction. (b) High-

resolution observation on a zirconium-containing

nanocrystalline ribbon, showing wall and modulated

domain contrast.

Figure 11

Domains on a sintered NdFeB magnet. (a) The

thermally demagnetized state on the basal plane, with

the axis of the textured magnet perpendicular to the

viewing surface. (b) The same state viewed parallel to the

axis reveals the high degree of texture in this material.

The grains are magnetostatically coupled as exchange

interaction is prevented by a nonmagnetic grain

boundary phase. (c) Domain-free, remanent state after

applying a saturation pulse [same location as (a)]. The

character of the branched domains visible in (a) is

displayed in (d) for a large-grained precursor material.

The twin grain boundary acts like a mirror for the

domains, so that ‘‘surface’’ domains (upper part) and

‘‘volume’’ domains (lower part) show up at the same

time (compare Fig. 1(f)).

852

Magnets, Soft and Hard: Magnetic Domains

high-coercivity materials, but the switching process

itself traverses a multidomain state (note that the

grain size is in the 10 mm range, which is much larger

than the single-domain size of about 100 nm).

If the domain walls are narrow due to high crys-

talline anisotropy, the walls may be effectively pinned

by a ﬁnely dispersed nonmagnetic phase. The alloy

(CoFe)

containing copper-rich Co

Sm precipi-

tates is an example for a high-quality permanent

magnet based on this principle. The precipitation

phase has a reduced magnetization and exchange

constant, so that the domain walls prefer to travel

along the platelet-shaped precipitates. The elimina-

tion of residual domains plays no role because these

domains are pinned (like all others) and the coercivity

is determined by the interaction between domain

walls and microstructure.

Note that both nucleation and pinning type mag-

nets rely on the high energy and narrow width of their

domain walls. A high wall energy makes nucleation

of new domains difﬁcult, a narrow wall width favors

an effective pinning of walls. The ﬁnely divided do-

mains in Fig. 11(a), which are of the two-phase

branched type discussed in Sect. 1, can be applied to

measure the wall energy. It was shown that the sur-

face domain width of such a pattern stays constant,

independent of crystal thickness, whereas the internal

domain width increases with thickness. This is deter-

mined by an equilibrium between stray ﬁeld energy

and wall energy. From the measurement of the sur-

face domain width, the domain wall energy can con-

veniently be determined.

3.2 Small-particle and Fine-crystalline Magnets

If a magnetic body becomes very small, it will not

contain domain walls and the particle becomes single-

domain. This occurs when the expense of wall energy

becomes greater than the gain of stray ﬁeld energy.

The critical single domain size depends on the

Q-parameter and the particle shape. The single

domain limit rises with increasing elongation of the

particle.

Permanent magnets can also be prepared from

such single-domain particles. If the individual parti-

cles consist of a low-anisotropy material, they must

have an elongated shape to remain single-domain.

For such large particles, the danger of thermally ac-

tivated spontaneous switching (superparamagnetism)

is absent. Elongated single-domain particles are

therefore thermally stable even if they are complete-

ly isolated. Classical examples of this permanent

magnet class are the Alnico alloys, consisting of ﬁne

ﬁlaments of a high-saturation FeCo alloy that are

embedded into a nonmagnetic NiAl matrix

(see Alnicos and Hexaferrites).

Modern small-particle magnets are based on high-

anisotropy materials that consist of single-domain

grains. A number of techniques can be used to prepare

ﬁne-crystalline magnets out of precursors such as

B: rapid quenching and subsequent crystalli-

zation, mechanical alloying (see Magnets: Mechani-

cally Alloyed), and the HDDR process (see Magnets:

HDDR Processed). All these methods generate a ﬁne

powder with a particle size around 100 mm, which

then has to be compacted into solid magnets. Oriented

magnets can be produced by ‘‘die-upsetting’’ (see

Textured Magnets: Deformation-induced). Also by the

HDDR process, an anisotropic powder can be pro-

duced which can be oriented in a magnetic ﬁeld.

But even nonoriented ﬁne-crystalline materials re-

main interesting as a relatively weak exchange inter-

action between very small grains can lead to an

enhanced remanence without a significant loss in

coercivity (exchange-enhanced ﬁne-crystalline mag-

nets). The coupling enhances the remanence above

the average M

¼0.5 M

of independently oriented

Figure 12

(a) Interaction domains observed on a surface

perpendicular to the preferred axis of textured, ﬁne-

grained NdFeB material prepared by melt spinning and

hot deformation. In the HDDR-processed material (b),

the interaction domain width depends on the degree of

texture. In (c), the interaction domains are observed

parallel to the texture axis in the same material as in (b).

(d) Interaction domains on the side face of an oriented

Alnico crystal.

853

Magnets, Soft and Hard: Magnetic Domains

uniaxial grains. A further possibility for achieving

high remanence in nonoriented small-grain materials

consists of adding a high-saturation soft magnetic

phase, which is strongly exchange-coupled to the ba-

sic hard magnetic phase if the extension of the soft

phase is sufﬁciently small (see Magnets: Remanence-

enhanced). The coercivity in these two-phase ﬁne-

crystalline magnets is dominated by the hard phase,

whereas the high remanence is primarily a conse-

quence of the soft phase.

The magnetization of the grains in ﬁne-grained

permanent magnets is strongly correlated. An irreg-

ular magnetic microstructure is displayed in the de-

magnetized state as shown in Fig. 12(a)–(c). The

black-and-white contrasts correspond to grain neigh-

borhoods that are magnetized along a certain net

magnetization direction. These patterns are predo-

minantly determined by dipolar interactions between

the ﬁne grains—an assumption which is justiﬁed by

the fact that this kind of domains is also observed in

small-particle magnets like Alnico (Fig. 12(d)) for

which the presence of exchange interaction between

the needles can be excluded. To indicate their special

nature, they were called ‘‘magnetostatic interaction

domains.’’ A characteristic feature of interaction

domains is their elongated shape extending along

the preferred axis of the material as is visible in

Fig. 12(c). The ‘‘domain boundaries’’ of the interac-

tion domains can move laterally to some extent by

the successive switching of neighboring grains (or

particles in case of Alnico), thus providing a certain

permeability for the demagnetized state.

3.3 Conclusions

The domain patterns in a variety of soft and hard

magnetic (mainly bulk) materials have been reviewed.

In both material classes, magnetic domains are

formed to reduce the magnetostatic energy of ﬁnite

samples. The interactions between physical micro-

structure, easy axes, applied ﬁelds, surface orienta-

tions, and mechanical stress leads to an enormous

variety of domain patterns, only a small fraction of

which could be presented by focusing on selected,

undisturbed samples.

Although in soft magnetic materials the avoidance

of magnetic charges is the dominating principle, the

domain arrangement is strongly inﬂuenced by an-

isotropy. The orientation of the main surfaces with

respect to the easy directions is decisive. If a surface

contains easy directions, it is no problem to reconcile

pole avoidance with avoiding anisotropy energy by

employing the surface-parallel easy directions only.

Grain-oriented transformer steel belongs to this class.

Here the degrees of freedom for domain formation

are strongly reduced and the basic domain patterns

are related to those of ideally oriented grains even in

the case of small misorientation.

Due to the near-single-crystal nature of this ma-

terial, its magnetic microstructure is conveniently ob-

servable and well understood, so that the losses of

transformer steel are steadily reduced by studying

and controlling the domain structure. Domain con-

trol is also possible in amorphous and nanocrystalline

ribbons if the inﬂuence of mechanical stress can be

avoided. For most other bulk soft magnetic materi-

als—the nonoriented polycrystalline SiFe, NiFe, and

ferrite materials—similar possibilities are not availa-

ble. Observing the complex, often heavily branched

domains at the surface of such materials offers only a

glimpse at the magnetization processes in the volume.

Domains in hard magnetic materials are only im-

portant during switching. In coarse-grained material

they are determined by a strong uniaxial anisotropy,

and in ﬁne-grained magnets the magnetostatic inter-

action between small grains results in an inhomoge-

neous magnetic microstructure called interaction

domains.

See also : Coercivity Mechanisms; Ferrite Magnets:

Improved Performance; Magnetic Films: Hard

Bibliography

Cullity B D 1972 Introduction to Magnetic Materials. Addison-

Wesley, Reading, MA

Chikazumi S 1997 Physics of Ferromagnetism. Clarendon,

Oxford

Hubert A, Scha

fer R 1998 Magnetic Domains. The Analysis of

Magnetic Microstructures. Springer, Berlin

O’Handley R 2000 Modern Magnetic Materials. Principles and

Applications. Wiley, New York

Van den Berg H 1986 Self-consistent domain theory in soft-

ferromagnetic media II. Basic domain structures in thin ﬁlm

objects. J. Appl. Phys. 60, 1104–13

R. Scha

fer

Institut fu

r Metallische Werkstoffe, Dresden, Germany

Magnets: Biomedical Applications

This article deals with magnetic substances used in-

side living beings, particularly humans. The applica-

tion of magnets in medicine has been known for more

than 2500 years. Probably the earliest document

dates from about 600 bc and concerns the extraction

of an iron arrow tip by means of an extracorporally

applied loadstone (Hirschberg 1899). Several publi-

cations of pre-Christian centuries described the ap-

plication of magnetic material as a remedy for a

variety of diseases (Ha

feli 1998). Nevertheless, it was

not before the middle of the twentieth century that

the number of different magnetic materials for bio-

medical applications considerably increased.

854

Magnets: Biomedical Applications

1. Natural Magnetic Substances in Humans

The existence of magnetic material, mainly magnetite

(Fe

), in living beings was found for the ﬁrst time

in 1962 (Lowenstam 1962) and later recognized as

being responsible for the orientation capabilities of a

number of animals. The discovery of small particles

consisting of magnetite and maghemite (g-Fe

)in

human tissue was reported for the ﬁrst time by

Kirschvink et al. (1992). There is evidence for some

inﬂuence of this material with reference to magnetic

resonance imaging (MRI), where it may be respon-

sible for contrast alterations. Human tissue is almost

completely diamagnetic. There are, however, some

paramagnetic substances with high values of suscep-

tibility: hemosiderin and ferritin. These substances

are responsible for storage and transport of iron in

organisms and can be used for the noninvasive ex-

amination of the iron content in the liver (Nielsen

et al. 1998). Finally, it is worth noting that the mag-

netic susceptibility of hemoglobin, an important part

of red blood cells, changes depending on its oxygen-

ation state. This behavior is used for the so-called

blood oxygenation level dependent (BOLD) contrast

in functional MRI (Schreiber et al. 1998).

2. Bulk Magnets

Important properties of bulk magnets positioned in-

side a living being concern the force or the torque

exerted in magnetic ﬁelds on appropriately designed

pieces of the material, the magnetic stray ﬁeld that

may be used for monitoring the position of the mag-

net, and the magnetic losses arising in alternating

magnetic ﬁelds. All the substances have to be non-

toxic. This can be achieved either by the choice of

nonpoisonous magnetic materials, as is the case with

magnetite, or by covering the magnet with suitable

coatings (plastic, gold ﬁlm, etc.).

2.1 Bulk Magnets for Force or Torque Applications

A magnetic ﬁeld may be generated by permanent

magnets inside the body. Another way is to use ﬁelds

generated outside the body by coils or permanent

magnets. An example of permanent magnets used

inside the body is the application in veterinary med-

icine (Fahlenbrach and Fahlenbrach 1970) where

cows are forced to swallow rods of Alnico, a perma-

nent magnetic material, covered with plastic to pre-

vent both the magnet from corrosion and tissues

from toxic effects. The magnet remains in one of the

stomachs and attracts steel parts often swallowed by

the animals together with green fodder or hay. In this

way the stomach and intestines are protected against

being damaged. The same principle is sometimes also

used in human medicine to remove pins, nails, and

other steel parts that have been swallowed (Ha

feli

1998). Another example concerns the ﬁxation of tem-

porary pacemaker electrodes by hand-held perma-

nent magnets in emergency cases (Paliege et al. 1979).

Pieces of gold-plated iron wires have been used as tips

of ﬂow-in stimulation catheters for this end. Inter-

esting experiments have been performed with certain

kinds of catheters that are able to move through

blood vessels driven by a combination of a d.c. mag-

netic ﬁeld and an a.c. ﬁeld. These parts consist of a

small permanent magnet (barium–ferrite cylinders of

a few cubic millimeters) and a ﬂexible plastic tail

(Frei et al. 1966).

In spite of this fascinating idea these catheters

could obviously not be introduced into general med-

ical practice. Guidance of small magnetic probes is

being investigated in terms of the so-called magnetic

stereotaxis, i.e., the minimally invasive manipulation

of a permanent magnet inside the human brain along

a path of minimum damage to a position where it can

be used to heat a well-deﬁned small region of tissue

(e.g., a brain tumor) or to act as a catheter for other

purposes. In vivo experiments have been successfully

performed with animals (Grady et al. 1990) and ap-

plication in humans, e.g., using multiple supercon-

ducting coils, looks promising (Meeker et al. 1996).

Equipment for generating ﬁelds outside the body is

generally less complicated. However, it was not until

the mid-1990s that a piece of permanent magnetic

material (Nd–Fe–B disk of 5 mm diameter and 2 mm

thickness) was positioned in the stomach and kept

there for several hours simply by the force exerted by

an external ﬁxed permanent magnet (Gro

ning et al.

1996). The magnetic disk covered by a layer of car-

nauba wax was part of a tablet designed for osmo-

tically controlled drug release. An interesting appli-

cation is the magnetic stoma seal for colostomy. It

consists of an implanted permanent magnetic ring

(Sm–Co covered with titanium) and a suitably

formed lid. At ﬁrst glance this system seems to be

very attractive. A critical review of about ﬁve years’

practice, however, showed a relatively high percent-

age of incontinence (Schwemmle 1982). Therefore,

the conventional technique is still widely used.

2.2 Bulk Magnets for Monitoring

For many years there have been attempts to inves-

tigate the motility of the gastrointestinal tract by

magnetic monitoring (Forsman 1998) with the aim of

avoiding exposure to x rays or other ionizing radi-

ation. However, it was not before 1997 that satisfying

results were presented. Weitschies et al. (1997) pub-

lished reliable images of magnetic marker monitoring

with a spatial resolution in the millimeter range and a

temporal resolution down to 4 ms. The technique uses

markers consisting of sucrose pellets coated with a

mixture of magnetite and polymethyl methacrylate

and enclosed in silicone rubber. After ingestion of

855

Magnets: Biomedical Applications

these markers the magnetic stray ﬁeld outside the

body is measured by means of a superconducting

quantum interference device (SQUID) and can be

used to calculate the actual position of the marker.

However, the technique is a rather expensive one

(SQUID array cooled by liquid helium, sophisticated

electronics, and a shielded room to reduce background

ﬁelds by several orders of magnitude). A simpler novel

technique is where an extracorporally applied pulsed

magnetic ﬁeld is used to repeatedly align the marker

with its magnetic axis. The stray ﬁeld is measured

during the impulse pauses after successively alternat-

ing pulses. In this way two essential advantages result:

the use of much cheaper conventional ﬁeld sensors

and working without any magnetic shielding, i.e., in a

normal environment (Andra

et al. 2000). It is worth

noting that the same principle can be used to monitor

other objects, e.g., the tip of a heart catheter.

2.3 Bulk Magnets for Tissue Heating

One kind of tumor therapy already introduced in

clinical practice is the combined treatment by radi-

ation and heat. Here, heating can be realized by eddy

currents induced in metallic pieces, so-called thermo-

seeds, implanted in the tumor region as an array of

appropriately positioned needles. Magnetic metals

offer the possibility of using magnetic losses with the

important advantage of having a mechanism for self-

regulation of the maximum temperature by means of

the appropriately chosen Curie temperature (Burton

et al. 1971). In contrast to most other medical appli-

cations of magnets, the materials of thermally self-

regulating implants are specially designed and consist

of nickel alloys with copper, silicon, and palladium,

or alloys of Fe–Pt and Pd–Co. A novel principle

(Cetas et al. 1998) makes use of the ﬁeld concentra-

tion due to the high permeability of a specially com-

posed ferrite material (e.g., containing manganese,

zinc, and copper) with a Curie temperature in the

range 45–60 1C. The rod-shaped ferrite core is sur-

rounded by a nonmagnetic electrically conductive

sheath (e.g., stainless steel), which simultaneously

provides biocompatibility.

3. Magnetic Par ticles

The biomagnetic application of magnetic particles has

been known since the late 1950s (Gilchrist et al. 1957)

but there is now renewed interest. The ﬁelds of interest

concern contrast media, vascular occlusion, localized

heating, targeting of drugs and radiation sources, and

cell separation. Most of the modern literature can be

found under the keyword ‘‘magnetic carriers.’’

3.1 Magnetic Contrast Agents

An early attempt to use magnetic powders as contrast

agents for x-ray investigations was made by Frei et al.

(1968). Satisfactory visualization was achieved of the

gastrointestinal tract by means of commercially avail-

able powders (e.g., consisting of magnesium ferrite)

even with human volunteers. These investigations

were not continued in spite of an advantage over

conventional contrast agents, namely the possibility

of manipulating the material by external ﬁelds during

the x-ray investigation. Other studies were concerned

with MRI of the abdomen using orally administered

superparamagnetic particles consisting of polymer

spheres surrounded by iron oxide crystals, each crys-

tal measuring less than 50 nm (Boudghe

ne et al.

1993). Dextran-coated superparamagnetic iron oxide

particles (presumably magnetite) can be used for cell

labeling (Yeh et al. 1993), particularly after coupling

to special monoclonal antibodies. Other magnetite

nanoparticles have already been approved for imag-

ing of liver metastases.

3.2 Vascular Occlusion

Several investigations have been performed with re-

spect to vascular occlusion by means of magnetic

particles. One of the ﬁrst publications deals with the

occlusion of intracranial aneurisms (Roth 1969). Iron

particles were used and a clinical experiment was

performed generating a magnetic thrombus with in-

jected particles exposed to a localized magnetic ﬁeld

of implanted magnets. Vascular occlusion with mag-

netic particles has also been investigated as a tech-

nique for selective infarction of tumors and organs.

Mosso and Rand (1973) injected mixtures of car-

bonyl–iron microspheres with liquid silicone into the

renal arteries of dogs. After a certain time, during

which the material was held in place by an extra-

corporally applied magnetic ﬁeld, vulcanization oc-

curred. Other authors have reported experiments

performed without silicone and also found agglo-

meration by means of the magnetic ﬁeld.

3.3 Localized Heating

Many attempts have been made to use the heating

of magnetic particles in high-frequency magnetic

ﬁelds for selective treatments of cancer cells within

unaffected normal tissue. Gilchrist et al. (1957) de-

scribed this idea and reported on promising ex-

perimental results obtained with g-Fe

particles

(20–100 nm diameter) administered by injection into

different kinds of tissue of dogs from where the par-

ticles were carried into the lymph nodes. The hope

was to heat the nodes and in this way to destroy

lymphatic metastases.

Similarly, the treatment of other tumors seemed to

be possible either by direct injection or by transport

via blood vessels, particularly the vessels formed by

neo-vascularization near growing tumors. A great

number of both in vitro and in vivo experiments has

856

Magnets: Biomedical Applications

been carried out since the pioneering work. Magnetic

particles of different size (diameters ranging from the

so-called multidomain state down to the superpara-

magnetic region) and different material (magnetite,

maghemite, magnetic glass-ceramic, Ba–Co–ferrite,

etc.) have been tested. The particles are covered or

composed with different organic material or used in

the bare state. Reviews of the results (Jordan et al.

1993, 1999, Hergt et al. 1998, Andra

1998) show that

clinical application cannot yet be applied and several

essential problems are still to be solved.

3.4 Magnetic Targeting of Drugs and

Radiation Sources

Magnetic drug targeting, i.e., guidance and retention

of drug-carrying magnetic particles by means of ex-

ternally applied magnetic ﬁelds, has been investigated

since the 1970s. The ﬁrst clinical experiences with hu-

man patients were reported by Kuznetsov et al.(1997)

who used Fe–C particles combined with anticancer

drugs. Lu

bbe and Bergemann (1997) used magnetic

carriers of multidomain particles of Fe

coated with

starch derivatives and loaded with epirubicin. A con-

centration of the particles was achieved by means of

suitably arranged permanent magnets outside the

body of the patient. Magnetic drug targeting may al-

so be used for the localized treatment of thrombosis,

as demonstrated by Inada et al.(1987)whoperformed

in vitro experiments with magnetic enzyme particles

consisting of magnetite and polyethylene glucol cou-

pled to urokinase. A further kind of magnetic target-

ing is the application of particles for the transport of

radioisotopes, e.g.,

Y, to the region where the radi-

ation is to be applied. This novel technique was tested

in animal experiments by Ha

feli et al. (1997) who used

microspheres (10–30 mm in diameter) consisting of a

mixture of polylactic acid and magnetite.

3.5 Magnetic Cell Separation

An already commercially introduced technology is

the selective cell separation by means of polymer

spheres containing small magnetic particles, so-called

magnetic beads. The surface of the beads is coated

with different kinds of monoclonal antibodies de-

signed for special selective binding with appropriately

prepared biological objects. In this way blood cells,

bacteria, or even specific nucleic acid sequences can

be isolated and manipulated in special magnetic sep-

aration equipment. The magnetic component of the

beads in most cases is magnetite. A review of this ﬁeld

can be found in Ugelstad et al. (1997).

See also: DNA Microarrays Using Magnetic Label-

ing and Detection; MRI Contrast Agents; Permanent

Magnetic Devices in Otiatria; SQUIDs: Biomedical

Applications

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Institut fu

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Jena, Germany

Magnets: Bonded Permanent Magnets

Bonded magnets are generally prepared by blending

coercive powder with a binder followed by compacting

or molding the material to the ﬁnal shape. Because

of the presence of the binder the energy product of

bonded magnets is always lower than that of their

sintered counterparts. However, two great advan-

tages of bonded magnets are their superior mechani-

cal properties and the fact that the magnets can be

prepared under net shape conditions. Bonded and sin-

tered magnets of ferrites have been known for many

decades. Both types of magnets have very low energy

products when compared to sintered rare earth-based

magnets. The excellent properties of rare earth-based

magnets are, however, accompanied by a high price.

The last decade of the twentieth century saw an

increased use of actuators and small electromotors

not only of low weight but also of low energy con-

sumption, as in battery-driven motors in particular.

There has also been an increased use of electronically

controlled, electrically actuated systems in automo-

biles, especially for critical vehicle control functions.

All this and the increased use of electromagnetic de-

vices in the electronic and electric industry has led to

an enhanced pressure to ﬁnd improved or alternative

permanent magnet materials lying between the low-

cost low-performance ferrite magnets and the high-

cost high-performance sintered NdFeB magnets.

Bonded NdFeB magnets meet most of the criteria

mentioned and their use is growing steadily. For this

reason bonded types of rare earth permanent mag-

nets will be discussed below in somewhat more detail.

1. Bonding Technologies

Bonded magnets are generally prepared by blending

coercive powder with a binder or by encapsulating

coercive powder in a binder, followed by compacting

or molding the material to the ﬁnal shape. The pow-

der of the magnetic materials used in bonded magnets

may be ferrites, alnico, samarium cobalt or neodym-

ium–iron–boron. In some cases also hybrid powders

are used.

Depending on the binder, the magnets produced

may be ﬂexible or rigid. Nitrile rubber or vinyl are

commonly used for the former type whereas for the

latter type nylon, teﬂon, polyester, or thermoset epox-

ies are employed.

There are four main manufacturing routes for

bonded magnets: calendering, injection molding, ex-

trusion, and compression bonding.

(i) Calendering involves processing the material

between rollers and leads to continuous strips of ad-

justable thickness and length. This process is sche-

matically shown in Fig. 1. The material used for

calendering is usually a mixture of the magnetic pow-

der and a thermoplastic binder. This mixture is pro-

cessed between rollers forming a continuous strip that

may be up to 100 m in length. Magnetic loading can

be up to 70% by volume.

(ii) The injection molding process is schematically

represented in Fig. 2. This process involves forcing

the heated mixture of magnetic powder and thermo-

plastic binder via tubes into a mold where it is al-

lowed to cool and harden. In this case too, magnetic

loading can reach 70%, especially when using powder

of spherical shape (Ma et al. 2002).

858

Magnets: Bon ded Permanent Magnets

(iii) Extrusion makes use of an oriﬁce through

which the compounded material is squeezed, as illus-

trated in Fig. 3. During the extrusion process the

oriﬁce is heated and the proﬁle is controlled as

the compounded material cools. Depending on the

binder, the cooled product can be ﬁrm, as for ﬂexible

end products, or rigid if a nonﬂexible end product is

desired. The loading can reach slightly higher values

(up to 75% by volume) than in the two previous

cases.

(iv) Compression bonding uses magnetic powder

blended with an epoxy that has been liqueﬁed by dis-

solution in an organic solvent. The latter is evaporated

during the blending process, leaving the magnetic

particles in an epoxy-encapsulated state. After drying,

the coated powder is ﬂowed into a conventional pow-

der press (Fig. 4) and compacted into the desired

shape. Finally, the compacted magnet is cured in an

oven at temperatures of around 150–175 1C. This

process can reach magnetic loadings up to 80%.

Because of the presence of the binder, the magnetic

remanence is always lower than that of the fully dense

magnetic material, as obtained, for instance, by sinte-

ring. The situation is even less favorable for the energy

product, being roughly proportional to the remanence

squared. There are, however, obvious advantages.

The major advantage is that bonded magnets can be

Figure 1

Schematic diagram showing the manufacturing of

magnetic strips by calendering (after Ormerod and

Constantinides 1997).

Figure 2

Schematic representation of the injection molding

process. This process involves forcing the heated mixture

of magnetic powder and thermoplastic binder via tubes

into a mold where it is allowed to cool and harden (after

Ormerod and Constantinides 1997).

Figure 3

Schematic representation of the extrusion process. The

compounded magnetic material is squeezed through an

oriﬁce. During the extrusion process the oriﬁce is heated

and the proﬁle is controlled as the compounded material

cools (after Ormerod and Constantinides 1997).

Figure 4

Diagram showing the compression bonding process. The

magnetic powder is blended with an epoxy that has been

dissolved in an organic solvent. The latter is evaporated

during the blending process, leaving the magnetic

particles in an epoxy-encapsulated state. The coated

powder is ﬂowed into a conventional powder press and

compacted into the desired shape. The compacted

magnet is ﬁnally cured in an oven (after Ormerod and

Constantinides 1997).

859

Magnets: Bonded Permanent Magnets