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

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in Pr

powders, which is the highest coercivity

reported for PrCo

-based magnets so far. A uniform

PrCo

microstructure with an average grain size of

about 15 nm is developed in the PrCo

-based powders.

The nanograined Sm

and PrCo

powders exhibit

an excellent thermal stability. Both have a coercivity

of about 5 kOe at 500 1C, which makes these nano-

structured magnets promising candidates for high-

temperature applications.

3. Conclusions

The microstructure and microchemistry of high-tem-

perature magnets can be ﬁnely tuned through adjust-

ments in the composition and processing parameters.

For bulk Sm(Co

bal

0.1

0.02

)

magnets with

z ¼7.5, the temperature coefﬁcient of coercivity

changes from a small positive to a large negative

value with increasing copper content. Changing the

ratio z and using an appropriate copper content lead

to a new magnet with a record high coercivity of

10.8 kOe at 500 1C. Its temperature coefﬁcient of co-

ercivity is only –0.03 1C

1

, which is much smaller

than that of the old commercial magnets. On the

other hand, mechanical milling has been found to be

an effective technique to produce high-performance

nanostructured RE–Co (RE ¼Sm, Pr) based pow-

ders. Magnetic properties of M

¼110.5 emug

1

¼66.2 emu g

1

, M

¼0.60, H

¼9.6 kOe, and

(BH)

¼10.8 MGOe have been obtained in Sm

powders which have an average grain size of about

30 nm. A high coercivity of 16.3 kOe along with a

high M

ratio of 0.66 and a medium-high max-

imum energy product of 11.6 MGOe have been ob-

tained in PrCo

powders with an average grain size of

about 15 nm.

Acknowledgment

This work was supported by the Air Force Ofﬁce of

Scientific Research through a MURI contract.

See also: Alnicos and Hexaferrites; Ferrite Magnets:

Improved Performance; Magnets: Sintered

Bibliography

Chen Z, Meng-burany X, Hadjipanayis G C 1999 High co-

ercivity in nanostructured PrCo

-based powders produced by

mechanical milling and subsequent annealing. Appl. Phys.

Lett. 75, 3165–7

Chen Z, Meng-burany X, Hadjipanayis G C 2000 Coercivity in

mechanically milled Pr–Co–Zr powders with TbCu

struc-

ture. J. Appl. Phys. 87, 5302–4

Chui S T 1999 Models for the pinning of domain walls in

Sm(Co, Fe, Cu, Zr)

type magnets at ﬁnite temperatures.

J. Appl. Phys. 85, 4394–6

Coehoorn R, de Mooji D B, de Waard C 1989 Meltspun per-

manent magnet materials containing Fe

B as the main phase.

J. Magn. Magn. Mater. 80, 101–4

Fischer R, Kronmu

ller H 1999 The role of the exchange inter-

action in nanocrystalline isotropic Nd

B-magnets.

J. Magn. Magn. Mater. 191, 225–33

Hadjipanayis G C 1996 In: Coey J M D (ed.) Rare-earth Iron

Permanent Magnets. Clarendon, Oxford, UK

Hadjipanayis G C 1999 Nanophase hard magnets. J. Magn.

Magn. Mater. 200, 373–91

Liu J F, Zhang Y, Dimitrov D, Hadjipanayis G C 1999 Micro-

structure and high temperature magnetic properties of

Sm(Co, Cu, Fe, Zr)

(z ¼6.7–9.1) permanent magnets.

J. Appl. Phys. 85, 2800–4

Manaf A, Buckley R A, Davis H A 1993 New nanocrystalline

high-remanence Nd–Fe–B alloys by rapid solidiﬁcation. J.

Magn. Magn. Mater. 128, 302–6

McCormick P G, Miao W F, Smith P A I, Ding J, Street R 1998

Mechanically alloyed nanocomposite magnets. J. Appl. Phys.

83, 6256–61

Ray A E 1984 The development of high energy product per-

manent magnets from 2:17 Re–TM alloys. IEEE Trans.

Magn. 20, 1614–8

Schultz L, Schnitzke K, Wecker J, Katter M, Kuhrt C 1991

Permanent magnets by mechanical alloying. J. Appl. Phys.

70, 6339–44

Strnat K J 1967 Legierungen des Kobalt mit seltenen

Erdmetallen, eine neue Gruppe aussichtsreicher

Dauermagnetwerkstoffe. Kobalt 36, 119–28

Strnat K J 1988 In: Wohlfarth E P, Buschow K H J

(eds.) Ferromagnetic Materials. North-Holland, Amsterdam

Chap. 4

Tang W, Zhang Y, Hadjipanayis G C 2000 Effect of Zr on the

microstructure and magnetic properties of Sm(Co

bal

0.1

0.088

)

8.5

. J. Appl. Phys. 87, 399–403

Velu E M T, Obermyer R T, Sankar S G, Wallace W E 1989

PrCo

-based high-energy-density permanent magnets.

J. Less-Common Met. 148, 67–71

G. C. Hadjipanayis

University of Delaware, Newark, Delaware, USA

Figure 6

Dark-ﬁeld TEM image of PrCo

powders.

870

Magnets: High-temperature

Magnets: Mechanically Alloyed

This contribution presents an overview of the ability

of the mechanical alloying process to prepare nano-

crystalline rare earth–transition metal (RE-TM) per-

manent magnet powders.

Reducing the grain size of the hard magnetic phase

is an effective way to develop coercivity in RE-TM

permanent magnets. Most suitable for the production

of ﬁne-grained magnets are nonequilibrium prepara-

tion techniques such as rapid quenching (RQ),

hydrogenation–disproportionation–desorption– recom-

bination (HDDR) (see Magnets: HDDR Processed )or

mechanical alloying (MA). The latter method has been

established by Schultz et al. (1987), where highly co-

ercive NdFeB powders were developed by mechanically

alloying the elemental constituents in a planetary ball

mill and performing a subsequent heat treatment.

Nowadays, mechanical alloying or mechanical

milling (MM, here the initial material is a prealloy)

is applied to all technologically interesting hard mag-

netic systems, of which the iron-based intermetallic

phases Nd

B, Sm

, and its modiﬁed ver-

sion Sm

are discussed in this contribu-

tion. For an overview of SmCo(Fe) prepared by MA,

see Wecker et al. (1991) and McCormick et al. (1998).

Each of the mentioned RE-TM hard magnetic phases

has its own advantages and disadvantages concerning

the intrinsic properties, the preparation routes or

simply the cost of the material (see Rare Earth Mag-

nets: Materials). In all cases, mechanical alloying re-

sulting in grain sizes as small as 20 nm leads to an

effective magnetic hardening. The nanocrystalline

microstructure of mechanically alloyed materials also

allows the preparation of exchange-coupled materi-

als, where neighboring grains are magnetically cou-

pled across the grain boundary (Neu et al. 1996). This

applies to single-phase material as well as to two-

phase material, where an additional soft magnetic

phase is coupled to the hard phase.

Magnetic materials prepared by MA are isotropic

powders. These can be epoxy bonded or cold pressed

to give compact magnets, but with a moderate re-

manence due to the low density and the isotropic easy

axis distribution. In some cases, fully dense magnets

can be prepared via hot pressing and texture can even

be induced by hot deformation (see Textured Mag-

nets: Deformation-induced), leading to highly reman-

ent magnets (Schultz et al. 1991). The properties,

which are very specific for the nanocrystalline micro-

structure, can also be achieved with rapid quenching.

Here, MA is a technologically easy and cost-effective

alternative for producing large quantities of powder.

1. The Principle of Mechanical Alloying

For MA, the starting material is sealed under an inert

atmosphere in a milling container together with milling

balls. The container movement can be a planetary

motion, a vibrational motion or a simple rotation in

the case of an industrial-sized attritor mill. In the

NdFeB system, the intimate mixing and repeated frac-

turing of the material during milling leads to powder

particles with a very ﬁne layered structure of neodym-

ium and iron and small embedded boron particles on a

length scale of a few nanometers. A subsequent heat

treatment causes a diffusional transformation towards

the thermodynamically more stable crystalline inter-

metallic Nd

B phase. The high activation mediat-

ed by the milling process (i.e., a large number of

dislocations and stress) allows this transformation to

take place at temperatures well below the melting

temperature and enables the production of nanocrys-

talline powders.

The actual thermodynamics and kinetics of the

process depend on composition, the energy input up-

on milling, and the choice of the initial material. For

higher energy input, the formation of an amorphous

phase, coexisting with a -Fe, has been observed upon

MA and MM. In all cases, however, the Nd

phase develops only after a heat treatment, so that

the optimum choice of annealing temperature T

ann

and time t

ann

is of crucial importance. Typically, for

low T

ann

or short t

ann

, an incomplete phase transfor-

mation limits H

, whereas large grains reduce H

for

too high T

ann

or too long t

ann

Similar considerations hold for other RE-TM

systems. Mechanical alloying of SmFe produces a

mixture of nanocrystalline iron and an amorphous

phase, which crystallizes into the 2:17-structure upon

heating (Schnitzke et al. 1990). The same behavior is

observed in SmFeGaC (Cao et al. 1996), whereas for

mechanical milling with higher energy input, full

amorphization is observed (Ding et al. 1993). In gen-

eral, the degree of transformation, the occurrence of

intermediate phases, and the resulting grain size de-

pend strongly on the annealing treatment.

2. Highly Coercive Powders and Magnets

Besides reducing the grain size, decoupling of the

hard magnetic grains is an effective approach to in-

crease H

. In the RE

B system this can be ob-

tained analogous to the case of sintered magnets

by choosing a RE content above the stoichiometric

value of 11.8 at%, leading to the formation of a

RE-rich paramagnetic grain boundary phase. High

coercive ﬁelds of m

¼2.0 T are reported for a

composition close to Nd

with a remanence

¼0.73 T (Schultz et al. 1991). The reported mag-

netic data refer to 100% density. The remanence

value is slightly below

is the saturation po-

larization) because of dilution with the additional

phase. By substituting dysprosium for neodymium,

values of m

as high as 5.6 T can be obtained

(Schultz 1992); the remanence then drops to 0.29 T.

871

Magnets: Mechanically Alloyed

Performing a die-upset experiment with a fully dense,

hot-pressed sample allows the development of texture,

and J

improves substantially (Fig. 1). With a

-based composition and additions of cop-

per and gallium, an MA magnet prepared in this way

exhibits J

¼1.31 T, an energy density (BH)

max

326 kJm

3

and m

¼1.2 T. These values compare

well with those of similar magnets prepared from rap-

idly quenched powders. It is again a property of the

very small grains that allows the effective texturing.

develops uniaxial magnetocrystalline

anisotropy only after modiﬁcation with interstitial

atoms like carbon or nitrogen, usually introduced by

a solid–gas reaction at temperatures around 500 1C

(Coey and Sun 1990). Schnitzke et al . (1990) prepared

powders with m

¼2.9 T by nitriding

nanocrystalline MA Sm

powders. The reman-

ence of 0.71 T is in accordance with J

¼1.54 T. Even

higher coercive ﬁelds of 4.4 T are reported by Liu

et al. (1994). A systematic study on the different cry-

stallographic modiﬁcations of the Sm

-nitrides

and their parent alloys is given by Teresiak et al.

(1999). The use of the Sm

-phase is, however,

limited by its low thermal stability. Above 550 1C,

disproportionates into SmN and a-Fe.

Often, small amounts of partly unnitrided soft

or soft a-Fe are present in the material.

For optimization, grain and particle sizes have to be

considered in the kinetics of the solid–gas reaction

(Coey and Smith 1999). The low thermal stability

also prohibits a subsequent hot compaction.

The stability of the carbonized or nitrided Sm

phase can be drastically improved by a substitution

of iron with gallium, silicon or aluminum. Nanocrys-

talline, highly coercive Sm

17x

was pre-

pared by MM with subsequent heat treatment, where

carbon had already been introduced in the prealloy

(Ding et al. 1994). The high stability also allows hot

pressing, and fully dense Sm

magnets

with coercive ﬁelds of 1.5 T and remanences of 0.5 T

can be prepared from MA powders (Cao et al. 1996).

Texture via die upsetting was not obtained.

3. Exchange-coupled Powders and Magnets

Exchange coupling across the grain boundaries leads

to an enhanced remanence compared to decoupled

single-domain grains due to spin rotation in the grain

boundary region (see Magnets: Remanence-en-

hanced). The effect increases with decreasing grain

size. Nanocrystalline single-phase Nd

B powders

with exact stoichiometric composition and controlled

grain size were prepared by mechanical milling and

subsequent heat treatment (Neu et al. 1999). The

grain-size dependence of the enhancement follows

the prediction of a newly developed micromagnetic

model (Neu et al. 1998).

In agreement with the consideration outlined

above, the highest reduced remanence J

¼0.56 is

found in the Nd

B powders with the smallest

average grain size of 33 nm. Smaller grains (21 nm)

were obtained in single-phase Nd

(FeCoGaNb)

powders and J

was further improved to 0.60.

Suess et al. (2000) report even higher reduced re-

manences of 0.68 in mechanically alloyed and hot

pressed Pr

B magnets. These experiments show

that exchange coupling, originally discovered in rap-

idly quenched ribbons, is as effective in MA or MM

magnet powders, when a sufﬁciently small grain size

is achieved.

If a second phase with higher J

, e.g., iron, is cou-

pled to the hard magnetic phase, the composite ma-

terial shows a collective magnetization behavior, a

further increased remanence, and a decreased but still

considerable coercive ﬁeld. Both the increase in J

and the decrease in H

scale with the amount of the

soft phase. If, however, owing to large soft grains,

the coupling is insufﬁcient, magnetization reversal is

initiated in the soft grains, coercivity is drastically

reduced, and the demagnetization curve shows a

two-step behavior. Milling of an iron-rich NdFeB

composition and subsequent heat treatment leads to

aNd

B þa-Fe composite with sufﬁciently small

soft grains of about 20–30 nm.

Some additional elements, which dissolve in iron

upon milling but are rejected upon annealing, can

slow down the iron diffusion and therefore further

reduce the iron grain size. Additional elements can

also change the crystallization temperature of the

2:14:1 phase and thereby inﬂuence the grain size.

Among the tested elements zirconium, niobium, and

molybdenum are found to improve the coercive ﬁeld

via a reduced a-Fe grain size (Crespo et al. 1997, Cui

et al. 2000). Important to the discussion of grain

sizes and their inﬂuence on the magnetic properties is

Figure 1

Hysteresis loop of a textured NdFeB magnet (MM3)

prepared via hot pressing and die-upsetting of

mechanically alloyed Nd

-based powder (after

Schultz et al. 1991). The magnet is measured parallel and

perpendicular to the press direction. For comparison,

the behavior of the annealed isotropic MA powder is

shown (MM1).

872

Magnets: M echanically Alloyed

consideration? of the degree of phase transformation.

Especially when the annealing is performed at low

temperatures, the advantage of small grain sizes is

overcome by a residual amount of amorphous soft

phase. There is no lower theoretical limit for grain

size of the soft phase to give optimum H

, but only

the experimental difﬁculty of obtaining both small

grains and a complete crystallization.

The substitution of cobalt for iron inﬂuences the

intrinsic and extrinsic magnetic properties. Milling

and annealing leads to the same iron:cobalt ratio in

soft and hard phase. Curie temperature and satura-

tion polarization in both phases are strongly im-

proved and the remanence is increased. Also J

increases, indicating a more effective coupling of the

grains, and the coercivity is rather constant. Upon

cobalt substitution, a smaller a-(Fe,Co) grain size has

been observed (You et al. 2000) and owing to the

decrease in the magnetocrystalline anisotropy for

both phases, an increase in the exchange length l

expected. This explains the improved coupling and

also the stability of H

, as the better coupling and the

decreased anisotropy level off.

In comparing MA with MM, the second process

often leads to better magnetic properties, attributed

to a more homogeneous microstructure with a small-

er grain size distribution (Miao et al. 1996). Conse-

quently, exchange-coupled NdFeB magnet powders

prepared by mechanical milling of a master alloy with

additional cobalt and grain reﬁning elements show

very attractive magnetic properties, with J

E1.2 T,

E0.5 T and (BH)

max

E150 kJm

3

(Fig. 2, Neu

1998). It is also possible to hot-press the as-milled

powder and to prepare fully dense magnets with

comparable properties (Wecker et al. 1995). As a rule

of thumb, in all these materials optimum energy den-

sities are obtained for roughly 20–40 vol.% soft

phase, which equals about 7–9 at.% neodymium.

Remanence enhancement in nanocrystalline SmFe-

nitrides occurs for a samarium content below 11 at.%.

MA of Sm

powders, subsequent annealing and

nitriding produces a two-phase mixture of Sm

and a-Fe with an iron grain size of about 20 nm. The

powders show exchange-coupling behavior with a

high remanence of J

¼1.13 T and a coercive ﬁeld of

¼0.39 T (Ding et al. 1993). As H

depends so

critically on the grain size of the soft phase, optimum

properties are only achieved in a very small window of

the annealing temperature. Successful attempts to re-

ﬁne the a-Fe grain size down to 10–20 nm were re-

ported by O’Donnel and Coey (1997) by adding 2

at.% zirconium or tantalum to a MA Sm

pow-

der. The addition of tungsten improves H

, but with-

out measurable effect on the average grain size

(Kaszuwara et al. 2000). Here, an increase of the uni-

axial anisotropy is assumed, as tungsten is observed to

dissolve in the Sm

phase. MA two-phase

/a-Fe magnets are discussed in Feutrill

et al. (1996) with an emphasis on the magnetization

behavior of exchange-coupled materials.

4. Conclusions

Mechanical alloying with subsequent heat treatment

results in nanocrystalline permanent magnet powders

with excellent properties. Hot compaction and hot

deformation enables the production of highly aniso-

tropic magnets. The most promising materials for

cost effective resin-bonded magnets are isotropic ex-

change-coupled nanocomposites with high re-

manences of J

X1.20 T and sufﬁcient coercive ﬁeld

¼0.5 T, which are based on NdFeCoB or PrFe-

CoB. The increase in the Curie temperature due to

cobalt allows their use at elevated temperatures. Al-

though diluted, these ‘‘lean’’ hard magnets based on

low RE-content can achieve remanences exceeding

0.8 T.

Likewise, the SmFeN/C powders offer Curie tem-

peratures above 450 1C, even without the expensive

cobalt. Remanence and energy density, especially in

the SmFeMC system, have still to be improved.

Bibliography

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Schultz L 1996 High-performance Sm

permanent

magnets made by mechanical alloying and hot pressing. J.

Phys. D: Appl. Phys. 29, 271–3

Coey J M D, Smith P A I 1999 Magnetic nitrides. J. Magn.

Magn. Mater. 200, 405–25

Coey J M D, Sun H 1990 Improved magnetic properties by

treatment of iron-based rare earth intermetallic compounds

in ammonia. J. Magn. Magn. Mater. 87, L251–4

Figure 2

Hysteresis loop of exchange-coupled Nd

magnet powder prepared by mechanical milling of a

stoichiometric master alloy blended with iron, cobalt,

and silicon powder. Milling and optimum annealing

leads to a nanocomposite of 70 vol.%

(Fe,Co)

B þ30 vol.% (a-(Fe, Co)) with grain sizes

of about 20 nm (after Neu 1998). m

¼0.5 T,

¼1.19 T, (BH)

max

¼149 kJm

3

873

Magnets: Mechanically Alloyed

Crespo P, Neu V, Schultz L 1997 Mechanically alloyed nano-

composite powders of Nd

B/a-Fe with additional ele-

ments. J. Phys. D: Appl. Phys. 30, 2298–303

Cui B Z, Sun X K, Liu W, Zhang Z D, Geng D Y, Zhao X G

2000 Effects of additional elements on the structure and

magnetic properties of Nd

B/a-Fe-type nanocomposite

magnets. J. Phys. D: Appl. Phys. 33, 338–44

Ding J, McCormick P G, Street R 1993 Remanence enhance-

ment in mechanically alloyed Sm

-nitride. J. Magn.

Magn. Mater. 124, 1–4

Ding J, Shen B, McCormick P G, Street R, Wang F 1994

Magnetic hardening of mechanically milled Sm

. J. Alloys Comp. 209, 221–4

Feutrill E H, McCormick P G, Street R 1996 Magnetization

behavior in exchange-coupled Sm

/a-Fe. J. Phys.

D: Appl. Phys. 29, 2320–6

Kaszuwara W, Leonowicz M, Kozubowski J A 2000 The effect

of tungsten addition on the magnetic properties and micro-

structure of SmFeN-a-Fe nanocomposites. Mater. Lett. 42,

383–6

Liu W, Wang Q, Sun X K, Zhao X, Zhao T, Zhang Z, Chuang

Y C 1994 Metastable Sm-Fe-N magnets prepared by me-

chanical alloying. J. Magn. Magn. Mater. 131, 413–6

McCormick P G, Miao W F, Smith P A I, Ding J, Street R 1998

Mechanically alloyed nanocomposite magnets. J. Appl. Phys.

83, 6256–61

Miao W F, Ding J, McCormick P G, Street R 1996 A com-

parative study of mechanically alloyed and mechanically

milled Nd

. J. Appl. Phys. 79, 2079–83

Neu V 1998 Exchange-coupled nanocrystalline Nd-Fe-B magnet

powders prepared by mechanical alloying. Dissertation, Uni-

versity of Technology, Dresden

Neu V, Hubert A, Schultz L 1998 Modelling of the enhanced

remanence of nanocrystalline, exchanged-coupled hard mag-

netic grains. J. Magn. Magn. Mater. 189, 391–6

Neu V, Klement U, Scha

fer R, Eckert J, Schultz L 1996 Re-

manence enhancement in mechanically alloyed two-phase

Nd-Fe-B magnetic material. Mater. Lett. 26, 167–70

Neu V, Schultz L, Bauer H-D 1999 Grain size dependence of

remanence enhancement and coercivity in nanocrystalline

Nd-Fe-B-powders. Nanostruct. Mater. 12, 769–74

O’Donnel K, Coey J M D 1997 Characterization of hard mag-

netic two-phase mechanically alloyed Sm

/a-Fe nano-

composites. J. Appl. Phys. 81, 6310–21

Schnitzke K, Schultz L, Wecker J, Katter M 1990 High

coercivity in Sm

magnets. Appl. Phys. Lett. 57, 2853–5

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ter. Sci. Forum 88–90, 687–94

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K-H 1999 Inﬂuence of nitrogenation on structure develop-

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J. Phys. D: Appl. Phys. 33, 926–31

V. Neu and L. Schultz

Institut fu

r Metallische Werkstoffe, Dresden, Germany

Magnets: Remanence-enhanced

Remanence-enhanced magnets are isotropic hard

magnetic materials with remanence exceeding one

half of the saturation magnetic polarization expected

for a randomly oriented assembly of noninteracting

single domain particles of uniaxial magnetic aniso-

tropy. Remanence enhancement is a result of strong

ferromagnetic coupling via intergrain exchange inter-

actions among spin magnetic moments belonging to

adjacent hard magnetic grains whose sizes are com-

parable to the ferromagnetic correlation length, or,

the ‘‘exchange length.’’ Typical exchange lengths in

ferromagnets are of the order of a few nanometers.

Therefore, remanence-enhanced magnets are nor-

mally nanocrystalline.

Typical remanence-enhanced magnets are rapidly

solidiﬁed Nd

B-based magnets for which the ra-

tio of remanence to saturation magnetic polarization

is about 0.6. Isotropic nanocomposite permanent

magnets comprising soft and hard magnetic phases

coupled via intergrain exchange interactions are also

regarded as remanence-enhanced magnets. In the

nanocomposite permanent magnets, the remanence

to saturation polarization ratio can be much larger

than in the single-phase nanocrystalline magnets be-

cause of the small and nonuniaxial magnetocrystal-

line anisotropy of the soft magnetic phase. Typical

examples of this type of magnet include Fe

B and a-Fe/Nd

B. The demagnetization

behavior of both types of remanence-enhanced mag-

nets has been successfully simulated using micromag-

netic calculations.

1. Remanence Enhancement in Nearly

Single-phase Hard Magnetic Materials

Magnetic hardness of most permanent magnetic ma-

terials originates from uniaxial anisotropy, either

shape anisotropy or magnetocrystalline anisotropy,

of at least one ferromagnetic component phase (see

Magnetic Anisotropy). The shape anisotropy arises

from the direction-dependent magnetostatic energy

caused by an elongated shape of a ferromagnetic

phase and is of the order of 10

1

MJm

3

. On the

874

Magnets: Remanence-enhanced

other hand, magnetocrystalline anisotropy arises

from the electrostatic energy associated with the elec-

tron charge distribution of the individual atom which

interacts with the electronic charge distribution asso-

ciated with the crystal. It becomes as large as

22 MJm

3

. Therefore, most modern permanent mag-

nets are composed of hard magnetic phases such as

MFe

(M ¼barium or strontium), Nd

, SmCo

, and Sm

which have a large

magnetocrystalline anisotropy.

In the following, an assembly of a large number of

single-domain hard magnetic particles of a ferromag-

netic material with a uniaxial anisotropy is considered,

the easy direction of magnetization being distributed

randomly in a three-dimensional space. When a suf-

ﬁciently large magnetic ﬁeld is applied in one direction

(z-axis), all magnetic moments of all particles will lie

on a hemisphere and then the applied ﬁeld is reduced

to zero: the assembly of particles has a remanent

magnetization of one half of the total saturation mag-

netization if there are no interactions among the mag-

netic moments of the particles (Fig. 1(a)), known as

the Stoner-Wohlfarth theorem (Stoner and Wohlfarth

1948). However, if there exist magnetic interactions,

either dipolar or exchange interactions, among mag-

netic moments of the particles, which cause a reduc-

tion of the total energy when adjacent particles align

their magnetic moment parallel, then the remanent

magnetization becomes larger than one half of the

total saturation magnetization (Fig. 1(b)). This phe-

nomenon is called remanence enhancement.

Intergranular exchange interactions occurs be-

tween atoms belonging to adjacent grains along

boundaries. The amount of energy associated with

this interaction is proportional to the contact area of

these grains. On the other hand, the magnetocrystal-

line anisotropy energy is proportional to the volume

of the grain. These two sources of energy are coun-

teracting with each other. When the local magnetic

anisotropy dominates, the effect of exchange cou-

pling can be neglected, and the magnet behaves like a

Stoner-Wohlfarth magnet. On the other hand, when

exchange coupling is very strong, the local magnetic

anisotropy is averaged out and the system behaves

like a soft magnetic material. Thus, there exists an

intermediate situation in which the system behaves as

a remanence-enhanced magnet.

The ratio of these two counteracting energy con-

tributions may be deﬁned as

Z ¼ K

V=J

S ð1Þ

where J

is the effective intergranular exchange con-

stant, (1/2)S the interface surface area per grain, K

the anisotropy constant, V the volume of a grain, ne-

glecting distributions of values of these quantities.

The parameter Z is proportional to the grain size.

Another parameter which depends on the microstruc-

ture of the material is J

, which depends on the atomic

and chemical structure of the grain boundaries.

Figure 2 shows calculated hysteresis loops of model

magnets composed of uniform cubic grains character-

ized by various values of the parameter Z.Thecalcu-

lation is based on the hypothesis that the magnetic

moment of each cubic cell is represented by a single

vector of length J

V,whereJ

is the saturation

magnetization per unit volume (Fukunaga and Inoue

1992). This ﬁgure clearly demonstrates that the reman-

ence ratio increases with increasing exchange inter -

actions (i.e., decreasing Z), whereas the coercivity

decreases, and that there exists an intermediate

strength of exchange coupling with which the mag-

netic properties in terms of both remanence and

coercivity can be optimized. With realistic values of

Figure 1

Schematic illustration of spin direction distribution of

(a) a Stoner-Wohlfarth magnet and (b) a remanence-

enhanced magnet.

Figure 2

Shapes of full hysteresis loops of single-phase isotropic

magnets with different exchange-coupling parameters

Z ¼K

V/J

S (after Fukunaga and Inoue 1992).

875

Magnets: Remanence-enhanced

and J

expected for rare earth intermetallic com-

pounds such as Nd

B, optimum magnetic proper-

ties are suggested to be obtained with grain sizes of

about 10 nm.

A micromagnetically more accurate description of

the mechanism of remanence enhancement needs

consideration of directional variation of atomic mag-

netic moments within a grain. Figure 3 shows the

results of two-dimensional ﬁnite-element calculations

of inhomogeneous regions along grain boundaries

of a model system with grain diameters of 10 nm

and 20 nm in the remanent state after magnetic

saturation in one direction (Schreﬂ et al. 1994). The

inhomogeneous regions refer to regions in which

magnetic moments are largely off from the local easy

direction of magnetization. This ﬁgure visualizes the

effect of grain size on the volume fraction of the in-

homogeneous regions in which magnetic moments are

inclined towards the common direction of magneti-

zation of the whole magnet.

2. Remanence Enhancement in Nanocomposite

Magnets

The class of permanent magnets called nanocom-

posite permanent magnets or ‘‘exchange-spring mag-

nets’’ (Kneller and Hawig 1991) shows a relatively

large remanence of 70–80% of saturation magneti-

zation. These magnets are sometimes considered as

remanence-enhanced magnets. One of the reasons

that this class of materials maintains such a large

fraction of ﬂux density in the remanent state is that

they contain a large fraction of nonuniaxial aniso-

tropic phases. The intention to compose a nanocom-

posite magnet is to increase J

by combining a hard

magnetic phase (phase 1) with other ferromagnetic

phases which have a large saturation magnetization

(phase 2). Phase 2 does not need to have a large uni-

axial anisotropy. Good magnetic properties may be

obtained by relying only on strong exchange coupling

between the component phases. For instance, b.c.c.-

Fe, which is an important component of nanocom-

posite magnets based on FeNdB or FeSmN systems,

has a cubic magnetocrystalline anisotropy.

The remanence ratio of a two-phase isotropic

nanocomposite magnet is given by

¼½v

þð1  v

Þm

=J

with

¼v

þð1  v

ÞJ

ð2Þ

where m

and m

are the remanence ratios of each

component, and J

and J

are the saturation mag-

netizations of phase 1 and phase 2, respectively. For

the case in which phase 1 is of uniaxial anisotropy

and phase 2 is of planar anisotropy, without any in-

teractions, m

¼0.5, and m

¼0.79. For cubic crystals

with /100S easy directions of magnetization (as

in b.c.c.-Fe), m

¼0.83 and for cubic crystals with

/111S easy directions of magnetization (as in f.c.c.-

Ni), m

¼0.86. Therefore, the remanence ratio is al-

ways larger than 0.5 for the case of nanocomposite

magnets (v

o1). A good nanocomposite magnet has

a remanence ratio even larger than the value given by

Eqn. (2) with the m

and m

values given above.

3. Processing Methods of Remanence-enhanced

Magnets

The nanoscopic structure which is required to realize

a measurable remanence enhancement is metastable

Figure 3

Inhomogeneous regions along grain boundaries in two-

dimensional model magnets with different grain sizes

using the magnetic parameters of Nd

B. The arrows

indicate the easy direction of magnetization. The shaded

areas denote the regions where the magnetic polarization

deviates from the local easy direction of magnetization

by more than 10 degrees (lighter gray) and 20 degrees

(darker gray). Periodic boundary conditions are applied

in the calculation (after Schreﬂ et al. 1994).

876

Magnets: Remanence-enhanced

in that it contains a high interfacial boundary energy.

Commonly used techniques to create such a micro-

structure utilize the instability of a metastable phase

such as an amorphous phase or a mixture of quasi-

equilibrium phases. Techniques to create such me-

tastable states include rapid solidiﬁcation, mechanical

alloying (see Magnets: Mechanically Alloyed), and

vapor deposition (see Magnetic Films: Hard). A short

heat treatment at sufﬁciently low temperatures trans-

forms the metastable state into another metastable

nanoscopic structure. Another mechanism which

may be utilized is the ‘‘spinodal decomposition’’

which is believed to be working in production of the

FeAlNiCo type of permanent magnets (see Alnicos

and Hexaferrites) and in the 2-17 type SmCoFeCuZr-

type magnets. However, the main body of existing

research has used the rapid solidiﬁcation technique

and, in many cases, a subsequent short annealing at

relatively low temperatures in the range between 600

and 700 1C is applied.

In the case of rapid solidiﬁcation, a continuous

strand of a molten alloy is poured onto a circumfer-

ential surface of a chilled roll. Pour rate of the molten

alloy is regulated by the diameter of an oriﬁce at the

bottom of a melt container and by keeping the head

pressure of the melt in a certain range. A continuous

operation of this process is possible if the melt pour

rate can be regulated within the optimum range by

supplying melt into the melt container.

The design of the alloy composition involves the

addition of small amounts of elements which signif-

icantly affect the kinetics of crystallization reactions.

Examples are aluminum, silicon, niobium and zirco-

nium in nanocrystalline Nd

B (Matsumoto et al.

1988) and in Fe/Nd

B nanocomposites, and cop-

per in Fe

B/Nd

B nanocomposites (Hirosawa

et al. 1998). Experiments have indicated that

B-based nanocrystalline or nanocomposite

magnets can be obtained by a single-stage rapid solid-

iﬁcation process without further heat treatment

(Manaf et al. 1991). This process is often called ‘‘di-

rect quenching.’’ Another route to produce nearly the

same microstructure is to rapidly solidify alloys at a

slightly larger cooling rate and apply a subsequent

heat treatment. This procedure is often referred as

‘‘overquenching and annealing.’’

The cooling rate during solidiﬁcation can be con-

trolled by adjusting both the melt pour rate and the

substrate (i.e., the chilled roll) speed. The industrially

preferred process is the overquenching and annealing

route because of a wider processing window in com-

parison to the direct quenching route. The a-Fe/

SmFe

N-type nanocomposites can be produced also

by the rapid solidiﬁcation and annealing route, and a

subsequent nitrogenation treatment, which is applied

in order to convert the soft magnetic SmFe

into the

hard magnetic SmFe

phase by a gas-phase inter-

stitial modiﬁcation of the lattice structure.

The mechanical alloying process is able to produce

a metastable mixture of phases and/or elements

having a large density of structural defects such as

grain boundaries and nonequilibrium vacancies

(Schultz et al. 1987). On annealing, such a structure

of a large internal energy can be transformed into

a nanocrystalline or nanocomposite structure. In

order to produce a bulk magnet, the resulting me-

chanically alloyed powder has to be compacted. The

metallic materials being processed must be protected

in an inert gas atmosphere in order to avoid severe

oxidation particularly in the case of rare earth mag-

nets. The mechanical alloying process, therefore, is

operated in batch. On a laboratory scale, this process

has been successfully used to produce nearly the

same material as obtained by the rapid solidiﬁcation

route.

4. Typical Magnetic Properties

of Remanence -enhanced Magnets

Some typical examples of remanence-enhanced

magnets include Nd

B, Fe

B/Nd

B, a-Fe/

B, and a-Fe/SmFe

. Single-phase SmCo

and Sm

-based materials are usually difﬁcult

to process into materials showing a marked reman-

ence enhancement because of the very large uniaxial

anisotropy which requires the grain sizes to be very

small. Table 1 gives a few examples of remanence-

enhanced magnets composed of nearly single phase

B-type compounds. Typical J

values are

0.6. For NdFeB and SmFeN nanocomposites, even

higher J

values of 0.7 to 0.8 are obtained (Table 2).

Figure 4 shows an example of the dependence of

Table 1

Magnetic properties and J

ratio of some Nd

B-based ribbon magnets.

Composition

(BH)

max

(kJ m

3

)

(kA m

1

)

(T)

(T) J

Reference

79.3

0.7

162 891 0.96 N/A ca. 0.6 Matsumoto et al. (1988)

11.8

81.0

5.7

1.7

144 874 0.925 1.527 0.606 Clemente et al. (1988)

7.5

1.5

153 962 0.926 1.607 0.576 Yamamoto et al. (1989)

174 1020 1.02 1.58 0.646 Bauer et al. (1996)

877

Magnets: Remanence-enhanced

remanence and intrinsic coercivity on grain size in

13.2

79.6

1.2

magnets (Manaf et al. 1991). A

visible remanence enhancement occurs at grain sizes

below 40 nm in this example.

Bibliography

Bauer J, Seeger M, Zern A, Kronmueller H 1996 Nanocrystal-

line FeNdB permanent magnets with enhanced remanence.

J. Appl. Phys. 80, 1667–73

Clemente G B, Keem J E, Bradley J P 1988 The microstructural

and compositional inﬂuence upon HREM behavior in

B. J. Appl. Phys. 64, 5299–301

Coehoorn R, de Mooij D B, Duchateau J P W B, Buschow K H

J 1988 Novel permanent magnetic materials made by rapid

quenching. J. Phys. C8, 669–70

Ding J, McCormick P G, Street R 1993 Remanence enhance-

ment in mechanically alloyed isotropic Sm

-nitride.

J. Magn. Magn. Mater. 124, 1–4

Fukunaga H, Inoue H 1992 Effect of intergrain exchange in-

teraction on magnetic properties in isotropic NdFeB mag-

nets: Jpn. J. Appl. Phys. 31, 1347–52

Hirosawa S, Kanekiyo H, Uehara M 1998 Magnetic properties

of Cr-doped Fe

B/Nd

B nanocomposite hard magnetic

materials. J. Magn. Soc. Jpn. 22 (Suppl. S1), 325–7

Kneller E F, Hawig R 1991 The exchange-spring magnet: a new

material principle for permanent magnets. IEEE Trans.

Magn. 27, 3588–600

Manaf A, Buckley R A, Davies H A, Leonowicz M 1991 En-

hanced magnetic properties in rapidly solidiﬁed NdFeB-

based alloys. J. Magn. Magn. Mater. 101, 360–2

Matsumoto F, Sakamoto H, Komiya M, Fujikura M 1988

Effects of silicon and aluminum additions on magnetic prop-

erties of rapidly quenched NdFeB permanent magnets. J.

Appl. Phys. 63, 3507–9

Schreﬂ T, Fidler J, Kronmuller H 1994 Remanence and co-

ercivity in isotropic nanocrystalline permanent magnets.

Phys. Rev. B 49, 6100–10

Schultz L, Wecker J, Hellstern E 1987 Formation and prop-

erties of NdFeB prepared by mechanical alloying and solid

state reaction. J. Appl. Phys. 53, 3583–5

Stoner E C, Wohlfarth E P 1948 A mechanism of magnetic

hysteresis in heterogeneous alloys. Philos. Mag. Trans.

R. Soc. 240, 599–644

Yamamoto H, Nagakura M, Ozawa Y, Katsuno T 1989 Mag-

netic properties of high-quality melt-spun Nd-Fe-Co-B-V

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nets and Their Applications Pt. 1. The Society of Non-

Traditional Technology, Tokyo, pp. 543–50

S. Hirosawa

Sumitomo Special Metals Company, Osaka, Japan

Magnets: Sintered

Sintered magnets are manufactured by powder met-

allurgy in many shapes and dimensions in large vol-

umes. Besides hard magnetic hexaferrites and Alnico

materials (see Alnicos and Hexaferrites), mainly rare

earth (RE)–transition metal (TM) alloys are applied.

By powder metallurgy the magnetocrystalline aniso-

tropy of hard hexaferrites, of samarium (Sm)–cobalt

(Co) compounds, SmCo

as well as modiﬁed Sm

(Co,

Cu, Fe, Zr)

, and of the neodymium (Nd)–iron (Fe)–

boron (B) compound, Nd

B, can be exploited in

order to achieve anisotropic magnets with a high

remanent polarization B

and a strong coercivity H

(see Rare Earth Magnets: Materials).

The basic principles of the production route,

in particular the beneﬁts of the different powder

compaction processes and their impact on the

Table 2

Magnetic properties and J

ratio of some nanocomposite magnet ribbons (NdFeB) and powder (SmFeN; a

demagnetization correction involved).

Composite phases and their

molar fractions

(BH)

max

(kJ m

3

)

(kA m

1

) J

(T) J

(T) Meas Eq. (2) Reference

(Fe

/Fe

/(Nd

93 240 1.2 1.6 0.75 0.75 Coehoorn et al. (1988)

/(Nd

185 416 1.25 1.78 0.70 0.60 Bauer et al. (1996)

/(Sm

2.6

)

205 310 1.42 1.75 0.81 0.63 Ding et al. (1993)

Figure 4

Dependence of remanence and intrinsic coercivity on

grain size in Nd

13.2

79.6

1.2

ribbon magnets (after

Manaf et al. 1991). ~ and  represent H

and J

on the

wheel side of ribbons and * and J represent H

and J

on the free side of ribbons, respectively.

878

Magnets: Sintered

dimensions and magnetic properties are reviewed in

this article.

1. Alloying and Powder Processing

Nd–Fe–B or Sm–Co alloys can be prepared from RE

metals, mainly neodymium and samarium, transition

metals, iron or cobalt, and master-alloys Fe–B,

Fe–Nb, or additions, e.g., aluminum, copper, zirco-

nium, etc. Since the RE metals are highly reactive,

alloying has to be performed in a vacuum induction

furnace. The melting cycle needs careful optimization

in order to minimize chemical reactions with the cru-

cible and to prevent oxidation by residual atmospheric

gases. The melted alloy is cast into ingots which have

a polycrystalline microstructure (see top of Fig. 1).

An alternative alloying route is the calciothermic

reduction diffusion (RD) process (Herget 1985). RE

oxides, TM powder, and the required additions are

mixed with calcium-granulate as a reducing agent.

The compacted powders are heated up to 1100 1Cina

vacuum, so that the calcium vapor reduces the RE

oxide to RE metal. The RE metal diffuses into the

TM powder and the RE–TM alloy results, mixed

with calcium oxides. After cooling to room temper-

ature the calcium oxides are leached by aqueous

solutions. Since the calciothermic RD process uses

RE oxides, the materials are more economic, but the

ﬁnal RE–TM alloy-powder contains more impurities

than an alloy melted in a vacuum induction furnace.

A recently developed alloying process is strip cast-

ing. The molten alloy is cast onto a rotating copper-

roll and is cooled at cooling rates of about

30 K min

1

to approximately 0.3 mm thick ﬂakes

(Hirose et al. 1998).

The cast ingots, alloy powder particles, or strip cast

ﬂakes have a polycrystalline microstructure resulting

in a random distribution of the easy-axes of the crys-

tals and hence of the magnetic moments. For achiev-

ing a well-deﬁned texture or easy-axis, respectively,

powder metallurgy is applied (Fig. 1). The alloys are

crushed and milled mechanically or by hydrogen de-

crepitation to a ﬁne alloy powder (Harris et al. 1994).

In order to get a perfect alignment in a magnetic ﬁeld,

the powder particles must consist of single crystals. In

general alloy powders with a particle size in the range

of 3–5 mm meet this condition well. For the produc-

tion of anisotropic magnets there exists different

processing routes that determine the magnetic prop-

erties, dimensional tolerances, and processing costs.

2. Compaction Methods

2.1 Cold Isostatically Pressed (CIP) Magnet Blocks

For the production of big blocks the alloy powder is

sealed in a rubber mold, aligned in a magnetic ﬁeld,

and pressed isostatically. Since the pressure increases

homogeneously from all directions, the alignment of

the powder particles is not disturbed significantly

during compaction (Fig. 1, left-hand side). The iso-

statically pressed blocks are sintered to more than

98% of the theoretical density and annealed in order

to optimize the magnetic properties.

Hence, isostatically pressed blocks have an excellent

texture. The alignment coefﬁcient amounts to about

98% (Rodewald et al. 2000). Due to the proper align-

ment of all magnetic moments, sintered Nd–Fe–B

magnets with a remanent polarization of 1.47 T and a

coercivity of 9.6 kA cm

1

(12 kOe) can be produced

resulting in a maximum energy density of 420 kJ m

3

(53 M GOe). Figure 2 presents the demagnetization

curves J(H)andB(H) of such sintered Nd–Fe–B mag-

nets at different temperatures. The temperature coef-

ﬁcients (TC) of the remanent polarization and of the

coercivity amount to 0.11% K

1

and 0.77% K

1

in the temperature range of 20–100 1C.

2.2 Axial Field Die-pressed (AP) Net-shape Parts

The manufacturing of parts from isostatically pressed

RE magnet blocks needs a lot of machining, which

Figure 1

Principles of the different processing routes of sintered

RE–TM magnets by powder metallurgy.

879

Magnets: Sintered