ft

Both ferromagnetic and ferrimagnetic materials are classified as either soft or
hard on the basis of their hysteresis characteristics. Soft magnetic materials are
used in devices that are subjected to alternating magnetic fields and in which energy
losses must be low; one familiar example consists of transformer cores. For this
reason the relative area within the hysteresis loop must be small; it is characteris-
tically thin and narrow, as represented in Figure 20.19. Consequently, a soft mag-
netic material must have a high initial permeability and a low coercivity. A material
possessing these properties may reach its saturation magnetization with a relatively
low applied field (i.e., is easily magnetized and demagnetized) and still has low
hysteresis energy losses.
The saturation field or magnetization is determined only by the composition
of the material. For example, in cubic ferrites, substitution of a divalent metal
ion such as for in will change the saturation magnetization.
However, susceptibility and coercivity which also influence the shape of the
hysteresis curve, are sensitive to structural variables rather than to composition.
For example, a low value of coercivity corresponds to the easy movement of do-
main walls as the magnetic field changes magnitude and/or direction. Structural
defects such as particles of a nonmagnetic phase or voids in the magnetic mate-
rial tend to restrict the motion of domain walls, and thus increase the coercivity.
Consequently, a soft magnetic material must be free of such structural defects.
Another property consideration for soft magnetic materials is electrical resis-
tivity. In addition to the hysteresis energy losses described above, energy losses may
result from electrical currents that are induced in a magnetic material by a mag-
netic field that varies in magnitude and direction with time; these are called eddy
currents. It is most desirable to minimize these energy losses in soft magnetic ma-
terials by increasing the electrical resistivity. This is accomplished in ferromagnetic
materials by forming solid solution alloys; iron–silicon and iron–nickel alloys are
examples. The ceramic ferrites are commonly used for applications requiring
soft magnetic materials because they are intrinsically electrical insulators. Their
applicability is somewhat limited, however, inasmuch as they have relatively small
(Hc),
FeO–Fe2O3Fe2
Ni2
96 • Chapter 20 / Magnetic Properties
Hard
Soft
H
B
Figure 20.19 Schematic magnetization curves
for soft and hard magnetic materials. (From
K. M. Ralls, T. H. Courtney, and J. Wulff,
Introduction to Materials Science and
Engineering. Copyright 1976 by John Wiley
& Sons, New York. Reprinted by permission
of John Wiley & Sons, Inc.)
©
soft magnetic
material
1496T_c20_76-113 10/31/05 16:31 Page 96 FIRST PAGES An Iron-Silicon Alloy That is Used in Transformer Cores
MATERIAL OF IMPORTANCE
As mentioned earlier in this section, trans-
former cores require the use of soft magnetic
materials, which are easily magnetized and de-
magnetized (and also have relatively high electri-
cal resistivities). One alloy commonly used for this
application is the iron–silicon alloy listed in Table
20.5 (97 wt% Fe-3 wt% Si). Single crystals of this
alloy are magnetically anisotropic, as are also single
crystals of iron (as explained above). Consequently,
energy losses of transformers could be minimized if
their cores were fabricated from single crystals such
that a [100]-type direction [the direction of easy
magnetization (Figure 20.17)] is oriented parallel to
the direction of an applied magnetic field; this con-
figuration for a transformer core is represented
schematically in Figure 20.20. Unfortunately, single
crystals are expensive to prepare, and, thus, this is
an economically unpractical situation. A better al-
ternative—one that is used commercially, being
more economically attractive—is to fabricate cores
from polycrystalline sheets of this alloy that are
anisotropic.
It is often the case that the grains in polycrys-
talline materials are randomly oriented, with the re-
sult that their properties are isotropic (Section 3.15).
However, one way of developing anisotropy in
polycrystalline metals is via plastic deformation, for
example by rolling (Section 11.4, Figure 11.8b);
rolling is the technique by which sheet transformer
cores are fabricated. A flat sheet that has been rolled
is said to have a rolling (or sheet) texture, or there is
a preferred crystallographic orientation of the grains.
For this type of texture, during the rolling operation,
for most of the grains in the sheet, a specific crys-
tallographic plane (hkl) becomes is aligned parallel
(or nearly parallel) to the surface of the sheet, and,
in addition a direction [uvw] in that plane lies par-
allel (or nearly parallel) to the rolling direction.
Thus, a rolling texture is indicated by the plane–
direction combination, (hkl)[uvw]. For body-
centered cubic alloys (to include the iron–silicon
alloy mentioned above), the rolling texture is (110)
[001], which is represented schematically in Figure
20.21. Thus, transformer cores of this iron–silicon
alloy are fabricated such the direction in which the
sheet was rolled (corresponding to a [001]-type di-
rection for most of the grains) is aligned parallel to
the direction of the magnetic field application.3
The magnetic characteristics of this alloy may
be further improved through a series of deforma-
tion and heat-treating procedures that produce a
(100)[001] texture.
Iron alloy core
Secondary
winding
Primary
winding
B field
Rolling plane
Rolling direction
[001] Direction[110] Plane
Figure 20.20 Schematic diagram of a transformer
core, including the direction of B field that is
generated.
Figure 20.21 Schematic representation of the
(110)[001] rolling texture for body-centered cubic iron.
3 For body-centered cubic metals and alloys, [100] and [001] directions are equivalent (Section 3.l0)—that is,
both are directions of easy magnetization.
20.9 Soft Magnetic Materials • 97
1496T_c20_76-113 10/31/05 16:31 Page 97 FIRST PAGES susceptibilities. The properties of a half-dozen soft magnetic materials are shown in
Table 20.5.
The hysteresis characteristics of soft magnetic materials may be enhanced for
some applications by an appropriate heat treatment in the presence of a magnetic
field. Using such a technique, a square hysteresis loop may be produced, which is
desirable in some magnetic amplifier and pulse transformer applications. In addi-
tion, soft magnetic materials are used in generators, motors, dynamos, and switch-
ing circuits.
20.10 HARD MAGNETIC MATERIALS
Hard magnetic materials are utilized in permanent magnets, which must have a high
resistance to demagnetization. In terms of hysteresis behavior, a hard magnetic
material has a high remanence, coercivity, and saturation flux density, as well as a
low initial permeability, and high hysteresis energy losses. The hysteresis character-
istics for hard and soft magnetic materials are compared in Figure 20.19. The two
most important characteristics relative to applications for these materials are the
coercivity and what is termed the “energy product,” designated as This
corresponds to the area of the largest B-H rectangle that can be constructed
within the second quadrant of the hysteresis curve, Figure 20.22; its units are kJ/m 3
(MGOe).4 The value of the energy product is representative of the energy required
(BH)max
(BH)max.
98 • Chapter 20 / Magnetic Properties
Table 20.5 Typical Properties for Several Soft Magnetic Materials
Initial Relative Saturation Hysteresis
Composition Permeability Flux Density B s Loss/Cycle Resistivity

Material (wt %) i [tesla (gauss)] [J/m3 (erg/cm3)] (
-m)
Commercial 99.95Fe 150 2.14 270 1.0  107
iron ingot (21,400) (2700)
Silicon–iron 97Fe, 3Si 1400 2.01 40 4.7  107
(oriented) (20,100) (400)
45 Permalloy 55Fe, 45Ni 2500 1.60 120 4.5  107
(16,000) (1200)
Supermalloy 79Ni, 15Fe, 75,000 0.80 — 6.0  107
5Mo, 0.5Mn (8000)
Ferroxcube A 48MnFe 2O4, 1400 0.33
40 2000
52ZnFe 2O4 (3300) (
400)
Ferroxcube B 36NiFe 2O4, 650 0.36
35 107
64ZnFe 2O4 (3600) (
350)
Source: Adapted from Metals Handbook: Properties and Selection: Stainless Steels, Tool Materials and Special-
Purpose Metals, Vol. 3, 9th edition, D. Benjamin (Senior Editor), American Society for Metals, 1980.
hard magnetic
material
4 MGOe is defined as
Furthermore, conversion from cgs–emu to SI units is accomplished by the relationship
1 MGOe  7.96 kJ/m3
1 MGOe  10 6 gauss-oersted
1496T_c20_76-113 10/31/05 16:42 Page 98 FIRST PAGES to demagnetize a permanent magnet; that is, the larger the harder is the
material in terms of its magnetic characteristics.
Again, hysteresis behavior is related to the ease with which the magnetic domain
boundaries move; by impeding domain wall motion, the coercivity and susceptibility
are enhanced, such that a large external field is required for demagnetization.
Furthermore, these characteristics are interrelated to the microstructure of the
material.
Concept Check 20.6
It is possible, by various means (i.e., alteration of microstructure and impurity ad-
ditions), to control the ease with which domain walls move as the magnetic field is
changed for ferromagnetic and ferrimagnetic materials. Sketch a schematic B-versus-H
hysteresis loop for a ferromagnetic material, and superimpose on this plot the loop
alterations that would occur if domain boundary movement were hindered.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site.)]
Conventional Hard Magnetic Materials
Hard magnetic materials fall within two main categories—conventional and high
energy. The conventional materials have values that range between about
2 and 80 kJ/m3 (0.25 and 10 MGOe).These include ferromagnetic materials—magnet
steels, cunife (Cu–Ni–Fe) alloys, alnico (Al–Ni–Co) alloys—as well as the hexagonal
ferrites (BaO–6Fe2O3). Table 20.6 presents some of the critical properties of several
of these hard magnetic materials.
The hard magnet steels are normally alloyed with tungsten and/or chromium.
Under the proper heat-treating conditions these two elements readily combine with
carbon in the steel to form tungsten and chromium carbide precipitate particles,
which are especially effective in obstructing domain wall motion. For the other metal
(BH )max
(BH)max
20.10 Hard Magnetic Materials • 99
B
H
Hd
Bd
Hd < (BH)max
Bd
(BH)max
Figure 20.22 Schematic
magnetization curve that
displays hysteresis. Within
the second quadrant are
drawn two B–H energy
product rectangles; the area
of that rectangle labeled
is the largest
possible, which is greater
than the area defined by
Bd–Hd.
(BH )max
1496T_c20_76-113 10/31/05 16:31 Page 99 FIRST PAGES alloys, an appropriate heat treatment forms extremely small single-domain and
strongly magnetic iron-cobalt particles within a nonmagnetic matrix phase.
High-Energy Hard Magnetic Materials
Permanent magnetic materials having energy products in excess of about 80 kJ/m 3
(10 MGOe) are considered to be of the high-energy type. These are recently
developed intermetallic compounds that have a variety of compositions; the two
that have found commercial exploitation are SmCo 5 and Nd 2Fe14B. Their magnetic
properties are also listed in Table 20.6.
Samarium–Cobalt Magnets
SmCo 5 is a member of a group of alloys that are combinations of cobalt or iron and
a light rare earth element; a number of these alloys exhibit high-energy, hard mag-
netic behavior,but SmCo 5 is the only one of commercial significance. The energy
products of these SmCo 5 materials [between 120 and 240 kJ/m 3 (15 and 30 MGOe)]
are considerably higher than the conventional hard magnetic materials (Table 20.6);
in addition, they have relatively large coercivities. Powder metallurgical techniques
are used to fabricate SmCo 5 magnets. The appropriately alloyed material is first
ground into a fine powder; the powder particles are aligned using an external mag-
netic field and then pressed into the desired shape. The piece is then sintered at an
elevated temperature, followed by another heat treatment that improves the mag-
netic properties.
Neodymium–Iron–Boron Magnets
Samarium is a rare and relatively expensive material; furthermore, the price of cobalt
is variable and its sources are unreliable. Consequently, the Nd 2Fe14B alloys have
become the materials of choice for a large number and wide diversity of applications
100 • Chapter 20 / Magnetic Properties
Table 20.6 Typical Properties for Several Hard Magnetic Materials.
Remanence Coercivity Curie
Br Hc (BH)max Temperature Resistivity
Composition [tesla [amp-turn/m [kJ/m3 Te

Material (wt %) (gauss)] (Oe)] (MGOe)] [C(F)] (
-m)
Tungsten 92.8 Fe, 0.95 5900 2.6 760 3.0  107
steel 6 W, 0.5 (9500) (74) (0.33) (1400)
Cr, 0.7 C
Cunife 20 Fe, 20 0.54 44,000 12 410 1.8  107
Ni, 60 Cu (5400) (550) (1.5) (770)
Sintered alnico 8 34 Fe, 7 Al, 0.76 125,000 36 860 —
15 Ni, 35 (7600) (1550) (4.5) (1580)
Co, 4 Cu,
5 Ti
Sintered ferrite 3 BaO–6Fe 2O3 0.32 240,000 20 450 104
(3200) (3000) (2.5) (840)
Cobalt rare earth 1 SmCo 5 0.92 720,000 170 725 5.0  107
(9200) (9,000) (21) (1340)
Sintered neodymium- Nd2Fe14B 1.16 848,000 255 310 1.6  106
iron-boron (11,600) (10,600) (32) (590)
Source: Adapted from ASM Handbook, Vol. 2, Properties and Selection: Nonferrous Alloys and Special-Purpose
Materials. Copyright 1990 by ASM International. Reprinted by permission of ASM International, Materials
Park, OH.
©
1496T_c20_76-113 10/31/05 16:31 Page 100 FIRST PAGES requiring hard magnetic materials. Coercivities and energy products of these
materials rival those of the samarium–cobalt alloys (Table 20.6).
The magnetization–demagnetization behavior of these materials is a function
of domain wall mobility, which, in turn, is controlled by the final microstructure—
that is, the size, shape, and orientation of the crystallites or grains, as well as the na-
ture and distribution of any second-phase particles that are present. Of course,
microstructure will depend on how the material is processed. Two different pro-
cessing techniques are available for the fabrication of Nd 2Fe14B magnets: powder
metallurgy (sintering) and rapid solidification (melt spinning). The powder metal-
lurgical approach is similar to that used for the SmCo 5 materials. For rapid
solidification, the alloy, in molten form, is quenched very rapidly such that either
an amorphous or very fine grained and thin solid ribbon is produced. This ribbon
material is then pulverized, compacted into the desired shape, and subsequently
heat treated. Rapid solidification is the more involved of the two fabrication
processes; nevertheless, it is continuous, whereas powder metallurgy is a batch
process, which has its inherent disadvantages.
These high-energy hard magnetic materials are employed in a host of different
devices in a variety of technological fields. One common application is in motors.
Permanent magnets are far superior to electromagnets in that their magnetic fields
are continuously maintained and without the necessity of expending electrical
power; furthermore, no heat is generated during operation. Motors using permanent
magnets are much smaller than their electromagnet counterparts and are utilized
extensively in fractional horsepower units. Familiar motor applications include the
following: in cordless drills and screw drivers; in automobiles (starting, window
winder, wiper, washer, and fan motors); in audio and video recorders; and in clocks.
Other common devices that employ these magnetic materials are speakers in audio
systems, lightweight earphones, hearing aids, and computer peripherals.
20.11 MAGNETIC STORAGE
Within the past few years, magnetic materials have become increasingly important
in the area of information storage; in fact, magnetic recording has become virtually
the universal technology for the storage of electronic information. This is evidenced
by the preponderance of audio tapes, VCRs, disk storage media, credit cards, and
so on. Whereas in computers, semiconductor elements serve as primary memory,
magnetic disks are used for secondary memory because they are capable of storing
larger quantities of information and at a lower cost. Furthermore, the recording and
television industries rely heavily on magnetic tapes for the storage and reproduc-
tion of audio and video sequences.
In essence, computer bytes, sound, or visual images in the form of electrical sig-
nals are recorded on very small segments of the magnetic storage medium—a tape
or disk.Transference to and retrieval from the tape or disk is accomplished by means
of an inductive read–write head, which consists basically of a wire coil wound around
a magnetic material core into which a gap is cut. Data are introduced (or “written”)
by the electrical signal within the coil, which generates a magnetic field across the
gap. This field in turn magnetizes a very small area of the disk or tape within the
proximity of the head. Upon removal of the field, the magnetization remains; that
is, the signal has been stored. The essential features of this recording process are
shown in Figure 20.23.
Furthermore, the same head may be utilized to retrieve (or “read”) the stored
information. A voltage is induced when there is a change in the magnetic field as
20.11 Magnetic Storage • 101
1496T_c20_76-113 10/31/05 16:31 Page 101 FIRST PAGES the tape or disk passes by the head coil gap; this may be amplified and then con-
verted back into its original form or character. This process is also represented in
Figure 20.23.
Recently, hybrid heads that consist of an inductive-write and a magnetoresistive
read head in a single unit have been introduced. In the magnetoresistive head, the
electrical resistance of the magnetoresistive thin film element is changed as a result
of magnetic field changes when the tape or disk passes by the read head. Higher sen-
sitivies and higher data transfer rates make magnetoresistive heads very attractive.
There are two principal types of magnetic media—particulate and thin film.
Particulate media consist of very small needle-like or acicular particles, normally
of -Fe 2 O 3 ferrite or CrO 2 ; these are applied and bonded to a polymeric film
(for magnetic tapes) or to a metal or polymer disk. During manufacture, these
particles are aligned with their long axes in a direction that parallels the direc-
tion of motion past the head (see Figures 20.23 and 20.24). Each particle is a
g
102 • Chapter 20 / Magnetic Properties
Recording medium
Recording
head
Signal
in Write
Signal
out
Read
Width
Gap
Figure 20.23 Schematic
representation showing how
information is stored and
retrieved using a magnetic
storage medium. (From J. U.
Lemke, MRS Bulletin, Vol. XV,
No. 3, p. 31, 1990. Reprinted with
permission.)
Figure 20.24 A scanning electron
micrograph showing the microstructure
of a magnetic storage disk. Needle-shaped
particles of -Fe2O3 are oriented and
embedded within an epoxy phenolic resin.
8000. (Photograph courtesy of P. Rayner
and N. L. Head, IBM Corporation.)
g
1496T_c20_76-113 10/31/05 16:31 Page 102 FIRST PAGES single domain that may be magnetized only with its magnetic moment lying along
this axis. Two magnetic states are possible, corresponding to the saturation mag-
netization in one axial direction, and its opposite. These two states make possi-
ble the storage of information in digital form, as 1’s and 0’s. In one system, a 1
is represented by a reversal in the magnetic field direction from one small area
of the storage medium to another as the numerous acicular particles of each such
region pass by the head. A lack of reversal between adjacent regions is indicated
by a 0.
The thin-film storage technology is relatively new and provides higher storage
capacities at lower costs. It is employed mainly on rigid disk drives and consists of
a multilayered structure. A magnetic thin-film layer is the actual storage compo-
nent (see Figure 20.25). This film is normally either a CoPtCr or CoCrTa alloy, with
a thickness of between 10 and 50 nm. A substrate layer below and upon which the
thin film resides is pure chromium or a chromium alloy. The thin film itself is
20.11 Magnetic Storage • 103
(b)
(a)
Figure 20.25 (a) A high-
resolution transmission
electron micrograph showing
the microstructure of a
cobalt–chromium–platinum
thin film that is used as a
high-density magnetic storage
medium. The arrow at the
top indicates the motion
direction of the medium.
500,000. (b) A
representation of the grain
structure for the electron
micrograph in (a); the arrows
in some of the grains indicate
the texture, or the direction
of easy magnetization. (From
M. R. Kim, S. Guruswamy,
and K. E. Johnson, J. Appl.
Phys., Vol. 74, No. 7, p. 4646,
1993. Reprinted with
permission.)
1496T_c20_76-113 10/31/05 16:31 Page 103 FIRST PAGES polycrystalline, having an average grain size that is typically between 10 and 30 nm.
Each grain within the thin film is a single magnetic domain, and it is highly desir-
able that grain shape and size be relatively uniform. For magnetic storage disks that
employ these thin films, the crystallographic direction of easy magnetization for
each grain is aligned in the direction of disk motion (or the direction opposite) (see
Figure 20.25). The mechanism of magnetic storage within each of these single-
domain grains is the same as for the needle-shaped particles, as described above—
that is, the two magnetic states correspond to domain magnetization in one direction
or its antiparallel equivalent.
The storage density of thin films is greater than for particulate media because
the packing efficiency of thin-film domains is greater than for the acicular particles;
particles will always be separated with void space in between. At the time of this
writing, storage densities for particulate media are on the order of bit/in. 2
For thin films, storage densities are approximately an order of
magnitude greater [i.e.,
Regarding specific magnetic properties, the hysteresis loops for these magnetic
storage media should be relatively large and square. These characteristics ensure
that storage will be permanent, and, in addition, magnetization reversal will result
over a narrow range of applied field strengths. For particulate recording media,
saturation flux density normally ranges from 0.4 to 0.6 tesla (4000 and 6000 gauss).
For thin films, will lie between 0.6 and 1.2 tesla (6000 and 12,000 gauss).
Coercivity values are typically in the range of to A/m (2000 to
3000 Oe).
20.12 SUPERCONDUCTIVITY
Superconductivity is basically an electrical phenomenon; however, its discussion has
been deferred to this point because there are magnetic implications relative to the
superconducting state, and, in addition, superconducting materials are used prima-
rily in magnets capable of generating high fields.
As most high-purity metals are cooled down to temperatures nearing 0 K, the
electrical resistivity decreases gradually, approaching some small yet finite value
that is characteristic of the particular metal. There are a few materials, however, for
which the resistivity, at a very low temperature, abruptly plunges from a finite value
to one that is virtually zero and remains there upon further cooling. Materials that
display this latter behavior are called superconductors, and the temperature at which
they attain superconductivity is called the critical temperature 5 The resistivity–
temperature behaviors for superconductive and nonsuperconductive materials are
contrasted in Figure 20.26. The critical temperature varies from superconductor to
superconductor but lies between less than 1 K and approximately 20 K for metals
and metal alloys. Recently, it has been demonstrated that some complex oxide
ceramics have critical temperatures in excess of 100 K.
At temperatures below the superconducting state will cease upon applica-
tion of a sufficiently large magnetic field, termed the critical field which de-
pends on temperature and decreases with increasing temperature. The same may
be said for current density; that is, a critical applied current density exists below