HARD MAGNETIC MATERIALS
SEMINAR, 11TH DECEMBER 2002
COURSE:
4H1609 FUNCTIONAL MATERIALS
PROF. ROLF SANDSTRÖM
PROF K. V. RAO
BY:
HELENE LÖFDAL
751012-0301
helen8@kth.se
ROLAND HOFSTETTER
780111-A175
m2hofrol@m.kth.se
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TABLE OF CONTENTS:
1.THEORIE .......................................................................................................................... 3
1.1Introduction ..................................................................................................................... 3
1.2Permanent Magnets ........................................................................................................ 4
1.3Characteristics ................................................................................................................. 5
2.HARD MAGNETIC ALLOYS ........................................................................................ 7
2.1Hirstorical ........................................................................................................................ 7
2.2Magnet Steels ................................................................................................................... 7
2.3AlNiCo Alloys .................................................................................................................. 8
2.4Ferrites ........................................................................................................................... 11
2.5Rare-Earth Alloys ......................................................................................................... 13
2.6 Nanocrystalline hard magnetic materials .................................................................. 16
3.APPLICATIONS ............................................................................................................. 17
3.1Principle of brushless DC motor .................................................................................. 18
3.2Principle of the synchronous motor ............................................................................. 18
3.3Magnetic Random Access Memories (MRAM) .......................................................... 19
4.CONCLUSION ................................................................................................................ 19
APPENDIX I ....................................................................................................................... 23
APPENDIX II ..................................................................................................................... 24
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1. THEORY
1.1 Introduction
Magnetic materials are from essential importance in many industrial and engineering
applications and have been researched for a long time. First permanent magnets have almost
exclusively been used for compass needles. At a later point the actual multitude of possible
applications has been discovered and today’s research focuses on new technologies like
nanostructured magnetic materials to achieve specific properties desired for industrial
applications.
Figure 1: Hysteresis loop for (a) soft magnetic material (b) hard magnetic material
This paper focuses on hard magnetic materials, which is one of the two subdomains in which
magnetic materials are classified. The main difference between soft and hard magnetic
materials is their behavior in absence of a magnetizing field. Its high remanence
magnetization and coercive field on the one hand and its low magnetic permeability on the
other characterize a hard magnetic material. A soft magnetic material is characterized by low
remanence magnetization and coercive field but a high permeability, which is desirable for
applications in which the material must be easily magnetized and demagnetized. This
fundamental difference is illustrated in figure 1.
Soft materials are thus used in variable or periodic magnetization application such as cores for
distribution power transformers, small electronic transformers as well as for stator and rotor
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materials for motors and generators [1]. Hard materials are rather used for applications in
which a constant (permanent) magnetic field is used (table 1 [2]).
Table 1: Overview over the major applications and their importance for Alnico and Ba
ferrite magnetic materials
1.2 Permanent magnets
When a bulk hard magnetic material is for the first time exposed to a magnetic field its
initially randomly magnetized domains grow and the domain moments are rotated to be
aligned with the applied external field at the saturation magnetization. Once that external field
is gone the magnetization is partially reverted but does not follow the magnetization curve
anymore. A remanent magnetization Br is present (figure 2 [2]) which means that a certain
amount of energy is stored in the material, which is now a permanent magnet. This energy,
assuming that the material is not demagnetized by an external field, remains stored
indefinitely in the magnet. This energy is always available for use and is not drained away by
repeated use, like the energy of a battery, because a magnet does no net work to its
surroundings [2]. The advantage of a permanent magnet rather than a magnetized soft
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material is that a constant magnetic field is generated without need of an external power
source and no heat is generated.
Figure 2: First magnetizing and demagnetizing curve of a hard
magnetic material, load line (0C) and operating point P
The two most widely used materials for permanent magnets are Alnico, an alloy series
consisting primarily of the elements Al, Ni and Co, and Barium ferrite, which is a ceramic
magnetic material. The properties of these important alloys will be discussed more in detail in
subsequent chapters.
1.3 Characteristics
The values of the remanent magnetization as well as the coercive field are characteristic
properties for a magnetic material in a closed circuit. Since a magnet is never used in a closed
circuit, the value of interest for the choice of a magnet for a specific application is the
generated demagnetizing field Hd. To generate such a field, the magnetic circuit must be open
and two free poles are created separated by an air gap. As soon as the magnetic material is not
a closed magnetic circle anymore, the operating point P is situated on the demagnetization
curve in the second quadrant of the hysteresis curve (figure 2). The magnetization at this point
is in any case lower than the remanence of the magnet.
P is the intersection of the demagnetization curve and the so-called load line (0C). Its slope
depends on various (primarily geometric) design parameters defined by the magnet designer.
An equation giving the relation between the desired field in the gap, Hg, as function of the
magnetic characteristics of the material used and its geometry can be applied:
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H g2Vg  ( Bm H m )Vm
(1.1)
where V is the volume and the indices g and m stand for gap and magnet respectively [2]. To
generate a given field Hg in an air gap of predefined dimensions Vg and minimizing the
material used, and thus the price of the magnet, the product (BmHm) has to be maximized. Or
in other words: if the design parameters have been optimized the operating point coincides
with the point on the demagnetization curve giving the maximum energy product (BH)max.
These demagnetization curves are represented in special diagrams so that the maximum
energy product of the various materials can be determined and compared at a glance (fig. 3
[2]). The thin lines represent constant values for the energy product in MGOe. In this example
material 1 has a significantly higher energy product than material 2.
Figure 3: The second quadrant of the hysteresis loop diagram of a hard magnetic material
allows the determination of the maximum energy product of a given alloy
The magnet designer needs to know the slope of the load line in order to determine the
operating point P. The slope of the load line is defined the following equation [2]:
Ag lm
Bm

H m Am l g
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(1.2)
where A and l are the cross-section and length of the magnet and the air gap respectively. This
slope depends exclusively on geometrical values of the circuit. Its values are indicated on the
left and top of the same diagram (fig. 3). The operation point is thus determined by
intersecting the line from the origin to the desired value of Bm/Hm with the demagnetization
curve.
As we have seen is the energy product one of the most important quality indices for magnetic
materials. It has to be mentioned that above made calculations were for an idealized magnetic
circuit. In reality not all field lines are straight lines (fringing) and there are also magnetic
losses along the magnetic material (leakage). For more details on losses refer to [2].
2. HARD MAGNETIC ALLOYS
2.1 Historical Background
The phenomenon of permanent magnetism has been known for more than a century. Until
about the end of the nineteenth century the only materials available for permanent magnets
apart from natural lodestone were hardened carbon steels [3]. Quite big efforts have been
made during the past century to better understand their characteristics in order to ameliorate
the latter and to find new alloys. Remarkable improvement has been made over the past
century to increase the energy product of the different alloys. The aim of this chapter is to
give an overview over some important permanent magnetic alloys.
2.2 Magnet Steels
The first for permanent magnets made Fe alloys containing besides carbon up to 6% W and
Cr (Table 2 [3]) showed no revolutionary improvement in the energy product. The discovery
of Co steels containing up to 40% of Co raised the energy product and coersivity
significantly. The steel containing 35% of Co as well as Cr and W reached a coercivity of as
high as 250 Oe and an energy product of 950 kGOe. The machining of steel containing high
amounts of cobalt is more difficult, but possible to be hot worked and machined in an
annealed state [3]. Forging and hot pressing are the typical processes followed by hardening
through quenching from hot temperature. The quenching temperature depends on the
composition of the respective steel. The main constituent of quenched steel is martensite;
undissolved carbides are often present. Martensite is a metastable phase in a state of high
residual stress, and the tempering treatment that follows quenching only partially relieves this
stress [2]. This metallurgical instability, the easy demagnetization and the brittleness of these
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steels gave them a bad reputation and are, in spite of the low cost, only used for particular
applications like hysteresis motors nowadays.
Table 2: Compositions and magnetic properties of selected permanent magnet steels
2.3 AlNiCo Alloys
Alnico alloys is a family of permanent magnet alloys based on the three ferromagnetic metals
Fe, Co and Ni containing smaller amounts of Al, Cu and other elements. The first alloy
containing 58% Fe, 30% Ni and 12% Al, was discovered in the early 1930s in Japan. That
alloy reached a coersivity of over 400 Oe and did not contain any Co. The coersivity almost
doubled in comparison to the best steel magnets.
Figure 4: Compositions of selected alnico alloys (in historical order)
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Isotropic Alnico contains about 12% of Co while field treated anisotropic Alnico alloys
contain 20 to 25% of Co. These alloys reached moderate coercivities of 580 to 780 Oe [3] and
were also called Alcomax. High coercivity alloys containing 30% and more Co were called
Hycomax. Others, primarily US manufacturers, only distinguish the different Alnico alloys by
numbers independent of their Co content.
Common composition ranges of Alnico are represented in figure 4 [2].
Alnico alloys have a stick like structure. The material is separated in a Fe and FeCo rich
strong magnetic phase, called ’ precipitate, and a NiAl rich weak or non-magnetic phase, 
matrix (figure 5 [4]). Both phases have a bcc structure.
Figure 5: Microstructure of anisotropic alnico (the
magnetization direction is in the plane of the sheet)
All Alnico alloys are too brittle to be cold worked. Large numbers of small magnets are
usually made by sintering which reduces slightly the magnetic properties but increases the
mechanical properties while bigger magnets are made by casting [3]. The diversity of alloys is
very high and the examples in figure 4 are just a small extract showing some alloys in
historical order. It is important to mention that the Al content is rather critical and is quite
constant over the different alloys showed in figure 4.
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Production
Alloys having good magnetic properties must be sintered until they have a density close to the
theoretical maximum density. The sintering process may take up to 24h if it is carried out as a
batch process in a good vacuum where heating and cooling are so slow. Alternatively, a pure
hydrogen atmosphere can be used to protect the material from oxygen or water vapor [3].
Grinding is the only surface finishing method for all types of Alnico alloys.
In order to increase and optimize the magnetic properties a three-stage heat treatment is
necessary [2]. For Alnico 1 to 4 the following process is applied:
1. Heat to 1250°C for a time sufficient to produce a homogeneous solid solution
2. Cool at a rate of about 1000°C/Sec to about 500°C
3. Tempering at 600°C for a few hours
Alnico 5 to 7 are treated the same way but the cooling process (2) is made in a magnetic field
of about 1000 Oe. The field treatment for Alnico 8 and 9 is made isothermally at 815°C for
about 10 to 20min, because these allows have to be held at a constant temperature for a longer
time.
Figure 6: Demagnetization curves for Alnico magnetic materials. Alnico 2 – 5 do not have a preferred
grain direction while Alnico 5 DG has been magnetized along the preferred grain direction
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In the beginning only small improvements in coercivity resulted from cooling in a magnetic
field. Later, additions of Ti were tried which have the effect that during cooling the
precipitations rods become longer and the structural anisotropy more important. The
coercivity for Ti alloys is significantly above the one of the earlier Alnico alloys. The magnet
has now an easy magnetization direction parallel to the direction of the applied field during
cooling because the precipitate rods form preferentially along the <100> direction of each
grain that is closest to the field direction, in order on minimize magneto static energy [2]. A
collection of demagnetization curves is given in figure 6 [2] where the difference between
field cooled Alnico 5 and Alnico 2 is significant. Not only has the Alnico 5 a higher
remanence but also a much higher energy product. It is important to note that a field cooling
only has a magnetizing effect if the field is applied below the Curie point of the alloy. Alloy
compositions have been modified to counteract this problem.
In high coercive alloys Ti is sometimes replaced by Nb and the magnetization is made
isothermally to achieve better results. The optimal values for treating temperatures and
durations are pretty critical and can in case of non-respect lead to very poor results. Some
high coercive alloys (e. g. 40% Co, 8% Ti) are so brittle that that breakage sometimes occurs
spontaneously. Many manufactures will not tackle this grade for the above-mentioned
problems [3].
The ease to magnetize as well as the small reversible changes of magnetic values with
temperature are big advantages of the Alnico alloys while the rather high Co content in many
of the alloys makes these alloys rather expensive [4].
2.4 Ferrites
Ferrite implies a material containing the ferrite group Fe2O3, which must not be confused with
the steel making term, which describes a bcc structure. The first artificially produced
permanent magnets were Co ferrites, which had a high coersivity but a rather low remanence
[3]. After yearlong production of soft magnetic ferrites in the 1940s, Philips discovered in the
early 1950s hard ferrites with the general formula MO.6(Fe2O3), where M stands for Ba, Sr or
Pb.
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Production:
The raw material is mixed and calcined at a temperature between 1000°C and 1350°C . The
calcined material is then crushed and milled, usually in a ballmill with water [3]. The grain
size is about 1m. The grain size is important for the grade of magnet being made.
To make isotropic magnets the powder is dried and pressed which is a rather simple process
and can easily be automated.
To make anisotropic material the powder is aligned by a magnetic field that is preferably
applied in the same direction as the pressure in the die the powder is compressed. The
alignment is favored by adding water when the powder is filled into the die. These compacts
are now sintered for a second time at a temperature between 1100°C and 1300°C before they
are cooled of very slowly to avoid cracking [3].
The quite high temperature ranges indicated above cover different elements. For Barium
ferrite, these temperatures are both at about 1200°C [2]. Barium ferrite has a hexagonal
structure and is usually formed to a cylindrical magnet having the easy axis parallel to the
cylinder axis.
Figure 7: Demagnetization curves for Indox 1 – 5
The large ions of O, Ba, Sr determine the dense hexagonal structure of the crystal lattice while
the smaller iron ions are located in interstitial sites [4]. The magnetic moments are aligned
parallel or anti-parallel along the c axis of the lattice. On average two of three magnetic
moments cancel each other out and one is externally active.
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The magnetic properties can be influenced by varying the heat treatment and milling
sequence. High sintering temperatures and light milling tend to produce high values of Br and
(BH)max but rather low coercivity. Lower sintering temperatures and excessive milling tend to
produce low densities, low Br and (BH)max but high coercivity [3].
This to a certain point mutually exclusive effect of high Br and high Hc is illustrated in figure
7 [2] for different alloys of barium ferrite.
Comparing the diagram in figure 7 to the one in figure 6 shows that the permeance coefficient
Bm/Hm at the maximum energy product is about 1 for barium ferrite (Indox 5), compared to
about 20 for Alnico 5. As a consequence, the cross sectional area of the ferrite has to be much
larger to achieve the same flux density in the air gap, because of the much lower flux density.
Due to the lower raw material cost, barium ferrite is much cheaper than Alnico and has a
lower density. Generally speaking, the two materials do not compete for the same application;
each is used where its special characteristics best fit it for the job [2].
Common applications of hard ferritic magnets are motors, loudspeakers and magnet systems
for holding applications [4].
Bonded Ferrites
So-called bonded ferrites can be made by bonding barium ferrite or strontium ferrite powder
in resins, plastics or natural rubber. If no special steps are taken the material is isotropic and
the energy product is unlikely to exceed 0.7MGOe [3]. If a good flexibility is required, for
applications like the gasket on refrigerator doors [2], the proportion of ferrite powder has to
be lowered. The energy product can in that case be significantly lower than the abovementioned value. Bonded ferrites have a relatively low service temperature and properties
changing with temperature [4].
2.5 Rare-Earth Alloys
Rare-earth element do not have their name because of the rare deposit but of the rather costand energy-intensive procedure to extract them as pure metals. Progresses made in these
separation and refining processes allow the use of such elements even for certain mass
product applications where the overall cost is an important factor [5].
A high progress in terms of magnetic properties have been made in the end of the 1960s with
the discovery of materials having significantly higher coercivity and energy product than the
classic permanent magnets Alnico and hard ferrite. Strnat and others discovered high
magnetic saturation polarization alongside great uniaxial magnetocrystalline anisotropy of
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some compounds of the type RCo5 [1], where R stands for rare earth. This promising family
of alloys was of great interest and SmCo5 has turned out to be a very interesting alloy, from
both, technical and economical viewpoints. SmCo5 has a high energy product of 22.5 MGOe
[1] with is about 2.5 times higher than for an Alnico 9 alloy. The anisotropic constant is with
5.5*107 ergs/cm3 extremely high. An alternative and also promising alloy was Sm2Co17
having a lower Co content and similar magnetic properties.
Figure 8: Fabrication process for the rare-earth hard
magnetic materials SmCo5, Sm2Co17 and NdFeB
In 1983, an other very important discovery were the Nd-Fe-B-base alloys which show higher
(BH)max values at room temperature than all other grades of permanent magnets [1]. The low
Curie temperature of 310°C has as a consequence that at temperatures higher than 120°C the
magnetic properties of SmCo5 are better; the (BH)max values as well as the remanence are
higher.
The SmCo5, Sm2Co17 as well as the Nd-Fe-B alloys are produced by a permanent magnet
sintering process, which is schematically represented in figure 8 [1].
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Figure 9: Comparison of the properties of the two rare-earth hard magnetic materials SmCo and NdFeB
Other astonishing high values for (BH)max have been obtained with Y-MM and Ce-MM,
where MM stands for Mischmetal, a solid solution containing about 50% rare earth material,
which is fairly cheap. The (Y-MM)Co5 alloy has en energy product of 22.5 MGOe [2], which
is as high as the value for SmCo5, and an anisotropic constant of 5.4*107 ergs/cm3.
A comparison of the most important characteristic values for SmCo and NdFeB magnets is
represented in figure 9 [5].
NdFeB can also be fabricated by melt-spinning where the molten alloy is ejected from the
melting cylinder on the surface of a water-cooled rotating wheel (for more information on
melt spinning refer to appendix II). The material is thus quenched at the rate of about one
million °C/sec forming a thin ribbon with an extremely fine crystal structure [5]. Different
techniques are used to form either an isotropic magnet, which can be magnetized along any
axis or an anisotropic magnet. The latter requires a two-stage hot-pressing procedure, where
the magnetic alignment is parallel to the pressing direction. (BH)max values of up to 40 MGOe
have been reported for a required magnetizing flux density of 2.5T.
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2.6 Nanocrystalline hard magnetic materials
Nanocrystalline magnetic materials consist of Nd-Fe-B alloys and are used in all types of
electric motors especially those like automotive sterling motors. Nanocrystalline magnets
allow reduction in weight and compactness, compared to conventional magnets. Stronger and
smaller permanent magnets allow the construction of smaller devices that consume less
energy.
The weak points of the currently produced NdFeB magnets are (i) their non-sufficient thermal
stability of the hard magnetic properties and (ii) a low corrosion resistance caused by the
presence of a Nd-rich phase. [7]
The first attempts to produce nanoscale microstructures to enhance the magnetic properties of
the Nb-Fe-B permanent magnetic materials used mechanical alloying of blended elemental
powders followed by heat treatment (Schultz et al. 1987). Since the grain structure so
obtained does not exhibit any crystallographic texture-and limits the energy product-special
processing methods such as die-upsetting were used by Schultz and coworkers (1989) to
provide the crystallographic anisotropy. While the coercivities of these nanocrystalline alloys
are high, the remanent magnetization is decreased. The hard magnetic alloys are not as
dependent on the grainsize as the soft magnetic materials. Two important requirements for
alloys to exhibit remanence enhancement are a nanocrystalline grain size and a degree of
coherence across interphase boundaries sufficient to enable adjacent phases to be exchangecoupled. [8]
Table 3 presents crystallographic and magnetic parameters for some magnets.
Composition
Crystal structure
Js [T]
HA [kAm-1]
JHc
[kAm-1]
(BH)max [kJm-3] Tc [K]
Alnico
Cubic
1.34
400
130
120
1070
SrFe12O19
Hexagonal
0.48
1500
280
55
720
SmCo5
Hexagonal
1.12
23000
1600
280
1020
Sm2Co17
Rhomboedral
1.28
5200
960
360
1195
Nd2Fe14B
Tetragonal
1.6
5680
1200
420
580
Sm2Fe17N3
Rhomboedral
1.5
11200
2240
380
743
Nd12.6(Fe,Co,Zr)81.4-
Tetragonal/cubic
1.62
5100
504
180
668
B6/-Fe
Table 3: Magnetic properties for magnetic materials: J s - saturation magnetic polarization, HA anisotropy field, JHc - intrinsic coercivity, (BH)max - maximum energy product, Tc - Curie temperature[8]
- 16 -
Recent studies have reported significantly higher values of Jr (remanent magnetic
polarization) in nanocomposite two-phase mixtures consisting of magnetically hard and soft
phases. Remanence enhancement has been reported in a variety of nanostructured Fe-rich rare
earth magnet alloys, prepared by the recrystallization either of melt-spun or of mechanically
alloyed materials. [7]
3. APPLICATIONS
Hard magnetic materials are used in, for example, synchronous and brushless motors,
automotive sterling-motors, electronic wristwatches, data recording material, generators.
Rare-earth magnets
DC and synchronous motors and generators.
AlNiCo
Are used in areas where very high operating temperatures are required.
Hard ferrites (ceramic magnets)
Loudspeakers, telephone receivers, motors, generators and other rotating devices. Holding
and clamping devices. Cheap, low density. They can be powered and included in a plastic
binder to form the so called "plastic magnets" which can be formed easily into desirable shape
[19]
Bonded magnets
DC motors; Brushless DC motors; Stepper motors; Magnetic torque couplers; Medical
magneto-therapeutic equipment; Automotive: ignition system components, fuel pumps,
position sensors, accelerometers, air bag sensors
Nanocrystalline magnets
Nd-Fe-B: All types of electric motors especially those like automotive sterling motors.
Reduction in weight and compactness makes them also appropriate when a smaller size is
received for example in medical devices, thin motors in implantable pumps and valves and
motors aiding eyelidmotions.
- 17 -
Fig.10 Loudspeaker[6]
3.1 Principle of the brushless DC motor
The brushless DC motor is the combination of a permanent excited synchronous motor and a
frequency inverter. The inverter has to replace the commutator of a conventional DC motor.
Fig. 11 and Fig. 12 show how a brushless DC motor can be derived from a mechanically
commutated DC motor with three armature slots. Its armature winding corresponds to a three
phase winding in delta connection. The commutator acts like a three phase frequency
converter. Stator (excitation) and rotor (armature) change places. [15]
Fig12. Principle of a permanent magnet excited
DC motor: wiring [15]
Fig.11 Principle of a permanent magnet excited
DC motor: construction[15]
3.2 Principle of the synchronous motor
Let’s take a dynamo for example. Inside the dynamo we find a static coil and a rotating
permanent ceramic magnet. The ceramic magnet has 8 poles and turns inside the coil (figure
13). By means of two claw rings, whose claws fit in the inside of the coil, the magnetic North- 18 -
and South-pole "appear" perpendicular to the coil. Turning the magnet makes the North- and
South-pole switch their position. And turning the magnet will induce an EMF in the coil.
When a power supply makes an ac current flow through the coil, the claws change their
North-South polarity continuously, attracting and repelling the poles of the ceramic magnet.
Fig. 13 Disassembled dynamo
3.3 Magnetic Random Access Memories (MRAM)
MRAMs are solid state non-volatile magnetic storage devices in which each bit of data is
stored on a small, elongated magnetoresistive sandwich element (figure 14). A typical
magnetoresistive (MR) sandwich consists of two magnetic layers of different coercivity, one
hard and one soft. The MRAM structure consists of an array of parallel word lines and
parallel sense lines. The MR elements are connected in series. Magnetic fields generated by
currents passed simultaneously through a sense line and a word line write the element at the
intersection of the two lines. To read, resistance changes in the sense line caused by a smaller
word line current are measured. [18]
- 19 -
Fig.14 Intersection of a sense line and a word line in a MRAM
4. CONCLUSION
Like in many other science research areas, the major part about magnet research is in the
nano-scale division, trying new alloys, new and better manufacturing-methods.
The market forces the industry to make smaller and more efficient devices. That drives
science to continue researching. A lot of new properties are discovered and are difficult to
predict from one alloy to another, that is why many time-consuming tests have to be done.
Still, the largest market-share is held by the conventional magnetic materials. Problems to be
solved are for example: Getting the magnetic materials to have a, for the application,
appropriate Curie-temperature. Giving the materials wanted magnetic properties and also
avoid brittleness.
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4. REFERENCES
[1] ‘Nd-Fe PERMANENT MAGNETS – Their Present and Future Applications’, I. V.
Mitchell, Elsevier Applied Science Publishers Ltd, England (1985)
[2] ‘Introduction to magnetic material’, B.D. Cullity, Addison-Wesley corp., Reading, MA,
1972, pp. 556 – 587
[3] ‘Permanent Magnets in Theory and Practice’, 2nd edition, M. McCraig, Pentech Press,
London (1987)
[4] From: http://mag-net.ee.umist.ac.uk/articles/a3.html
‘Permanent Magnets and their Applications’, W. Strass, WIDIA Magnettechnik, Essen and
Prof. J. B Chalmers, Dept. of Electrical engineering and electronics, UMIST, Manchester
(2002)
[5] Selecting the correct magnetic material by Carl R. Tufts, Electrical Electronics Insulation
Conference, 1995, and Electrical Manufacturing & Coil Winding Conference. Proceedings ,
1995, p. 66
[6] http://ece.gmu.edu/~pceperle/st3/ece305~1.htm
Loudspeakerfigure.
[7] www.mse.put.poznan.pl/research_project.html
[8]http://www.wtec.org/loyola/nano/06_03.htm
Ferromagnetic nanostructured materials
([9] http://mag-net.ee.umist.ac.uk/reports/P3/p3_2.html)
Backwards extruding)
[10] http://www.appliedmagnettechnology.com/Tech/rapidquenched_magnetic_powders.htm
[11] http://sakai.elec.hkg.ac.jp/la.htm
Laser ablation photo
[12] http://ablation.magnet.fsu.edu/
Laser ablation text
[13] http://www.biomicro.uc.edu/ECES608/Lecture%209%20%20Electrodeposition%20and%20Micromolding.pdf
Electrodeposition
[14] http://cost523.epfl.ch/eoi2000f.html
Planar flow casting
[15] http://www.s-line.de/homepages/bosch/sensorless/node9.html
Brushless motor picture
[16] http://www.tms.org/Meetings/Annual-97/Program/Sessions/TAS8.html
Meltspinning lite text
[17] http://www.stoner.leeds.ac.uk/techniques/Meltspinning.htm
Meltspinning bilder+text
[18]http://rleweb.mit.edu/Publications/pr144/23-2.htm; MRAM
[19] Magnetic Materials 4.2.8. p 81
- 21 -
Appendix I [5]:
- 22 -
Appendix II:
MANUFACTURING METHODS
Meltspinning
This method is also used for the manufacturing of giant magneto resistant materials (GMR)
High speed; glassy, postannealing grainboundaryproblem. Low speed; nanodispersed
immiscibles no postannealing.
The rapidly solidified Nd-Fe-B powders form the entire basis of the bonded magnet industry.
At present the rapid solidification is carried out exclusively by melt spinning (figure 15), a
technique in which a stream of molten alloy is directed onto the outer surface of a rapidly
spinning wheel, producing flake like particles having highly stable and magnetically hard
microstructure. These particles are comminuted into powder before being processed into
bonded magnets. These powders are magnetically isotropic. The bonded magnets made using
these powders range in energy product of 5 MGOe for injection molded magnets to about 12
MGOe for compression molded varieties [16]. Figure 16 shows a meltspun ribbon.
Figure 16: Meltspun ribbon
Figure 15: Principle of meltspinning
[17]
[17]
Centrifugal Atomization
Technology of manufacturing rapid-quenched magnetic powders (RQMP) based on methods
of centrifugal atomization or spraying of melt followed by rapid quenching on cooled shield.
- 23 -
Figure 17: Centrifugal Atomizer consisting of: 1-feeder, 2-rotating water cooling cup;
3-water cooled screen/crystallizer, 4- collecting bin, 5-rotating spindle [10]
Quenched powder
* Vacuum induction melting, ingot production
* Preparation for atomization
* Centrifugal atomization
* Crystallization annealing
* Magnetic separation
Bonded magnet production based on BZMP
* Powder and binder mixing
* Pressing of a product
* Polymerization
* Impregnation and coating
* Drying
This technology is providing the preparation of magnetic powders of wide variety of material
coercivity conditions. BZMP (NdFeB powders) manufactured by new method is characterized
by stable granulometric composition, high-level of uniformity of alloying component
distribution in powder particles and their fine-grained structure. This facilitates the forming of
material coercivity condition.
Comparing melt-spinning and centrifugal atomization
Method of centrifugal atomization (CA) is featured by cheaper, simpler and more productive
equipment than in melt-spinning technology (M). Productivity of CA 5-7 times as much as in
M.
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CA technology is free from processing problems inherent for M technology.
Controlling capability of thickness of flakes produced by CA method regardless of quenching
rate, in comparison with M methods, allows for obtaining the optimal quenched flakes in wide
range of thickness (5 - 50 m) where nanocrystalline and amorphous powders can coexist.
Effective using the controlled crystallization annealing to have powders with wide
combination of magnetic and other physical properties. [10]
Planar flow casting
Planar flow casting is performed in vacuum (or inert atmosphere) and gives continuous
ribbons. The ribbon thickness will be of the order of 50 nm in all cases. In rapidly quenched
ribbons, a metastable solid solution of the two metals is usually found, with only a small
fraction of nanometer-size segregated transition-metal particles.
Heat treatment of as-prepared ribbons to induce a massive segregation of nanometer-sized
transition-metal particles. Both conventional (furnace) anneals and rapid heating (Joule selfheating) will be exploited in order to optimize particle size (which must be as low as possible
[<5 nm]) and standard deviation of size distribution (which also must be as low as possible
[10%]). [14]
Laserablation
Figure 19: Laser impact on material
Figure 18: laserablation installation
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A high intensity pulsed laser (Nd:YAG or excimer) is focussed onto a metallic or oxide target
in a growth chamber (figures 18, 19). The beam melts the target locally and a plume of atoms
and ions is given off from the target. These atoms land on a substrate which is mounted on a
heater in the growth chamber, resulting in a thin film of material. The ablated materials
deposit on a substrate under controlled conditions of temperature and pressure, forming a thin
film. If the substrate has a uniform crystal lattice structure, the film may be induced to grow
on top of it as a crystal with a preferred orientation. The quality of the film can be controlled
by changing the substrate material, the substrate temperature, and the pressure of molecular
oxygen or argon in the growth chamber. Depending on whether one is growing high
temperature superconductors or dielectric insulators, these growth conditions can vary
enormously. After growth, these films are characterized structurally, electrically and
magnetically.[12]
Electrodeposition
Electrodeposition is based on electrolysis and is used for producing additive metal structures
on a substrate. The object to be plated must be metallized and/or masked to define regions of
metal deposition. It is an inexpensive method and a surface around 1mm can be produced.
[13]
Other methods for producing amorphous materials
Evaporation and liftoff
Etching of a sputtered film
Irradiation
Ion beam mixing
Ion implantation
Solid state diffusion
Ball milling
Sputtering (diode, RF or triode)
Glow Discharge Decomposition
Chemical Vapor Decomposition
Laser glazing
Splat-cooling (e.g. piston and anvil method) [17]
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