Magnetic properties of materials

1
Magnetic
Properties
of
Materials

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Magnetic field lines of force around a
current loop and a bar magnet
Magnetic Dipoles

3
Applied Magnetic Field
• Created by current through a coil:
• Relation for the applied magnetic field, H:
L
I
N
H 
applied magnetic field
units = (ampere-turns/m)
current
Applied
magnetic field H
current I
N = total number of turns
L = length of each turn

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Response to a Magnetic Field
• Magnetic induction results in the material
• Magnetic susceptibility, c (dimensionless)
current I
B = Magnetic Induction (tesla)
inside the material
c measures the
material response
relative to a vacuum.
H
B
vacuum c = 0
c > 0
c < 0

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Magnetic Field Vectors
Flux Density Relative Permeability
(Magnetization)
Magnetic Susceptibility

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Magnetic Units and Conversion Factors for the SI and cgs–emu
Systems
Magnetic Field Vectors

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Origins of Magnetic Moments
• Measures the response of electrons to a magnetic field.
• Electrons produce magnetic moments:
• Net magnetic moment:
--sum of moments from all electrons.
magnetic moments
electron
nucleus
electron
spin

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Types of Magnetism
Magnetic induction
B (tesla)
Strength of applied magnetic field (H)
(ampere-turns/m)
vacuum (c = 0)
-5
diamagnetic (c ~ -10 )
(1)
e.g., Al2O3, Cu, Au, Si, Ag, Zn
ferromagnetic e.g. Fe3O4, NiFe2O4
ferrimagnetic e.g. ferrite(), Co, Ni, Gd
(3)
(c as large as 106 !)
(2) paramagnetic
e.g., Al, Cr, Mo, Na, Ti, Zr
(c ~ 10-4)
permeability of a vacuum:
(1.26 x 10-6 Henries/m)
H
B o

c

 )
1
(

Magnetic Moments for 3 Types
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No Applied
Magnetic Field (H = 0)
Applied
Magnetic Field (H)
(1) diamagnetic
none
opposing
(2) paramagnetic
random
aligned
(3) ferromagnetic
ferrimagnetic
aligned
aligned

- Diamagnetic materials are attracted towards
regions where the field is weak
- Relative permeability µr is less than unity
- Negative magnetic susceptibility

Diamagnetism and Paramagnetism
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Schematic representation of the flux density B versus the magnetic field strength H
for diamagnetic and paramagnetic materials. Diamagnetism and paramagnetism
materials are considers to non magnetic because they exhibit magnetization only
when in the presence of an external field.

Diamagnetism and Paramagnetism
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Room-Temperature Magnetic Susceptibilities for Diamagnetic and Paramagnetic
Materials

Domains and Hysteresis
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Schematic depiction of domains in a
ferromagnetic material; arrows represent
atomic magnetic dipoles. Within each
domain, all dipoles are aligned, whereas
the direction of alignment varies from one
domain to another.

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The gradual change in magnetic dipole orientation across a domain wall

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• As the applied field (H) increases...
--the magnetic moment aligns with H.
Applied Magnetic Field (H)
Magnetic
induction
(B)
0
Bsat
H = 0
H
H
H
H
H
• “Domains” with
aligned magnetic
moment grow at
expense of poorly
aligned ones!

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Magnetic flux density versus the magnetic
field strength for a ferromagnetic material
that is subjected to forward and reverse
saturations (points S and S’). The
hysteresis loop is represented by the solid
curve; the dashed curve indicates the initial
magnetization. The remanence Br and the
coercive force Hc are also shown.

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A hysteresis curve at less than saturation
(curve NP) within the saturation loop for a
ferromagnetic material. The B–H behavior
for field reversal at other than saturation is
indicated by curve LM.

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Comparison of B-versus-H
behaviors for ferromagnetic and
diamagnetic/ paramagnetic
materials (inset plot). Small B
fields are generated in materials
that experience only
diamagnetic/paramagnetic
behavior, which is why they are
considered to be non-magnetics

21
Permanent Magnets
Adapted from Fig. 20.14,
Callister 7e.
Applied Magnetic
Field (H)
1. initial (unmagnetized state)
B
large coercivity
--good for perm magnets
--add particles/voids to
make domain walls
hard to move (e.g.,
tungsten steel:
Hc = 5900 amp-turn/m)
• Hard vs Soft Magnets
small coercivity--good for elec. motors
(e.g., commercial iron 99.95 Fe)
Applied Magnetic
Field (H)
B
Soft
• Process:
2. apply H, cause
alignment
4
Negative H needed to demagnitize!
. Coercivity, HC
3. remove H, alignment stays!
=> permanent magnet!

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Magnetization curves for soft and
hard magnetic materials.
• Ferromagnetic is 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
• Losses must be low, one familiar example is
transformer cores.
• Relative area within the hysteresis loop must be
small (it is characteristically thin and narrow)
• A soft magnetic 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 lowhysteresis energy losses.
Soft Magnetic Materials

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Another property consideration for soft magnetic materials is electrical resistivity.
Energy losses may result from electrical currents that are induced in a magnetic
material by a magnetic 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 materials 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.
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
addition, soft magnetic materials are used in generators, motors, dynamos, and
switching circuits.
Soft Magnetic Materials

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Typical Properties for Several Soft
Magnetic Materials

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Hard Magnetic Materials
• Hard magnetic materials are utilized in permanent magnets
• High resistance to demagnetization.
• High remanence, coercivity, and saturation flux density, as well as a low
initial
permeability, and high hysteresis energy losses.
Schematic magnetization curve
displays hysteresis. The second
quadrant are drawn two B–H energy
that product rectangles; the area of
that rectangle labeled (BH)max which
is greater than the area defined by
Bd–Hd.

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Summary
• A magnetic field can be produced by:
-- putting a current through a coil.
• Magnetic induction:
-- occurs when a material is subjected to a magnetic field.
-- is a change in magnetic moment from electrons.
• Types of material response to a field are:
-- ferri- or ferro-magnetic (large magnetic induction)
-- paramagnetic (poor magnetic induction)
-- diamagnetic (opposing magnetic moment)
• Hard magnets: large coercivity.
• Soft magnets: small coercivity.

Magnetic properties of materials

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