Download magnetic field strength, H

MAGNETISM
Can be defines as:
Phenomenon by which materials assert an attractive or repulsive
force or influence on other materials
Magnetic Materials includes -iron, some steels, lodestone minerals
Principle applied in medicine- Magnetic Resonance Imaging
From:http://en.wikipedia.
org/wiki/Image:Modern_
3T_MRI.JPG
Magnetic Dipoles
Magnetism force moving electrically charged particles
Magnetic dipoles is similar to electric dipoles
Represented by small bar of magnet with north and south poles
(also represented by arrow)
Within magnetic field, the force of the field exerts a torque that
tends to orient the dipoles with the field
Magnetic Dipole Moment, μm
un
m
A
 m  IAun
I = circulating current
un= unit vector coming out
from area A
I
Fig. 8.1: Definition of a magnetic dipole moment.
From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)
http://Materials.Usask.Ca
B
m

A
I

B
Fig. 8.2: A magnetic dipole moment in an external field
experiences a torque.
From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)
http://Materials.Usask.Ca
From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)
http://Materials.Usask.Ca
Origin of Magnetic Moments
Each electron in an atom has magnetic moments that originate from
two sources:
•One is related to its orbital motion around the nucleus; as a
moving charge, electron
-small current loop,
-generating a very small magnetic field,
-have a magnetic moment along its axis of rotation
•The other magnetic moment originates from this electron spin, which is
directed along the spin axis
Spin magnetic moments may be only in an “up” direction or in an
antiparallel “down” direction
Do you know?
Magnetic resonance (MR) imaging is founded on the
manipulation of magnetic dipole moments in such a way
that signals generated from these interactions that can
be translated into visual images of the body.
Figure Typical MR images of the head (left), neck
(middle) and kidneys (right).
Magnetic Field Vectors
Magnetic Field Vectors
•Magnetic Field Strength, H
•Magnetic Flux Density, B
•Magnetic Permeability, 
•Magnetization, M
•Magnetic Susceptibility, χm
Magnetic Field Strength
• The externally applied magnetic field, i.e. the
magnetic field strength, H.
If the magnetic field is generated by solenoid consisting
of:
N= closely spaced turns,
l =length,
I= current magnitude
The units of H are amperes per meter.
Magnetic Flux Density
Magnetic flux density, B, represents the
magnitude of the internal field strength within a
substance that is subjected to an H field.
Both B & H are field vectors, being characterized
not only by magnitude, but also by direction in
space.
The magnetic field strength and flux density are
related according to:
The units for B are teslas
Magnetic Permeability
Magnetic permeability is define as the magnetic
field per unit magnetizing field
B

H
The permeability has dimensions of webers per
amperemeter (Wb/A-m) or henries per meter (H/m).
In a vacuum,
where o is the permeability of a vacuum,
4 10-7 (1.257  10-6) H/m.
Magnetic Permeability
Relative permeability μr of a medium is the fractional
increase in the magnetic field with respect to the field in
free space when a material medium is introduced.
B
B
r 

B0  0 H
Magnetization
Another field quantity, M, called the magnetization of
the solid, is defined by the expression
In the presence of an H field, the magnetic moments
within a material tend to become aligned with the field
& to reinforce it by virtue of their magnetic fields; the
term oM
The magnitude of M is proportional to the applied field as follows:
Magnetic Susceptibility
Magnetic susceptibility χm indicates the ease with
which the material becomes magnetized under an
applied magnetic field
M  H
m
m is unitless
Bohr Magneton
Represented by:
B = Bohr Magneton = 9.27 x 10-24 A m2
•Bohr magneton (B ) is a useful elementary
unit of magnetic moment on the atomic scale.
It is equal to the magnetic moment of one electron
spin along an applied magnetic field B =eħ/2me
e is the elementary charge
is the reduced Planck’s constant
me is the electron rest mass
Diamagnetism and Paramagnetism
Diamagnetism = a form of magnetism that is non-permanent and occurs only in
the applied field with the direction opposite the applied field
Note:r < 1 (slightly) : m is negative and in the order of 10-5
Paramagnetism = magnetism does not exist with absence of H (random
arrangement of dipoles moments), but exist under applied field (H)
Note: r > 1 : m is small and positive in the order of 10-5 to 10-2
Diamagnetic and Paramagnetic materials  “non-magnetic”
Diamagnetism : Paramagnetism : Ferromagnetism
B vs H for diamagnetic and paramagnetic materials
Diamagnetism and Paramagnetism
Room Temperature m for Diamagnetic and Paramagnetic Materials
Diamagnetics
Paramagnetics
Material
m
Material
m
Aluminum Oxide
Copper
Gold
Mercury
Silicon
Silver
Sodium Chloride
Zinc
-1.81x10-5
-0.96x10-5
-3.44x10-5
-2.85x10-5
-0.41x10-5
-2.38x10-5
-1.41x10-5
-1.56x10-5
Aluminum
Chromium
Chromium Chloride
Manganese Sulfate
Molybdenum
Sodium
Titanium
Zirconium
2.07x10-5
3.13x10-4
1..51x10-3
3.70x10-3
1.19x10-4
8.48x10-6
1.81x10-4
1.09x10-4
Ferromagnetism
Ferromagnetism = phenomenon in certain (metallic) materials that possess a
permanent magnetic moment in the absence of H
Domain = Area (volume) of a material that the mutual spin alignment exist
Saturation Magnetization (Ms)  The maximum possible magnetization
Antiferromagnetism and Ferrimagnetism
Antiferromagnetism = the alignment of the spin moments of the neighboring
atoms or ions in exactly opposite direction
MnO = Antiferroelectric
Mn2+  Spin-origin magnetic moment  Align antiparallel in crystal structure
O2-  No net magnetic moment  Cancellation of ms, ml
Antiferromagnetism and Ferrimagnetism
Ferrimagnetism = a permanent magnetization in materials that is very similar to
ferromagnetism but originates from different source of the net magnetic moment
Ferrimagnetic Material : Cubic Ferrites : MFe2O4 : M = one of the metallic elements
Prototype  Fe3O4 (magnetite or lodestone)  Inverse Spinel Structure
Fe2+ O2- - (Fe3+)2 (O2-)3
O2- = Magnetically neutral
Fe2+ = Net spin magnetic moment = 4B
Fe3+ = Net spin magnetic moment = 5B
Ferrofluid
• Ferrofluids or Magnetic Fluids are fluids with
magnetic nanoparticle suspended in a liquid
medium
• The particles are generally coated to prevent
magnetostatic interactions which would cause the
particles to cluster together
Fromhttp://www.ucl.ac.uk/
~ucfbpmb/ferrofluid%
20copy.jpg
Biomedical Applications of Magnetic
Materials
• Abnormalities in body tissues and organs can be
detected on the basis of the production of crosssectional images using Magnetic Resonance Imaging
(MRI)
• Chemical analysis of body tissues is also possible
using Magnetic Resonance Spectroscopy (MRS).
• Magnetic Drug Targeting- applies nanoparticles to
target drugs and genes to specific sites in vivo
– Using this method, this can enhance drug and gene
uptake at the sites
– Also known as magnetic target carriers (MTC)
A chitosan "mothership" capsule (light blue)
attaches and delivers drug-filled vesicles (dark blue)
to a tumor. This capsule may be targeted to tumor
cells either by antibodies (the Y- shaped spines) on
its outer surface or by magnetic nanoparticles (dark
red) inside. These two targeting systems effectively
act as navigators, taking the capsules "along for the
ride" to precise locations where the drugs are
needed. Dowling et al. has found, they can then be
guided to specific locations in the body with an
electromagnetic field.
www.bioe.umd.edu/fischell-fellowship/dowling.html
Magnetic force bioreactor for tissue
engineering
• The magnetic force bioreactor is designed to apply forces
directly to the cell membrane by coupling biocompatible
magnetic nano- and microparticles to the membrane
surface
From-http://www.maths.nottingham.ac.uk/personal/pmzsjf/Image2.gif
Other applications:
•MRI Contrast Enhancement
•MR delivers excellent soft-tissue contrast, however,
assistance from contrast media (which usually from
paramagnetic agent) is done to obtain better image.
Figure shows brain images both before and after
contrast allow disruptions in the blood-brain
barrier to be investigated
From: www.hull.ac.uk/mri/lectures/gpl_page.html