Magnetism at the Nanoscale Level: Nanomagnetism

1
Magnetism at the Nanoscale Level Nanomagnetism
Review A current loop (enclosed area A, current
I) generates a magnetic moment µ given by µ
IA
I
A
2
Magnetism at the Nanoscale Level Nanomagnetism
Review A current loop (enclosed area A, current
I) generates a magnetic moment µ given by µ
IA
µ
I
A
µ is a vector
3
Magnetism at the Nanoscale Level Nanomagnetism
Review A current loop (enclosed area A, current
I) generates a magnetic moment µ given by µ
IA
µ
I
A
µ is a vector
µ produces a magnetic field similar to that of a
very small bar magnet (what is called a dipole
magnetic field)
N
S
µ
4
What does this have to do with magnetism at the
atomic and nano levels? Electrons orbit nuclei
and atoms also can have magnetic moments,
Electron charge
Orbital angular momentum of the electron
Electron mass
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What does this have to do with magnetism at the
atomic and nano levels? Electrons orbit nuclei
and atoms also can have magnetic moments,
Electron charge
Orbital angular momentum of the electron
Electron mass
There is also a spin magnetic moment due to the
spin of electrons (a purely quantum phenomenon),
With mS 1/2
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Atoms with unfilled energy levels (shells) often
have magnetic momentsespecially those with
unfilled inner energy levels, such as some
transition metals in the Periodic Table.
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Illustrations of various arrangements of
individual atomic magnetic moments that
constitute (a) paramagnetic, (b) ferromagnetic,
(c) ferrimagnetic and (d) antiferromagnetic
materials.
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So, some atoms behave like small bar magnets
(whether due to µ or µZ).
Consider a small sample of magnetic materials
nano-sized or larger. What happens when an
external magnetic field is applied?
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So, some atoms behave like small bar magnets
(whether due to µ or µZ).
Consider a small sample of magnetic materials
nano-sized or larger. What happens when an
external magnetic field is applied?

• Magnetic moments, like bar magnets, align in
  the external field remember how magnetic compass
  needles align in the Earths field. A
  materials residual magnetic field left after the
  external field is removed (if any is left) is
  called the remnant magnetic field.
• The coercive field is the magnetic field that
  is required to undo the remnant magnetization.

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For nano-sized magnets, the remnant magnetization
often increases with decreasing particle size.
At right the dependence of the remnant
magnetization on particle size (d) for
Nd-B-Fe. from Poole Owens Intro to
Nanotechnology (figure 7.5)
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The magnitude of the coercive field decreases as
particle size decreases.
At right the dependence of the coercive magnetic
field on particle size (d) for Nd-B-Fe. from
Poole Owens Intro to Nanotechnology (figure 7.6)
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Ferrofluids (or magnetofluids) suspensions of
nano-sized ferromagnetic particles.
First discovered in the 1940s (when particles
were in the micron range), ferrofluid particles
are now nano-sized (5 to 20 nm). The behavior of
ferrofluids in external magnetic fields is
analogous to that of liquid crystals in external
electric fields.
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Magnetization curve for a ferrofluid made of
magnetite (Fe3O4) nanoparticles 1 Oersted 10-4
Tesla. From Poole Owens Intro to
Nanotechnology (figure 7.22). NB Earths surface
magnetic field is 30 to 60 10-6 T.
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(A paramagnetic character, but with more
complexity because the nano-ferromagnets are
suspended in a liquid.)
Magnetization curve for a ferrofluid made of
magnetite (Fe3O4) nanoparticles 1 Oersted 10-4
Tesla. From Poole Owens Intro to
Nanotechnology (figure 7.22). NB Earths surface
magnetic field is 30 to 60 10-6 T.
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Some of this complexity is seen in a ferrofluids
physical behavior in an external magnetic field
A picture taken through an optical microscope of
chains of magnetic nanoparticles formed in a thin
film of ferrofluid when the magnetic field is
parallel to the plane of the film. (This
behavior is analogous to liquid crystals in an
electric field.)
Poole Owens Intro to Nanotechnology (figure
7.23).
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Details of ferrofluid chains as a function of
applied magnetic field strength 1 Oe 10-4 T.
Poole Owens Intro to Nanotechnology (figure
7.24both).
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Looking at ferrofluid chain ends An optical
microscope picture of ends of chains of magnetic
nanoparticles in a ferrofluid film when the
magnetic field is perpendicular to the surface of
the film. The magnetic field strength is large
enough to form a hexagonal lattice (look at the
pattern carefully).
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Looking at ferrofluid chain ends An optical
microscope picture of ends of chains of magnetic
nanoparticles in a ferrofluid film when the
magnetic field is perpendicular to the surface of
the film. The magnetic field strength is large
enough to form a hexagonal lattice (look at the
pattern carefully).
Poole Owens Intro to Nanotechnology (figure
7.25)
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A ferrofluid applicationas a vacuum seal between
moving mechanical parts.
Illustration of the use of a ferrofluid as a
vacuum seal on a rotating magnetically permeable
shaft mounted on the pole pieces of a permanent
magnet. Poole Owens Intro to Nanotechnology
(figure 7.29).
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Magnetite (Fe3O4)
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Magnetite (Fe3O4)
A crystal in what is called a cubic spinel
crystal structure.
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Magnetite (Fe3O4)
A crystal in what is called a cubic spinel
crystal structure.
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Magnetite (Fe3O4) Fe3(Fe3Fe2)(O2-)4 This
structure is made up of 2 interlocking
sublatticies A (Fe3) and B (Fe3Fe2).
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Magnetite (Fe3O4) Fe3(Fe3Fe2)(O2-)4 This
structure is made up of 2 interlocking
sublatticies A (Fe3) and B (Fe3Fe2).
Fe (A)
5µB
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Magnetite (Fe3O4) Fe3(Fe3Fe2)(O2-)4 This
structure is made up of 2 interlocking
sublatticies A (Fe3) and B (Fe3Fe2).
Fe (B)
4 µB or 5µB
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Magnetite (Fe3O4) Fe3(Fe3Fe2)(O2-)4 This
structure is made up of 2 interlocking
sublatticies A (Fe3) and B (Fe3Fe2).
Fe (B)
Fe (A)
5µB
4 µB or 5µB
On average there is about one-half electron spin
per structure (the crystals unit cell),
producing a net magnetic moment.
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Magnetite (Fe3O4) Fe3(Fe3Fe2)(O2-)4 This
structure is made up of 2 interlocking
sublatticies A (Fe3) and B (Fe3Fe2).
8.396 Å
Fe (B)
Fe (A)
5µB
4 µB or 5µB
On average there is about one-half electron spin
per structure (the crystals unit cell),
producing a net magnetic moment.