Todays objectives Magnetic Properties II

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Todays objectives - Magnetic Properties II

• Temperature dependence of saturation
  magnetization
• Be able to sketch B vs. H for a ferromagnet and
  describe why it is hysteretic and nonlinear.
• Lots and lots of domains
• Be able to sketch and describe hysteresis loops
  for soft and hard magnets. How are each applied?
  How are each optimized?
• Sketch resistance vs. Temp for a superconductor.
  Why is there a transition, and what are the
  magnetic properties above and below TC?

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Temperature dependence

• The saturation magnetization is a measure of the
  maximum magnetization (assumes perfect alignment
  of all individual atomic magnetic dipoles).
• As temperature increases, Ms diminishes,
  decreasing to 0 at a critical temperature (Tc).
• Beyond Tc a ferromagnet becomes paramagnetic.
• Beyond Tn a ferrimagnet becomes paramagnetic.
• The same is true for antiferromagnetic materials.

Ferromagnets are usually metals. Ferrimagnets are
usually ceramics.
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Temperature dependence
ferromagnetic
TC or Tn
anti-ferromagnetic
T0K
paramagnetic
ferrimagnetic
Above a critical temperature called the Curie
point (TC), ferro- and ferrimagnetic materials no
longer possess a spontaneous magnetization. They
become PARAMAGNETIC. So do anti-ferromagnetic
materials.
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Lots and lots of domains
Domains form for a reason in ferro- and
ferrimagnetic materials. They are not random
structures.
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Browns Paradox lots of questions???
Take a simple iron needle. Iron is
ferromagnetic, it should possess a spontaneous
magnetization. The name ferromagnetic means
magnetic like iron. It should attract another
iron needle depending on the orientation of the
magnetization vector. But it does not Browns
Paradox. Why??? Only if the magnetic iron is
magnetized by a permanent magnet or an
electromagnet, it will attract other pieces of
iron. But this attraction disappears in a short
while. Why??? How come lodestone (Fe3O4) can stay
magnetic for much longer times?
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Why do Domains Form?
HD
MS
HD
Domains form to minimize (and in some cases to
completely eliminate) demagnetization fields
(HD). They are not random structures.
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Magnetic Domains

• In reality, a ferro- or ferrimagnet is comprised
  of many regions (domains) with mutual alignment
  of the individual atomic magnetic dipole moments.
• These domains are not necessarily aligned with
  respect to each other.
• Domain walls between the domains are
  characterized by a gradual transition from one
  orientation to the next.
• The overall magnetization of the material (M) is
  the vector sum of the magnetization vectors for
  all of the individual domains.
• If not magnetized, the overall magnetization is
  simply zero.

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Domain Walls
Bloch Wall
Bloch Wall and Nèel Wall
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Domain orientation (poling)

• Ferromagnets are simply considered to have
  extremely high and linear permeabilities (the
  same is true for susceptibilities).
• But, this simple picture ignores the domain
  structure of magnetic materials.

• Reality is more complex
• Initially, domains are randomly oriented and B0.
• Application of an external field (H) grows any
  domains with a similar orientation as H,
  shrinking the others.
• Eventually, only a single domain remains.
• Ultimately, something near the saturation
  magnetization is reached (Ms or Bs).

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Magnetic Hysteresis

• Once a magnetic material is saturated, decreasing
  H again does not return M (or B) to the same
  position.
• This hysteresis in the magnetic response is
  related to
• a) the mechanism (the last domain switched may
  not be the first to switch back the other
  direction)
• b) drag of domain wall motion
• For no external magnetic field, a remanent
  induction (Br) will remain.
• Some domains remain aligned in the old
  direction.
• A negative field, the Coercive Field (Hc),
  must be applied to eliminate all Br.
• The opposite mechanism occurs for increasing the
  external field after total saturation in the
  reverse direction.

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Partial hysteresis

• If the applied external field sweeps through a
  portion of the hysteresis loop, there will be
  some finite hysteresis in the B response even if
  the field does not reach the coercive field
• due to the same mechanisms as cause hysteresis in
  general
• domain wall drag, and the order of domain
  reorientation.

-Hc
Hc
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Unmagnetized vs. magnetized

• HExternal magnetic field (magnetic field
  strength).
• Bmagnetic induction (magnetic flux density).
• µpermeability (depends on the material, often
  referred to in terms of the relative permeability
  or the susceptibility)
• This equation for the magnetic induction is
  explicitly for an unmagnetized ferromagnet.
• Mmagnetization, representing the magnetic
  moments within a material in the presence of a
  magnetic field of strength H.
• Once the material has been poled, though, the
  equation must be modified.
• Br accounts for any remanent magnetic induction
  (domain orientation).

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Magnet types

• Magnets are categorized depending on the shape of
  the magnetic hysteresis loop.
• Soft magnet narrow in H
• Hard magnet broad in H
• The area of the loop represents energy lost in
  moving the domain walls as the magnet is poled
  from one extreme to the other and back again.
• Energy may also be lost due to local electric
  currents generated within the material caused by
  the external field.
• AC electric field causes a magnetic field, and
  vice versa.

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Soft magnets

• Strong induction for a relatively weak external
  field.
• High saturation field (Bs), High permeability
  (µ), low coercive field (Hc)
• Therefore a low energy loss per poling cycle.
• Applied when rapid, lossless switching is
  required usually subjected to ac magnetic
  fields
• Transformer cores

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Soft magnet optimization

• Saturation field is determined by the composition
• Coercivity is a function of structure (related to
  domain wall motion)
• For the best soft magnet, minimize defects such
  as particles or voids as they restrict domain
  wall motion.
• Lower energy loss per loop if non-conducting (no
  eddy currents).
• Form a solid solution such as Fe-Si or Fe-Ni to
  improve resistivity by a factor of 4 or 5 (from
  110-7 to 410-7).
• Use a ceramic ferrite to improve resistivity by
  10 to 14 orders of magnitude (insulators instead
  of metals).
• (MnFe2O4, ZnFe2O4 2000), (NiFe2O4, ZnFe2O4 107)

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Hard magnets

• High saturation induction, remanence, and
  coercivity.
• High hysteresis losses
• It is hard to repole a hard magnet.
• Book talks about the energy productforget the
  definitionit simply allows us to describe how
  strong the magnet is in terms of the amount of
  energy required to repole it.
• Standard and high energy hard magnets.
• Standard are simple tungsten steel FeNiCu alloys
• High energy hard magnets are 100 times
  stronger.
• SmCo5, Nd2Fe14B is the most common

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Hard magnet optimization

• As for soft magnets, the microstructure is
  related to the energy required to move magnetic
  domains and thus how hard the magnet is.
• Now, though, we want a wide hysteresis loop so we
  may want to
• Introduce defects such as second phase particles.
• Optimize size, shape, and orientation of
  crystallites in a polycrystalline magnet.
• Have a conducting material (eddy current losses).

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Magnetic hard drives

• The magnetic disk has a soft magnet (easy to pole
  and repole with little energy loss.
• The read/write head is a hard magnet, or an
  electromagnet.
• Concept is the same as for an audio tape or video
  tape.
• Magnetics have thus far ruled for computer hard
  drives.
• Flash (solid state, Si based) is coming on strong
• Ferroelectrics are also increasingly being
  applied
• Thermo-mechanical methods may also be used in the
  future

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Microstructure

• Magnetic recording media used to include needle
  shaped particles.
• Now, extremely flat thin films are used to
  diminish surface roughness.

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Magnetic Storage Media
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More magnetic domains
http//www.veeco.com/nanotheatre/nano_view.asp?Cat
ID3page2recs20CP
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Many thanks to Prof. Barry Wells, UConn-Physics.
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WHAT IS SUPERCONDUCTIVITY??
For some materials, the resistivity vanishes at
some low temperature they become superconducting.
Superconductivity is the ability of certain
materials to conduct electrical current with no
resistance. Thus, superconductors can carry large
amounts of current with little or no loss of
energy.
Type I superconductors pure metals, have low
critical field, sudden transition from super to
normal conductivity. Type II superconductors
primarily of alloys or intermetallic compounds,
gradual transition from super to normal.
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HISTORY
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APPLICATIONS Power
The cable configuration features a conductor made
from HTS wires wound around a flexible hollow
core. Liquid nitrogen flows through the core,
cooling the HTS wire to the zero resistance
state. The conductor is surrounded by
conventional dielectric insulation. The
efficiency of this design reduces losses.
Superconducting Transmission Cable From American
Superconductor.
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APPLICATIONS Medical

• Superconducting coils can carry a lot of current.
  
• They thus produce a very strong and uniform
  magnetic field inside the patient's body.

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MEISSNER EFFECT
When you place a superconductor in a magnetic
field, the field is expelled below TC.
B
B
T gtTc
T lt Tc
Magnet
Below TC, the superconductor is diamagnetic, so
fields within it are opposite to that of the
magnetic field to which it is exposed.
Superconductor
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A superconductor displaying the MEISSNER EFFECT
If the temperature increases the sample will lose
its superconductivity and the magnet cannot float
on the superconductor.
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APPLICATIONS Superconducting Magnetic Levitation
The Yamanashi MLX01MagLev Train
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1-2-3 Superconductors (YBa2Cu3O7-x)
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Superconductivity

• There are some limitations, though
• In addition to temperature sensitivity,
  superconductivity is also a function of current
  density and the external magnetic field.
• The material goes non-superconducting if TC, HC,
  or JC are exceeded.

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Magnet type review
Hard vs Soft Ferro or Ferri-magnets
Adapted from Fig. 20.16, Callister 6e. (Fig.
20.16 from K.M. Ralls, T.H. Courtney, and J.
Wulff, Introduction to Materials Science and
Engineering, John Wiley and Sons, Inc., 1976.)
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)
small coercivity--good for elec. motors (e.g.,
commercial iron 99.95 Fe) and hard drive
media. --remove defects to make domain wall
motion as easy as possible.
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SUMMARY

• Temperature dependence of saturation
  magnetization
• Be able to sketch B vs. H for ferromagnets and
  describe why it is hysteretic and nonlinear.
• Be able to sketch and describe hysteresis loops
  for soft and hard magnets. How are each applied?
  How are each optimized?
• Sketch resistance vs. Temp for a superconductor.
  Why is there a transition, and what are the
  magnetic properties above and below TC?

Reading for next class
Optical Properties, Chapter sections 21.1-4