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Title: Todays objectives Magnetic Properties II


1
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?

2
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.
3
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.
4
Lots and lots of domains
Domains form for a reason in ferro- and
ferrimagnetic materials. They are not random
structures.
5
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?
6
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.
7
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.

8
Domain Walls
Bloch Wall
Bloch Wall and Nèel Wall
9
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).

10
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.

11
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
12
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).

13
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.

14
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

15
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)

16
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

17
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).

18
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

19
Microstructure
  • Magnetic recording media used to include needle
    shaped particles.
  • Now, extremely flat thin films are used to
    diminish surface roughness.

20
Magnetic Storage Media
21
More magnetic domains
http//www.veeco.com/nanotheatre/nano_view.asp?Cat
ID3page2recs20CP
22
Many thanks to Prof. Barry Wells, UConn-Physics.
23
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.
24
HISTORY
25
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26
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.
27
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.

28
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
29
A superconductor displaying the MEISSNER EFFECT
If the temperature increases the sample will lose
its superconductivity and the magnet cannot float
on the superconductor.
30
APPLICATIONS Superconducting Magnetic Levitation
The Yamanashi MLX01MagLev Train
31
1-2-3 Superconductors (YBa2Cu3O7-x)
32
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.

33
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.
34
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
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