Introduction to Mt il Si & E i i Materials Science

Microsoft PowerPoint - MSE-20-Magnetic Properties(42)-091209 [ ]Introduction to M t i l S i & E i i Materials Science & Engineering
Chapter 20Chapter 20. MAGNETIC PROPERTIESM GNE O E E
How do we measure the magnetic properties?
Wh t th t i f ti ?What are the atomic reasons for magnetism?
How are magnetic materials classified?
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Areal density of magnetic HDD as a function of calendar yearf f y
May 2006 MRS Bulletin
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Price per gigabyte of data stored on a HDD as a function of calendar yeary
projection
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Applications
Transformers.
To focus/deflect electron beams (e.g., TEM).
Medical diagnostic devices: MRI.
B h Th h f i f Bohr magneton - The strength of a magnetic moment of an electron (μB) due to electron spin. Magnetic permeability The ratio between inductance or Magnetic permeability - The ratio between inductance or magnetization and magnetic field. It is a measure of the ease with which magnetic flux lines can ‘‘flow’’ through a material.m g f f g m Magnetization - The total magnetic moment per unit volume. Magnetic susceptibility - The ratio between magnetization and the g p y g applied field. Hard magnet – Ferromagnetic materials are often used to enhance the magnetic flux density (B) produced when an electric current is passed through the material. Soft Magnetic Materials - Applications include cores for electromagnets, electric motors, transformers, generators, and other electrical equipment
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F l f h f Ferromagnetism - Alignment of the magnetic moments of atoms in the same direction so that a net magnetization remains after the magnetic field is removedthe magnetic field is removed. Domains - Small regions within a single or polycrystalline material in which all of the magnetization directions are aligned. Saturation magnetization - When all of the dipoles have been aligned by the field, producing the maximum magnetization. Remanance - The polarization or magnetization that remains in a material after it has been removed from the field. H st sis l p Th l p t d t b m n ti ti n in Hysteresis loop - The loop traced out by magnetization in a ferromagnetic or ferrimagnetic material as the magnetic field is cycled.y Curie temperature - The temperature above (Tc) which ferromagnetic or ferrimagnetic materials become paramagnetic.
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IntroductionIntroduction
Magnetic Moment, MagnetizationMagnetic Moment, Magnetization
Di i P i F iDi i P i F i3
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5 ApplicationsApplications
Magnetic Moments - Basic Concepts M ti f t lli h d ti l sMagnetic force -> controlling charged particles Magnetic dipole
(Fi 20 1) (Fig. 20-2)
(Fig. 20-1) ( g )
Bohr magneton μB :
μ = × ⋅ h
The strength of a magnetic moment of an electron due to electron spin.
2m Spin magnetic moments of each electron = ±μB
Orbital magnetic moments of each electron = ml μB
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Net magnetic moment = sum of moments from all electrons http://bp.snu.ac.kr
Origin of Magnetic Moments Magnetic moment of each electron has two sources:Magnetic moment of each electron has two sources:
– spin magnetic moments + permanent orbital √ Spinning around an axisp g
One of two directions - up or down direction. √ Orbital moment around nucleus
Moving electron can be considered as a small current loop.
(Fig. 20-4)
(a)The spin of the electron produces a magnetic field with a direction dependent on the quantum number ms.
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p q s (b) Electrons orbiting around the nucleus create a magnetic
field around the atom. http://bp.snu.ac.kr
Magnetic Moment
A group of elements, transition metals, have an inner energy level that is not completely filledenergy level that is not completely filled.
Related to Table 20 4Related to Table 20-4
[Ar]3d64s2[Ar]3d 4s [Ar]3d74s2
Magnetic Moment
Cu has a completely filled 3d shell, and thus does not display a net moment.display a net moment.
The e–s in the 3d level of transition elements do not t th sh lls i i s F M th fi st fi s enter the shells in pairs. For Mn, the first five e–s
have the same spin.
Only after half of the 3d-level is filled, pairs with opposing spins form.pp g p
Therefore, each atom in a transition metal has a permanent magnetic moment which is related to the permanent magnetic moment, which is related to the number of unpaired electrons each atom behaves as i di l
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Applied magnetic field H
N = total number of turns L = length of each turn
(Fig. 20-3)
current I
B H iB = H in vacuum
M i i d i l i h i l Response to a Magnetic Field
Magnetic induction results in the material.
B = Magnetic Induction (tesla) inside the material
current I B = μH μ: permeability
M ti s s tibilit (di si l ss)
B = μH μ: permeability
Magnetic susceptibility, χ (dimensionless)
B χ > 0 vacuum χ = 0 χ < 0 (Fig. 20-6)
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Magnetization B = H permeability of a vacuum = 1
B = μH μ: permeability Permeability – Degree to which a material can
be magnetizedμ μ p y B = μH = H + M M = χH χ: magnetic susceptibility
be magnetized. - B field can be induced under the
external H field. M = χH χ: magnetic susceptibility
χ = μ-1
H : magnetic field (strength)
B : magnetic flux density
M : magnetization A current passing through a coil sets up a magnetic field H with a flux density Bmagnetic field H with a flux density B.
The flux density is higher l
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The effect of the core material on the flux density
Paramagnetism
Ferromagnetismg
Diamagnetism
Th ti t th The magnetic moment opposes the field in diamagnetic materials.
Applied Magnetic Field (H)
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μ < 1 μ ≅ 0.99999 negative χ
B = μH = H + M = H + χH B < H Cu, Ag, Au, and Al2O3 are diamagnetic @ RT, g, , 2 3 g Superconductors have perfect diamagnetism @ below Tc
(Table 20-2)
Paramagnetism The effect is lost as soon as the magnetic field is removed.
μ ≅ 1 00001 ~ 1 001μ ≅ 1.00001 ~ 1.001 Both diamagnetic and paramagnetic materials are nonmagnetic.
Occurs in materials with unpaired electrons.
A net magnetic moment due to e’ spin is associated with each g p atom.
When H field is applied, the dipoles line up with the field pp , p p causing a positive magnetization.
Because the dipoles do not interact, extremely large p , m y g magnetic fields are required to align all of the dipoles.
Found in Al, Ti, and Cu alloys.
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Paramagnetism
Ferromagnetism Spontaneous magnetization
Transition metals: Fe, Co, Ni, rare earth (Gd, Nd, etc.) Alignment of an appreciable fraction of molecular magnetic moment in some favorable direction in crystal χ large
Related to unfilled 3d and 4f shells Ferromagnetic transition temperature (Curie temperature)
Ferromagnetism
Caused by the unfilled energy levels in the 3d level of Fe, Ni, & Co.
Permanent magnetic moments result due to uncancelled e- spins
caused by the e- structure.y
In ferromagnetic materials, the permanent unpaired dipoles easily
li i h h i d H d h h i i line up with the imposed H, due to the exchange interaction, or
mutual reinforcement of the dipoles.
Coupling interactions develop alignment of net spin magnetic
moments of adjacent atoms even when no external field actingmoments of adjacent atoms, even when no external field acting.
Large magnetizations are obtained even for small magnetic fields.
23 μ = very high, 106 http://bp.snu.ac.kr
Ferromagnetism Paramagnetic above Curie temperature.
Increased thermal motion can randomize the directions of the moments.
B = H + M (Fi 20 10)B = H + M (Fig. 20-10) _______
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Produced by the solidification process, for example.y p p
Have a change in crystal orientation across them.
Impede dislocation motionsImpede dislocation motions.
grain grain boundarie
Ferroelectric Domain Boundaries Domain - a small region of the material in which the direction of electric polarization or magnetization remains the same
ex) ferromagnets (Fe, Co, Ni) vs. ferroelectric (PZT, BaTiO3)
(Fig. 20-12)
(Fig. 20-11)
Ferromagnetic Domains
When H is imposed on the material,
d i th t l li d domains that are nearly lined up
with the field, grow at the expense g p
of unaligned domains.
domain walls must movedomain walls must move.
H provides the force required for
this movement.
dipoles of neighboring atoms.
within the grain structure of a ferromagnetic material, even in
the absence of an external field.
Domains are regions in the material in which all of the dipoles g p
are aligned.
In a material that has never been exposed to a ma netic field In a material that has never been exposed to a magnetic field,
the individual domains have a random orientation. The net
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Ferromagnetic Materials
As the applied field (H) increases the magnetic moment aligns with Hthe magnetic moment aligns with H.
Bsat
“Domains” with aligned c B) H H
Domains” with aligned magnetic moments grow t th f l ag
ne ti
c ct
io n
a in
du c
(Fig. 20-13)
Soft magnetic materials:
domains can get reoriented with small Happlieddomains can get reoriented with small Happlied
Small remanence desired, so that no magnetization remains
h H dwhen Happlied is removed
Small hysteresis loop minimizes energy losses Br
(e.g., metallic glasses)
Rapid response to high frequency magnetic fields HRapid response to high-frequency magnetic fields. Hc
(Fig. 20-14)
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Magnetic Storage di di(Fi 20 23)
- apply magnetic field H & align domains (i e magnetize the medium)
recording mediumInformation is stored: (Fig. 20-23)
domains (i.e., magnetize the medium) - detect a change in the magnetization
recording head
Two media types: Particulate: needle shaped Fe2O3 Thi fil C PtC C C T ll-Particulate: needle-shaped γ-Fe2O3 mag. moment along axis. (tape, floppy)
-Thin film: CoPtCr or CoCrTa alloy Domains are ~ 10-30 nm (hard drive)
~2.5μm ~60nm
38 = critical temperature
Advances in Superconductivity
This research area was stagnant for many years. √ Everyone assumed T was about 23 K√ Everyone assumed Tc,max was about 23 K.
√ Many theories said you couldn’t go higher.
1987- new results published for Tc > 30 K √ High temperature superconductors - ceramics (mostly oxides)√ g mp p m (m y )
√ Started enormous race
Tl2Ba2Ca2Cu3Ox Tc = 122 K
Highest T = 133 K in ambient pressure 39
Highest Tc = 133 K in ambient pressure
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l dnormal superconductor
(Fig. 20-28)
This is why a superconductor will float above a magnet (levitation)
Superconducting Materials
Nb 9.2 1930
Nb3Sn 18.1 1954
Nb3(Al3/4Ge1/4) 20-21 1966
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HgBa Ca Cu O 133 1993
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Hg0.8Pb0.2Ba2Ca2Cu3Ox 133 1994 1900 2000‘20 ‘40 ‘60 ‘80
20 20 K
1900 200020 40 60 80 q He 4 K
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Summary A magnetic field can be produced by a currentA magnetic field can be produced by a current. Magnetic induction: √ h t i l i bj t d t ti fi ld√ occurs when a material is subjected to a magnetic field. B = μH = H + M = H + χH
T f t i l t fi ld Types of material response to a field are: √ ferromagnetic (large magnetic induction)
( d )√ paramagnetic (poor magnetic induction) √ diamagnetic (opposing magnetic moment)
Hard magnets: large coercivity Soft magnets: small coercivity
Problems from Chap. 20 http://bp.snu.ac.kr Prob 20-4 Prob 20-10 Prob 20-15 Prob 20-16 (only for ferromagnetic)
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Prob. 20-4 Prob. 20-10 Prob. 20-15 Prob. 20-16 (only for ferromagnetic)
Prob. 20-24 Prob. 20-27 http://bp.snu.ac.kr-2009-12-09

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Introduction to Mt il Si & E i i Materials Science