1. Chapter 20: Magnetic Properties
ISSUES TO ADDRESS...
• How do we measure magnetic properties?
• What are the atomic reasons for magnetism?
• How are magnetic materials classified?
• Materials design for magnetic storage.
• What is the importance of superconducting magnets?

2. Applied Magnetic Field
• Created by current through a coil:
Applied
magnetic field H
current I
N = total number of turns
L = length of each turn
• Relation for the applied magnetic field, H:
applied magnetic field
units = (ampere-turns/m)
current

3. Response to a Magnetic Field
• Magnetic induction results in the material
current I
B = Magnetic Induction (tesla)
inside the material
• Magnetic susceptibility, c (dimensionless)
c measures the
material response
relative to a vacuum. H B
vacuum c
= 0
> 0
< 0

4. Magnetic Susceptibility
• Measures the response of electrons to a magnetic field.
• Electrons produce magnetic moments:
Adapted from Fig. 20.4, Callister 7e.
magnetic moments
electron
nucleus
spin
• Net magnetic moment:
--sum of moments from all electrons.
• Three types of response...

5. 3 Types of Magnetism Magnetic induction B (tesla)
permeability of a vacuum:
(1.26 x 10-6 Henries/m)
Magnetic induction
B (tesla)
ferromagnetic e.g. Fe3O4, NiFe2O4
ferrimagnetic e.g. ferrite(), Co, Ni, Gd
(3) ( c
as large as 10 6 !)
(2)
paramagnetic
e.g., Al, Cr, Mo, Na, Ti, Zr ( c
~ 10 -4 )
vacuum ( c
= 0) -5
diamagnetic ( c
~ -10 )
(1)
e.g., Al 2 O 3
, Cu, Au, Si, Ag, Zn
Strength of applied magnetic field (H)
(ampere-turns/m)
Plot adapted from Fig. 20.6, Callister 7e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 7e.

6. Magnetic Moments for 3 Types
No Applied
Applied
Magnetic Field (H = 0)
Magnetic Field (H)
(1) diamagnetic
none
opposing
Adapted from Fig. 20.5(a), Callister 7e.
Adapted from Fig. 20.5(b), Callister 7e.
(2) paramagnetic
random
aligned
Adapted from Fig. 20.7, Callister 7e.
(3) ferromagnetic
ferrimagnetic
aligned

7. Ferro- & Ferri-Magnetic Materials
• As the applied field (H) increases...
--the magnetic moment aligns with H. B
sat H
Adapted from Fig , Callister 7e. (Fig adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) H H
• “Domains” with
aligned magnetic
induction (B) H
Magnetic
moment grow at
expense of poorly H
aligned ones!
H = 0
Applied Magnetic Field (H)

8. Permanent Magnets • Process: • Hard vs Soft Magnets B B
2. apply H, cause
alignment
3. remove H, alignment stays!
=> permanent magnet!
Adapted from Fig , Callister 7e. 4
Negative H needed to demagnitize!
. Coercivity, HC
Applied Magnetic
Field (H)
1. initial (unmagnetized state)
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)
• Hard vs Soft Magnets
small coercivity--good for elec. motors
(e.g., commercial iron Fe)
Adapted from Fig , Callister 7e. (Fig from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.)
Applied Magnetic
Field (H) B
Hard
Soft

9. Magnetic Storage • Information is stored by magnetizing material.
• Head can...
-- apply magnetic field H &
align domains (i.e.,
magnetize the medium).
-- detect a change in the
magnetization of the
medium.
recording head
recording medium
Adapted from Fig , Callister 7e. (Fig from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)
Image of hard drive courtesy Martin Chen.
Reprinted with permission
from International Business Machines Corporation.
• Two media types:
-- Particulate: needle-shaped
g-Fe2O3. +/- mag. moment
along axis. (tape, floppy)
Adapted from Fig , Callister 7e. (Fig courtesy P. Rayner and N.L. Head, IBM Corporation.)
~2.5 mm
Adapted from Fig (a), Callister 7e. (Fig (a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, )
~120 nm
--Thin film: CoPtCr or CoCrTa
alloy. Domains are ~ nm!
(hard drive)

10. Superconductivity Hg
Copper (normal)
4.2 K
Adapted from Fig , Callister 7e.
Tc = temperature below which material is superconductive
= critical temperature

11. Limits of Superconductivity
26 metals + 100’s of alloys & compounds
Unfortunately, not this simple:
Jc = critical current density if J > Jc not superconducting
Hc = critical magnetic field if H > Hc not superconducting
Hc= Ho (1- (T/Tc)2)
Adapted from Fig , Callister 7e.

12. Advances in Superconductivity
This research area was stagnant for many years.
Everyone assumed Tc,max was about 23 K
Many theories said you couldn’t go higher
1987- new results published for Tc > 30 K
ceramics of form Ba1-x Kx BiO3-y
Started enormous race.
Y Ba2Cu3O7-x Tc = 90 K
Tl2Ba2Ca2Cu3Ox Tc = 122 K
tricky to make since oxidation state is quite important
Values now stabilized at ca. 120 K
Suddenly everyone was doing superconductivity. Everyone was doing similar work, making discoveries, & rushing to publish so they could claim to have done it first. Practically, daily new high temp. records were set.

13. Meissner Effect Superconductors expel magnetic fields
This is why a superconductor will float above a magnet
normal
superconductor
Adapted from Fig , Callister 7e.

14. Current Flow in Superconductors
Type I current only in outer skin
- so amount of current limited
Type II current flows within wire M H
Type I
Type II
complete diamagnetism
mixed state
HC1
HC2 HC
normal

15. Superconducting Materials
CuO2 planes X
linear chains Ba Y Ba
(001) planes
YBa2Cu3O7
Vacancies (X) provide electron coupling between CuO2 planes.

16. Summary • A magnetic field can be produced by: • Magnetic induction:
-- putting a current through a coil.
• Magnetic induction:
-- occurs when a material is subjected to a magnetic field.
-- is a change in magnetic moment from electrons.
• Types of material response to a field are:
-- ferri- or ferro-magnetic (large magnetic induction)
-- paramagnetic (poor magnetic induction)
-- diamagnetic (opposing magnetic moment)
• Hard magnets: large coercivity.
• Soft magnets: small coercivity.
• Magnetic storage media:
-- particulate g-Fe2O3 in polymeric film (tape or floppy)
-- thin film CoPtCr or CoCrTa on glass disk (hard drive)

17. ANNOUNCEMENTS
Reading:
Core Problems:
Self-help Problems: