Magnetic properties

2
• Created by current through a coil:
• Relation for the applied magnetic field, H:
applied magnetic field
units = (ampere-turns/m)
current
APPLIED MAGNETIC FIELD

3
• Magnetic induction results in the material
• Magnetic susceptibility, χ (dimensionless)
χ measures the
material response
relative to a vacuum.
RESPONSE TO A MAGNETIC FIELD

4
• Measures the response of electrons to a magnetic
field.
• Electrons produce magnetic moments:
• Net magnetic moment:
--sum of moments from all electrons.
• Three types of response...
Adapted from Fig. 20.4, Callister 6e.
MAGNETIC SUSCEPTIBILITY

5
permeability of a vacuum:
(1.26 x 10-6 Henries/m)
Plot adapted from Fig. 20.6, Callister 6e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 6e.
3 TYPES OF MAGNETISM

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Adapted from Fig. 20.5(a), Callister 6e.
Adapted from Fig. 20.5(b), Callister 6e.
Adapted from Fig. 20.7, Callister 6e.
MAGNETIC MOMENTS FOR 3 TYPES

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• As the applied field (H) increases...
--the magnetic moment aligns with H.
Adapted from Fig. 20.13, Callister 6e. (Fig. 20.13 adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.)
FERRO- & FERRI-MAGNETIC MATERIALS

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)
8
• Process:
• Hard vs Soft Magnets
small coercivity--good for elec. motors
(e.g., commercial iron 99.95 Fe)
Adapted from Fig. 20.14, Callister 6e.
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.)
PERMANENT MAGNETS

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• Information is stored by magnetizing material.
Simulation of hard drive courtesy Martin Chen.
Reprinted with permission
from International Business Machines Corporation.
• Head can...
--apply magnetic field H &
align domains (i.e.,
magnetize the medium).
--detect a change in the
magnetization of the
medium.
• Two media types:
--Particulate: needle-shaped
γ-Fe2O3. +/- mag. moment
along axis. (tape, floppy)
--Thin film: CoPtCr or CoCrTa
alloy. Domains are ~ 10-30nm!
(hard drive)
Adapted from Fig. 20.18, Callister 6e. (Fig. 20.18 from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)
Adapted from Fig. 20.19, Callister 6e. (Fig. 20.19 courtesy P. Rayner and N.L. Head, IBM Corporation.)
Adapted from Fig. 20.20(a), Callister 6e. (Fig. 20.20(a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, 1993. )
MAGNETIC STORAGE

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• A magnetic field can be produced by:
--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 γ-Fe2O3 in polymeric film (tape or floppy)
--thin film CoPtCr or CoCrTa on glass disk (hard drive)
Note: For materials selection cases related to a magnet coil, see slides 22-11 to 22-15.
SUMMARY