�MAGNETIC PROPERTIES

ISSUES TO ADDRESS...

• How do we measure magnetic properties?

1

• What are the atomic reasons for magnetism?

• Materials design for magnetic storage.

• How are magnetic material classified?

Chapter 18-

APPLIED MAGNETIC FIELD

2

• Created by current through a coil:

• Relation for the applied magnetic field, H:

applied magnetic field

units = (ampere-turns/m)

current

Chapter 18-

RESPONSE TO A MAGNETIC FIELD

3

• Magnetic induction results in the material

• Magnetic susceptibility, χ (dimensionless)

χ measures the

material response

relative to a vacuum.

Chapter 18-

MAGNETIC SUSCEPTIBILITY

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.

Chapter 18-

3 TYPES OF MAGNETISM

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.

Chapter 18-

MAGNETIC MOMENTS FOR 3 TYPES

6

Adapted from Fig. 20.5(a), Callister 6e.

Adapted from Fig. 20.5(b), Callister 6e.

Adapted from Fig. 20.7, Callister 6e.

Chapter 18-

FERRO- & FERRI-MAGNETIC MATERIALS

7

• 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.)

Chapter 18-

PERMANENT MAGNETS

large coercivity

--good for perm magnets

--add particles/voids to

make domain walls

hard to move (e.g.,

tungsten steel:

H_c = 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.)

Chapter 18-

MAGNETIC STORAGE

9

• 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. )

Chapter 18-

SUMMARY

10

• 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 20-11 to 20-15.

Chapter 18-