MAGNETIC PROPERTIES ISSUES TO ADDRESS... • How do we measure magnetic properties? • What are the atomic reasons for magnetism? • How are magnetic material classified? • Materials design for magnetic storage.

Chapter 20- 1

Magnetic field (H) exists at 1. Loop of current

2. Around a magnet

APPLIED MAGNETIC FIELD • Created by current through a coil: Applied magnetic field H

N turns total L = length Length of wire each turn

current I

• Relation for the applied magnetic field, H: H=

NI L

applied magnetic field units = (ampere-turns/m)

current

H – applied magnetic field Regulated by value of N, I, L

RESPONSE TO A MAGNETIC FIELD • Magnetic induction results in the material B = Magnetic Induction (tesla) inside the material

B – the magnetic field that is induce inside the solid

current I

• Magnetic susceptibility, χ (dimensionless)

B

χ>0 vacuum χ = 0 χ>1 :a good magnetic solid = magnetic susceptibility B = (1 + χ)µ o H µr = (1+χ) ; µ = µrµ0 = (1+χ)µ0

Values of susceptibility of diamagnetic and paramagnetic is low

•

RESPONSE TO A MAGNETIC FIELD Magnetic induction results in the material B = Magnetic Induction (tesla) inside the material

current I

• Magnetic susceptibility, χ (dimensionless)

B

χ>0 vacuum χ = 0 χ permanent magnet!

Adapted from Fig. 20.14, Callister 6e.

Applied Magnetic Field (H)

4. Coercivity, Hc: 1. initial (unmagnetized state) Negative H needed to demagnitize!

Soft

Har d

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)

Har d

B

• Hard vs Soft 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., Applied 1976.) Magnetic

Field (H)

small coercivity--good for elec. motors (e.g., commercial iron 99.95 Fe) Chapter 20- 8

Periodic Table

The magnetic field results from spin of unpaired electrons in 3d level.

Exchange Energy: Eex = -2Je S1 S2; is negative for suitable d orbital radius

Je Co

+

Fe

0 Mn

-

Ni Gd

r rd

Cr

Fig. 8.20: The exchange intergral as a function of r/r d , where r is the interatomic distance and r d the radius of the d-orbit (or the average dsubshell radius). Cr to Ni are transition metals. For Gd the x-axis is r/r f where r f is the radius of the f-orbit. From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002) http://Materials.Usask.Ca

Prototype of Ferrimagnetic: Fe3O4 = Fe2+Fe3+2O2-4 and MFe2O4

O - 32 atom in 8 FCC units

Magnetic Moment caused by incomplete cancellation of contributions from Fe2+

Spinel: AB2O4 FCC – anion (O) 1/8 Tetrahedral: A 2/4 Octrahedral: B Ex: MgAl2O4

Spinel (inverse): B(AB)O4 FCC – antion (O) 1/8 Tetrahedral: B 2/4 Octrahedral: A,B Ex: FeMgFeO4

1 elementary cell = 1 FCC unit of O One unit cell = 8 elementary cell

Interstitial sites in FCC

Tetrahedral Octahedral

In one FCC, there are: 4 Octahedral sites 8 Tetrahedral sites

SUMMARY • 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.

Chapter 20- 10