Chapter 21: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

21-1Anderson 205-

Applied Magnetic Field• Created by current through a coil

H =NILAmpere-turns

m

Appliedmagnetic field H

current I

N turns totalL = length of each turn

• Relation for applied magnetic field

units:

21-2Anderson 205-

Fig. 21.3(a), Callister

• Magnetic induction results

Material Response to a Magnetic Field H

current I

B = Magnetic Induction (tesla) inside the material

• Magnetic susceptibility, χ (dimensionless)

H

B

vacuum χ = 0

χ > 0

χ < 0

χ measures

response relativeto a vacuum

21-3Anderson 205-

Fig. 21.3(b), Callister

Magnetic Susceptibility, χ• Measures response of electrons to magnetic field

• Electrons produce magnetic moments

magnetic moments

electron

nucleus

electron

spin

• Net magnetic moment: sum of moments from all electrons

• Three types of response...

21-4Anderson 205-

Fig. 21.4, Callister

Three Types of Magnetism

Strength of applied magnetic field (H)

diamagnetic (χ ~ -10-5)

paramagnetic (χ ~ 10-3)

ferromagnetic e.g., Fe3O4, NiFe2O4

vacuum (χ = 0)

B = (1 + χ)µoH

ferrimagnetic e.g., ferrite(α), Co, Ni, Gd

(1)

(2)

(3)

21-5Anderson 205-

Fig. 21.6, Callister

Ampere-turnsm

permeability of a vacuum

(1.26·10-6Henries/m)

e.g., Al2O3, Cu, Au, Si, Ag, Zn

e.g., Al, Cr, Mo, Na, Ti, Zr

(χ as large as 106 !)

Magnetic Moments in the Three TypesNo Applied

Magnetic Field (H = 0)Applied

Magnetic Field (H)

(1) diamagnetic

(3) ferromagnetic

(2) paramagnetic

ferrimagnetic

21-6Anderson 205-

Fig. 21.5(a)Callister

Fig. 21.5(b)Callister

Fig. 21.7Callister

Ferro/ferri magnetic Materials

• As applied field (H) increases, magnetic moment aligns with H

Applied Magnetic Field (H)

H

H

H

HH

H = 0

0

Bsat

• “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

21-7Anderson 205-

Fig. 21.13 Callister

4. Coercivity, Hc: Negative

H needed to demagnitize! 1. initial (unmagnetized state)

2. apply H, causealignment3. remove H, alignment stays!

-> permanent magnet!

Applied MagneticField (H)

B

• Hard vs Soft Magnets

Applied MagneticField (H)

B

large coercivity -good for permanent magnets)-add particles/voids to make domain walls hard to move-e.g., tungsten steel Hc = 5900 amp-turn/m

small coercivity -good for electric motors-e.g., commercial iron 99.95Fe

Permanent Magnets

21-8Anderson 205-

Based on Fig. 21.14 Callister

Based on Fig. 21.16 Callister

Magnetic Storage

• Information stored by magnetizing material

Head can...-apply a magnetic field H and align domains (i.e., magnetize) the recording medium-can detect a change in magnetization of the medium

• Two media types... -particulate

needle-shaped γ-Fe2O3

+/- magnetic moment along axis

-thin film

~2.5µm ~60nm

CoPtCr or CoCrTa alloydomains ~ 10-30nm!

21-9Anderson 205-

Based onFig. 21.19Callister

Based on Fig. 21.20Callister

Fig. 21.18Callister