# Chapter 18: Magnetic Properties - PowerPoint PPT Presentation

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Chapter 18: Magnetic Properties. ISSUES TO ADDRESS. • What are the important magnetic properties ?. • How do we explain magnetic phenomena?. • How are magnetic materials classified?. • How does magnetic memory storage work?.

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Chapter 18: Magnetic Properties

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### Chapter 18: Magnetic Properties

• What are the important magnetic properties?

• How do we explain magnetic phenomena?

• How are magnetic materials classified?

• How does magnetic memory storage work?

• What is superconductivity and how do magnetic fields effect the behavior of superconductors?

### Generation of a Magnetic Field -- Vacuum

• Computation of the applied magnetic field, H:

• Created by current through a coil:

B0

N = total number of turns

 = length of each turn (m)

I= current (ampere)

H

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

B0 = magnetic flux density in a vacuum (tesla)

I

• Computation of the magnetic flux density in a vacuum, B0:

B0=0H

permeability of a vacuum(1.257 x 10-6 Henry/m)

### Generation of a Magnetic Field -- within a Solid Material

• Relative permeability (dimensionless)

• A magnetic field is induced in the material

B

B = Magnetic Induction (tesla) inside the material

applied

magnetic field H

B=H

permeability of a solid

current I

### Generation of a Magnetic Field -- within a Solid Material (cont.)

• B in terms of Hand M

B=0H +0M

B

> 0

cm

vacuum cm = 0

< 0

cm

H

• Magnetization

M=mH

Magnetic susceptibility (dimensionless)

• Combining the above two equations:

B=0H +0 mH

=(1 + m)0H

permeability of a vacuum:

(1.26 x 10-6 Henry/m)

cm is a measure of a material’s magnetic response relative to a vacuum

### Origins of Magnetic Moments

• Magnetic moments arise from electron motions and the spins on electrons.

magnetic moments

electron

electron

spin

nucleus

Adapted from Fig. 18.4, Callister & Rethwisch 3e.

electron orbitalmotion

electron spin

• Net atomic magnetic moment:

-- sum of moments from all electrons.

• Four types of response...

### Types of Magnetism

(3) ferromagnetic e.g. Fe3O4, NiFe2O4

(4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd

cm

(2) paramagnetic (

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

~ 10-4)

cm

(

as large as

106

!)

cm

(1) diamagnetic (

~ -10-5)

cm

vacuum

(

= 0)

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

B (tesla)

H (ampere-turns/m)

Plot adapted from Fig. 18.6, Callister & Rethwisch 3e. Values and materials from Table 18.2 and discussion in Section 18.4, Callister & Rethwisch 3e.

(2) paramagnetic

random

aligned

Adapted from Fig. 18.5(b), Callister & Rethwisch 3e.

(3) ferromagnetic

(4)ferrimagnetic

Adapted from Fig. 18.7, Callister & Rethwisch 3e.

aligned

aligned

### Magnetic Responses for 4 Types

No Applied

Applied

Magnetic Field (H = 0)

Magnetic Field (H)

(1) diamagnetic

opposing

Adapted from Fig. 18.5(a), Callister & Rethwisch 3e.

none

### Domains in Ferromagnetic & Ferrimagnetic Materials

H

H

H

• “Domains” with

aligned magnetic

H

moment grow at

expense of poorly

aligned ones!

H

H = 0

• As the applied field (H) increasesthe magnetic domains change shape and size by movement of domain boundaries.

B

sat

Adapted from Fig. 18.13, Callister & Rethwisch 3e. (Fig. 18.13 adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.)

induction (B)

Magnetic

0

Applied Magnetic Field (H)

### Hysteresis and Permanent Magnetization

Stage 2. Apply H, align domains

Stage 3. Remove H, alignment

remains! => permanent magnet!

Stage 4. Coercivity, HCNegative H needed to demagnitize!

Stage 6. Close the hysteresis loop

Stage 5. Apply -H, align domains

• The magnetic hysteresis phenomenon

B

Adapted from Fig. 18.14, Callister & Rethwisch 3e.

H

Stage 1. Initial (unmagnetized state)

### Hard and Soft Magnetic Materials

Hard

Soft

Hard magnetic materials:

-- large coercivities

-- used for permanent magnets

inhibit domain wall motion

-- example: tungsten steel --

Hc = 5900 amp-turn/m)

B

H

Soft magnetic materials:

-- small coercivities

-- used for electric motors

-- example: commercial iron 99.95 Fe

Adapted from Fig. 18.19, Callister & Rethwisch 3e. (Fig. 18.19 from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.)

10

### Magnetic Storage

recording medium

Adapted from Fig. 18.23, Callister & Rethwisch 3e. (Fig. 18.23 from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)

-- Particulate: needle-shaped

g-Fe2O3. magnetic moment

aligned with axis (e.g., tape).

--Thin film: CoPtCr or CoCrTa

alloy. Domains are ~ 10-30 nm

wide (e.g., hard drive).

Adapted from Fig. 18.25(a), Callister & Rethwisch 3e. (Fig. 18.25(a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, 1993. )

Adapted from Fig. 18.24, Callister & Rethwisch 3e. (Fig. 18.24 courtesy P. Rayner and N.L. Head, IBM Corporation.)

~2.5mm

~120nm

• Information can be stored using magnetic materials.

-- “write” - i.e., apply a magnetic field and align domains in small regions of the recording medium

-- “read” - i.e., detect a change in the magnetization in small regions of the recording medium

• Two media types:

### Superconductivity

Found in 26 metals and hundreds of alloys & compounds

• TC= critical temperature

= temperature below which material is superconductive

Mercury

Copper (normal)

Fig. 18.26, Callister & Rethwisch 3e.

4.2 K

### Critical Properties of Superconductive Materials

TC = critical temperature - if T > TC not superconductingJC = critical current density - if J > JC not superconductingHC= critical magnetic field - if H > HCnot superconducting

Fig. 18.27, Callister & Rethwisch 3e.

### Meissner Effect

• Superconductors expel magnetic fields

• This is why a superconductor will float above a magnet

normal

superconductor

Fig. 18.28, Callister & Rethwisch 3e.

• Research in superconductive materials was stagnant for many years.

• Everyone assumed TC,max was about 23 K

• Many theories said it was impossible to increase TC beyond this value

• 1987- new materials were discovered with TC > 30 K

• ceramics of form Ba1-x Kx BiO3-y

• Started enormous race

• Y Ba2Cu3O7-xTC = 90 K

• Tl2Ba2Ca2Cu3Ox TC = 122 K

• difficult to make since oxidation state is very important

• The major problem is that these ceramic materials are inherently brittle.

### Summary

• A magnetic field is produced when a current flows through a wire coil.

• Magnetic induction (B):

-- an internal magnetic field is induced in a material that is situated within an external magnetic field (H).

-- magnetic moments result from electron interactions with the applied magnetic field

• Types of material responses to magnetic fields are:

-- ferrimagnetic and ferromagnetic (large magnetic susceptibilities)

-- paramagnetic (small and positive magnetic susceptibilities)

-- diamagnetic (small and negative magnetic susceptibilities)

• Types of ferrimagnetic and ferromagnetic materials:

-- Hard: large coercivities

-- Soft: small coercivities

• Magnetic storage media:

-- particulate g-Fe2O3 in polymeric film (tape)

-- thin film CoPtCr or CoCrTa (hard drive)