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# Chapter 18: Magnetic Properties - PowerPoint PPT Presentation

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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• 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?

• 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

• 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...

(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.

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

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)

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

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

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:

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

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.

• 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-x TC = 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.

• 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)