ch-18 [compatibility mode]

7/28/2019 Ch-18 [Compatibility Mode]

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ISSUES TO ADDRESS...

How do we measure magnetic properties?

What are the atomic reasons for magnetism?

Chapter 18: Magnetic Properties

Chapter 18 - 1

Materials design for magnetic storage.

How are magnetic materials classified?

What is the importance of superconducting magnets?


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Created by current through a coil:Applied Magnetic Field

Applied

magnetic field H

current I

N= total number of turnsL = length of each turn

Chapter 18 - 2

Relation for the applied magnetic field, H:

L

INH

=

applied magnetic fieldunits = (ampere-turns/m)

current


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Magnetic induction results in the materialResponse to a Magnetic Field

B= Magnetic Induction (tesla)

inside the material

Chapter 18 - 3

Magnetic susceptibility, (dimensionless)

current I

measures thematerial responserelative to a vacuum.

H

B

vacuum = 0 > 0

< 0


4/16

Measures the response of electrons to a magnetic field.

Electrons produce magnetic moments:

Magnetic Susceptibility

magnetic moments

electron electron

Chapter 18 - 4

Net magnetic moment:--sum of moments from all electrons.

Three types of response...

From Fig. 18.4Callisters MaterialsScience andEngineering,Adapted Version.

nucleus spin


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3 Types of Magnetism

Magnetic induction

B(tesla) ferromagnetic e.g. Fe3O4, NiFe2O4ferrimagnetic e.g. ferrite(), Co, Ni, Gd

(3)

( as large as 106 !)

permeability of a vacuum:(1.26 x 10-6 Henries/m)

HBo

+= )1(

Chapter 18 - 5

Plot adapted from Fig. 18.6, Callisters MaterialsScience and Engineering, Adapted Version.

Values and materials from Table 18.2 and discussion inSection 18.4, Callisters Materials Science andEngineering, Adapted Version.

Strength of applied magnetic field (H)(ampere-turns/m)

vacuum ( = 0)

-5diamagnetic ( ~ -10 )(1) e.g., Al2O3, Cu, Au, Si, Ag, Zn

(2) paramagnetice.g., Al, Cr, Mo, Na, Ti, Zr

( ~ 10-4)


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Magnetic Moments for 3 Types

From Fig. 18.5(a),Callisters MSEAdapted Version.

No AppliedMagnetic Field (H= 0)

AppliedMagnetic Field (H)

(1) diamagneticnone

opposing

Chapter 18 - 6

From Fig. 18.5(b),Callisters MSEAdapted Version.

(2) paramagnetic

random

aligned

From Fig. 18.7,Callisters MSEAdapted Version.

(3) ferromagneticferrimagnetic

aligned

aligned


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As the applied field (H) increases...--the magnetic moment aligns with H.

From Fig. 18.13,Callisters MaterialsScience andEngineering, AdaptedVersion.(Fig. 18.13 adapted from

Ferro- & Ferri-Magnetic Materials

(B)

Bsat

H

H

Chapter 18 - 7

. . ya an . ew-

Hughes, Metals,Ceramics, andPolymers, CambridgeUniversity Press, 1974.)

Applied Magnetic Field (H)

Magnetic

induction

0

H= 0

H

H

aligned magneticmoment grow atexpense of poorlyaligned ones!


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From Fig. 18.14,Callisters MSE Adapted

Version.

Permanent Magnets

Applied MagneticField (H)

1. initial (unmagnetized state)

B Process: 2. apply H, causealignment

4

Negative H needed to demagnitize!

. Coercivity, HC

3. remove H, alignment stays!=> permanent magnet!

Chapter 18 - 8

large coercivity--good for perm magnets

--add particles/voids to

make domain wallshard to move (e.g.,tungsten steel:Hc = 5900 amp-turn/m)

Hard vs Soft Magnets

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

From Fig. 18.19, CallistersMSE Adapted Version.(Fig. 18.19 from K.M. Ralls,T.H. Courtney, and J. Wulff,Introduction to MaterialsScience and Engineering, JohnWiley and Sons, Inc., 1976.)

Applied MagneticField (H)

B

So

ft


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Information is stored by magnetizing material.

Image of hard drive courtesy

Head can...-- apply magnetic field H &

align domains (i.e.,magnetize the medium).-- detect a change in the

magnetization of the recording head

recording medium

From Fig. 18.23, Callisters MSE

Magnetic Storage

Chapter 18 - 9

art n en.

Reprinted with permissionfrom International BusinessMachines Corporation.

.

Two media types:

Adapted Version. (Fig. 18.23 from J.U.

Lemke, MRS Bulletin, Vol. XV, No. 3, p.31, 1990.)

-- Particulate: needle-shaped-Fe2O3. +/- mag. moment

along axis. (tape, floppy)From Fig. 18.24,Callisters MSEAdapted Version.(Fig. 18.24courtesy P. Raynerand N.L. Head, IBMCorporation.)

~2.5m

From Fig. 18.25(a),Callisters MSE AdaptedVersion. (Fig. 18.25(a) fromM.R. Kim, S. Guruswamy,and K.E. Johnson, J. Appl.Phys., Vol. 74 (7), p. 4646,1993. )

~120nm

--Thin film: CoPtCr or CoCrTaalloy. Domains are ~ 10 - 30 nm!

(hard drive)


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Superconductivity

Copper(normal)

Hg

Chapter 18 -10

Tc= temperature below which material is superconductive

= critical temperature

4.2 KFrom Fig. 18.26

Callisters Materials Scienceand Engineering, AdaptedVersion.


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Limits of Superconductivity

26 metals + 100s of alloys & compounds

Unfortunately, not this simple:

Jc= critical current density if J> Jcnot superconductingHc= critical magnetic field if H> Hcnot superconducting

Chapter 18 - 11

Hc= Ho(1- (T/Tc)2)

From Fig. 18.27Callisters MaterialsScience and Engineering,Adapted Version.


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Advances in Superconductivity

This research area was stagnant for many years.

Everyone assumed Tc,maxwas about 23 K

Many theories said you couldnt go higher 1987- new results published for Tc> 30 K

ceramics of form Ba1-x Kx BiO3-y

Chapter 18 -12

Started enormous race. Y Ba2Cu3O7-x Tc= 90 K

Tl2Ba2Ca2Cu3Ox Tc= 122 K

tricky to make since oxidation state is quite important

Values now stabilized at ca. 120 K


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Meissner Effect

Superconductors expel magnetic fields

Chapter 18 -13

This is why a superconductor will float above amagnet

normal superconductor From Fig. 18.28Callisters MaterialsScience and Engineering,Adapted Version.


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Current Flow in Superconductors

Type I current only in outer skin

- so amount of current limited

Type II current flows within wire

Chapter 18 -14

M

H

Type I

Type II

completediamagnetism mixed

state

HC1 HC2HCnormal


15/16

Superconducting Materials

X

X

X

X

X

Ba BaY

CuO2 planes

linearchains

Cu

OCu

Chapter 18 -15

Vacancies (X) provide electron coupling between CuO2 planes.

X

X

YBa2Cu

3O

7

(001) planes


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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)

Summary

Chapter 18 -16

-- paramagnetic (poor magnetic induction)-- diamagnetic (opposing magnetic moment)

Hard magnets: large coercivity. Soft magnets: small coercivity.

Magnetic storage media:-- particulate -Fe2O3 in polymeric film (tape or floppy)

-- thin film CoPtCr or CoCrTa on glass disk (hard drive)

ch-18 [compatibility mode]

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