Chapter 21: 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?

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

Chapter 21 - 1 Magnetic medium layer is usually a cobalt-based alloy

Hard disk drive for computer

Chapter 21 - 2 1. Introduction 2. Basic Concepts 3. Diamagnetism and Paramagnetism 4. Ferromagnetism 5. Antiferromagnerism and Ferrimagnetism 6. The Influence of Temperature on Magnetic Behavior 7. Domains and Hysteresis 8. Magnetic Anisotropy 9. Soft Magnetic Materials 10.Hard Magnetic Materials 11.Magnetic Storage 12.Superconductivity

Chapter 21 - 3 1. Introduction: Magnetism • Magnetism is a class of physical phenomena that are mediated by magnetic fields. – Exert an attractive or repulsive force or influence on other materials – Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. • Magnetic devices: – Compass, power generators, transformers, electrical motors, radio, telephone, computer, video, … • Magnets: iron, some steels, lodestone, … • All substances affected by magnetic field

Chapter 21 - 4 2. Basic Concepts

Magnetic dipole moment

Magnetic field lines of force

Fig_21_02

Fig_21_01 Chapter 21 - Generation of a Magnetic Field - Vacuum

• 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 I (tesla) • Computation of the applied magnetic field, H:

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

B0 = μ0H permeability of a vacuum (1.257 x 10-6 Henry/m) Chapter 21 - 6 Generation of a Magnetic Field -- within a Solid Material

• A magnetic field is induced in the material B B = Magnetic Induction (tesla) applied inside the material magnetic field H B = μH permeability of a solid current I

• Relative permeability (dimensionless)

Chapter 21 - 7 Generation of a Magnetic Field - within a Solid Material (cont.)

• Magnetization M = cmH Magnetic susceptibility (dimensionless)

• B in terms of H and M B = μ0H + μ0M • Combining the above two equations:

B = μ0H + μ0 cmH B cm > 0 = (1 + cm)μ0H permeability of a vacuum: vacuum cm = 0 (1.26 x 10-6 Henry/m) c m < 0 cm is a measure of a material’s magnetic response relative to a H vacuum

Chapter 21 - 4 Table_21_01

Chapter 21 - Origins of Magnetic Moments • Magnetic moments arise from electron motions and the spins on electrons.

magnetic moments

electron electron

nucleus spin

electron orbital electron motion spin • Net atomic magnetic moment: -- sum of moments from all electrons.

• Four types of response...

Chapter 21 - 10 Bohr magnetron

• The most fundamental magnetic moment -24 2 – μB , of magnitude 9.27 x10 A.m .

• For each electron in an atom, the spin

magnetic moment is ± μB (plus for spin up, minus for spin down).

• Furthermore, the orbital magnetic moment

contribution is equal to ml μ B, ml being the magnetic quantum number of the electron.

Chapter 21 - 11 3. Types of Magnetism

B = (1 + cm)μ0H (3) Ferromagnetic e.g. ferrite(α) Fe, Co, Ni, Gd

(Magnetic flux density) (4) ferrimagnetic e.g. Fe3O4, NiFe2O4 c 6 ( m as large as 10 !)

-4

(2) paramagnetic ( cm ~ 10 ) (tesla)

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

vacuum (c m = 0) -5

(1) diamagnetic (c m ~ -10 ) e.g., Al2O3, Cu, Au, Si, Ag, Zn H (ampere-turns/m) (Magnetic field strength)

Chapter 21 - 12 Table_21_02

Chapter 21 - Magnetic Responses for 4 Types No Applied Applied

Magnetic Field (H = 0) Magnetic Field (H)

(1) diamagnetic

none

opposing

(2) paramagnetic

aligned

random

(3) ferromagnetic

(4) ferrimagnetic aligned aligned

Chapter 21 - 14 4. Ferromagnetism

• Metals possess a permanent magnetic moment in the absence of an external field and manifest very large and permanent magnetizations. – displayed by the transition metals iron (as BCC -ferrite), cobalt, nickel, and gadolinium (Gd) – Net magnetic moment per atom for Fe, Co and Ni are

2.22, 1.72 and 0.6 μB • Magnetic susceptibilities as high as 106 are possible for ferromagnetic materials. – result from atomic magnetic moments due to uncanceled electron spins as a consequence of the electron structure.

Chapter 21 - 15 5. Antiferromagnetism (MnO) and Ferrimagnetism (Fe3O4)

Fig_21_08 Fig_21_09

Table_21_03 Chapter 21 - Table_21_04

Chapter 21 - 6. Temperature on magnetic behavior

• Curie temperature (Tc): with increasing temp., the saturation magnetism decrease gradually and then abruptly drops to zero.

Chapter 21 - 18 7. Domains and Hysteresis • As the applied field (H) increases the magnetic domains change shape and size by movement of domain boundaries. B sat

H

) H B “ ” H • Domains with Fig_21_12 aligned magnetic H moment grow at

Magnetic expense of poorly induction( aligned ones! H

0 Applied Magnetic Field (H)

H = 0 Chapter 21 - 19 7. Hysteresis and Permanent Magnetization • The magnetic hysteresis phenomenon

B Stage 2. Apply H, Stage 3. Remove H, alignment align domains remains! => permanent magnet!

H Stage 4. Coercivity, HC Stage 1. Initial (unmagnetized state) Negative H needed to demagnitize!

Stage 6. Close the Stage 5. Apply -H, hysteresis loop align domains

Chapter 21 - 20 8. Magnetic Anisotropy • The dependence of magnetic behavior on crystallography orientation

Fig_21_18

Chapter 21 - 21 Fig_21_17 9-10. Hard and Soft Magnetic Materials

Hard magnetic materials: B -- large coercivities -- used for permanent magnets -- add particles/voids to

inhibit domain wall motion

-- example: tungsten steel -- Soft Hc = 5900 amp-turn/m) H Soft magnetic materials: -- small coercivities -- used for electric motors -- example: commercial iron 99.95 Fe

Chapter 21 - 22 Chapter 21 - 23 Chapter 21 - 24 11. Magnetic Storage • Digitized data in the form of electrical signals are transferred to and recorded digitally on a magnetic medium (tape or disk) • This transference is accomplished by a recording system that consists of a read/write head -- “write” or record data by applying a magnetic field that aligns domains in small regions of the recording medium -- “read” or retrieve data from medium by sensing changes in magnetization

Fig. 21.23, Callister & Rethwisch 9e.

Chapter 21 - 25 Magnetic Storage Media Types • Hard disk drives (granular/perpendicular media): -- CoCr alloy grains (darker regions)

separated by oxide grain boundary Fig. 21.24, Callister

segregant layer (lighter regions) & Rethwisch 9e. nm

(Courtesy of Seagate -- Magnetization direction of each Recording Media) grain is perpendicular to plane of 80 disk

• Recording tape (particulate media):

nm nm

~ 500 ~ ~500

-- Acicular (needle-shaped) -- Tabular (plate-shaped) ferromagnetic metal alloy ferrimagnetic barium-ferrite particles particles Chapter 21 - 26 12. Superconductivity

Found in 26 metals and hundreds of alloys & compounds Mercury

Copper (normal)

4.2 K

• TC = critical temperature = temperature below which material is superconductive

Chapter 21 - 27 Critical Properties of Superconductive Materials

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

Chapter 21 - 28 Meissner Effect

• Superconductors expel magnetic fields

normal superconductor

• This is why a superconductor will float above a magnet

Chapter 21 - 29 Chapter 21 - 30 Advances in Superconductivity

• 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-xKxBiO3-y – Started enormous race

• YBa2Cu3O7-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.

Chapter 21 - 31 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 γ-Fe2O3 in polymeric film (tape) -- thin film CoPtCr or CoCrTa (hard drive) Chapter 21 - 32