Ferromagnetism

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© 1998-2008 by Glenn Elert -- A Work in Progress
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Discussion

introduction

This section is intended to be a discussion of magnetism on the small scale, not just the magnetism of iron, nickel, and cobalt (although that will be its primary focus). Perhaps it should be titled "micromagnetism".

Everything's due to electron spin. Well, almost everything.

Types of Magnetic Behavior (in order of decreasing strength)
type spin alignment examples
ferromagnetic all spins align parallel to one another iron, cobalt, nickel, gadolinium, dysprosium, heusler alloys
ferrimagnetic most spins parallel to one another, some spins antiparallel magnetite (Fe3O4), yttrium iron garnet (YIG)
antiferromagnetic periodic parallel-antiparallel spin distribution chromium, FeMn, NiO
paramagnetic spins tend to align parallel to an external magnetic field oxygen, sodium, aluminum, calcium, uranium
diamagnetic spins tend to align antiparallel to an external magnetic field nitrogen, copper, silver, gold, water, organic compounds
superdiamagnetic all spins align antiparallel to an external field meisner effect in superconductors

alloys

Steel …

Basic Types of Steel (in order of increasing carbon content)
type composition magnetic? other characteristics
ferritic stainless Fe, Cr, C yes moderate corrosion resistance,
moderate durability
austenitic stainless Fe, Cr, Ni, C no high corrosion resistance,
moderate durability
martensitic stainless Fe, Cr, C yes moderate corrosion resistance,
high durability
nonstainless (high carbon) Fe, C yes low corrosion resistance,
high durability

Alloys made expressly for permanent magnets …

Ferromagnetic alloys made entirely of nonferrous metals …

magnetic recording

the basic mechanism


[magnify — rather large svg]

media formats

media shapes

substrates

ferromagnetic material

Common Materials Used for Magnetic Tape
type bias material comments
I "normal" gamma ferric oxide
(γ-Fe2O3)		 first commercially manufactured in 1937
II "high" chromium dioxide
(CrO2)		 later replaced by layers of ferric oxide (Fe2O3) and cobalt (Co) with similar magnetic characteristics
III   ferric chrome
(FeCr)		 quickly became obsolete
IV "metal" finely ground metallic iron later replaced by mixtures of finely ground iron and cobalt
n/a   barium ferrite
(BaFe12O19)		 magnetic stripes on bank and credit cards, high coercivity, less susceptible to accidental erasure

transition temperatures

The Curie temperature is named for the French physicist Pierre Curie (1859-1906), who discovered the laws that relate some magnetic properties to change in temperature in 1895.

The antiferromagnetic equivalent of the Curie Temperature is called the Néel Temperature in honor of the French physicist Louis Néel (1904-2000), who successfully explained antiferromagnetism in 1936.

Curie temperatures of selected
ferromagnetic materials		   Néel temperatures of selected
antiferromagnetic materials
elements TC (K)   material TN (K)
iron 1043   CoCl2 25
cobalt 1404   CoF2 38
nickel 628   CoO 291
gadolinium 289   chromium 475
erbium 32   Cr2O3 307
dysprosium 155   erbium 80
      FeCl2 70
ferrous compounds TC (K)   FeF2 79 - 90
barium ferrite 720   FeO 198
strontium ferrite 720   FeMn 490
Alnico 1160   α-Fe2O3 953
Alumel 436   MnF2 72 - 75
Mutamel 659   MnO 122
Permalloy 869   MnSe 173
Trafoperm 1027   MnTe 310 - 323
NdFeB 580   NiCl2 50
SmCo5 990   NiF2 78 - 83
Sm2Co17 1070   NiFeO 180
      NiO 533 - 650
nonferrous compounds TC (K)   TiCl3 100
CrO2 390   UCu5 15
CuAlMn3 ? V2O3 170
LaxCa1–xB6 900      
MnAs 318    
MnBi 633      
polymerized C60 ~500    

animal magnetism (magnetotaxis?)

Summary

Problems

practice

  1. Write something.
    • Answer it.
  2. Write something.
    • Answer it.
  3. Write something.
    • Answer it.
  4. Write something.
    • Answer it.

numerical

  1. problems

Resources

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