Magnetism

1
Magnetism

• GLY 4200
• Fall, 2012

2
Early Observations of Magnetism

• Ancient Greeks, especially those near the city of
  Magnesia, and Chinese, observed natural stones
  that attracted iron
• The naturally magnetic stones are called
  lodestone
• The Chinese discovered a steel needle stroked by
  a lodestone became magnetic, and, if suspended,
  pointed N-S

3
What is Magnetism?

• Although discovered relatively early in mans
  history, and exploited, the causes of magnetism
  were not understood

4
Orbiting Electrons

• Moving electrical currents generate magnetic
  forces
• This includes electrons orbiting and spinning
  around a nucleus
• Each orbiting electron possesses a magnetic
  moment equal to 1 Bohr magnetron (µB), or 0.927 x
  10-23 Am2 (Amps meter2)

5
Isolated Ions

• Net magnetic moment is equal to the sum of
• Orbital contributions
• Spin contributions
• Filled orbitals give a net contribution of zero
  to the magnetic moment since the two electrons
  orbit and spin oppositely

6
Net Magnetic Moments

• Generated only in atoms or ions with incomplete
  electronic shells
• Most important subshells likely to be
  incompletely filled are the 3d (first transition
  row) and the 4f (rare earth elements)
• The second and third transition rows (4d and 5d
  electrons) also produce magnetic moments but the
  elements, and hence the minerals, are rare

7
Three d Electrons

• Three d electrons have large spin and relatively
  low orbital contributions to magnetic moments
• In compounds the orbital contribution is affected
  by, and largely negated by, bonding to other ions
• Since the 4s electrons are outside the 3d, the 3d
  electrons are partially shielded and the orbital
  contribution will not be entirely negated

8
Spin Contribution

• Spin contribution is largely responsible for the
  3d electrons contribution to the magnetic moment
  and is proportional to the number of unpaired d
  electrons

9
Four f Electrons

• In 4f electron containing elements, the electrons
  are well-shielded by outer electrons
• The 4f electrons are not involved in bonding and
  both orbital and spin effects contribute to the
  total magnetic moment

10
Magnetic Susceptibility

• An aggregate of ions or atoms may behave much
  differently than an individual ion
• Magnetic Susceptibility is the ratio of induced
  magnetization to the strength of the external
  magnetic field causing the induced magnetization
• Magnetic susceptibility may be grouped into
  different classes of behavior

11
Diamagnetism

• Minerals possessing ions with totally paired
  electron spins
• No transition elements are present, and the net
  magnetic moment is zero
• In a strong magnetic field diamagnetic materials
  exhibit a small negative magnetic susceptibility,
  which means they are weakly repelled from the
  magnet

12
Paramagnetism

• Transition metals ions are present but the
  magnetic moments are randomly distributed
• Net field is zero, although an external field
  will produce some alignment of dipoles, which
  disappears when the external field is removed
• Alignment of the magnetic dipoles produces a
  small positive magnetic susceptibility and these
  minerals are attracted to a magnet in a strong
  magnetic field
• Example olivine (Mg, Fe)2SiO4

13
Ferromagnetism

• Adjacent moments are aligned
• After an external field is applied the dipoles
  interact and the field remains locked in
• The magnetism is due to unbalanced electron spin
  in the inner orbits of the elements concerned
• The ionic spacing in ferromagnetic crystals is
  such that very large forces, called exchange
  forces, cause the alignment of all atoms to give
  highly magnetic domains

14
Making a Magnet

• In unmagnetized metal these domains are randomly
  oriented
• After a strong magnetic field is applied the
  domains align and the material remains a strong
  magnet after the external field is removed
• Examples of ferromagnetic materials are the
  metals cobalt and nickel, and alloys such as
  alnicol

15
Curie Temperature

• Upon heating the domains may become randomly
  aligned once again
• This transition to a paramagnetic state is called
  the Curie temperature, after Pierre Curie, who
  was instrumental in elucidating the behavior of
  paramagnetic materials
• In metallic iron the Curie temperature is 770
  C

16
Antiferromagnetism

• Alternate atoms have oppositely directed moments
• Magnetic susceptibility is low but increases with
  increasing T up to the Néel temperature
• Above this temperature the susceptibility falls
  and the material is paramagnetic
• Examples include Cr metal, and compounds like
  MnO, MnS, and FeO

17
Louis E.F. Néel

• The Néel temperature is named after L.E.F. Néel,
  who discovered the phenomenon of the transition
  from antiferromagnetism to paramagnetism in 1930
• Born 1904, died 2000
• Nobel prize in physics, 1970

18
Ferrimagnetism

• Adjacent atoms have antiparallel alignment, but
  the magnitude of the magnetic moments of
  different ions is different
• Cancellation is incomplete and strong magnetism
  may exist
• Alternatively, the number of magnetic moments
  aligned in one direction may be different than in
  another direction
• Ferrimagnetic materials may have magnetism
  similar to that of ferromagnetic materials

19
Incorrect Identification

• Some minerals have been incorrectly described in
  the literature as being ferromagnetic when in
  fact they are ferrimagnetic
• Examples include ilmenite FeTiO3, magnetite
  (Fe3O4 or Fe2Fe23O4) and pyrrhotite (Fe1-xS, x
  0.0 ? 0.2)
• Curie temperature for magnetite is 85 C, much
  lower than for metallic iron

20
Magnetic Separation

• Magnetic separation, based on the differing
  magnetic susceptibilities of different minerals,
  is used in processing minerals since many
  minerals, especially those containing iron, are
  attracted to or repelled from a magnet in a
  strong magnetic field
• Magnetic separation is used in both laboratory
  and commercial scales for mineral separation
• Picture Frantz laboratory magnetic mineral
  separator

21
Aerial Remote Sensing of Magnetism

• An airplane flies over an area towing a
  magnetometer, which measures local perturbations
  of the earth's magnetic field
• These aircraft fly low (100 to 300 meters) and
  use highly sensitive magnetometers

22
Diagram of Magnetometer Tow

• Catalina aircraft fitted with a magnetometer

23
Sulfide Ore Bodies

• Many sulfide ore bodies are associated with
  magnetite and, although the magnetite itself may
  have no economic value, the sulfides often are
  valuable
• This method is rapid and relatively cheap,
  especially in areas of rough terrain
• Any positive magnetic anomalies must be verified
  by subsequent geophysical and geochemical
  exploration

24
Paleomagnetism

• Ferrimagnetic minerals are permanently magnetized
• Study of natural remnant magnetism of rocks
  yields a record of the earths magnetic field
  through time
• This reveals polarity reversals, and can aid in
  the study of plate motions

25
Polarity Reversal Record

• Paleomagnetic record of 0-4 MYBP