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Magnetism

GLY 4200 Fall, 2012

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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

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What is Magnetism?

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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 BC

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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

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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

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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

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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 Fe2+Fe2

3+O4) and pyrrhotite (Fe1-xS, x = 0.0 → 0.2)

• Curie temperature for magnetite is 85 B C, much lower than for metallic iron

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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

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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

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Diagram of Magnetometer Tow

• Catalina aircraft fitted with a magnetometer

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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

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Paleomagnetism

• Ferrimagnetic minerals are permanently magnetized

• Study of natural remnant magnetism of rocks yields a record of the earth’s magnetic field through time

• This reveals polarity reversals, and can aid in the study of plate motions

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Polarity Reversal Record

• Paleomagnetic record of 0-4 MYBP