magnetism - a mysterious force of nature and some of its consequences
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Magnetism - a mysterious force of nature and some of its consequences. Peter Day Royal Institution and UCL, London. Magnets in Nature A magnetic rock – lodestone (Fe 3 O 4 ). The name ‘magnet’ comes from Magnesia in Greece, where lodestone is found. Picture from Wikipedia. - PowerPoint PPT PresentationTRANSCRIPT
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Magnetism
- a mysterious force of natureand some of its consequences
Peter DayRoyal Institution and UCL,
London
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Magnets in NatureA magnetic rock – lodestone
(Fe3O4)
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The name ‘magnet’ comes from Magnesia in Greece, where lodestone is found
Picture from Wikipedia
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Magnets in NatureMagnetite particles in
bacteria
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In 1851 Michael Faraday gave the Christmas Lectures for young people at the Royal Institution
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He wanted to illustrate chemistry and physics with an everyday (1851) object
What did he choose?
A candle!
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The Christmas Lectures today
What everyday object would Faraday use now?
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An iPod??
And how does it work?
MAGNETISM!
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The first serious experimentalwork on magnetism:William Gilbert, 1628
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Beating wrought iron induces magnetism
Magnetic domains align in the earth’s field when the material is malleable
From Gilbert, de Magnete
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Some other insights from Gilbert
Lodestone loses its magnetism on heating: “fire destroys the magnetic virtues in a stone, not because it takes away any parts specially attractive, but because the consuming force of the flame mars by the demolition of the material the form of the whole”
Only a few solids are magnetic “why has nature been so stingy as to provide only a small number…”
A piece of wrought iron has a North and South pole and, if cut in half, each of the fragments has, too
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Atomic magnets- how they line up
Ferromagnet
Antiferromagnet
Ferrimagnet
Canted antiferromagnet
Helimagnet
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Further insights came from this man
Michael FaradayMagnetism and electricity- electromagnetic induction electric motors and generators
Lines of magnetic flux- concept of ‘fields’ (Maxwell)
Paramagnetism and diamagnetism- universal property of matter
Magnetism and light- light as electromagnetic radiation
1791- 1867
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Moving a magnet through a coil generates electricity
Faraday’s sketch from his notebook
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The apparatus and how it works
From the Faraday Museum,Royal Institution, London
The world’s first electricity generator!
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Electromagnetic rotation
Faraday’s notebook
Experimental setup
The world’s first electric motor!
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Lines of magnetic flux
Iron filings experiment Published sketch
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Two demonstration experiments
Electromagnet attracts manyobjects – even through glass!
Throwing iron filings at a box containing a hidden magnet
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Paramagnetism and diamagnetism
Search for diamagnetism:Faraday’s ‘great electromagnet’
Magnetic flux lines inpara- & diamagnets
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The world’s biggest superconducting electromagnet
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Faraday’s ‘Magnetic Laboratory’
Watercolour by Charlotte Moore
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Magnetism and light
Faraday’s notebook Arrangements of magnet pole-pieces and glass
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A modern magneto-optic light modulator based on the Faraday
effect
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What kind of materials behave as ferro- or
ferrimagnets?• A few metallic elements – iron, nickel
• Intermetallic compounds – LaCo5
• Ternary oxides (ferrimagnets) – Magnetite (Fe3O4), Garnets (Y3Fe5O12),
Magnetoplumbites (PbFe12 O19)
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These are all opaque metals or semiconductors
Are there any magnets that are electrical insulators and hence
transparent? Very few!
In 1976 we made some – they were also soluble because they contain molecules
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A2CrCl4: transparent ferromagnets
Bellitto and Day 1976
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Solvent-soluble ferromagnets (CnH2n+1NH3)2CrX4
Bellitto and Day 1978
Magnetisation
Hysteresis
Crystal structure
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Prussian Blue (first made in 1704) was first shown to be a ferromagnet in the 1970s
Guedel et al, 1973; Mayoh and Day, 1975
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The first purely organic ferromagnet p-nitrophenyl-nitronylnitroxide
Kinoshita et al 1991
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Single molecule magnets
Fe8 Mn12
Wieghardt 1984Gatteschi 1993
Lis 1980Sessoli & Gatteschi 1993
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Mn12 is a ‘hard’ magnet
-3 -2 -1 0 1 2 3
-20
-10
0
10
20
T=2.1K
MA
GN
ET
IZA
TIO
N ( B
)
MAGNETIC FIELD (T)
Bistability : in zero field the magnetisation can be positive or negative depending on the sample history
Coercive Field
Remnant Magnetisation
Sessoli & Gatteschi 1993
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0
0.2
0.4
0.6
0.8
1
0.01 0.1 1 10 100 1000
B
B
B
B
B
B
B
B
B
B
B
B
B
B
M/M
s
t (s)
1.5 K
1.6 K
1.7 K
1.8 K
1.9 K
2.8 K
2.7 K
2.6 K
2.5 K2.3 K
2.2 K
2.1 K
2.4 K
2.0 K
Magnetisation vs. time at low temperature
Mn12
The magnetisation relaxes very slowly at low temperature
Can this molecule be used to store information??
Wernsdorfer et al 2000
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Another way to store information using magnetism
Grunberg 1986
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The two magnetic arrangements have different electrical
resistance
Giant magnetoresistance
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And this is the basis of the iPod!
Three cheers for magnetism!