Introduction Force exerted by a magnetic field Current loops, torque, and magnetic moment

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Magnetism. Introduction Force exerted by a magnetic field Current loops, torque, and magnetic moment Sources of the magnetic field Atomic moments Magnetism in materials Types of magnetic material Hard disks Tipler Chapters 28,29,37. Dr Mervyn Roy, S6. Introduction. - PowerPoint PPT Presentation

Text of Introduction Force exerted by a magnetic field Current loops, torque, and magnetic moment

Slide 1Sources of the magnetic field
Atomic moments
(Shen Kua, China)
1200’s Compass revolutionises exploration by sea
1600’s William Gilbert discovers the Earth is a natural magnet
1800’s Connection between electricity and magnetism (Faraday, Maxwell)
Magnetism
~0.3 Gauss = 3£10-5 T ( 1 T = 1 N / (A m) )
0.5 to 1 T
Magnetism
force acts at right angles to both v and B
+ve charge
– no effect from B
– B induces circular motion
align n of current loop with B
Torque,
B fields exert forces on current carrying wires
current, i - moving charges.
l
B field exerts a force on a current carrying wire
i
i
n
B
F1
F2
currents produce a field
permeability of free space
field from current element:
i dl
atomic ‘current’
Orbital moment
Electron also has intrinsic angular momentum, ‘spin’
Spin moment
Total moment:
Use ‘LS’ coupling scheme (J = L-S , L+S)
Full electron shells have zero net orbital and spin angular momentum
For partially filled shells:
Spin
Magnetic
n=1
l=0
Eg. Iron [Ar] 4s2 3d6
Filled shells up to [Ar] don’t contribute. Filled 4s has zero ang. mom.
3d6 has
Moments in bulk materials
Typically in bulk materials the orbital moment is quenched (QM result).
The spin moment can give us a rough idea of ‘how magnetic’ a material is.
When considering the magnetic properties of a material we can think of the material as being made from a large number of current loops – atomic moments.
The question is: how are each of these moments oriented?
- It depends on the magnetic exchange interaction!
4 classes of material
Diamagnetic moments are zero
Ferromagnetic moments align
distance
exchange
Magnetism
magnetisation = net magnetic moment per unit volume
Material with a magnetisation M has an associated field
Applied fields tend to magnetise a material (align moments). Then, total field:
In para/diamagnetic materials, M proportional to
typically small ~ 10-5 but - as large as ~103 to105 in ferromagnets (not constant)
If all moments in material have aligned – material is saturated
Magnetism
atoms have zero angular momentum – ie. no permanent moment
When field applied, M is small and in opposite direction to Bapp
small and negative (superconductor = perfect diamagnet )
Paramagnets
atoms have angular momentum and permanent moments
When field applied small fraction of moments align, small and >0
Moments would ‘like’ to align but get randomised by thermal motion
Magnetisation depends on applied field and temperature
B
oM
Bapp
B
oM
Bapp
M
Bapp
Ms
Magnetism
Atoms have large permanent moments
Moments align in small fields. Alignment can persist when field is removed.
large, positive and field dependent,
Region over which moments are aligned is called a Magnetic Domain
Black = , White =
(www.aps.org/units/dmp/gallery/domains.cfm)
Domain structure in Fe thin film imaged with PEEM at DIAMOND
100 nm
40 nm
energy lost during magnetisation cycle = area enclosed by hysteresis curve
In magnetically soft materials Br is small
not much energy is dissipated during a cycle
Bapp
B
Use hard or soft ferromagnetic material depending on the application
Br
Bc
Bc
Magnetism
head flying height < 20 nm!
“1” stored as field reversal
Magnetism
- nanostructured films?
Hard Disks
Use weaker magnetic signals - but then:
- need smoother platter surfaces (nm) – glass?
Limit is set by exchange interaction / domain size. Manipulate this?
Use ordered array of individual nanoparticles – but then need to overcome super-paramagnetic limit
- stabilise iron nanocluster moment with Cr shell?
STM of Fe nanoclusters
Fe / Co nanostructured film
LUMPS
0
1
2
3
4
0
0.2
0.4
0.6
0.8
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