Magnetic Materials Science How magnets help us explore and
record the world Caroline Ross Professor, Materials Science and
Engineering, MIT
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My Aim Today - show how we design (magnetic) materials for
particular functions - show one persons random walk through science
and engineering. natural lodestone a hard drive
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What do you think of when you think of a magnet? N S solenoid
fridge magnets? electromagnets? compass needles? toys? Maxwells
equations? 78-yr old magnetic man
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William Gilbert 1600 De Magnete, Magneticisque Corporibus, et
de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on
That Great Magnet the Earth) Gilbert, a physician, postulated that
the earth was a giant magnet
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Where does the magnetism come from? Spinning electron has 1 B
(Bohr magneton) of magnetic moment. Iron atom (Z = 26) 1s 2 2s 2 2p
6 3s 2 3p 6 3d 6 4s 2 The 3d electrons are mostly unpaired: 5 have
spin up, one has spin down. So we might expect 4 B. Iron metal The
3d and 4s bands hybridize. We have 3d 7.05 4s 0.95. We end up with
2.2 B per Fe. The moments all align parallel in the crystal.
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Meteorite, FeNi (a ferromagnet) Lodestone, Fe 3 O 4 (a
ferrimagnet) What are the naturally occuring magnetic materials?
The strong tendency to magnetic alignment is due to a quantum
mechanical exchange interaction.
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What are non-magnetic materials? CoO, Cr (an antiferromagnet
Neel, 1948) Water, Silicon, Gold, etc. (a diamagnet Faraday, 1845)
Organometallics, Oxygen, Al, Mn anything with an unpaired spin but
no magnetic order is a paramagnet
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A paradox - How can a magnetic material not have a magnetic
moment? Pierre-Ernest Weiss, 1865-1940 proposed domains in 1907
Magnetic materials can form domains, so the net magnetism cancels
out. fys.uio.no commons.wikimedia.org
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When we apply a magnetic field, we move the domain walls, and
we get magnetic hysteresis
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Whats my job as a materials scientist? We want to control the
properties of magnets and find ways of manufacturing them This
means designing new magnetic materials. Here are four applications
we will discuss today: Permanent magnets for motors, and soft
magnets for transformers MRI contrast agents and hyperthermia
treatments for cancer Hard disk drives and tapes Nanoelectronics
beyond CMOS A ferrofluid
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Soft magnets for transformer cores Michael Faraday and James
Henry in 1831 discovered electromagnetic induction, the basis of a
transformer. Induction: A changing magnetic flux causes a voltage
in a coil
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A transformer core needs a soft magnet that has little
hysteresis, because its magnetization needs to change frequently -
Fe-3%Si, - amorphous Fe alloys, - soft cubic ferrites (Zn,Mn)Fe 2 O
4
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Permanent magnets for DC motors When the coil is powered, a
magnetic field is generated around the armature. The left side of
the armature is pushed away from the left magnet and drawn toward
the right, causing rotation. A moving electron (or a current) in a
magnetic field experiences a force F = BiL.
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A DC motor needs a hard magnet Ferrite Alnico Samarium-cobalt
Neodymium-iron-boron Alnico is ~58%Fe, 30%Ni, 12%Al and some Co.
Cooling from 1250C makes the body-centered cubic alloy undergo
spinodal decomposition to give 30 nm wide [100]- oriented FeNiCo
rods in a nonmagnetic matrix SmCo 5 and Nd 2 Fe 14 B have strong
crystal anisotropy making them magnetically hard. 200 nm
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Medical Magnets Magnetic resonance imagingHyperthermia therapy
The magnetic moments of H in water are aligned by a strong magnetic
field. An rf field tuned to the resonance frequency of the protons
flips their magnetic moments. As they relax, photons are produced
which are characteristic of the tissue. Paramagnetic Gd 3+ or Mn,
or superpara- magnetic iron oxide, act as contrast agents. Magnetic
particles, typically iron oxide, exposed to an ac field produce
heat by hysteresis loss. Warming tumor cells to ~42C causes them to
be more susceptible to e.g. chemotherapy.
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How do we make magnetic nanoparticles? Fe 2+ + Fe 3+ + OH - Fe
3 O 4 + H 2 O Surface coating necessary to: prevent agglomeration
evade opsonization and recognition by bodys reticulo-endothelial
system improve monodispersity provide functional groups for further
derivation, when necessary Columbia OBrien in solution; with
ligands to coat surface
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Magnetic Data Storage The first magnetic data storage, 1899 The
Telegraphone recorded data on a wire using induction, and read it
back using a loudspeaker. The first magnetic recording is of
Emperor Franz Josef of Austria at the 1900 World Exposition.
http://www.youtube.com/watch?v=pzrB_pwi2TM&noredirect=1
Valdemar Poulson
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What Emperor Franz Joesph said "...Erfindung hat mich sehr
interessiert, und ich danke sehr fr die Vorfhrung derselben." (the
sentence seems to be a bit cut off at the beginning) Translated: "I
found (this) invention very interesting and thank a lot for its
demonstration."
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Data Storage on Disks 1956 IBM RAMAC fifty 24 inch platters,
with a total capacity of five million characters, $50k
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Seagate ST4053 40 MByte 5 1/4 inch, full-height "clunker circa
1987, $435 Hitachi Ultrastar 3.5 drive, 2014, 4 TB $350 Data
Storage Density on Disks and Flash Memory Hard drive Flash memory 1
Tb/in 2
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A beautiful piece of engineering Hard drives combine mechanics
and electromagnetism. Disks spin ~10,000 rpm, store a bit in ~20 nm
x 80 nm area, and read back at ~6 GHz. The head flies a few nm
above the surface.
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Perpendicular magnetic recording Si/Ti/CoCrPt Perpendicular
magnetic anisotropy allows high density. First products in 2007.
Demos now ~1 Tbit/in 2. CoCrPt alloy with c-axis out of plane.
Requires a soft magnetic underlayer.
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Conclusions Magnetic materials have come a long way, from
natural magnets (meteorites and lodestone) to a vast range of
magnetic materials -hard and soft magnets for motors and
transformers -nanoparticle magnets for MRI and cancer treatments
-precisely engineered thin film magnets for data storage in hard
disks -multifunctional magnets for magnetoresistive, magnetooptical
or multiferroic devices They all show the relationship between the
properties of a material, its structure, and its processing, that
is the core of materials science and engineering. My email:
[email protected] N S