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Magnetic Materials Science How magnets help us explore and record the world Caroline Ross

Magnetic Materials Science How magnets help us explore and record the world Caroline Ross Professor, Materials Science and Engineering, MIT. My Aim Today show how we design (magnetic) materials for particular functions show one person’s random walk through science and engineering.

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Magnetic Materials Science How magnets help us explore and record the world Caroline Ross

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  1. Magnetic Materials Science How magnets help us explore and record the world Caroline Ross Professor, Materials Science and Engineering, MIT

  2. My Aim Today • show how we design (magnetic) materials for particular functions • show one person’s random walk through science and engineering. a hard drive natural lodestone

  3. What do you think of when you think of a magnet? fridge magnets? electromagnets? compass needles? toys? Maxwell’s equations? N S 78-yr old ‘magnetic man’ solenoid

  4. 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

  5. Where does the magnetism come from? Spinning electron has 1 µB (Bohr magneton) of magnetic moment. Iron atom (Z = 26) 1s2 2s2 2p6 3s2 3p6 3d6 4s2 The 3d electrons are mostly unpaired: 5 have ‘spin up’, one has ‘spin down’. So we might expect 4µB. Iron metal The 3d and 4s bands hybridize. We have 3d7.05 4s0.95. We end up with 2.2µB per Fe. The moments all align parallel in the crystal.

  6. What are the naturally occuring magnetic materials? Meteorite, FeNi (a ferromagnet) The strong tendency to magnetic alignment is due to a quantum mechanical ‘exchange interaction’. Lodestone, Fe3O4 (a ferrimagnet)

  7. What are “non-magnetic” materials? Water, Silicon, Gold, etc. (a diamagnet –Faraday, 1845) Organometallics, Oxygen, Al, Mn … anything with an unpaired spin but no magnetic order is a paramagnet CoO, Cr (an antiferromagnet –Neel, 1948)

  8. A paradox - How can a magnetic material not have a magnetic moment? fys.uio.no commons.wikimedia.org Magnetic materials can form domains, so the net magnetism cancels out. Pierre-Ernest Weiss, 1865-1940 proposed domains in 1907

  9. When we apply a magnetic field, we move the domain walls, and we get magnetic hysteresis…

  10. What’s 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

  11. “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

  12. 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)Fe2O4

  13. 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.

  14. 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 1250˚C makes the body-centered cubic alloy undergo spinodal decomposition to give 30 nm wide [100]-oriented FeNiCo rods in a nonmagnetic matrix SmCo5 and Nd2Fe14B have strong crystal anisotropy making them magnetically hard. 200 nm

  15. Medical Magnets Magnetic resonance imaging Hyperthermia 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 Gd3+ 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 ~42˚C causes them to be more susceptible to e.g. chemotherapy.

  16. How do we make magnetic nanoparticles? Fe2+ + Fe3+ + OH- Fe3O4 + H2O in solution; with ligands to coat surface Columbia – O’Brien Surface coating necessary to: • prevent agglomeration • evade opsonization and recognition by body’s reticulo-endothelial system • improve monodispersity • provide functional groups for further derivation, when necessary

  17. 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

  18. What Emperor Franz Joesph said • "...Erfindung hat mich sehr interessiert, und ich danke sehr für die Vorführung 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."

  19. Data Storage on Disks 1956 – IBM RAMAC fifty 24 inch platters, with a total capacity of five million characters, $50k

  20. Data Storage Density on Disks and Flash Memory 1 Tb/in2 Hard drive Flash memory Seagate ST4053 40 MByte 5 1/4 inch, full-height "clunker” circa 1987, $435 Hitachi Ultrastar 3.5” drive, 2014, 4 TB $350

  21. 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.

  22. Perpendicular magnetic recording Si/Ti/CoCrPt • Perpendicular magnetic anisotropy allows high density. First products in 2007. Demos now ~1 Tbit/in2. • CoCrPt alloy with c-axis out of plane. Requires a soft magnetic underlayer.

  23. 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: caross@mit.edu N S

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