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. natural lodestone a hard drive

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

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

Where does the magnetism come from? Spinning electron has 1 µ 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 4µ B. Iron metal The 3d and 4s bands hybridize. We have 3d s We end up with 2.2µ B per Fe. The moments all align parallel in the crystal.

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’.

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

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

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

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

“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

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

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.

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 SmCo 5 and Nd 2 Fe 14 B have strong crystal anisotropy making them magnetically hard. 200 nm

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 ~42˚C causes them to be more susceptible to e.g. chemotherapy.

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 body’s reticulo-endothelial system improve monodispersity provide functional groups for further derivation, when necessary Columbia – O’Brien in solution; with ligands to coat surface

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. Valdemar Poulson

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."

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

Seagate ST 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

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.

Perpendicular magnetic recording Si/Ti/CoCrPt Perpendicular magnetic anisotropy allows high density. First products in Demos now ~1 Tbit/in 2. CoCrPt alloy with c-axis out of plane. Requires a soft magnetic underlayer.

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 N S