Emerging Technologies of Computation

Montek Singh

COMP790-084Sep 6, 2011

Basics of magnetism

Nanomagnets and their coupling

Next class:◦ Challenges and Benefits◦ Open questions

Today: Magnetic Quantum Dot Cellular Automata (MQCA)

Basics of magnetism

Magnetic domains

Magnetic domains regions within magnetic material

with uniform magnetization… … separated by “domain walls” magnetic moments all exactly

aligned responsible for magnetic behavior of “ferromagnetic” materials: iron, nickel, etc.

What causes them? magneto-static energy is minimized

Other energy terms: magnetoelastic anisotropy magnetocrystalline anisotropy Zeeman energy

Basics of magnetism Magnetic domains

◦ more domains reduces magnetostatic energy

◦ but increases wall energy

Single Domain (SD)◦ 10 to 100nm is a good size

Magnetization◦ … under externally applied magnetic field (H)◦ domains change shape and magnetic moments re-orient

walls shift or disappear◦ even physical shape of material can change somewhat!

called magnetostriction useful in actuators and sensors

◦ in strong enough field, all domains aligned minimizes Zeeman energy

Basics of magnetism

Shape-induced magnetic anisotropy◦ long grains tend to have magnetic moments

along the longer dimension◦ magnetized grain produces internal magnetic field

that counters external field internal field is anisotropic

Magnetization axis can be determined by fabricated shape◦ gives us two states!◦ comes in handy for magnetic storage!

Basics of magnetism

Nanomagnets

Two types of coupling◦ ferromagnetically ordered coupling

collinear along the long axes states line up

◦ antiferromagnetically ordered coupling side-by-side, in parallel favors antiparallel: up-down-up-down…

Nanomagnets

MQCA: majority gate

MQCA: majority gate