montek singh comp790-084 sep 6, 2011. basics of magnetism nanomagnets and their coupling next...
Post on 19-Dec-2015
216 views
TRANSCRIPT
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
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