Magnetism
Magnetism is a powerful force that causes certain items to be attracted to refrigerators. ~ American writer Dave Barry
Magnetism is a field of attraction between particles. Most materials are influenced to some extent by magnetic fields.
The magnetic behavior of crystalline materials is highly sensitive to the lattice constant. (The lattice constant characterizes the physical dimensions of unit cells in crystal.) ~ Japanese physicist Hideaki Sakai
There are 3 known magnetic states. All appear in crystals. Ferromagnetism – the magnetism of magnets and compass needles – has been known for over a millennium.
The 2nd state of magnetism is antiferromagnetism: where the ionic magnetic fields of metals cancel each other out, owing to complementary electron spins. Antiferromagnetism was discovered in the 1950s. It is the basis for read-heads in computer hard-disk drives.
Having no fondness for heat, both ferromagnetism and antiferromagnetism exhibit their talents only when cooled below a critical temperature.
The 3rd state of magnetism is in quantum spin liquids (QSL), discovered in 2012. QSL is the liquid-like magnetism of quantum entanglement. The state is called liquid because it is disordered compared to the spin state of crystalline ferromagnetism. QSL and ferromagnetism are analogous to the states of water and ice.
Quantum spin liquids cannot be described by the broken symmetries associated with conventional ground states. In fact, the interacting magnetic moments in these systems do not order but are highly entangled with one another over long ranges.
A key feature of spin liquids is that they support exotic spin excitations carrying fractional quantum numbers. In a spin liquid, the atomic magnetic moments are strongly correlated, but do not order or freeze even as the temperature goes to zero. ~ Chinese American physicist Tian-Heng Han et al
