If J is greater than 0 in
If J is greater than 0 in the energy function negative J times the sum of the quantity S-sub-i S-sub-i-plus-one, these materials follow a model solved by Lars Onsager; those S terms signify spins. They cause a crackling noise when placed near loudspeakers in an effect named for Barkhausen. They are made up of similarly-aligned domains which grow smaller with temperature, losing their namesake property above the Curie point; those domains of dipole moments remain aligned even if the external field is removed. For 10 points, name these strongest and most commonly used metals whose north and south poles attract, exemplified by cobalt, nickel, and iron.
If J is greater than 0 in the energy function negative J times the sum of the quantity S-sub-i S-sub-i-plus-one, these materials follow a model solved by Lars Onsager; those S terms signify spins.
ferromagnets [accept ferromagnetism; accept Ising model until "these materials" is read]