Paramo

Meet Paramo, a frosty cube full of tiny magnets who lines up better the colder she gets.

In a paramagnetic material each atom acts like a tiny magnet. An outside magnetic field tries to line them all up. Heat jostles them about and scrambles their directions. The overall magnetisation comes from the contest between the two.

Pierre Curie found in 1895 that the magnetisation is proportional to the field and inversely proportional to the temperature, M = CB/T, where C is the Curie constant of the material. Halve the temperature and the magnetisation doubles.

Liquid oxygen at 90 K is paramagnetic enough to cling between the poles of a strong magnet, while oxygen gas at room temperature feels only a tiny pull. The law fails at very low temperatures, once nearly all the tiny magnets are already lined up.

Curie's law is used to measure very low temperatures in laboratories, by reading how magnetised a paramagnetic salt has become.