Neel temperature
/ NAY-ell TEM-pruh-cher /
An antiferromagnet keeps its order as a hidden, up-down-up-down checkerboard of moments. Like every kind of magnetic order, it cannot survive too much heat. There is a temperature above which the neat alternating pattern dissolves into random tumbling — the Neel temperature, named for Louis Neel, who explained antiferromagnetism.
It is the antiferromagnet's exact counterpart to the Curie temperature. Below it, the exchange interaction holds neighboring moments locked in their opposing arrangement, each pointing against the next. As temperature climbs, heat erodes that discipline, and right at the Neel temperature the alternating order collapses to zero. Above it, the moments survive but jostle at random, and the material becomes an ordinary paramagnet — magnetically the same on the outside as it was below, since antiferromagnetic order never showed externally anyway.
The Neel temperature matters because it pins down where antiferromagnetic order lives, which is crucial for the antiferromagnets buried inside spintronic devices and high-temperature superconductors. The subtle catch is that the transition is nearly invisible from the outside — there is no net magnetism appearing or vanishing — so finding the Neel temperature usually takes neutron beams or careful measurement of how susceptibility peaks, not a simple test with a magnet.
Common manganese oxide is antiferromagnetic only when colder than about minus 151 degrees Celsius — its Neel temperature. Warm it past that and its hidden up-down spin pattern melts away, even though nothing visible changes to a magnet held nearby.
Manganese oxide's hidden antiferromagnetic order vanishes above its Neel temperature.
Curie temperature and Neel temperature describe the same kind of event — magnetic order dissolving into thermal disorder — but for different orderings. Curie is for order with net magnetism (ferro- and ferrimagnets); Neel is for the self-cancelling order of antiferromagnets.