Magnetism comes from the tiny magnetic fields produced by spinning electrons inside atoms. In an antiferromagnetic material, these atomic magnets naturally line up in opposite directions. Every up magnet sits right next to a down magnet.
This alternating pattern means the magnetic forces cancel each other out perfectly. The solid has zero overall magnetism even though its atoms are highly magnetic. Strong internal forces called exchange interactions keep this neat pattern locked in place.
However, adding heat to the solid creates thermal energy. This thermal energy causes the atoms to vibrate and shake violently. As the solid gets hotter, the shaking starts to overpower the internal magnetic forces.
When the solid hits the Neel temperature, the thermal energy completely destroys the neat pattern. The atomic magnets break free and point in totally random directions. The solid enters a new state called paramagnetism.
This sudden change is similar to how ice melts into water at a specific temperature. The material changes its physical state because heat overcomes the organizing forces. In this new state, the random magnets can be temporarily aligned by an outside magnetic field.