Some atoms have a quantum degree of freedom, the spin, which acts like a small magnet. The nature of the interactions between atoms defines the organization of spins and ultimately gives matter its magnetic properties. In the ferromagnetic state, this interaction tends to align the spins in the same direction, producing magnetization as in our usual magnets.
But the interaction can also lead to the anti-alignment of the spins: it is then an antiferromagnetic state that is achieved, without overall magnetization although the spins are perfectly ordered. In “spin glasses” the interactions are random, freezing the spins in disordered configurations and producing a magnetic analogue of the glasses. In any case, these magnetic states suddenly disappear when the materials are heated and the thermal energy exceeds the interaction energy.