Certain metallic alloys have magnetic interactions which oscillate in value as a function of the separation of the different atoms.
These alloys can be created in an amorphous state --- the positions of the atoms are random and disordered like a glass, rather than regular and ordered like a crystal.
The magnetic interactions between spins are thus sometimes ferromagnetic, sometimes antiferromagnetic. This is what is known as a spin glass.
Spin glasses have a number of physically and mathematically interesting properties, and are the subject of much research.