(1) The exchange interactions within its molecules are extremely strong, and the molecules can be seen as objects with definite spin

(2) The distance between molecules is relatively long, and the interaction is weak, which can be regarded as independent identical molecules

(3) The overall properties of crystals are the superposition of identical molecular properties

Thus, the magnetic properties of crystals can exhibit the magnetic properties of individual molecules. So, we only study the magnetism of one molecule in a crystal to determine its magnetism. We refer to this type of research object as "single molecule magnets" (SMMs).

(4) When the SMM crystal is at a very low temperature, a stepped shape appears in the hysteresis loop

This expresses a quantum phenomenon where the physical property "magnetization" of macroscopic objects undergoes "quantization", so this phenomenon is a "macroscopic quantum phenomenon".