Now we know why an external magnetic field can generate net magnetism in aluminum. But this is not enough to explain why there is attraction between two substances? To answer this question, we need to use a concept called exchange energy. Exchange energy is a physical quantity in quantum mechanics that describes the strength of the interaction between two adjacent electron spins. Simply put, the exchange energy determines whether the spin of two adjacent electrons tends to be parallel or antiparallel.

For the small magnetic dipoles inside aluminum, in the absence of an external magnetic field, exchange energy causes them to tend towards parallel arrangement; When there is an external magnetic field, exchange energy makes them tend to align with the direction of the external magnetic field. This forms a stable state.

Between two pieces of material? If both materials are excited by an external magnetic field and have the same direction, there will be an exchange energy between them, causing the small magnetic dipoles between them to tend to align in parallel. This forms a more stable state. This state is equivalent to a decrease in the total energy between two pieces of material, and a decrease in energy means that there is a force pulling them closer, which is what we perceive as attraction.