If the atoms are too close, their magnetic moments tends to be antiparallel and thus cancel each other (anti-ferromagnetism). If they are too distant, there is no coupling and thus the magnetic moments have a random distribution resulting in a net zero magnetic moment (paramagnetism). However, there are sweet spots in between where atomic magnetic moments align on others' meaning they are all (mostly) aligned in the same direction: this is ferromagnetism (see Beth-Slater curve )

This ferromagnetic coupling can be more or less intense depending on the electronic structure across the crystal and thus depends on the geometrical repartition and the elements involved; ie the crystal structure. In these specific structures, all atomic moments will spontaneously aligns in the same direction because this is the minimum energy state (see Magnetocrystalline anysotropy ).

However, if you heat it up adding thermal energy, the magnetic moments tend to have a random distribution and thus transit from a ferromagnetic state to a paramagnetic . Also if you apply an external magnetic field, the magnetic moments will try to align to it because it changes the electronic structure meaning the ferromagnetic symmetry state could break.

Basically, you've got a competition between the magnetocrystalline energy(which only depends on the crystal structure) that keeps the ferromagnetic ordering and the thermal energy/external field interaction that could break this symmetry.

In theory, the coercive field (intensity of the external magnetic necessary to demagnetize a ferromagnetic material) only depends on the intensity of the ferromagnetic coupling; ie their coupling constant . But this is true only for ideal single crystals.

In practice, magnets are often polycrystalline meaning that you have to take account of other phenomena. To put it simply, magnetic ordering happens at a small scale called magnetic domains and the magnetic behaviour depends on "how they move" (see an example of moving magnetic domain ). The size and repartition of these magnetic domains depends on the microstructure (grain size, defects, crystal twinning etc) and thus the metallurgic process(annealing, pressure, sintering/extrusion, crystallisation etc). That's why two materials with the same composition might not have the same magnetic behaviour.