What causes the difference in magnetic permeability between materials? Why do some materials (like iron) have greater magnetic permeability than others (like aluminum)?   We don't need to consider negative permeability here.
Is it that more of the atoms have electrons that polarize their spin and some atoms don't polarize at all?  If so, why would that be?
Or is it that within each atom, there is a greater level of polarization of electron spin? That is, the electrons don't get quite straight.
 A: Every subatomic particle in atoms has a magnetic dipole moment. Generally, these particles pairwise form closed magnetic loops and do not contribute to a magnetic dipole moment of the atom.
Permanent magnets are those in which the remaining unpaired particles in the material are more or less aligned in the same direction. More or less because they are influenced by the common magnetic field of all subatomic particles and, furthermore, in solids their rotational and motional mobility is limited.
There are a lot more reasons for the different magnetic behavior of materials:

*

*Depending on the number of unpaired particles in atoms, molecules and compounds and

*on their positions relative to each other,

*further depending on the Curie temperature and

*on crystallisation in Weiss domains,

the self-holding of the aligned magnetic dipoles changes from material to material. All this influences the permeability of the material.
A: The point is that spins are interacting with each other as well as with the external field. Plus, they get disturbed by the statistical motion of the particles. That is the reason for example, why ferromagnetism breaks down above a certain temperature. Furthermore, due to the Pauli principle, spins tend to pair with respect to opposite axes, which cancels magnetic moments.
So both of your own answers are kind of right: 1) less or more atoms tend to participate in the mutual alignment of spins, depending on temperature, and 2) spins contribute to varying degree to the magnetic moment of each atom, depending on the Pauli principle/the electron configuration.
