We extend a microscopic theory of polarization and magnetization to include the spin degree of freedom of the electrons. We include a general spin orbit coupling and Zeeman interaction term to account for the modifications to the dynamics upon treating the electrons as spinful particles. We find a contribution to the magnetization due to the intrinsic angular momentum of the electrons. Additionally, the charge current gains a component transverse to both this intrinsic magnetization and the electric field of the crystal lattice. The microscopic polarization and magnetization fields are introduced throughout an extended system using a set of orthogonal orbitals associated with each site. As well free charge and current density fields are introduced associated with charge movement from site to site. The sites act as natural expansion points for the microscopic fields allowing for the evaluation of multipole moments associated with the polarization and magnetization fields. Associated with the dipole moments are the respective macroscopic polarization and magnetization fields, from which we can extract various response tensors. We focus on topologically trivial insulators in the limit of uniform fields to recover the magnetoelectric polarizability (MP) tensor, which contains the accepted expression for the orbital magnetoelectric polarizability (OMP) tensor as well as an added explicitly spin dependent contribution. This general framework can then be extended to finite frequency responses.
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