This chapter affords a brief summary of the quantum theory of angular momentum. As the angular momentum of a charged particle is proportional to its magnetization, this subject is at the core of the… Expand

The total spin S is a good quantum number in problems of interacting spins. We have shown that for rather general antiferromagnetic or ferrimagnetic Hamiltonians, which need not exhibit translational… Expand

The two-dimensional Ising model for a system of interacting spins (or for the ordering of an AB alloy) on a square lattice is one of the very few nontrivial many-body problems that is exactly soluble… Expand

The original edition of "The Theory of Magnetism" developed the various relevant topics using modern methods adapted for the many-body problem. It presented and taught the fermionic field theory… Expand

Luttinger’s exactly soluble model of a one-dimensional many-fermion system is discussed. We show that he did not solve his model properly because of the paradoxical fact that the density operator… Expand