[′hīz·ən·bərg ′thē·ə·rē əv ‚fer·ō′mag·nə‚tiz·əm] (solid-state physics) A theory in which exchange forces between electrons in neighboring atoms are shown to depend on relative orientations of electron spins, and ferromagnetism is explained by the assumption that parallel spins are favored so that all the spins in a lattice have a tendency to point in the same direction.