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It is suggested that the observed lowering of the spin-orbit parameters in transition-ion complexes is caused by a screening effect which expands the 3d wave functions. Evidence in support of this suggestion is given by neutron diffraction form-factor measurements on Mn ++ salts. The theory of the transferred hyperfine interaction between the electron spin of Mn ++ and surrounding F 19 nuclei is discussed, and the complications introduced into the theory when dealing with other ions are described. The theory is found to be unsatisfactory because it is not possible to treat the interaction via the Is orbitals of F" with sufficient accuracy.

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The direct exchange integral which occurs in the Heisenberg theory of ferromagnetism is evaluated for all internuclear spacings. We rind that it is always positive, whereas Bethe originally suggested it would be positive only at large spacing and more recently it has been suggested that the integral should always be negative. However, at the observed internuclear separation the magnitude calculated is of the order of 70 times too small to explain the experimentally determined exchange constant in ferromagnetic metals, and we therefore conclude that direct exchange is not responsible for ferromagnetism in these metals.