Determination of the Ferromagnetic Exchange Energy Constant in Ferromagnetic Ni and Ni–Cu Films by Means of Domain Wall Width Measurements

A method for measuring the ferromagnetic exchange energy constant A is being proposed. From A the exchange integral J, which is the energy due to exchange coupling between nearest neighbors, can also be determined. The method depends upon relating A to the image width of a Néel-type domain wall observed in an electron microscope by Lorentz microscopy. An expression was found which relates the constants of the film (the exchange energy constant A, the saturation magnetization Ms and the anisotropy constant K) to the domain wall width. Measurements were made in a Philips EM100B electron microscope on vacuum-deposited films of 200 Å thickness. Values of A and J for Ni were found to be A = 0.46×10−6 erg/cm and J = 5.0×10−14 erg. Due to a rapid decrease in wall image contrast as Cu was added, observations were made only up to 3.7 wt% Cu. In the range of zero to 3.7 wt% Cu, A shows an initial sharp increase up to 0.3 wt% Cu then decreases fairly gradually. J also has an initial sharp increase, but then increases only quite slowly above 0.3 wt% Cu.