Ferromagnetic Domains in Thin Single-Crystal Nickel Platelets

Abstract

Ferromagnetic domain observations have been made on single-crystal nickel platelets, ranging from 900 Å to over 10 μ in thickness, that have been grown nonepitaxially by the hydrogen reduction of the bromide. The platelets are generally rectangles or right triangles with (001) faces, 〈100〉 sides or legs, and edges from several microns up to 3 mm in length. Because their perfection is close to that of metal whiskers, the intrinsic coercive force for domain wall motion is minute (<0.01 Oe). Thus the domain configurations, observed by Lorentz microscopy and the Bitter technique, are in stable or metastable equilibrium. For platelets less than 1.0 μ thick, the magnetization lies in the plane along 〈110〉 directions, these being the projections of the 〈111〉 directions of minimum anisotropy energy. Echelons form the common closure structure near the edges. Some 180° walls appear to be of a combined Néel-Bloch type. At a thickness of 1.0 μ a striped powder pattern appears within the planar domains, with a 0.7 μ spacing between stripes. Near 2 μ the pattern changes to a coarser one in which the stripes are not parallel to the planar magnetization. Above about 7 μ the surface pattern is essentially that for the bulk crystal. Near 1100 Å, Néel walls and spotted transition walls are observed. Cross-tie walls and individual cross-tie spots appear with unusual clarity in single-crystal nickel-cobalt platelets (7% Co). Their creation and transformation are readily observable. Striped and planar domain patterns similar to those for nickel appear on fcc cobalt platelets.