The Magnetic Properties of Evaporated Nickel and Iron Films

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Abstract
Coercive force, retentivity, and hysteresis loop forms for evaporated films of Ni and Fe.—Films of nickel and iron, produced by evaporation at low pressure, were studied with the object of verifying the reported existence of an abrupt change of coercive force at a critical thickness, and also to discover the cause of certain peculiar hysteresis loop forms. Critical thickness was found for iron films at approximately 50 mμ, in satisfactory agreement with Sorensen. At this thickness the coercive force changes abruptly from the high value of approximately 100 for thinner films. The observations were made upon films deposited on a base of aluminum foil,.0025 cm in thickness. The foil was heated previous to and during deposit. No such critical thickness was observed in nickel. Peculiar hysteresis loops were found. Nickel films unheated have very narrow loops, with magnetic induction nearly proportional to the field up to 139 gauss, where the induction is one-third to one-half that of the metal in bulk. Films heated previous to and during deposit give a magnetic induction at 139 gauss that is approximately three times as great, or like that of metal in bulk. But the retentivity and coercive force increase five-fold and become less like those of the metal in bulk. In similarly heated films of iron, these last two magnetic properties are also much greater than with metal in bulk. The peculiarities of the films are probably caused by the nature of the crystalline state. To what extent the phenomena depend upon the presence of the aluminum base is yet to be ascertained. The present view is that the presence of gas alters the crystal growth. On the whole the experiments seem to emphasize the importance of the influence of the crystalline state upon magnetic properties.