Heat Effects in the Magnetization of Silicon Iron

Abstract

A brief description is given of the method used by Bates and his co-workers which permits the measurement of the thermal changes which accompany the step-by-step magnetization of ferromagnetic metals. The method has now been used to examine the behavior of 0.5 and 4 percent silicon iron in the untreated and annealed states when magnetized along the virgin curve and in cycles with fields up to 380 oersteds. In this work, some twenty copper-constantan thermocouples generate currents when a magnetic field acting on the ferromagnetic specimen is changed, and these are transmitted ballistically via a toroidal Mu Metal transformer to a highly sensitive moving coil galvanometer of long period. As the thermal changes in the present work were very small, a new technique had to be developed to deal with them. The measured thermal changes are plotted as functions both of the intensity of magnetization and of the effective magnetic field. The several errors and experimental difficulties, including asymmetry in thermal readings, stray induction effects, heating due to eddy currents, and thermal and electromagnetic disturbances experienced during the measurements are briefly discussed, and the methods used to overcome them are briefly outlined.The results are examined in the light of current domain theory and a theoretical procedure developed by Stoner and Rhodes, in which the thermal contribution resulting from change in intrinsic magnetization, often described as the magnetocaloric effect, is substracted from the recorded thermal change, and the residue is considered in detail. Another procedure recently applied to results for nickel was also examined, but it was found inapplicable to the silicon iron data. An unusual depression or dip is found in the curves of thermal change plotted as a function of the applied field when the region of low field values is reached upon reducing the field from the maximum used in describing a hysteresis cycle. This observation provides evidence in support of Kittel's theory of flux closure which Bozorth has used to explain the low remanence of such alloys which exhibit little magnetostriction and magnetic anisotropy and small internal strain. It is suggested that the strange depression may be correlated with some peculiar domain boundaries recorded by the Bitter powder technique on the (100) surface of a single crystal of silicon iron when it is demagnetized either thermally or in the above manner. These boundaries "wriggle" in an extraordinary manner which suggests that they separate regions either of flux closure in opposite directions or domains whose magnetization vectors meet head-on.