Effect of the Geometry and Microstructural Heterogeneity of Ferritic Elements on the Magnetic Hysteresis Loop

Abstract

This study focuses on toroidal elements having dimensions 3.1 mm x 1.5 mm x 1.4 mm and composition, mol.%: 37.6 Fe₂O₃; 38.1 MnO; 9.3 MgO; 11.7 ZnO; 3.3 CaO. It is shown that increase of the inner-to-outer diameter ratio of the toroids leads to higher squareness of the hysteresis loop. The geometry of the samples and gradients of coercive force H_c caused by structural heterogeneity influence the shape and squareness of the hysteresis loop. In the absence of oxidation, the surface layers of elements have lower H_c than inner ones because their magnetic reversal begins under smaller fields, promoting smooth transition from the horizontal to falling region of the hysteresis loop. The nonequilibrium oxidation of elements when they are cooled down to the hardening temperature leads to increase in H_c: to a greater extent on the surface and to a smaller extent within the toroids. As a result, the upper half of the falling branch of the hysteresis loop becomes more abrupt while the lower more in-clined. Isothermal holding at the hardening temperature reduces the gradients of He and the squareness of hysteresis loop.