Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 152

Measured curves of magnetization M versus applied magnetic field H (or magnetic flux density B vs H) for diamagnetic and paramagnetic materials are straight lines and they retrace themselves as H is increased or decreased. In comparison, magnetization curves for multi‐domain ferromagnetic and ferrimagnetic materials are not straight lines and they do not retrace themselves as H is increased or decreased (Figure 4.21). This lack of reversibility is a well‐known phenomenon called hysteresis. Another feature of the magnetization curves for ferromagnetic and ferrimagnetic materials is that they show a remanent magnetization (point C or point D) even though there is no applied field. This is the phenomenon of permanent magnetism. Hysteresis is due to the presence of multiple magnetic domains within the material. The motion of the domain boundaries and the reorientation of the domain directions are not fully reversible. The domains do not return completely to their original arrangement but, instead, some alignment of the magnetic dipoles remains even when the magnetic field is completely removed.

Figure 4.21 Magnetization curve for a ferromagnetic or ferrimagnetic material showing a hysteresis loop caused by domain motion.