Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 147

The magnetic response of a material depends on its atomic structure and, in the case of some materials, on its temperature, although the effect of temperature is not normally relevant to the application of biomaterials. We can view a magnet as a dipole with a certain magnetic moment in much the same way as we view an electric dipole with an electric dipole moment (Chapter 2). The magnetic response of a material has its origins in the tiny magnetic dipole moments associated with the individual electrons of its constituent atoms or ions.

There are two contributions to the magnetic dipole moment of the electron (Figure 4.18). One is due to the intrinsic spin of the electron analogous to a charged body spinning about its own axis. The other is due to its orbital motion around the nucleus. Each contribution is quantized, that is, it depends on

 The spin quantum number ms of the electron, which can have values of +1/2 (sometimes referred to as spin up) and −1/2 (spin down)

 The magnetic orbital quantum orbital quantum number ml of the electron, which can have values from −l to +l, where l is the angular momentum (azimuthal) quantum number.

Figure 4.18 Schematic diagram illustrating the two contributions to the magnetic dipole moment of an electron in an atom due to its intrinsic spin and its orbital motion around the nucleus.

The important quantity is the Bohr magneton μ_B, the magnetic moment of an unpaired electron, equal to 9.27 × 10⁻²⁷ J/T. For each electron in an atom, for example, the intrinsic spin magnetic moment is +μ_B (spin up) or −μ_B (spin down), whereas the orbital magnetic moment is m_lμ_B.