Magnetic Properties

Every substance has some magnetic character, and its origin lies in the electrons. Each electron behaves like a tiny magnet, with a magnetic moment arising from two motions (Fig. 1.35):

• its orbital motion around the nucleus, and
• its spin about its own axis.

Being a charged particle in motion, the electron acts like a small current loop with a magnetic moment. Thus every electron has both a permanent spin and an orbital magnetic moment. This moment is tiny and is measured in the Bohr magneton ($\mu_B$):

On the basis of their magnetic behaviour, substances fall into five categories (the alignment of domain moments for the last three is shown in Fig. 1.36):

• (i) Paramagnetic — weakly attracted by a magnetic field and magnetised along it, but they lose their magnetism once the field is removed. This arises from one or more unpaired electrons. Examples: O$_2$, Cu$^{2+}$, Fe$^{3+}$, Cr$^{3+}$.
• (ii) Diamagnetic — weakly repelled by a magnetic field and weakly magnetised in the opposite direction. This occurs when all electrons are paired, so their magnetic moments cancel. Examples: H$_2$O, NaCl, C$_6$H$_6$.
• (iii) Ferromagnetic — very strongly attracted and can be permanently magnetised. The metal ions group into small regions called domains, each acting as a tiny magnet. Normally the domains are randomly oriented and cancel out, but in a field they all line up (Fig. 1.36a) giving a strong effect that persists even after the field is removed. Examples: iron, cobalt, nickel, gadolinium and CrO$_2$.
• (iv) Antiferromagnetic — the domains are arranged so that they are oppositely oriented and cancel each other's moments (Fig. 1.36b). Example: MnO. …