Magnetic behavior of insulators

Insulators in which atoms have completely filled electronic shells, such as the noble elements or the purely ionic alkali halides (composed of atoms from columns I and VII of the Periodic Table) are actually the simplest cases the atoms or ions in these solids differ very little from isolated atoms or ions, because of the stability of the closed electronic shells. The presence of the crystal produces a very minor perturbation to the atomic configuration of electronic shells. The magnetic behavior of noble element solids and alkali halides predicted by the analysis at the individual atom or ion level is in excellent agreement with experimental measurements. 

In insulating solids whose magnetic behavior arises from individual ions or atoms, there is a common feature in the response to an external magnetic field H, known as the Curie law. This response is measured through the magnetic susceptibility, which according to the Curie law is inversely proportional to the temperature. The magnetic susceptibility is defined as 

This is exactly the form of the Curie law, that is, the susceptibility is inversely proportional to the temperature. In order for this derivation to be valid, we have to make sure that w is much smaller than unity. From the definition of w we obtain the condition 

Typical values of the magnetic moment mo (see following discussion) are of order the Bohr magneton, which is defined as 

The values of the total spin 5, orbital angular momentum L and total angular momentum J for the state of the atom or ion determine the exact value of the magnetic moment mo. The interaction energy of an electron in a state of total spin S and orbital angular momentum L, with an external magnetic field H, to lowest order in the field is given by 

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These expressions are known as the Lande g-factors in the context of the total angular momentum of atoms or ions with partially filled electronic shells, which are relevant to the magnetic behavior of insulators. 

Fluorides have long been known to provide useful materials for the checking and improvement of magnetic models1-4. Since they are generally strong insulators, their magnetic behavior can be thus interpretated more easily than that of oxides and sulfides, due to the absence of electronic delocalization. 

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