Editing Paramagnetism (section)

===Systems with interactions===
[[File:Para-ferro-anti.jpg|thumb|300px|Idealized Curie–Weiss behavior; N.B. T<sub>C</sub>=θ, but ''T''<sub>N</sub> is not θ. Paramagnetic regimes are denoted by solid lines. Close to ''T''<sub>N</sub> or ''T''<sub>C</sub> the behavior usually deviates from ideal.]]

As stated above, many materials that contain d- or f-elements do retain unquenched spins. Salts of such elements often show paramagnetic behavior but at low enough temperatures the magnetic moments may order. It is not uncommon to call such materials 'paramagnets', when referring to their paramagnetic behavior above their Curie or Néel-points, particularly if such temperatures are very low or have never been properly measured.  Even for iron it is not uncommon to say that ''iron becomes a paramagnet'' above its relatively high Curie-point. In that case the Curie-point is seen as a [[phase transition]] between a ferromagnet and a 'paramagnet'. The word paramagnet now merely refers to the linear response of the system to an applied field, the temperature dependence of which requires an amended version of Curie's law, known as the [[Curie–Weiss law]]:

:<math>\mathbf{M} = \frac{C}{T- \theta}\mathbf{H}</math>

This amended law includes a term θ that describes the exchange interaction that is present albeit overcome by thermal motion. The sign of θ depends on whether ferro- or antiferromagnetic interactions dominate and it is seldom exactly zero, except in the dilute, isolated cases mentioned above.

Obviously, the paramagnetic Curie–Weiss description above ''T''<sub>N</sub> or ''T''<sub>C</sub> is a rather different interpretation of the word "paramagnet" as it does ''not'' imply the ''absence'' of interactions, but rather that the [[magnetic structure]] is random in the absence of an external field at these sufficiently high temperatures. Even if ''θ'' is close to zero this does not mean that there are no interactions, just that the aligning ferro- and the anti-aligning antiferromagnetic ones cancel. An additional complication is that the interactions are often different in different directions of the crystalline lattice ([[anisotropy]]), leading to complicated [[magnetic structure]]s once ordered.

Randomness of the structure also applies to the many metals that show a net paramagnetic response over a broad temperature range. They do not follow a Curie type law as function of temperature however; often they are more or less temperature independent. This type of behavior is of an itinerant nature and better called Pauli-paramagnetism, but it is not unusual to see, for example, the metal [[aluminium]] called a "paramagnet", even though interactions are strong enough to give this element very good electrical conductivity.