Paramagnetism

A paramagnetic material is one whose atoms/molecules DO carry a nonzero net magnetic dipole moment individually (an unpaired-electron atom, as in section 11.3.1), but, in the absence of an external field, thermal agitation keeps these moments randomly oriented, so their vector sum -- the material's net magnetization -- is zero (Fig. 11.7(a)). When a sufficiently strong external field is applied, especially at low temperature (which reduces the randomising effect of thermal motion), a growing fraction of the atomic moments align themselves along the field direction (the orientation of lower potential energy), producing a net magnetization; Fig. 11.7(b) and (c) show this progression from a weak to a strong applied field, and Fig. 11.8 shows the resulting concentration of field lines drawn into the material.

Because an aligned paramagnetic material reinforces rather than opposes the applied field, paramagnetic materials are ATTRACTED toward the stronger part of a non-uniform field -- the opposite of diamagnetic behaviour. If a paramagnetic liquid is placed in a U-tube manometer with a magnet near one arm, the liquid RISES into that arm (again, the opposite of the diamagnetic case). Metals such as magnesium, lithium, molybdenum and tantalum, and compounds such as MnSO$_4\cdot$H$_2$O and oxygen gas, are examples of paramagnetic materials. When the external field is removed, thermal agitation once again randomises the moment directions and the net magnetization returns to zero -- unlike ferromagnetism (section 11.5.3), paramagnetism leaves no memory of the applied field. …