Susceptibility

At temperatures above T_C, where the ferromagnetic order collapses, and the material becomes paramagnetic, the atomic moments of a few Bohr magnetons experience random thermal fluctuations. Although M_s is zero, an applied field can induce some alignment of the atomic moments, leading to a small magnetization M which varies linearly with H, except in very large fields or very close to the Curie point. The susceptibility, defined as

χ = M/H, .......(1)

is a dimensionless quantity, which diverges as T → T_C from above. Above T_C it often follows a Curie–Weiss law

χ = C/(T − T_C), ........(2)

where C is known as the Curie constant. Its value is of order 1 K.
The magnetic response to an applied field of materials which do not order magnetically may be either paramagnetic or diamagnetic. In isotropic paramagnets, the induced magnetization M is in the same direction as H, whereas in diamagnets it is in the opposite direction. Superconductors exhibit diamagnetic hysteresis loops below their superconducting transition temperature T_sc, and their susceptibility can approach the limiting value of −1.
The susceptibility of many paramagnets follows a Curie law,

χ = C/T, .........(3)

but for some metallic paramagnets and almost all diamagnets χ is independent of temperature. The sign of the room-temperature susceptibility is indicated on the magnetic periodic table (Table A, see endpapers) and the molar susceptibility χ_mol is plotted. There it is appropriate to look at the molar susceptibility because some of the elements are gasses at room temperature. A cubic metre of a solid contains roughly 10⁵ moles, so χ_mol is approximately five orders of magnitude less than χ. From Table A, it can be seen that the transition metals are paramagnetic, whereas main group elements are mostly diamagnetic.

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