Néel effect

In superparamagnetism (a form of magnetism), the Néel effect appears when a superparamagnetic material in a conducting coil is subject to varying frequencies of magnetic fields. The non-linearity of the superparamagnetic material acts as a frequency mixer, with voltage measured at the coil terminals. It consists of several frequency components, at the initial frequency and at the frequencies of certain linear combinations. The frequency shift of the field to be measured allows for detection of a direct current field with a standard coil.

In 1949 French physicist Louis Néel (1904-2000) discovered that when they are finely divided, ferromagnetic nanoparticles lose their hysteresis below a certain size; this phenomenon is known as superparamagnetism. The magnetization of these materials is subject to the applied field, which is highly non-linear. This curve is well described by the Langevin function, but for weak fields it can be simply written as:

${\displaystyle M(H)=\chi _{0}H+N_{e}H^{3}+\varepsilon (H^{3})}$,

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