Hopkinson effect

The sharp increase in a ferromagnetic material’s magnetic permeability as it approaches its Curie temperature from below, often producing a peak just before ferromagnetism disappears.

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Curie temperature: The temperature at which a ferromagnetic or ferrimagnetic material loses spontaneous magnetic order. The Hopkinson peak occurs just below this transition, where magnetic order is weakening.
Ferromagnetism: Magnetic ordering in which neighboring moments align spontaneously, producing net magnetization below a transition temperature. The effect occurs in ferromagnets as their spontaneous order approaches its disappearance.
Ferrite: A ceramic material composed mainly of iron oxide combined with other metal oxides, often with ferrimagnetic properties. Many ferrites display a measurable permeability maximum near their Curie temperatures.
John Hopkinson: An English engineer and physicist known for work on electromagnetism, magnetic materials, and electrical engineering. The effect bears Hopkinson’s name, reflecting his early investigations of magnetic behavior.
Magnetic permeability: A measure of how a material responds magnetically to an applied magnetic field. The effect is defined by a sharp rise in this material property.
Ferrimagnetism: Magnetic order in which opposing sublattices have unequal moments, leaving a net magnetization. Ferrites can show a Hopkinson peak near their Curie transition.
Magnetic temperature sensor: A sensor that infers temperature from a temperature-dependent magnetic property. A permeability peak can provide a characteristic temperature marker in suitable materials.
Pierre Curie: A French physicist whose research included magnetism, piezoelectricity, and the temperature dependence of magnetic properties. Curie’s work established the transition temperature central to describing the effect.
Magnetic susceptibility: The ratio describing how much magnetization a material develops in response to an applied magnetic field. Its temperature dependence helps explain the permeability maximum near the transition.
Magnetization: The magnetic moment per unit volume of a material. Permeability describes response through magnetization under an applied field.