Ferromagnétisme

Magnetic domains are regions within ferromagnetic materials where atomic magnetic dipoles align parallel to each other due to strong quantum mechanical coupling. In materials like iron, nickel, and cobalt, each domain has a net dipole moment. Without an external field, domains are randomly oriented, but under an applied field, they align and grow in size, producing substantial magnetization much larger than paramagnetic materials.

Ferromagnets retain magnetization due to hysteresis, a property where reducing the external field to zero does not return magnetization to zero. The aligned domains remain oriented even without the applied field. This persistence occurs because the quantum mechanical coupling holding domains in alignment is strong enough to resist thermal agitation at room temperature, allowing permanent magnetization to persist.

The hysteresis loop is a closed curve showing the relationship between applied magnetic field and magnetization in ferromagnets. As the field increases, magnetization rises until magnetic saturation. When the field reverses, domains slowly flip, reaching zero magnetization at the coercive field. Continuing the reversal completes the loop, demonstrating the material's history-dependent magnetization behavior.

Soft ferromagnets have narrow hysteresis loops where magnetization disappears when the external field is removed, making them ideal for electromagnets and transformer cores. Hard ferromagnets have broader loops and retain magnetization even after field removal, making them suitable for permanent magnets like alnico used in compass needles. The difference lies in domain stability and coercive field strength.

The Curie temperature is the threshold above which ferromagnets lose their permanent magnetic properties and become paramagnets. Increased thermal motion at higher temperatures disrupts and randomizes domain orientation and size. For iron, the Curie temperature is 1043 K. Some elements and alloys have Curie temperatures below room temperature, making them ferromagnetic only at lower temperatures.

Ferromagnets exhibit magnetic susceptibility of 10³ to 10⁴ and relative permeability of 1,000 to 100,000 because their aligned domains contain enormous numbers of atoms with parallel dipole moments. This collective alignment produces a net magnetic dipole moment far exceeding paramagnetic materials. The strong quantum mechanical coupling prevents thermal agitation from misaligning domains, maintaining high induced magnetization.

Ferromagnets can be demagnetized through hard blows or heating. Mechanical shock disrupts domain alignment, while increased thermal motion randomizes domain orientation and size. Heating a ferromagnet above its Curie temperature causes it to transition into a paramagnetic state, permanently losing its permanent magnetic properties until cooled below the Curie temperature.