ferromagnetic materials

Meaning: In ferromagnetic materials such as iron, cobalt and nickel, atoms have unpaired electrons whose magnetic moments line up with their neighbors in small regions called domains. In an external field, domains aligned with it grow and others rotate, so the material is strongly attracted and the net field inside becomes much larger. When the field is removed, some alignment can remain, so the material can become a permanent magnet. Heating above the Curie temperature destroys the alignment, and the material then behaves as paramagnetic.

Equation: B = μ₀μr n I for a long solenoid with a core, where B is the magnetic field (T), μ₀ = 4π × 10⁻⁷ T·m/A, μr is relative permeability (unitless, far above 1 for ferromagnets), n is turns per unit length (m⁻¹), and I is current (A).

On the MCAT: Passages put an iron core in a solenoid or electromagnet and ask how the field changes (it increases). Questions also ask you to rank the strength of attraction (ferromagnetic > paramagnetic > diamagnetic) or to predict that heating a magnet past its Curie temperature removes its magnetism.

Common confusion: Being attracted to a magnet does not make a material ferromagnetic. Paramagnetic materials are weakly attracted and keep no magnetization once the field is removed. Diamagnetic materials, with all electrons paired, are weakly repelled. Domains and retained magnetization define ferromagnetism.