22.2 Ferromagnets and Electromagnets

Ferromagnetic Materials

Ferromagnetic materials are the ones that behave like what most people picture when they think of "magnets." They can be strongly attracted to magnetic fields, and more importantly, they can stay magnetized even after the external field is removed. Iron, nickel, and cobalt are the classic examples. Understanding how these materials work also sets up the key idea behind electromagnets, where electric currents create controllable magnetic fields.

Ferromagnets vs. Other Magnetic Materials

Most materials respond only weakly to magnetic fields. Ferromagnetic materials are different in several ways:

• High magnetic susceptibility: They're easily magnetized by an external magnetic field. Even a relatively weak external field can produce a strong response.
• Magnetic hysteresis: They retain their magnetization after the external field is removed. This is why a permanent magnet stays magnetic on its own. Hysteresis refers to the fact that the magnetization "lags behind" changes in the applied field.
• High magnetic permeability: Magnetic field lines pass through ferromagnetic materials much more easily than through air or most other substances.
• Spontaneous magnetic moment: The magnetic moments of individual atoms (think of each atom as a tiny magnetic dipole) tend to align with their neighbors, even without an external field pushing them to do so. This spontaneous alignment is what makes ferromagnetism special.

Magnetic Domains and Magnetization

If all the atomic magnetic moments in a piece of iron naturally align, why isn't every piece of iron a magnet? The answer is magnetic domains.

• A magnetic domain is a small region within a ferromagnetic material where all the atomic magnetic moments point in the same direction.
• In an unmagnetized piece of iron, the domains themselves point in random directions. Their fields cancel out, so the material has no net magnetic field overall.
• When you apply an external magnetic field, two things happen:
  1. Domains already aligned with the field grow larger, expanding at the expense of neighboring domains that point in other directions.
  2. The boundaries between domains, called domain walls, shift to accommodate this growth. Domain walls move in whatever way minimizes the overall magnetic energy of the system.
• Magnetic saturation is the point where all domains have aligned with the external field. Once you reach saturation, increasing the external field further produces no additional magnetization since there are no remaining misaligned domains left to flip.

Magnetic Properties and Electromagnets

Curie Temperature

Every ferromagnetic material has a Curie temperature, a specific critical temperature above which it loses its ferromagnetic behavior entirely.

• Below the Curie temperature, atomic magnetic moments stay aligned within domains, and the material behaves as a ferromagnet.
• Above the Curie temperature, thermal energy is strong enough to disrupt that alignment. The material becomes paramagnetic, meaning it responds weakly to external fields and can no longer hold permanent magnetization.
• For iron, the Curie temperature is about 1043 K (770 °C). Each ferromagnetic material has its own value.
• As temperature rises toward the Curie point, ferromagnetic properties gradually weaken. Magnetic susceptibility, permeability, and spontaneous magnetization all decrease before vanishing at the Curie temperature.
• This matters for real-world design: if you're choosing a ferromagnetic material for a motor or sensor, you need to make sure it won't reach its Curie temperature during normal operation.

Electromagnets and the Electricity-Magnetism Connection

An electromagnet uses electric current to generate a magnetic field. Unlike a permanent magnet, its field can be turned on, turned off, or adjusted in strength.

The basic principle comes from Ampère's law: a current-carrying wire produces a magnetic field around it. For a long straight wire, the magnetic field strength at a distance $r$ from the wire is:

$H = \frac{I}{2\pi r}$

where $I$ is the current and $H$ is the magnetic field strength.

A practical electromagnet takes this further by winding the wire into a coil (a solenoid) around a ferromagnetic core:

1. Electric current flows through the coil, generating a magnetic field along the axis of the solenoid.
2. The ferromagnetic core inside the coil concentrates and greatly enhances that field, thanks to its high permeability.
3. The result is a magnetic field much stronger than the coil alone could produce.

You can increase an electromagnet's field strength in three ways:

1. Increase the number of turns in the coil (more loops of wire means a stronger cumulative field).
2. Increase the current flowing through the coil.
3. Use a core material with higher magnetic permeability (a better ferromagnetic material concentrates the field more).

Because you control the current, you control the field. This makes electromagnets essential in electric motors, generators, MRI machines, and magnetic levitation systems.

Additional Magnetic Concepts

• Ferrimagnetism is similar to ferromagnetism, but the atomic magnetic moments only partially align. Some point in opposite directions, so the net magnetization is weaker than in a true ferromagnet. Ferrites used in some electronics are ferrimagnetic.
• Magnetic anisotropy means a material's magnetic properties depend on direction. Magnetization "prefers" to align along certain crystal axes, which influences how easily the material can be magnetized in different orientations.