Questions about ferromagnetism

What is the difference between "ferromagnetic" and "magnetic" materials?

The terms "magnetic" and "ferromagnetic" are often used as synonyms, although this is not entirely correct from a technical point of view.

It is a "ferromagnetic" material or object if a magnet adheres to it. So if a magnet sticks to a hammer, the hammer can be described as ferromagnetic. In common parlance, this is often incorrectly referred to as a magnetic object.

Which objects are attracted by a magnet?

Magnets only attract ferromagnetic materials (see explanation above). It is irrelevant whether it is a ferrite magnet, a neodymium magnet or an electromagnet.

These include:
- Iron (Fe)
- Nickel (Ni)
- Cobalt (Co)

Note: We are often asked whether stainless steel or aluminium are ferromagnetic, i.e. whether they are attracted by a magnet.

Aluminium belongs to the paramagnetic metals and is only very weakly attracted by a magnet, so it is not suitable for a magnet to adhere to.

With stainless steel, this question cannot be answered in general terms, as there are many different types and compositions of stainless steel. If it is ferromagnetic stainless steel, it is also called ferritic steel. In contrast, austenitic steel is usually not ferromagnetic.

There are also some rare earths, some of which are only ferromagnetic at much lower temperatures. At room temperature, these are only paramagnetic (see explanation below). They are therefore only very weakly attracted by magnets:
- Terbium (Tb)
- Erbium (Er)
- Gadolinium (Gd)
- Holmium (Ho)
- Dysprosium (Dy)

Ferromagnetism

Ferromagnetic materials (iron, cobalt, nickel at room temperature, or special alloys) special alloys) are materials that exhibit spontaneous magnetisation in an external magnetic field and are thus attracted by the magnetic field. Here, elementary magnets already present in the ferromagnetic material are aligned by the external magnetic field. When the external magnetic field is switched off or the ferromagnetic material is removed from the magnetic field, the alignment of the elementary magnets disappears except for a residual magnetisation (remanence). With magnetically soft materials, this remanence is very small. In magnetically hard materials, a considerable remanence can remain (see also hysteresis of magnets).
The actual cause of ferromagnetism cannot, strictly speaking, be described classically; rather, quantum mechanics is required for this, more precisely the so-called exchange interaction, which is responsible for the  parallel alignment of the electron spins in ferromagnetic materials. However, as electrons with the same spin must differ in their local state (Pauli principle), an alignment of the spins is accompanied by an increase in kinetic energy. Only if the reduction in potential energy due to the exchange interaction exceeds the increase in kinetic energy can the electron spins as carriers of the magnetic moments be rectified and the material thus become ferromagnetic. This is the reason why the vast majority of known chemical elements do not exhibit ferromagnetism. As already mentioned, only iron, cobalt and nickel exhibit this ferromagnetic property as pure chemical elements at room temperature.
In contrast, all elements and substances exhibit so-called diamagnetic properties.

Paramagnetism

In the case of paramagnetic substances, an external magnetic field partially aligns the unordered magnetic moments of the molecules. The degree of alignment and thus the induced magnetic field is proportional to the applied magnetic field (magnetic permeability >1), but this field disappears immediately after the external magnetic field is switched off.
Like ferromagnetic substances, paramagnetic substances are drawn into the area of higher field strength, i.e. into the magnetic field.
However, paramagnetism is significantly weaker than ferromagnetism.
Paramagnetism only occurs in elements/molecules with unpaired electrons and a magnetic moment.
The causal physical explanation is again provided by quantum mechanical processes.

Diamagnetism

Diamagnetism occurs in all elements and substances. However, as diamagnetism is orders of magnitude weaker than ferro- and paramagnetism, it is only measurably detectable in substances that do not exhibit ferro- and paramagnetism.
Diamagnetic substances are pushed out of a magnetic field in the direction of a lower field strength. Diamagnets are characterised by a magnetic permeability <1 (negative magnetic susceptibility). This means that the proportionality factor between the induced and the external field is <1.
For comparison: for paramagnetic substances it is >1, for ferromagnets the relationship is not linear.
Like ferromagnetism, diamagnetism can only be explained by quantum mechanical effects.