By Chris Deziel, updated March 24 2022
Magnetism and electricity are deeply intertwined phenomena that can often be viewed as two sides of the same coin. The magnetic behavior of metals originates from the arrangement of electrons in their atomic shells.
Every element possesses magnetic characteristics, though most are subtle and not readily apparent. Metals that attract magnets share a common trait: unpaired electrons in their outermost shells. This electronic configuration is the key driver of magnetism.
Metals that can be permanently magnetized are called ferromagnetic. The list is short, and the term derives from the Latin word for iron, ferrum.
In contrast, paramagnetic materials become temporarily magnetized when exposed to a magnetic field. The class includes not only metals but also covalent molecules such as oxygen (O₂) and various ionic solids.
Anything that is neither ferromagnetic nor paramagnetic is diamagnetic. Diamagnetic substances exhibit a slight repulsion to magnetic fields, so a conventional magnet does not pull on them. In reality, all materials display diamagnetism to some degree.
According to the accepted atomic model, the nucleus contains positively charged protons and electrically neutral neutrons, held together by the strong nuclear force. Surrounding the nucleus is a cloud of electrons occupying discrete energy levels or shells. These electrons are responsible for the magnetic properties of an atom.
When an electron orbits the nucleus, it produces a changing electric field that, by Maxwell’s equations, generates a magnetic field. The field’s magnitude equals the area inside the orbit multiplied by the current. Each electron contributes a minuscule current, and the resulting magnetic moment is measured in Bohr magnetons. In a typical atom, the magnetic fields of all orbiting electrons cancel out, leaving a net zero moment.
Beyond orbital motion, electrons possess an intrinsic property called spin, which is crucial in determining magnetic behavior. Spin is not a classical rotation but an intrinsic angular momentum. Electrons with spin “up” have positive spin, while those with spin “down” have negative spin.
Because spin tends to be unbalanced, it often produces a net magnetic moment in an atom, whereas orbital contributions may cancel. Thus, spin dominates over orbital motion in shaping magnetic properties.
Electrons occupy shells in spin‑up and spin‑down pairs, typically resulting in zero net magnetic moment. The outermost, or valence, shell determines an element’s magnetic character. An unpaired electron in this shell creates a net magnetic moment, rendering the element magnetic; fully paired valence electrons lead to diamagnetism.
This rule holds for most elements, though certain transition metals like iron (Fe) have valence electrons that can reside in lower energy shells.
Because every electron loop generates a magnetic field, all materials exhibit diamagnetism. When an external magnetic field is applied, the induced currents oppose the field—a consequence of Lenz’s Law. This weak repulsion is present in every substance, but is often too slight to detect without sensitive equipment.
The total magnetic moment, J, equals the sum of orbital and spin angular momentum. When J = 0, the atom is non‑magnetic; when J ≠ 0, it is magnetic, requiring at least one unpaired electron.
Examples of diamagnetic metals include:
In a strong magnetic field, a diamagnetic object such as a gold bar will orient itself perpendicular to the field lines, demonstrating its subtle resistance.
Metals with at least one unpaired outer‑shell electron are paramagnetic. They align with an external magnetic field but lose this alignment once the field is removed. Common paramagnetic metals include:
Although they are not attracted by a permanent magnet, their induced magnetic moments can be detected with sensitive instruments.
Paramagnetism is not exclusive to metals. Molecules like O₂ exhibit it, while non‑metals such as calcium are also paramagnetic. A classic demonstration involves placing liquid oxygen between the poles of a powerful electromagnet; the oxygen climbs the poles and vaporizes, forming a visible gas cloud. The same experiment with liquid nitrogen, which is diamagnetic, shows no movement.
Ferromagnetic elements become magnetized in an external field and retain that magnetization afterward. The key is the presence of multiple unpaired electrons and the formation of magnetic domains. When a magnetic field is applied, domains align, and the alignment persists even after the field is removed—a phenomenon known as hysteresis, which can last for years.
Ferromagnetic elements include:
High‑performance permanent magnets are typically rare‑earth magnets. Neodymium magnets (NdFeB) and samarium‑cobalt magnets (SmCo) combine a ferromagnetic core with a paramagnetic rare‑earth element. Ferrite (iron oxide) and alnico (AlNiCo) magnets are also ferromagnetic but generally weaker.
Every magnetic material has a characteristic temperature, the Curie point, above which its magnetic order collapses. For iron, the Curie point is 1,418 °F (770 °C); for cobalt, it’s 2,050 °F (1,121 °C). Above these temperatures, the material becomes paramagnetic or diamagnetic. Cooling below the Curie point restores ferromagnetism.
Magnetite (Fe₃O₄) is often described as ferromagnetic but is, in fact, ferrimagnetic. Its crystal structure contains two interpenetrating lattices—octahedral and tetrahedral—with opposing but unequal magnetic moments, resulting in a net magnetic moment. Other ferrimagnetic materials include yttrium iron garnet and pyrrhotite.
Below a material’s Néel temperature, certain metals, alloys, and ionic solids transition from paramagnetic to antiferromagnetic, losing their response to external magnetic fields. In antiferromagnetism, neighboring spins align antiparallel, canceling each other out.
Néel temperatures can be extremely low (≈ –150 °C) or near room temperature, depending on the compound. Only a few elements, such as chromium and manganese, exhibit antiferromagnetism. Notable antiferromagnetic compounds include manganese oxide (MnO), some forms of iron oxide (Fe₂O₃), and bismuth ferrite (BiFeO₃).
As temperature rises, antiferromagnetic order weakens, reaching a peak paramagnetic response at the Néel temperature before thermal agitation diminishes alignment.
While most everyday metals are ferromagnetic or paramagnetic, understanding these magnetic classifications reveals why certain metals remain unaffected by conventional magnets.
