Magnetic Atom

Intuition

Not every atom is magnetic. Carbon, oxygen, copper — all “ordinary” atoms — contain electrons whose spins pair up and cancel: each pair carries opposite spin, contributes zero net magnetic moment, and the atom is magnetically silent. Magnetic atoms are the exceptions: their electron shells contain unpaired spins that don’t have partners to cancel against. Iron, cobalt, and nickel are the famous examples — their 3d shell hosts several unpaired electrons, and the resulting net moment is what ultimately makes a fridge magnet stick to a fridge.

A subtle but important point follows: when current flows through a ferromagnetic wire, the wire does not demagnetize. The electrons that carry the current are different from the electrons that carry the magnetism. Sorting these two populations is the conceptual key to spintronics.

Formal Definition

An atom is magnetic when its electron configuration leaves a non-zero total electronic angular momentum, so that its atomic magnetic moment ${\mathbf{\mu }}_{\text{atom}}\ne \vec{0}$.

Equivalently, in the simplest spin-only picture: a magnetic atom is one with at least one unpaired electron.

In a metal, the atom’s electrons fall into two populations:

• Bound (core / localized) electrons — tied to the nucleus, sitting in inner shells (e.g. 3d in Fe/Co/Ni, 4f in rare earths). These carry the magnetism.
• Conduction electrons — delocalized over the lattice (e.g. 4s in Fe); they carry the electrical current.

Key Results

1. Origin: unpaired spins in inner shells

In iron (Z = 26), the electronic configuration is

$$[\mathrm{Ar}]3d^{6}4s^2.$$

By Hund’s first rule, the 3d electrons maximize total $S$: four of the six 3d electrons occupy different orbitals with parallel spin, leaving $\sim 4$ unpaired spins per atom and an atomic moment of roughly $2$–$2.2{\mu }_B$ in metallic Fe. Pair them all up — as happens for C or O in typical bonds — and the atomic moment vanishes.

The same mechanism is responsible for the magnetic moments of Co, Ni (3d transition metals) and Gd, Dy, etc. (4f rare earths).

2. Why a current does not erase the magnetism

A common confusion: if “electrons are moving through the wire,” why doesn’t the magnetism flow away? Because two different electron populations live side by side:

• The bound 3d electrons stay locked in their atomic shells. The nucleus and these inner-shell electrons remain fixed in the lattice (they only vibrate thermally), so the atomic magnetic moment stays put.
• The conduction 4s electrons move through the lattice and carry the current. They feel the local exchange field of the bound moments, which polarizes them and causes spin-dependent scattering — the microscopic origin of giant magnetoresistance in a spin valve and of the spin-polarized current in spintronics.

This separation of roles — localized moments + itinerant carriers — is the s–d model of itinerant ferromagnetism, and it is what makes spintronics work.

Summary

• An atom is magnetic when at least one of its electrons is unpaired (typically in a partially filled 3d or 4f shell).
• The magnetism resides in the bound electrons, which stay fixed with the nucleus in the lattice.
• The current is carried by the conduction electrons, which are spin-polarized by the bound moments but never erase them.
• Therefore a ferromagnet can simultaneously be a magnet and a conductor — the prerequisite for every spintronic device.

Connections

• magnetic-moment — the vector quantifying the atomic magnetism
• magnetization — collective field built from atomic moments
• spin — intrinsic angular momentum of the electron (stub)
• spin-polarized-current — current shaped by the bound moments (stub)
• spin-valve — device exploiting spin-dependent scattering

References

• J. M. D. Coey, Magnetism and Magnetic Materials (Cambridge, 2010), Ch. 3–4.
• N. W. Ashcroft & N. D. Mermin, Solid State Physics, Ch. 31–32.
• S. Blundell, Magnetism in Condensed Matter (Oxford, 2001).