Light reflected from a magnet comes back with its polarisation – the direction its waves vibrate in, which polarised sunglasses filter – slightly turned, by an amount that follows the magnetisation. Because it needs nothing but a focused laser beam, it can read the magnetic state of a far narrower than a hair without touching it.
Going deeper
Light reflected from a magnet comes back with its polarisation slightly turned, by an amount that follows the magnetisation. A focused laser therefore reads the magnetic state of a flake a few micrometres across, without contacts – and the shape of the loop tells how many layers are ordered and how they couple.
Why the polarisation turns
In a magnetised material, left- and right-circularly polarised light see slightly different refractive indices, because ties the electronic transitions to the magnetisation. Linearly polarised light is a combination of the two, so on reflection the two components return with different phases and amplitudes: the plane of polarisation rotates by the Kerr angle and the light picks up some ellipticity.
The rotation is small – millidegrees to a degree – but it is proportional to the magnetisation, and it reverses when the magnetisation does. Measuring it with a balanced detector and a modulated beam brings the sensitivity far below a single atomic layer’s worth of moment.
What it did for 2D magnets
A single flake a few micrometres across contains far too few for a conventional . Magneto-optics solved that: the beam can be focused onto the flake itself, and the signal comes only from the illuminated spot. This is how in CrI3 and in thin Cr2Ge2Te6 was established in 2017.
The loop shape carries more than a yes or no. In CrI3 a monolayer gives a square loop with remanence at zero field, a bilayer gives almost no signal until a field flips one layer, and a trilayer gives a stepped loop – the layer-by-layer antiferromagnetic coupling read directly. Scanning the beam maps domains, and reflective magnetic does the same job by measuring absorption instead of rotation.
What it cannot do
The signal measures a projection of the magnetisation along the light path, so the geometry matters: the polar configuration senses out-of-plane magnetisation, and longitudinal or transverse arrangements are needed for in-plane magnets such as CrSBr. An antiferromagnet with no net moment gives no first-order signal at all, which is why and other probes are used there.
It is also not an absolute measurement. The Kerr angle depends on the material’s optical constants, the wavelength, the and interference in the layers beneath, so converting rotation into a magnetic moment requires modelling. The laser heats the sample too, which can shift the apparent , so power dependence should be checked before quoting one.
For specialists
Rotation and ellipticity of reflected polarised light in a magnetised medium (the Kerr effect), and the differential absorption of circular polarisations (RMCD). Sensitivity down to a single layer is what established magnetic order in CrI3 and Cr2Ge2Te6, and both give layer-resolved loops, domain images and the sign of without any contacts.