Scanning Tunneling Microscopy (STM) allows the investigation of the electronic nature of materials near the atomic scale.  Using a magnetic probe tip the STM can be made sensitive to the spin of the tunneling electrons as well.

The basic working principle is well-known as spin-valve effect: since any ferromagnetic material exhibits a spin imbalance at the Fermi-level EF and the electron spin has to be preserved in an elastic tunneling process the conductivity of a tunnel junction must depend on the relative magnetization direction between tip and sample.

Using non-magnetic probe tips coated by a thin (typically < 10 atomic layers) film of magnetic material researchers are able to measure both the in-plane as well as the out-of-plane magnetization component of the sample, respectively. The smallest magnetic features observed so-far were domain walls with a width of 0.6 nm in ferromagnetic iron films in W(110) and the atomic scale antiferromagnetic structure of a manganese monolayer.
 

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