Phys. Rev. Lett. 118, 097203 (2017)

In condensed matter, structural inhomogeneities may favour the nucleation of topological defects within ordered phases. Fast thermal quenching can generate defect pairs with opposite topological character even in the absence of structural disorder, bearing intriguing similarities with the predictions of cosmological theories. However, although this latter scenario has been verified experimentally in several classes of materials, the observation of quench-induced defect pairs in magnets is still missing.

Now, Eggebrecht et al. report on the generation of metastable magnetic textures in Fe/Si3N4 bilayers at room temperature. Using laser pulses with duration ∼100 fs, the researchers heat the Fe layer locally, inducing a ferromagnet-to-paramagnet transition in μm2-sized areas. After the light pulse, the underlying Si3N4 substrate induces thermal quenches with ultrahigh rates ∼1012 K s−1. The researchers then image the magnetic configuration of the iron layer by means of Lorentz electron microscopy before and after the laser pulse.

As long as the laser fluence is above a threshold value, a quench-induced generation of a dense, glass-like network of vortices and antivortices with fluid-like spatial pair correlations is observed (pictured). The observed nanoscale texture is stable over months, even though further sub-threshold laser pulses favour the annihilation of defect pairs.