Field-induced vortex-like textures as a probe of the critical line in reentrant spin glasses

We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe$$_{1-x}$$ 1 - x Mn$$_{x}$$ x )$$_{75}$$ 75 P$$_{16}$$ 16 B$$_{3}$$ 3 Al$$_{3}$$ 3 (“a-Fe$$_{1-x}$$ 1 - x Mn$$_{x}$$ x ”). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the so-called “reentrant” spin-glass (RSG) phase, up to the critical concentration $$x_{\mathrm{C}} \approx 0.36$$ x C ≈ 0.36 which separates the RSG and “true” spin glass (SG) within the low temperature part of the magnetic phase diagram of a-Fe1−xMnx. These textures systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample ($$x = 0.41$$ x = 0.41 ), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the critical line between the RSG and SG phases.