Quasi-two-dimensional character of the magnetic order-disorder transition in YMn$_6$Sn$_6$

The magnetic structure of YMn$_6$Sn$_6$ helimagnet has been studied by small-angle neutron scattering in the temperature range from 25 to 360 K. Two peaks with centers at 1.7 and 0 nm$^{-1}$ have been observed in the dependences of the intensity of neutron scattering on the momentum transfer. An increase in the temperature is accompanied by a decrease in the intensity of the first peak and an increase in the intensity of the second peak. In addition, the shape of the peak with the center at 1.7 nm$^{-1}$ changes from Gaussian to Lorentzian. This change can be explained by the fact of the magnetic order-disorder transition from the helimagnet to the state with spatial ferromagnetic fluctuations occurs in YMn$_6$Sn$_6$ in the interval of 250–360 K. The anisotropy of the shape along the momentum transfer is observed for the peak with the center at 1.7 nm$^{-1}$. This, as well as the compatible variation of the FWHM and intensity of the peak with the center at 1.7 nm$^{-1}$ with the temperature, indicates the quasi-two-dimensional character of the occurring magnetic transition.