Chiral criticality in the doped helimagnets Mn${}_{1-y}$Fe${}_y$Si

The critical spin fluctuations in Mn${}_{1-y}$Fe${}_y$Si compounds have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering, and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous yet well distinguishable crossovers: (i) from paramagnetic to partially chiral and (ii) from partially chiral to highly chiral fluctuating state. The temperature crossovers are associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.

Figure 1

The sample with concentration $y=0.08$. (a) The temperature dependencies $\chi$ (red squares), $d\chi /\mathit{dT}$ (blue circles), and $d^2\chi /{\mathit{dT}}^2$ (dashed line) in the magnetic field $H=50$ mT; four different temperature regions are shown (S, HCF, PCF, and P; see explanations in text). Four SANS maps corresponding to the shadowed/colored areas on the temperature scale are: (b) $T<T_c$, (c) $T_c⩽T⩽T_k$, (d) $T_k⩽T⩽T_{\mathit{DM}}$, and (e) $T>T_{\mathit{DM}}$.

Figure 2

The inverse correlation length $\kappa$ (squares) and the polarization $P_s$ (circles) as a function of the temperature $T-T_c$ for Mn${}_{0.92}$Fe${}_{0.08}$Si. Straight lines are guides to the eyes.

Figure 3

The relaxation rate of the fluctuations $\Gamma$ as a function of the temperature $T-T_c$ for Mn${}_{0.92}$Fe${}_{0.08}$Si.

Figure 4

ac-susceptibility measurements. The same temperature dependencies as in Fig. 1a but for other three concentrations: $y=0.09,0.10$, and $0.12$ on the upper, the middle, and the lower graphs, respectively. Four different temperature regions, S, HC, PC and P, are also shown, colored the same as in Fig. 1a.

Figure 5

Concentration dependencies of the critical temperature $T_{\mathrm{c}}$ and the crossovers points $T_{\mathit{DM}}$ and $T_k$ for Mn${}_{1-y}$Fe${}_y$Si compounds. The inset shows the temperature dependencies $\chi$, $d\chi /\mathit{dT}$, and $d^2\chi /{\mathit{dT}}^2$ in the magnetic field $H=50$ mT for the sample with $y=0.10$.

Figure 6

The temperature dependence of resistivity $\rho$ and its first derivative $d\rho /\mathit{dT}$ for Mn${}_{0.92}$Fe${}_{0.08}$Si.