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Critical fluctuations beyond the quantum phase transition in Dzyaloshinskii-Moriya helimagnets Mn_(1-x)Fe_xSi
Polarized small-angle neutron scattering was used for studying of critical fluctuations in the
Dzyaloshinskii–Moriya helimagnets Mn_(1 – x)Fe_xSi with x = 0.10, 0.15, 0.20. The Mn_(1 – x)Fe_xSi compounds are helically ordered below Tc and show a helical fluctuation regime above Tc in a wide range up to TDM. The critical
temperatures Tc and TDM decrease with x and tend to 0 at x = 0.11 and 0.17, respectively. It was shown
experimentally that three samples reveal properties of fluctuations in different regimes. The sample with x =
0.10 provides sharp narrow peak in polarized SANS maps for temperatures near Tc and in the ordered phase,
whereas in the one with x = 0.15 critical fluctuations are suppressed by the disorder which destroys long-range
magnetic order, corresponding scattering peaks being substantially wider than for x = 0.10. For the sample
with x = 0.20 even at lowest temperatures only traces of the half-moon scattering patterns are visible. The
degree of the scattering polarization is close to 1 for all three samples meaning that the corresponding helical
fluctuations are chiral. The Mn_(1 – x)Fe_xSi compounds represent an example of the system where ferromagnetic
exchange approaches zero but Dzyaloshinskii–Moriya interaction is finite and provides chiral rotation of
spins in magnetic fluctuations. We argue at the qualitative level that observed peculiarities can be attributed
to defect antiferromagnetic bonds which are added to the system by Fe ions and lead to finite correlation
length of the spiral at small temperatures for x > x_c.