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Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe1−xCoxSi series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co and Mn. Our findings indicate that the sign of the DMI in Fe1−xCoxSi is controlled by the Co composition x. We have been able to directly measure the change of the link between structure and magnetism in this helimagnetic B20 alloy.PACS numbers: 61.12.Ex, Scattering of polarized neutrons on chiral magnetic structures allows one to determine the absolute magnetic configuration, thus left-and right-handed helices can be easily distinguished [1]. On the other hand, knowing the magnetic configuration, one can analyse the polarization of a scattering beam [2]. Similar effects could also help to manipulate spin polarization of an electron current providing that the electrons interact with the known chiral magnetic structure.The ability to manipulate the electron spin is a necessary component for the spintronics [3], thus magnetic chiral organic molecules [4] or large scale magnetic structures have been proposed as such tools [5]. However, the question how to get the magnetic structure of a necessary chirality for spintronics applications is still open.Here we address the question for the case of Fe 1−x Co x Si solid solutions which, for certain compositions, show chiral (spiral) magnetic ordering [6][7][8].The structural chirality in monosilicides of 3d-metals is solely controlled by crystal growth [9]. A link between the structural and magnetic chiralities is provided by the Dzyaloshinskii-Moriya interaction (DMI) and has been experimentally proved for many monosilicides of 3dmetals [9][10][11][12].

The magnetic system of the Mn1−xFexGe solid solution is ordered in a spiral spin structure in the whole concentration range of x ∈ [0 ÷ 1]. The close inspection of the small-angle neutron scattering data reveals the quantum phase transition from the long-range ordered (LRO) to short range ordered (SRO) helical structure upon increase of Fe-concentration at x ∈ [0.25 ÷ 0.4]. The SRO of the helical structure is identified as a Lorentzian contribution, while LRO is associated with the Gaussian contribution into the scattering profile function. The scenario of the quantum phase transition with x as a driving parameter is similar to the thermal phase transition in pure MnGe. The quantum nature of the SRO is proved by the temperature independent correlation length of the helical structure at low and intermediate temperature ranges with remarkable decrease above certain temperature TQ. We suggest the x-dependent modification of the effective RudermanKittel-Kasuya-Yosida exchange interaction within the Heisenberg model of magnetism to explain the quantum critical regime in Mn1−xFexGe. The cubic B20-type compounds (MnSi, etc) are well known for the incommensurate magnetic structures with a very long period appeared due to noncentrosymmetric arrangement of magnetic atoms. It is widely recognized that the helix spin structure is built on the hierarchy of interactions: ferromagnetic exchange interaction, antisymmetric Dzyaloshinskii-Moryia interaction (DMI), and the anisotropic exchange interaction [1,2]. It is also known that the substitution of manganese by iron in the isostructural solid solutions Mn 1−x Fe x Si suppresses the helical spin state [3]. The neutron scattering studies [4,5] together with magnetic data and specific heat measurements [3,6,7] discovered a quantum critical point (QCP) corresponding to the suppression of the spin spiral phase with long-range order (LRO) in Mn 1−x Fe x Si. This QCP located at x c1 ≈ 0.11 − 0.12 is, however, hidden by a short-range order of the spin helix (SRO) [5][6][7] that agrees well with the theoretical models [8,9]. This SRO phase, sometimes referred as chiral spin liquid [8], which is destroyed at the second QCP x c2 ≈ 0.24. Thus it has been shown that Mn 1−x Fe x Si undergoes a sequence of the two quantum phase transitions [7].The real breakthrough in understanding of the experimental facts mentioned above has been done via scrutinizing the Hall effect in Mn 1−x Fe x Si [10]. It was found that the substitution of Mn with Fe results rather in the hole doping opposite to the natural expectations on the electron doping. The two groups of the charge carriers contribute to the Hall effect and the ratio between them changes the sign of the Hall effect constants at x c1 ≈ 0.11, what is definitely associated with the QCP in these compounds. Despite the fact that the solid solutions of Mn 1−x Fe x Si are often considered as itinerant magnets [8,9], recent magnetic resonance and magnetoresistance studies [11,12] favor the alternative explanation based on the Heisenberg localized magn...

We have studied the spin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet FeGe in a temperature range from 225 K up to TC ≈ 278.7 K by small-angle neutron scattering. The method we have used is based on [S. V. Grigoriev et al. Phys. Rev. B 92 220415(R) (2015)] and was extended here for the application in polycrystalline samples. We confirm the validity of the anisotropic spin-wave dispersion for FeGe caused by the Dzyaloshinskii-Moriya interaction. We have shown that the spinwave stiffness A for FeGe helimagnet decreases with a temperature as A(T ) = 194(1 − 0.7(T /TC ) 4.2 ) meVÅ 2 . The finite value of the spin-wave stiffness A = 58 meVÅ 2 at TC classifies the order-disorder phase transition in FeGe as being the first order one.

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Synchrotron diffraction as a function of temperature and pressure, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. Similar but less pronounced behavior has also been observed in Mn1−xCoxGe and Mn1−xFexGe solid solutions. Spin density fluctuations and Mn spin state instability are discussed as possible candidates for the observed effects.

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