Hidden quantum phase transition in Mn1−xFexGe evidenced by small-angle neutron scattering

Abstract

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...

Magnetic small-angle neutron scattering

“…Since the system is strongly anisotropic with the moments confined in the (001) plane, the phase transition is considered to be quasi two-dimensional. A qualitatively similar behavior was observed by Altynbaev et al, 2016 in powder samples of B20 MnGe (crystallite size 1 µm).…”

Topological transitions among skyrmion- and hedgehog-lattice states in cubic chiral magnets

“…Thus, the DMI may not be the primary origin of the short-period helical structure in 3D HL. Instead, the magnetic frustration or Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction 30–33 causing competing ferromagnetic and antiferromagnetic EXIs can be a possible mechanism. Possibly related to such conduction-electron mediated exchange interactions, we found that the strong Hubbard- U implemented in the band structure calculation favors a short-period helical structure (Supplementary Fig.…”

Complex magnetism of B20-MnGe: from spin-spirals, hedgehogs to monopoles

“…The main finding from these works is that D is too small when combined with A to explain the experimentally observed short period of the magnetic structure, which is given by λ ≈ 4πA/D. Experimentally, the magnetic structure has been interpreted as consisting of helical spirals 21,[25][26][27]29 as in other B20 compounds, with the short period ascribed to competing long-range magnetic interactions 26,29 . Transport signatures and LTEM imaging strongly back a more complex magnetic structure, which has initially been interpreted as a conventional twodimensional skyrmion lattice 19,20,31,33 , and afterwards as a three-dimensional skyrmion-antiskyrmion or hedgehogantihedgehog lattice 28,30,32,35 .…”