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Register a new userEvolution of Helimagnetic Correlations when approaching the Quantum Critical Point of Mn$_{1-x}$Fe$_x$Si
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We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^*$ and the quantum critical concentration $x_C sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^*$, does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral RKKY interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^*$.
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We present a comprehensive small angle neutron scattering study of the doping dependence of the helimagnetic correlations in Mn$_{1-x}$Fe$_{x}$Si. The long-range helimagnetic order in Mn$_{1-x}$Fe$_x$Si is suppressed with increasing Fe content and disappears for $x$ $>$ $x^*$ $approx$ 0.11, i.e. well before $x_C$ $approx$ 0.17 where the transition temperature vanishes. For $x$ $>$ $x^*$, only finite isotropic helimagnetic correlations persist which bear similarities with the magnetic correlations found in the precursor phase of MnSi. Magnetic fields gradually suppress and partly align these short-ranged helimagnetic correlations along their direction through a complex magnetization process.
This study presents the effect of local electronic correlations on the Heusler compounds Co$_2$Mn$_{1-x}$Fe$_x$Si as a function of the concentration $x$. The analysis has been performed by means of first-principles band-structure calculations based on the local approximation to spin-density functional theory (LSDA). Correlation effects are treated in terms of the Dynamical Mean-Field Theory (DMFT) and the LSDA+U approach. The formalism is implemented within the Korringa-Kohn-Rostoker (KKR) Greens function method. In good agreement with the available experimental data the magnetic and spectroscopic properties of the compound are explained in terms of strong electronic correlations. In addition the correlation effects have been analysed separately with respect to their static or dynamical origin. To achieve a quantitative description of the electronic structure of Co$_2$Mn$_{1-x}$Fe$_x$Si both static and dynamic correlations must be treated on equal footing.
We report a comprehensive small-angle neutron scattering~(SANS) study of Mn$_{1-x}$Fe$_{x}$Si at zero magnetic field. To delineate changes of magneto-crystalline anisotropies (MCAs) from effects due to defects and disorder, we recorded complementary susceptibility and specific heat data, and investigated selected compositions of Mn$_{1-x}$Co$_{x}$Si. For all systems studied the transition temperature and magnetic phase diagrams evolve monotonically with composition consistent with literature. The SANS patterns of the magnetic order recorded under zero-field cooling display strong changes of the directions of the intensity maxima and smeared out intensity distributions as a function of composition. We show that cubic MCAs account for the complex evolution of the SANS patterns, where for increasing $x$ the character of the MCAs shifts from terms that are fourth-order to terms that are sixth order in spin--orbit coupling. The magnetic field dependence of the susceptibility and SANS establishes that the helix reorientation as a function of magnetic field for Fe- or Co-doped MnSi is dominated by pinning due to defects and disorder. The presence of thermodynamic anomalies of the specific heat at the phase boundaries of the skyrmion lattice phase in the doped samples and properties observed in Mn$_{1-x}$Co$_{x}$Si establishes that the pinning due to defects and disorder remains, however, weak and comparable to the field scale of the helix reorientation. The observation that MCAs, that are sixth order in spin-orbit coupling, play an important role for the spontaneous order in Mn$_{1-x}$Fe$_{x}$Si and Mn$_{1-x}$Co$_{x}$Si, offering a fresh perspective for a wide range of topics in cubic chiral magnets such as the generic magnetic phase diagram, the morphology of topological spin textures, the paramagnetic-to-helical transition, and quantum phase transitions.
Separating between ordinary (OHE) and anomalous (AHE) Hall effect in the paramagnetic phase of Mn$_{1-x}$Fe$_{x}$Si reveals OHE sign inversion associated with the hidden quantum critical (QC) point $x^*sim0.11$. The semimetallic behavior at intermediate Fe content leads to verifiable predictions in the field of fermiology, magnetic interactions and QC in Mn$_{1-x}$Fe$_{x}$Si. The change of electron and hole concentrations is considered as a driving force for tuning the QC regime in Mn$_{1-x}$Fe$_{x}$Si via modifying of RKKY exchange interaction within the Heisenberg model of magnetism.
We have measured de Haas-van Alphen oscillations of Cr$_{1-x}$V$_x$, $0 le x le 0.05$, at high fields for samples on both sides of the quantum critical point at $x_c=0.035$. For all samples we observe only those oscillations associated with a single small hole band with magnetic breakdown orbits of the reconstructed Fermi surface evident for $x<x_c$. The absence of oscillations from Fermi surface sheets most responsible for the spin density wave (SDW) in Cr for $x>x_c$ is further evidence for strong fluctuation scattering of these charge carriers well into the paramagnetic regime. We find no significant mass enhancement of the carriers in the single observed band at any $x$. An anomalous field dependence of the dHvA signal for our $x=0.035$ crystal at particular orientations of the magnetic field is identified as due to magnetic breakdown that we speculate results from a field induced SDW transition at high fields.
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