Suppression of the bulk high spin–low spin transition by doping the chiral magnet MnGe

Abstract : In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds Mn 0.86 Co 0.14 Ge and Mn 0.9 Rh 0.1 Ge, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures (≈19 GPa, to be compared with ≈13 GPa). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys. In itinerant magnets, the strong sensitivity of the magnetic moment to fine details of the band structure may result in magnetic states which become energetically equivalent for certain values of the lattice constant. This can, for instance, lead to electronic transitions, between a high spin (HS) state (with a large specific volume) toward a low spin (LS) state (with smaller volume), that can be observed by varying either temperature or pressure. One of the most spectacular consequences of such phenomenon is believed to be the Invar effect [1,2], discovered by Guillaume in Fe-Ni alloys [3], with numerous industrial applications. Other examples of HS-LS transitions can be found in molecular compounds containing transition metals atoms [4] (Co, Fe, Mn), such as the Prussian blues analogs [5], due to the strong sensitivity of the crystal field to external parameters (pressure, temperature, or light). Helical magnets with a noncentrosymmetric space group, such as MnSi or FeGe, are textbook examples of itinerant magnetism, hosting skyrmion lattices and being highly sensitive to pressure and chemical substitution. Their helimagnetic ground states are built upon a hierarchical energy scheme involving ferromagnetic (FM) exchange, Dzyaloshinskii-Moriya (DM) interaction, and crystalline anisotropy energies [6]. It collapses under pressure, yielding non-Fermi liquid behavior and partial ordering of fluctuating magnetic moments [7,8]. In this family, MnGe stands as an exception. Its short helical period of ≈30 Å [9,10] cannot be explained by a bare competition between a FM exchange and DM interactions , since it would require an unphysically large spin-orbit coupling. Recent experiments suggest the presence of a 3d soliton lattice without need of a magnetic field [11], possibly triggered by topological chiral interactions [12]. Strikingly, * nicolas.martin@cea.fr when applying pressure, the magnetic order and local moment of MnGe collapse in two steps, through HS and LS states [13], followed by a zero spin (ZS) state [14]. This peculiar behavior was predicted by ab initio calculations [15], showing rigid shifts of the spin-split bands upon compression. The HS-LS transition is associated with irreversibilities of the lattice constant [14], strongly recalling Invar anomalies [16,17]. Under chemical substitution of Mn for 3d-Co or 4d-Rh atoms, helical order in MnGe strongly changes, showing the onset of very long period structures above a certain doping level (x > 0.3 and 0.5 for Rh and Co, respectively), with characteristics similar to certain cholesteric liquid crystals [18]. These substitutions yield either a compression (Co) or a dilatation (Rh) of the cubic lattice constant a. At lower doping, when MnGe helical order is preserved, one could then expect that the chemical pressure resulting from the substitution would either enhance (Co) or counteract (Rh) the effect of the applied pressure. This should yield a shift of the HS-LS transition (situated at p C1 ≈ 6 GPa in pure MnGe) toward lower (Co) or higher (Rh) pressures, depending on the nature of the substituting ion. In order to check the above scenario, we have used high-pressure neutron diffraction to study two samples with low doping level, namely, Mn 0.86 Co 0.14 Ge and Mn 0.9 Rh 0.1 Ge. From the lattice constants at ambient pressure and at T = 1.5 K (a = 4.767 Å for Mn 0.86 Co 0.14 Ge and 4.794 Å for Mn 0.9 Rh 0.1 Ge [18]), one should expect pressure shifts of −1.8 GPa for Co doping and +0.9 GPa for Rh doping with respect to MnGe (a = 4.785 Å). Strikingly, instead of a rigid shift of the critical pressure, we observe a complete smearing of the transitions in both cases. The helical period and ground state magnetic moment gradually decrease without showing any critical behavior in the studied pressure range (i.e., up to 9 GPa). Moreover, the magnetic moments of the two compounds vary in very similar ways and do not universally 2469-9950/2019/100(6)/060401(5) 060401-1
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N. Martin, M. Deutsch, T. Hansen, M. Fernandez-Diaz, L. Fomicheva, et al.. Suppression of the bulk high spin–low spin transition by doping the chiral magnet MnGe. Physical Review B : Condensed matter and materials physics, American Physical Society, 2019, 100 (6), pp.060401. ⟨10.1103/PhysRevB.100.060401⟩. ⟨hal-02276287⟩

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