Long-period helical structures and twist-grain boundary phases induced by chemical substitution in the Mn1−x(Co,Rh)xGe chiral magnet

Abstract

We study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for non magnetic 3d-Co and 4d-Rh ions. At high doping levels, we observe spin helices with very long periods -more than ten times larger than in the pure compound-and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at x > ∼ 0.45 and > ∼ 0.25 for Co-and Rh-doping, respectively. In analogy with observations made in cholesteric liquid crystals, we suggest that it reveals the presence of magnetic twist-grain-boundary phases, involving a dense short-range correlated network of screw dislocations. The dislocation cores are described as smooth textures made of non-radial double-core skyrmions.

Theoretical and experimental study of high-pressure synthesized B20-type compounds Mn1− x (Co,Rh) x Ge

“…In paper [14], the analysis of the experimental data for two compositions, Mn 0.4 Co 0.6 Ge and Mn 0.5 Co 0.5 Ge, has also demonstrated the presence of two-peak feature of SANS intensity in the temperature range from 5 K to 80 K. This feature has been described in [14] as consisting of main and satellite peaks. The authors [13], however, explain the second peak as a manifestation of the TGB phase. We join the latter opinion and consider the magnetic structure of Mn 1−x Co x Ge in this region as the LPH+TGB coexistence.…”

“…Unfortunately, there are no magnetization measurements for Mn 1−x Co x Ge. The available SANS results [13] obtained for Co/Rh doping up to x ≤ 0.5 produce only the magnetic moment at Mn site (µ Mn ) on the assumption that Co/Rh and Ge atoms bear no moment.…”

“…More recently, the characterization of magnetic structure in Mn 1−x Co x Ge at 0 ≤ x ≤ 0.9 has been performed using the SQUID magnetometer and SANS measurements [14]. Here, we applied a more sensitive experimental technique, which allowed us to refine the shape of the χ(T ) dependencies [13], which are more than one order of magnitude larger than in pure MnGe and the Mn-rich side (x ≤ 0.25) of the Mn 1−x (Co,Rh) x Ge series [13,14].…”

“…Mn1−xRhxGe Figure 9a displays the T -x magnetic phase diagram for Mn 1−x Rh x Ge constructed using the results of our previous susceptibility measurements at 0 ≤ x ≤ 1 [8], with taking account of the SANS results at 0 ≤ x ≤ 0.5 [13]. Two main regions of dissimilar magnetic behavior are filled with different colours.…”

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

“…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.…”