Structural and magnetic chiralities are found to coexist in a small group of materials in which they produce intriguing phenomenologies such as the recently discovered skyrmion phases. Here, we describe a previously unknown manifestation of this inte
rplay in MnSb2O6, a trigonal oxide with a chiral crystal structure. Unlike all other known cases, the MnSb2O6 magnetic structure is based on co-rotating cycloids rather than helices. The coupling to the structural chirality is provided by a magnetic axial vector, related to the so-called vector chirality. We show that this unique arrangement is the magnetic ground state of the symmetric-exchange Hamiltonian, based on ab-initio theoretical calculations of the Heisenberg exchange interactions, and is stabilised by out-of-plane anisotropy. MnSb2O6 is predicted to be multiferroic with a unique ferroelectric switching mechanism.
The magnetic structure for the newly discovered iron-arsenide compound CaFeAs has been studied by neutron powder diffraction. Long-range magnetic order is detected below 85K, with an incommensurate modulation described by the propagation vector k=(0,
$delta$,0), $deltasim$ 0.39. Below $sim$ 25K, our measurements detect a first-order phase transition where $delta$ locks into the commensurate value 3/8. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density-waves with magnetic moments directed along the textit{b}-axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular the drop in resistivity at the lock-in transition.
The magnetic properties of RMn2O5 multiferrroics as obtained by unpolarized and polarized neutron diffraction experiments are reviewed. We discuss the qualitative features of the magnetic phase diagram both in zero magnetic field and in field and ana
lyze the commensurate magnetic structure and its coupling to an applied electric field. The origin of ferrolectricity is discussed based on calculations of the ferroelectric polarization predicted by different microscopic coupling mechanisms (exchange striction and cycloidal spin-orbit models). A minimal model containing a small set of parameters is also presented in order to understand the propagation of the magnetic structure along the c-direction.
Precise magnetic structures of RMn2O5, with R= Y, Ho, Bi in the commensurate/ferroelectric regime, have been determined by single-crystal neutron diffraction. For each system, the integrated intensities of a large number of independent magnetic Bragg
reflections have been measured, allowing unconstrained least-squares refinement of the structures. The analysis confirms the previously reported magnetic configuration in the ab-plane, in particular the existence of zig-zag antiferromagnetic chains. For the Y and Ho compounds additional weak magnetic components parallel to the c-axis were detected which are modulated in phase quadrature with the a-b components. This component is extremely small in the BiMn2O5 sample, therefore supporting symmetric exchange as the principal mechanism inducing ferroelectricity. For HoMn2O5, a magnetic ordering of the Ho moments was observed, which is consistent with a super-exchange interaction through the oxygens. For all three compounds, the point symmetry in the magnetically ordered state is m2m, allowing the polar b-axis found experimentally.
