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abstracts
Incommensurate helical (or cycloidal) magnetic structure may have left- and right-wound states (helicity), which are in principle equally populated in a magnet with inversion symmetry. In addition, for a Heisenberg triangular antiferromagnet, clockwise and counter-clockwise rotations of the 120 degree spin structure provide another intriguing degree of freedom. Hence, a triangular magnet that has incommensurate helical ordering along the stacking direction will show intriguing interplay of the helicity (of the helical structure) and chirality (in the triangular plane). Such phenomenon is, however, rarely studied in the past since only one example, the Ba3NbFe3Si2O14 langathite, has been known to date [1]. In this work, we study MnSb2O6, which consists of distorted triangular lattice stacking along the c-axis [2,3]. MnSb2O6 belongs to the space group P321, and hence lacks inversion symmetry. Due to this fact, unique selection of the helicity and chirality may be expected. However, the earlier studies were carried out using unpolarized neutron diffraction with mostly the powder sample, and thus helicity and chirality selection cannot be concluded. Here, we have performed single-crystal diffraction experiment using polarized neutrons in addition to the unpolarized ones, and have succeeded in determination of the magnetic structure of MnSb2O6. The resulting magnetic structure is nearly cycloidal with the magnetic modulation vector q = (0, 0, 0.182) (see figure below). The spin rotation plane is, however, inclined from the ac-plane toward the b-axis for approximately 30 degrees. Polarization analysis indicates that both the helicity of the (nearly-) cycloidal structure and chirality of the in-plane 120 degree structure are uniquely selected. The 30 degree inclination from the ac-plane is a key finding of this work, allowing new kind of multiferroicity in this material.
abstracts
Since the discovery, research on iron-based superconductivity (SC) has become one of the main streams in condensed matter physics [1]. The interplay between structure, magnetism and SC is one of most intriguing subjects of this field. The common structural feature is the presence of square planar sheets of Fe atoms coordinated tetrahedrally by pnictogens or chalcogens. They have been, at the early stage, realized in the ZrCuSiAs (1111), ThCr2Si2 (122), anti-PbO (11) and Cu2Sb (111) structures. To gain further insight into the mechanism of the SC and variation of magnetic orders, investigation of Fe-based compounds with a separate spatial dimension is important. This is because the dimensionality should influence magnetism and can control itinerancy of electrons by changing Fermi surface topology. As spin ladders in copper oxides shed a new light on the mechanism of SC, a study on an analogue with ladder geometry among Fe-based compounds is highly desired. Here we report our recent studies of iron-based ladder compounds AFe2X3 (A = K, Rb, Cs, Ba; X = S, Se, Te) [2,3]. Crystal structure is novel, comprising of FeX4 tetrahedra with channels which host A atoms, and four-fold coordinated Fe2+ ions form two-leg ladder geometry. Unlike most of parent compounds of the Fe-based SCs, the ladder compounds are insulating down to the lowest measured temperature. Through bulk properties and neutron diffraction measurements, a variety of magnetic structures and low dimensional characteristics were elucidated. These would provide a clue of the SC realized in a separate dimension systems. The description of theory that accounts for the observed magnetic structures will be also presented.
