We report powder and single crystal neutron diffraction measurements of the magnetic order in AMnBi2 (A = Sr and Ca), two layered manganese pnictides with anisotropic Dirac fermions on a Bi square net. Both materials are found to order at TN ≈ 300 K in k = 0 antiferromagnetic structures, with ordered Mn moments at T = 10 K of approximately 3.8 µB aligned along the c axis. The magnetic structures are Néel-type within the Mn-Bi layers but the inter-layer ordering is different, being antiferromagnetic in SrMnBi2 and ferromagnetic in CaMnBi2. This allows a meanfield coupling of the magnetic order to Bi electrons in CaMnBi2 but not in SrMnBi2. We find clear evidence that magnetic order influences electrical transport. First principles calculations explain the experimental observations and suggest that the mechanism for different inter-layer ordering in the two compounds is the competition between the anteiferromagnetic superexchange and ferromagnetic double exchange carried by itinerant Bi electrons.

By combining bulk properties, neutron diffraction and non-resonant X-ray diffraction measurements, we demonstrate that the new multiferroic Cu3Nb2O8 becomes polar simultaneously with the appearance of generalised helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins -an observation that cannot be reconciled with the "conventional" theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically-induced structural chirality. [5] are cycloidal multiferroics, so called because their magnetic structures can be described as incommensurate circular (or elliptical) modulations with the wavevectors in the plane of rotation of the spins. In cycloidal multiferroics, the non-collinear magnetic configuration itself is established by the competition between nearest and next-nearest neighbor interactions. The coupling to the crystal structure occurs through the spin-orbit interaction, making it energetically favorable to develop local Dzyaloshinskii-Moriya (DM) vectors, associated with a local polarization. In simple, high-symmetry cases, the electrical polarization is perpendicular both to the magnetic propagation vector and to the normal to the plane of rotation of the spins, since the following formula holds [6]: P = λk m × (s 1 × s 2 ), where s 1 and s 2 are two adjacent spins along the propagation direction k m and λ is a coupling constant. In more complex, lower symmetry cases, the electrical polarization need not be perpendicular to the propagation vector, which, in turn, need not be contained in the plane of rotation of the spins (generic helicoidal structures). However, it is a strong prediction of the cycloidal multiferroics model that if all the spins rotate in a common plane, then the electrical polarization must be strictly contained within that plane.In this letter, we present a new multiferroic, with chemical formula Cu 3 Nb 2 O 8 and centrosymmetric triclinic symmetry (space group P1) in the paramagnetic phase. Using magnetic neutron powder diffraction, magnetic susceptibility, heat capacity, electrical polarization and non-resonant X-ray magnetic scattering measurements, we show that Cu 3 Nb 2 O 8 orders magnetically at T N ∼ 26 K, and develops an electrical polarization below a second magnetic transition at T 2 ∼ 24 K. In the polar phase below T 2 , a coplanar helicoidal magnetic structure is stabilized with propagation vector k m = (0.4876, 0.2813, 0.2029) in a general direction in reciprocal space. Strikingly, the electrical polarization in Cu 3 Nb 2 O 8 (with a magnitude of 17.8 µCm −2 ) is almost exactly perpendicular to the plane of rotation of the spins, in clear contradiction with the predictions of the cycloidal multiferroics model. We conclude that the electrical polarization in Cu 3 Nb 2 O 8 must arise through coupling of the chiral component of the magnetic structure with the crystal structure, ra...

We report neutron inelastic scattering measurements on the stoichiometric iron-based superconductor LiFeAs. We find evidence for (i) magnetic scattering consistent with strong antiferromagnetic fluctuations, and (ii) an increase in intensity in the superconducting state at low energies, similar to the resonant magnetic excitation observed in other iron-based superconductors. The results do not support a recent theoretical prediction of spin-triplet p-wave superconductivity in LiFeAs, and instead suggest that the mechanism of superconductivity is similar to that in the other iron-based superconductors.

Coherent lattice vibrations are excited and probed with pulses of 10 fs duration in LaMnO(3). The measured frequencies correspond to those of Jahn-Teller stretching and of out-of phase rotations of the oxygen octahedra. Surprisingly, the amplitude and damping rate of both modes exhibit a sharp discontinuity at the Néel temperature, highlighting nontrivial coupling between light, lattice, and magnetic structure. We explain this effect by applying the Goodenough-Kanamori rules to the excited state of LaMnO(3), and note that charge transfer can invert the sign of the semicovalent exchange interaction, which in turn perturbs the equilibrium bond lengths.

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.