Multiferroic behavior in the new double-perovskite Lu2_2MnCoO6_6

We present a new member of the multiferroic oxides, Lu$_2$MnCoO$_6$, which we have investigated using X-ray diffraction, neutron diffraction, specific heat, magnetization, electric polarization, and dielectric constant measurements. This material possesses an electric polarization strongly coupled to a net magnetization below 35 K, despite the antiferromagnetic ordering of the $S = 3/2$ Mn$^{4+}$ and Co$^{2+}$ spins in an $\uparrow \uparrow \downarrow \downarrow$ configuration along the c-direction. We discuss the magnetic order in terms of a condensation of domain boundaries between $\uparrow \uparrow$ and $\downarrow \downarrow$ ferromagnetic domains, with each domain boundary producing a net electric polarization due to spatial inversion symmetry breaking. In an applied magnetic field the domain boundaries slide, controlling the size of the net magnetization, electric polarization, and magnetoelectric coupling

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2_2S4_4

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc$_2$S$_4$ by directly observing the 'spiral surface' - a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the $J_1$-$J_2$ Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc$_2$S$_4$, and also demonstrate a new way to realize a magnetic vortex lattice.Comment: 10 pages, 11 figure

New Insights on the Reversible Lithiation Mechanism of TiO 2

International audienceOperando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements provide new insights on the mechanism of lithium insertion into TiO2(B). The investigation of the evolution of electronic, long-range, and local structure during electrochemical cycling indicates a purely monophasic insertion mechanism upon lithium insertion, while global and local structure are only slightly modified. While XRD reflects an anisotropic lattice expansion, EXAFS reveals a wide distribution of Ti–O bond length, in line with the presence of two distinct distorted octahedral Ti environments, in agreement with previous DFT calculations. Upon lithium insertion, these Ti–O coordination shells undergo significant modifications which are enhanced once the insertion of 0.4 Li is exceeded, connoting a two regime process that is in good agreement with the electrochemical signature of this material. DFT calculations and local chemical bond analyses were coupled with experimental results, thus providing additional insights into the structural response of TiO2(B) upon lithiation