Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites

The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with temperature in YMnO$_3$ using neutron total scattering techniques, and interpret them with the help of first-principles calculations. The results show that, at room temperature, the local and average structures are consistent with the established ferroelectric $P6_3cm$ symmetry. On heating, both local and average structural analyses show striking anomalies from $\sim 800$ K up to the Curie temperature consistent with increasing fluctuations of the order parameter angle. These fluctuations result in an unusual local symmetry lowering into a \textit{continuum of structures} on heating. This local symmetry breaking persists into the high-symmetry non-polar phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure

Local atomic and magnetic structure of dilute magnetic semiconductor (Ba,K)(Zn,Mn)2_2As2_2

We have studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba,K)(Zn,Mn)$_2$As$_2$ through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. We detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of $\lesssim 5$ \AA, resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment of Mn spins along the crystallographic $c$-axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. We discuss these results in the context of other experiments and theoretical studies on this system

Magnetic and structural properties of thin films and nanoparticles studied by scattering methods

The present work concerns the magnetic and structural proprieties of magnetic thin films and magnetic nanoparticles studied by scattering methods. The structural properties of epitaxially grown Fe/Cr/Fe trilayer were studied with anomalous x-ray scattering. Two different x-ray energies have been used; (i) one E$_{1}$=5985eV to match the maximum contrast of the Fe/Cr interface close to the Cr absorption K-edge and (ii) a second one E$_{2}$=6940eV where the Fe/Cr interface displays the lowest contrast. The specular reflectivity and longitudinal diffuse scans together with omega scans for both energies were measured. The simulations within the frame of Distorted Wave Born Approximation (DWBA) allowed us to describe quantitatively the morphology of each interface. The roughness, Hurst parameter and the thickness of every layer as well as an oxidation effect at the surface of the sample are derived. The strength and limitations of the method are discussed. The influence of confinement on the magnetic behavior, was investigated in MnO embedded in a porous glass. We studied the magnetic order and phase transition of MnO nanoparticles by polarized neutron scattering. The nanopores filled with MnO are wormlike structures with a typical diameter of 7nm. From the temperature dependence of the magnetic ($\frac{1}{2}$ $\frac{1}{2}$ $\frac{1}{2}$) Bragg intensity we obtained that for the MnO nanoparticles the phase transition is continuous with a Néel temperature T$_{N}$=122K. This is in contrast to bulk MnO, which exhibits a first order phase transition at 118K. Furthermore, we observed that a part of the MnO nanoparticle material remains disordered even at 10K, which indicates frustration at the interface MnO to glass. The continuous character of the transition and the unusual temperature dependence with a reduced order parameter was described and modelled by surface induced disorder. In order to study the spin-canting effect in magnetic nanoparticles, we explored a synthesis route of Co nanoparticles in which we employed water-in-oil microemulsions that are stabilized by the nonionic surfactants of the ethoxylated alkyl- or arylether series such as C$_{12}$E$_{5}$ and Igepal CO520 and contain the NaBH$_{4}$-solution. Co nanoparticle formation is then induced by injecting a solution of Co(AOT)$_{2}$ in hexane. We found out that the water inside the microemulsion leads already to oxidation of the particles. [...