Valerio Scagnoli

Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties.

Charge ordering transition in GdBaCo2O5: Evidence of reentrant behavior

We present a detailed study on the charge ordering transition in a GdBaCo2O5.0 system by combining highresolution synchrotron powder/single-crystal diffraction with electron paramagnetic resonance experiments as a function of temperature. We found a second-order structural phase transition at T-CO=247 K (Pmmm to Pmma) associated with the onset of long-range charge ordering. At T-min approximate to 1.2T(CO), the electron paramagnetic resonance linewidth rapidly broadens, providing evidence of antiferromagnetic spin fluctuations. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo2O5.0 sets in at approximate to T-CO. Pair distribution function analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above T-CO the local structure is consistent with a fully random occupation of Co2+ and Co3+ in a 1:1 ratio and with a complete charge ordering below T-CO. Below T approximate to 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of charge ordering. This result is supported by the weakening of superstructure reflections and the temperature evolution of electron paramagnetic resonance linewidth that is consistent with paramagnetic reentrant behavior reported in the GdBaCo2O5.5 parent compound.

Three-dimensional magnetization structures revealed with X-ray vector nanotomography

In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities—Bloch points—occur. These were predicted more than fifty years ago but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure and a twisted state that appears to correspond to an ‘anti-Bloch point’. Our imaging method enables the nanoscale study of topological magnetic structures in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications.

Magnetic order dynamics in optically excited multiferroic TbMnO3

We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales.

Acentric magnetic and optical properties of chalcopyrite (CuFeS2)

The absence of spatial inversion symmetry at both local (point group 4) and global (crystal class (4)2m) levels greatly influences the electronic properties of chalcopyrite (CuFeS(2)). The predicted dichroic signals (natural circular, non-reciprocal and magneto-chiral) and resonant, parity-odd Bragg diffraction patterns at space-group forbidden reflections portray the uncommon, acentric symmetry. Despite extensive experimental investigations over several decades, by mineralogists, chemists and physicists, there is no consensus view about the electrical and magnetic properties of chalcopyrite. New spectroscopic and diffraction data, gathered at various temperatures in the vicinity of the copper and iron L(2,3) edges, provide necessary confidence in the magnetic motif used in our analytic simulations of x-ray scattering. With the sample held at 10 and 65 K, our data establish beyond reasonable doubt that there is no valence transition, and ordering of the copper moments as the origin of the low-temperature phase (T(c) ≈ 53 K) is ruled out.

Crystal symmetry lowering in chiral multiferroic Ba3TaFe3Si2O14 observed by x-ray magnetic scattering

Chiral multiferroic langasites have attracted attention due to their doubly chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft x-ray diffraction study of the multiferroic Ba3TaFe3Si2O14 at the FeL2,3 and oxygenK edges. BelowTN (≈27K)we observe the satellite reflections (0,0,τ ), (0,0,2τ ), (0,0,3τ ), and (0,0,1 − 3τ) where τ ≈ 0.140 ± 0.001. The dependence of the scattering intensity on x-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane (ˆc axis) magnetic dipole while the (0,0,2τ ) originates from the electron density distortions accompanying magnetic order.We observe dissimilar energy dependencies of the diffraction intensity of the purelymagnetic odd-harmonic satellites at the Fe L3 edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase.

Coherent Acoustic Perturbation of Second-Harmonic-Generation in NiO

We investigate the structural and magnetic origins of the unusual ultrafast second-harmonic-generation (SHG) response of femtosecond-laser-excited nickel oxide (NiO) previously attributed to oscillatory reorientation dynamics of the magnetic structure induced by d-d excitations. Using time resolved x-ray diffraction from the (3/2 3/2 3/2) magnetic planes, we show that changes in the magnitude of the magnetic structure factor following ultrafast optical excitation are limited to Delta / = 1.5% in the first 30 ps. An extended investigation of the ultrafast SHG response reveals a strong dependence on wavelength as well as characteristic echoes, both of which give evidence for an acoustic origin of the dynamics. We therefore propose an alternative mechanism for the SHG response based on perturbations of the nonlinear susceptibility via optically induced strain in a spatially confined medium. In this model, the two observed oscillation periods can be understood as the times required for an acoustic strain wave to traverse one coherence length of the SHG process in either the collinear or anticollinear geometries.

Correlated electron systems: Emitting electrons through phonons

Ultrashort laser pulses create strain waves that generate highly mobile charges at an oxide interface. These charges propagate into the oxide layer destroying its antiferromagnetic ordering and insulating properties, providing insight into the physics of metal–insulator transitions.

Emitting electrons through phonons: Correlated electron systems

Ultrashort laser pulses create strain waves that generate highly mobile charges at an oxide interface. These charges propagate into the oxide layer destroying its antiferromagnetic ordering and insulating properties, providing insight into the physics of metal–insulator transitions.

Emitting electrons through phonons

Ultrashort laser pulses create strain waves that generate highly mobile charges at an oxide interface. These charges propagate into the oxide layer destroying its antiferromagnetic ordering and insulating properties, providing insight into the physics of metal–insulator transitions.