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Ensembles of identical atoms exhibit peculiar collective properties in their interaction with radiation depending on geometry and environment where they are embedded in. A remarkably clean and versatile platform to study collective effects in resonant light scattering are M\"ossbauer nuclei placed in planar x-ray waveguides. Here we conceive and demonstrate experimentally distinct temporal emission characteristics in these systems, ranging from a tunable accelerated exponential decay all the way to a pronounced oscillatory emission pattern, depending on the waveguide geometry and mode of excitation. The observed temporal and spatial emission characteristics of the collectively excited nuclear state in the waveguide -- the nuclear exciton -- are well reproduced by a unified theoretical model. Our findings pave the way for applications ranging from fundamental studies of cooperative emission at hard x-ray frequencies up to new methods of narrowband x-ray control via the engineering of collective radiation patterns.

We present a study of the control of electric field induced strain on the magnetic and electrical transport properties in a magneto-elastically coupled artificial multiferroic Fe3O4/BaTiO3 heterostructure. In this Fe3O4/BaTiO3 heterostructure, the Fe3O4 thin film is epitaxially grown in the form of bilateral domains, analogous to a-c stripe domains of the underlying BaTiO3(001) substrate. By in-situ electric field dependent magnetization measurements, we demonstrate the extrinsic control of the magnetic anisotropy and the characteristic Verwey metal-insulator transition of the epitaxial Fe3O4 thin film in a wide temperature range between 20-300 K, via strain mediated converse magnetoelectric coupling. In addition, we observe strain induced modulations in the magnetic and electrical transport properties of the Fe3O4 thin film across the thermally driven intrinsic ferroelectric and structural phase transitions of the BaTiO3 substrate. In-situ electric field dependent Raman measurements reveal that the electric field does not significantly modify the anti-phase boundary defects in the Fe3O4 thin film once it is thermodynamically stable after deposition and that the modification of the magnetic properties is mainly caused by strain induced lattice distortions and magnetic anisotropy. These results provide a framework to realize electrical control of the magnetization in a classical highly correlated transition metal oxide., Comment: 11 pages, 5 figures

We report the structural and magnetic properties of four-layer Aurivillius compound Bi$_5$FeTi$_3$O$_{15}$ (BFTO) at high hydrostatic pressure conditions. The high-pressure XRD data does not explicitly show structural phase transitions with hydrostatic pressure, however the observed changes in lattice parameters indicate structural modifications at different pressure values. In the initial pressure region values, the lattice parameters $\textit{a}$- and $\textit{b}$- are nearly equal implying a quasi-tetragonal structure, however as the pressure increases $\textit{a}$- and $\textit{b}$- diverges apart and exhibits complete orthorhombic phase at pressure values of about $\geq$8 GPa. Principal component analysis of high pressure Raman measurements point out an evident change in the local structure at about 5.5 GPa indicating that the evolution of the local structure under applied pressure seems to not follow crystallographic changes (long range order). Nuclear forward scattering (NFS) measurement reveal the development of magnetic ordering in BFTO at 5K with high pressures. A progressive increase in magnetic order is observed with increase in pressure at 5K. Further, NFS measurements carried out at constant pressure (6.4GPa) and different temperatures indicate that the developed magnetism disappears at higher temperatures (20K). It is attempted to explain these observations in terms of the observed structural parameter variation with pressure., Comment: 8 pages, 5 figures

The microstructures and interfaces of two-phase vertically aligned nanocomposite (VAN) thin films play a key role in the design of spintronic device architectures and their multifunctional properties. Here, we show how the microstructures in self-assembled VAN thin films of La0.7Sr0.3MnO3:NiO (LSMO:NiO) can be effectively tuned from nano-granular to nano-columnar, and to nano-maze by controlling the number of laser shots from the two constituent phase targets in the pulsed laser deposition (PLD) film growth. The observed microstructural induced strain is found to significantly enhance the magnetoresistance in a very broad temperature range between 10-240 K and to modulate the in-plane exchange bias (EB), with the largest EB value observed in the maximally strained heterostructures. Most interestingly, a unique perpendicular exchange bias (PEB) effect is also observed for these heterostructures with an enhanced PEB field of up to 230 Oe. X-ray magnetic circular dichroism and training effect measurements demonstrate that the observed EB is disorder-induced and arises due to the pinning of NiO uncompensated moments at the disordered interface which is ferromagnetically coupled with LSMO. Furthermore, systematic changes in the electronic structure across the vertical interface related to a variation of the Mn3+/Mn4+ content arise as a consequence of out-of-plane tensile strain., Comment: 19 pages, 8 figures, Supporting information

PbTiO3 has the highest tetragonal distortion (c/a=1.064) and highest spontaneous polarization among perovskite titanates. But, it is hazardous and hence one needs to reduce Pb content by substituting or reducing Pb content for use in applications. Pb(1-x)(K0.5Sm0.5)xTiO3 (0&x&0.5) perovskite powders were synthesized by sol-gel process, where Pb2+ was replaced by a combination of K+10.5Sm+30.5 (equivalent charge and comparable ionic radius) providing an excellent substitution model to study changes in structural and electrical properties. Vibrational properties and dielectric properties are modified with substitution. A polar tetragonal to a nearly nonpolar cubic phase transition decreases to lower temperatures with substitution due to reduces the lattice strain with substitution. Ferroelectricity is retained even for x=0.5, which has a nearly cubic phase and makes the material technologically important., Comment: 13 pages, 6 figures

The present work reports the preparation of polycrystalline multiferroic $GdFeO_3$ (GdFO) and characterization with x-ray diffraction (XRD), magnetization, temperature dependent Raman spectroscopy, temperature and magnetic field dependent $^{57}Fe$ M$\ddot{o}$ssbauer spectroscopy measurements. The sample is found to be phase pure from Rietveld refinement of XRD pattern. The M$\ddot{o}$ssbauer spectra measured in the presence of external magnetic field show the signatures of field induced spin reorientation transition, which are corroborated by magnetization measurements. From the temperature dependent variation of internal hyperfine field, N$\grave{e}$el transition temperature ($T_{N,Fe}$) of 672.5$\pm$0.2 K and critical exponent ($\beta$) of 0.333$\pm$0.003 is obtained. Temperature dependent (300 - 760 K) Raman spectroscopy measurements show the signatures of spin-phonon coupling and local structural re-arrangement across $T_{N,Fe}$., Comment: 8 pages, 12 figures