Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

Magnetoelectric transfer of a domain pattern.

The utility of ferroic materials is determined by the formation of domains and their poling behavior under externally applied fields. For multiferroics, which exhibit several types of ferroic order at once, it is also relevant how the domains of the coexisting ferroic states couple and what kind of functionality this might involve. In this work, we demonstrate the reversible transfer of a domain pattern between magnetization and electric-polarization space in the multiferroic Dy0.7Tb0.3FeO3. A magnetic field transfers a ferromagnetic domain pattern into an identical ferroelectric domain pattern while erasing it at its magnetic origin. Reverse transfer completes the cycle. To assess the generality of our experiment, we elaborate on its conceptual origin and aspects of application.

Metastable disordered phase in flash-frozen Prussian Blue analogues.

A new metastable phase in flash-frozen disordered Prussian blue analogues is reported. The phase is characterised by the appearance of diffuse scattering clouds and the reduction of the local structure symmetry: from cubic to a tetragonal or lower space group. The phase transition is characterised by the translational modulation of the structure and is likely caused by the freezing of the water confined in the pores of the structure.

Emerging spin-phonon coupling through cross-talk of two magnetic sublattices.

Many material properties such as superconductivity, magnetoresistance or magnetoelectricity emerge from the non-linear interactions of spins and lattice/phonons. Hence, an in-depth understanding of spin-phonon coupling is at the heart of these properties. While most examples deal with one magnetic lattice only, the simultaneous presence of multiple magnetic orderings yield potentially unknown properties. We demonstrate a strong spin-phonon coupling in SmFeO3 that emerges from the interaction of both, iron and samarium spins. We probe this coupling as a remarkably large shift of phonon frequencies and the appearance of new phonons. The spin-phonon coupling is absent for the magnetic ordering of iron alone but emerges with the additional ordering of the samarium spins. Intriguingly, this ordering is not spontaneous but induced by the iron magnetism. Our findings show an emergent phenomenon from the non-linear interaction by multiple orders, which do not need to occur spontaneously. This allows for a conceptually different approach in the search for yet unknown properties.

Magnetoelectric coupling of domains, domain walls and vortices in a multiferroic with independent magnetic and electric order.

Magnetically induced ferroelectrics exhibit rigidly coupled magnetic and electric order. The ordering temperatures and spontaneous polarization of these multiferroics are notoriously low, however. Both properties can be much larger if magnetic and ferroelectric order occur independently, but the cost of this independence is that pronounced magnetoelectric interaction is no longer obvious. Using spatially resolved images of domains and density-functional theory, we show that in multiferroics with separately emerging magnetic and ferroelectric order, the microscopic magnetoelectric coupling can be intrinsically strong even though the macroscopic leading-order magnetoelectric effect is forbidden by symmetry. We show, taking hexagonal ErMnO3 as an example, that a strong bulk coupling between the ferroelectric and antiferromagnetic order is realized because the structural distortions that lead to the ferroelectric polarization also break the balance of the competing superexchange contributions. We observe the manifestation of this coupling in uncommon types of topological defects like magnetoelectric domain walls and vortex-like singularities.

Direct observation of polar tweed in LaAlO3.

Polar tweed was discovered in mechanically stressed LaAlO3. Local patches of strained material (diameter ca. 5 µm) form interwoven patterns seen in birefringence images, Piezo-Force Microscopy (PFM) and Resonant Piezoelectric Spectroscopy (RPS). PFM and RPS observations prove unequivocally that electrical polarity exists inside the tweed patterns of LaAlO3. The local piezoelectric effect varies greatly within the tweed patterns and reaches magnitudes similar to quartz. The patterns were mapped by the shift of the Eg soft-mode frequency by Raman spectroscopy.

Interplay of chemical structure and magnetic order coupling at the interface between Cr2O3 and Fe3O4 in hybrid nanocomposites.

Hybrid nanocomposites based on ferrimagnetic (FiM) Fe3O4 and magnetoelectric antiferromagnetic (AFM) Cr2O3 nanocrystals were synthesized to offer a particular three-dimensional (3D) interface between the two oxides. This interface favours an intermixing process (demonstrated by combining Raman spectroscopy and magnetization measurements) that determines the final magnetic behavior.

Structures and magnetism of the rare-earth orthochromite perovskite solid solution LaxSm1-xCrO3.

A new mixed rare-earth orthochromite series, LaxSm1-xCrO3, prepared through single-step hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by the hydrothermal treatment of amorphous mixed-metal hydroxides at 370 °C for 48 h. Transmission electron microscopy (TEM) reveals the formation of highly crystalline particles with dendritic-like morphologies. Rietveld refinements against high-resolution powder X-ray diffraction (PXRD) data show that the distorted perovskite structures are described by the orthorhombic space group Pnma over the full composition range. Unit cell volumes and Cr-O-Cr bond angles decrease monotonically with increasing samarium content, consistent with the presence of the smaller lanthanide in the structure. Raman spectroscopy confirms the formation of solid solutions, the degree of their structural distortion. With the aid of shell-model calculations the complex mixing of Raman modes below 250 cm(-1) is clarified. Magnetometry as a function of temperature reveals the onset of low-temperature antiferromagnetic ordering of Cr(3+) spins with weak ferromagnetic component at Néel temperatures (TN) that scale linearly with unit cell volume and structural distortion. Coupling effects between Cr(3+) and Sm(3+) ions are examined with enhanced susceptibilities below TN due to polarization of Sm(3+) moments. At low temperatures the Cr(3+) sublattice is shown to undergo a second-order spin reorientation observed as a rapid decrease of susceptibility.