Using a time-resolved optically-pumped scanning optical microscopy technique we demonstrate the laser-driven excitation and propagation of spin waves in a 20-nm film of a ferromagnetic metallic alloy Galfenol epitaxially grown on a GaAs substrate. In contrast to previous all-optical studies of spin waves we employ laser-induced thermal changes of magnetocrystalline anisotropy as an excitation mechanism. A tightly focused 70-fs laser pulse excites packets of magnetostatic surface waves with an e −1 -propagation length of 3.4 µm, which is comparable with that of permalloy. As a result, laser-driven magnetostatic spin waves are clearly detectable at distances in excess of 10 µm, which promotes epitaxial Galfenol films to the limited family of materials suitable for magnonic devices. A pronounced in-plane magnetocrystalline anisotropy of the Galfenol film offers an additional degree of freedom for manipulating the spin waves' parameters. Reorientation of an in-plane external magnetic field relative to the crystallographic axes of the sample tunes the frequency, amplitude and propagation length of the excited waves. arXiv:1904.05171v2 [cond-mat.str-el]

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Polarization states and physical properties of ferroelectrics depend on the mechanical boundary conditions due to electrostrictive coupling between electric polarization and lattice strains. Here, we describe theoretically both equilibrium thermodynamic states and electric permittivities of ferroelectric nanocrystals subjected to the elastic three-dimensional (3D) clamping by a surrounding dielectric material. The problem is solved by the minimization of a special thermodynamic potential that describes the case of an ellipsoidal ferroelectric inclusion embedded into a linear elastic matrix. Numerical calculations are performed for BaTiO3, PbTiO3, and Pb(Zr0.5Ti0.5)O3 nanoparticles surrounded by silica glass. It is shown that, in the case of BaTiO3 and PbTiO3, elastic 3D clamping may change the order of a ferroelectric phase transition from first to second. Furthermore, the mechanical inclusion–matrix interaction shifts the temperatures of structural transitions between different ferroelectric states and even eliminates some ferroelectric phases existing in stress-free BaTiO3 and Pb(Zr0.5Ti0.5)O3 crystals. Another important effect of elastic clamping is the lowering of the symmetry of ferroelectric states in ellipsoidal inclusions, where orthorhombic and monoclinic phases may form instead of the tetragonal and rhombohedral bulk counterparts. Finally, our thermodynamic calculations show that the dielectric responses of studied perovskite ferroelectrics are sensitive to matrix-induced clamping as well. For instance, dielectric peaks occurring at structural transitions between different ferroelectric phases in BaTiO3 appear to be much higher in spherical inclusions than in the freestanding crystal. Predicted clamping-induced enhancement of certain dielectric responses at room temperature indicates that composite materials comprising nanocrystals of perovskite ferroelectrics are promising for device applications requiring the use of high-permittivity dielectrics.

Ferroelectric crystallites embedded into a dielectric matrix experience temperature-dependent elastic strains caused by differences in the thermal expansion of the crystallites and the matrix. Owing to the electrostriction, these lattice strains may affect polarization states of ferroelectric inclusions significantly, making them different from those of a stress-free bulk crystal. Here, using a nonlinear thermodynamic theory, we study the mechanical effect of elastic matrix on the phase states of embedded single-domain ferroelectric nanocrystals. Their equilibrium polarization states are determined by minimizing a special thermodynamic potential that describes the energetics of an ellipsoidal ferroelectric inclusion surrounded by a linear elastic medium. To demonstrate the stability ranges of such states for a given material combination, we construct a phase diagram, where the inclusion's shape anisotropy and temperature are used as two parameters. The 'shape-temperature' phase diagrams are calculated numerically for PbTiO and BaTiO nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. The developed phase maps demonstrate that the joint effect of thermal stresses and matrix-induced elastic clamping of ferroelectric inclusions gives rise to several important features in the polarization behavior of PbTiO and BaTiO nanocrystals. In particular, the Curie temperature displays a nonmonotonic variation with the ellipsoid's aspect ratio, being minimal for spherical inclusions. Furthermore, the diagrams show that the polarization orientation with respect to the ellipsoid's symmetry axis is controlled by the shape anisotropy and the sign of thermal stresses. Under certain conditions, the mechanical inclusion-matrix interaction qualitatively alters the evolution of ferroelectric states on cooling, inducing a structural transition in PbTiO nanocrystals and suppressing in BaTiO inclusions some transformations occurring in their bulk counterpart. The constructed phase maps open the possibility to calculate dielectric properties of strained PbTiO and BaTiO nanocrystals and ferroelectric nanocomposites comprising such crystallites.

Using advanced micromagnetic simulations, we describe the coupled elastic and magnetic dynamics induced in ferromagnet/normal metal bilayers by shear waves generated by the attached piezoelectric transducer. Our approach is based on the numerical solution of a system of differential equations, which comprises the Landau-Lifshitz-Gilbert equation and the elastodynamic equation of motion, both allowing for the magnetoelastic coupling between spins and lattice strains. The simulations have been performed for heterostructures involving a Fe81Ga19 layer with the thickness ranging from 100 to 892 nm and a few-micrometer-thick film of a normal metal (Au). We find that the traveling shear wave induces inhomogeneous magnetic dynamics in the ferromagnetic layer, which generally has an intermediate character between coherent magnetization precession and the pure spin wave. Owing to the magnetoelastic feedback, the magnetization precession generates two additional elastic waves (shear and longitudinal), which propagate into the normal metal. Despite such complex elastic dynamics and reflections of elastic waves at the Fe81Ga19|Au interface, periodic magnetization precession with the excitation frequency settles in the steady-state regime. The results obtained for the magnetization dynamics at the Fe81Ga19|Au interface are used to evaluate the spin current pumped into the Au layer and the accompanying charge current caused by the inverse spin Hall effect. The calculations show that the dc component of the charge current is high enough to be detected experimentally even at small strains ∼10−4 generated by the piezoelectric transducer.