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Register a new userNonreciprocal propagation of surface acoustic wave in Ni/LiNbO3
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We have investigated surface acoustic wave propagation in Ni/LiNbO$_3$ hybrid devices. We have found the absorption and phase velocity are dependent on the sign of wave vector in a device, which indicates the nonreciprocal propagation characteristic of systems with time reversal and spatial inversion simultaneously broken symmetries. The nonreciprocity is reversed by the 180$^circ$ rotation of magnetic field. Nonreciprocity seems largely dependent on the shape of ferromagnetic Ni film. The origin of these observations is ascribed to film shape dependent magnetoelastic coupling.
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One of the most fundamental forms of magnon-phonon interaction is an intrinsic property of magnetic materials, the magnetoelastic coupling. This particular form of interaction has been the basis for describing magnetic materials and their strain related applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with a unprecedented ratio up to 100$%$ rectification at the theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling, but also justify the feasibility of the magneto-rotation coupling based application.
Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.
Surface Acoustic Wave (SAW) resonances were imaged within a closed domain in the ferroelectric LiTaO$_3$ via scanning Microwave Impedance Microscopy (MIM). The MIM probe is used for both SAW generation and measurement, allowing contact-less measurement within a mesoscopic structure. Measurements taken over a range of microwave frequencies are consistent with a constant acoustic velocity, demonstrating the acoustic nature of the measurement.
Surface acoustic wave (SAW) devices based on thin films of ZnO are a well established technology. However, SAW devices on bulk ZnO crystals are not practical at room temperature due to the significant damping caused by finite electrical conductivity of the crystal. Here, by operating at low temperatures, we demonstrate effective SAW devices on the (0001) surface of bulk ZnO crystals, including a delay line operating at SAW wavelengths of {lambda} = 4 and 6 {mu}m and a one-port resonator at a wavelength of {lambda} = 1.6 {mu}m. We find that the SAW velocity is temperature dependent, reaching $v simeq 2.68$ km/s at 10mK. Our resonator reaches a maximum quality factor of $Q_i simeq 1.5times 10^5$, demonstrating that bulk ZnO is highly viable for low temperature SAW applications. The performance of the devices is strongly correlated with the bulk conductivity, which quenches SAW transmission above about 200 K.
The equilibrium ground-state structure of LiNbO3 in the paraelectric and ferroelectric phases is fully optimized in a first-principles calculation using the full-potential linearized augmented plane wave method. The equilibrium volume, c/a ratio and all (four, in the ferroelectric phase) internal parameters are found to be in good agreement with the experimental data. Frozen phonon calculations are performed for TO-Gamma phonons corresponding to the A1 and A2 irreducible representations of the R3c space group in the ferroelectric phase. The comparison with available experimental frequencies for the A1 modes is satisfactory (including the 6Li isotope effect), and the displacement patterns are unambiguously attributed. For the (Raman inactive) A2 modes, phonon frequencies and eigenvectors are predicted.
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