Spin current driven by ultrafast magnetization of FeRh.

Laser-induced ultrafast demagnetization is an important phenomenon that probes arguably the ultimate limits of the angular momentum dynamics in solid. Unfortunately, many aspects of the dynamics remain unclear except that the demagnetization transfers the angular momentum eventually to the lattice. In particular, the role and origin of electron-carried spin currents in the demagnetization process are debated. Here we experimentally probe the spin current in the opposite phenomenon, i.e., laser-induced ultrafast magnetization of FeRh, where the laser pump pulse initiates the angular momentum build-up rather than its dissipation. Using the time-resolved magneto-optical Kerr effect, we directly measure the ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure. A strong correlation between the spin current and the magnetization dynamics of FeRh is found even though the spin filter effect is negligible in this opposite process. This result implies that the angular momentum build-up is achieved by an angular momentum transfer from the electron bath (supplier) to the magnon bath (receiver) and followed by the spatial transport of angular momentum (spin current) and dissipation of angular momentum to the phonon bath (spin relaxation).

Electric and Magnetic Tuning of Gilbert Damping Constant in LSMO/PMN-PT(011) Heterostructure.

Electric field control of magnetodynamics in magnetoelectric (ME) heterostructures has been the subject of recent interest due to its fundamental complexity and promising applications in room temperature devices. The present work focuses on the tuning of magnetodynamic parameters of epitaxially grown ferromagnetic (FM) La0.7Sr0.3MnO3(LSMO) on a ferro(piezo)electric (FE) Pb(Mg0.33Nb0.67)O3-PbTiO3(PMN-PT) single crystal substrate. The uniaxial magnetic anisotropy of LSMO on PMN-PT confirms the ME coupling at the FM/FE heterointerface. The magnitude of the Gilbert damping constant (α) of this uniaxial LSMO film measured along the hard magnetic axis is significantly small compared to the easy axis. Furthermore, a marked decrease in the α values of LSMO at positive and negative electrical remanence of PMN-PT is observed, which is interpreted in the framework of strain induced spin dependent electronic structure. The present results clearly encourage the prospects of electric field controlled magnetodynamics, thereby realising the room temperature spin-wave based device applications with ultra-low power consumption.

Electric-Field Control of Propagating Spin Waves by Ferroelectric Domain-Wall Motion in a Multiferroic Heterostructure.

Magnetoelectric coupling in multiferroic heterostructures offers a promising platform for electric-field control of magnonic devices based on low-power spin-wave transport. Here, electric-field manipulation of the amplitude and phase of propagating spin waves in a ferromagnetic Fe film on top of a ferroelectric BaTiO3 substrate is demonstrated experimentally. Electric-field effects in this composite material system are mediated by strain coupling between alternating ferroelectric stripe domains with in-plane and perpendicular polarization and fully correlated magnetic anisotropy domains with differing spin-wave transport properties. The propagation of spin waves across the strain-induced magnetic anisotropy domains of the Fe film is directly imaged and it is shown how reversible electric-field-driven motion of ferroelectric domain walls and pinned anisotropy boundaries turns the spin-wave signal on and off. Furthermore, linear electric-field tuning of the spin-wave phase by altering the width of strain-coupled stripe domains is demonstrated. The results provide a new route toward energy-efficient reconfigurable magnonics.

Cation-Deficiency-Induced Crystal-Site Engineering for ZnGa2O4:Mn2+ Thin Film.

Zn-deficient spinel-type ZnGa2O4:Mn2+ phosphor thin films were prepared using pulsed laser deposition. With an increase (decrease) in the Zn deficiency (concentration) of the films, changes in lattice constant, optical band gap, and photoluminescence spectra were observed. All films without γ-Ga2O3:Mn showed green luminescence attributable to the transition from the 4T1 state to the 6A1 state. In addition, the spectral shape changed depending on the temperature. The luminescence spectra have two peaks resulting from the Mn2+ ions located in the tetrahedral and octahedral sites. These peaks had different thermal quenching temperatures, which were around 320 and 260 K, respectively. Therefore, the spectral shape changed with increasing temperature. The spectral shape also depended on the Zn concentration. With an increase (decrease) in the Zn concentration (deficiency) of the films, the intensity of emission from Td increased in comparison with that from Oh. Therefore, the position of Mn2+ was controlled by Zn deficiency similarly to the effect of crystal-site engineering.

Enhancement of Ultrahigh Rate Chargeability by Interfacial Nanodot BaTiO3 Treatment on LiCoO2 Cathode Thin Film Batteries.

Nanodot BaTiO3 supported LiCoO2 cathode thin films can dramatically improve high-rate chargeability and cyclability. The prepared BaTiO3 nanodot is <3 nm in height and 35 nm in diameter, and its coverage is <5%. Supported by high dielectric constant materials on the surface of cathode materials, Li ion (Li+) can intercalate through robust Li paths around the triple-phase interface consisting of the dielectric, cathode, and electrolyte. The current concentration around the triple-phase interface is observed by the finite element method and is in good agreement with the experimental data. The interfacial resistance between the cathode and electrolyte with nanodot BaTiO3 is smaller than that without nanodot BaTiO3. The decomposition of the organic solvent electrolyte can prevent the fabrication of a solid electrolyte interface around the triple-phase interface. Li+ paths may be created at non solid electrolyte interface covered regions by the strong current concentration originating from high dielectric constant materials on the cathode. Robust Li+ paths lead to excellent chargeability and cyclability.

Electric-field control of magnetism via strain transfer across ferromagnetic/ferroelectric interfaces.

By taking advantage of the coupling between magnetism and ferroelectricity, ferromagnetic (FM)/ferroelectric (FE) multiferroic interfaces play a pivotal role in manipulating magnetism by electric fields. Integrating the multiferroic heterostructures into spintronic devices significantly reduces energy dissipation from Joule heating because only an electric field is required to switch the magnetic element. New concepts of storage and processing of information thus can be envisioned when the electric-field control of magnetism is a viable alternative to the traditional current based means of controlling magnetism. This article reviews some salient aspects of the electric-field effects on magnetism, providing a short overview of the mechanisms of magneto-electric (ME) coupling at the FM/FE interfaces. A particular emphasis is placed on the ME effect via interfacial magneto-elastic coupling arising from strain transfer from the FE to FM layer. Recent results that demonstrate the electric-field control of magnetic anisotropy, magnetic order, magnetic domain wall motion, and etc are described. Obstacles that need to be overcome are also discussed for making this a reality for future device applications.

Comparative study of phase transitions in BaTiO3 thin films grown on (001)- and (110)-oriented SrTiO3 substrate.

An increase of the Curie temperature, TC, was demonstrated for BaTiO3 (BTO) epitaxial thin films grown by a pulsed laser deposition technique on (001) and (110) SrTiO3 (STO) substrates. The T(C) values for the BTO films on (001) and (110) STO were determined, from the temperature dependence of the lattice constants, to be 720 and 550 K, respectively, both of which are higher than that for bulk BTO. The increase of T(C) for the film on (001) BTO is suggested to stem from a large tetragonal deformation with a tetragonality of ~1.02 due to a two-dimensional clamping effect, which persists even in the paraelectric phase. That for the (110) BTO film, on the other hand, could be caused by a one-dimensional clamping effect due to uniaxial strain.

Ferroelectricity of Li-doped silver niobate (Ag, Li)NbO3.

Phase evolution in (Ag(1-x)Li(x))NbO(3) (ALN) solid solution was investigated by the x-ray diffraction technique, dielectric and polarization measurements. It is shown that a small substitution of Ag with Li gives rise to an orthorhombic-rhombohedral structural transformation in ABO(3)-perovskite silver niobate at room temperature. Structural refinements indicate that both the A- and B-site displacements contribute to the spontaneous polarization of the ferroelectric phase with symmetry R3c. Increasing Li-concentration enhances the ferroelectric rhombohedral distortion, resulting in an increase of the para-ferroelectric phase transition temperature and the polarization of the solid solutions.