RESUMO
Magnetic tunnel junction (MTJ) is a leading contender for next generation high-density nonvolatile memory technology. Fast and efficient switching of MTJs between different resistance states is a challenging problem, which can be tackled by using an unconventional stimulus-a femtosecond laser pulse. Herein, we report an experimental study of the laser-induced magnetization dynamics in a Co20Fe60B20/MgO/Co20Fe60B20 (CoFeB/MgO/CoFeB) MTJ with ultrathin CoFeB electrodes possessing perpendicular magnetic anisotropy (PMA). In addition to ultrafast demagnetization, a femtosecond laser pulse gives rise to a decaying magnetization precession in the thinner CoFeB layer subjected to an in-plane magnetic field, while the magnetization of the thicker CoFeB layer remains aligned with the applied field. Remarkably, the precession frequency demonstrates a strong and nonlinear rise with increasing pump fluence, which stems from the complete laser-induced suppression of PMA in the 1.2 nm-thick CoFeB electrode reached at a moderate fluence of about 1.8 mJ cm-2 at room temperature. This important feature signifies that the laser excitation of such an electrode can enable an ultrafast transition from a perpendicular-to-plane to an in-plane magnetization orientation in the absence of a magnetic field and reveals the feasibility of the laser-driven switching of MTJ between different states. The revealed gradual quenching of PMA with increasing fluence is explained by the laser-induced heating of the MTJ, which affects the interfacial magnetic anisotropy stronger than the shape anisotropy. Interestingly, at low fluences, the values of interfacial anisotropy and saturation magnetization altered by the laser excitation scale with each other as expected for the two-site anisotropic exchange interaction, but the scaling exponent increases significantly at moderate fluences, which enables the realization of a laser-induced spin reorientation transition.
RESUMO
Ferroelectric nanocomposites are intriguing nonhomogeneous materials, which may have unusual phase states and specific physical properties useful for practical applications. Here we theoretically describe dielectric properties of nanocomposites comprising single-domain ferroelectric nanocrystals embedded into a linear dielectric medium. First, small-signal intrinsic permittivities of spheroidal PbTiO3 and BaTiO3 crystallites surrounded by an isotropic matrix with linear elastic properties are calculated with the aid of a nonlinear thermodynamic theory. It is shown that thermal stresses caused by differences in thermal expansion between the inclusions and the matrix may strongly influence the intrinsic permittivities of ferroelectric nanocrystals. Second, macroscopic dielectric responses of ferroelectric-dielectric composites are evaluated in the Maxwell Garnett approximation. For effective permittivities of such composites, generalized relations are derived, which allow for both the shape and dielectric anisotropies of ferroelectric nanocrystals. Numerical calculations of the effective permittivities are performed for composites comprising PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. For nanocomposites involving randomly oriented and similarly aligned spheroidal inclusions, temperature dependences of the effective permittivities are determined with the account of phase transitions occurring in strained ferroelectric nanocrystals and suppression of their dielectric responses due to the depolarizing-field effect created by low-permittivity matrix. Our theoretical calculations show that effective permittivities of composites comprising needle-like or disc-shaped ferroelectric inclusions could strongly exceed the permittivity of the matrix. Most importantly, we predict that BaTiO3-K2O-SiO2 composites with such inclusions should exhibit a broad dielectric peak around the room temperature, which is associated with shifted structural transitions in strained BaTiO3 nanocrystals. The presented theory provides guidelines for the development of ferroelectric nanocomposites with enhanced dielectric responses, which could be suitable for applications in supercapacitors and other advanced electronic devices.
RESUMO
Among recently discovered ferroelectricity-related phenomena, the tunnelling electroresistance (TER) effect in ferroelectric tunnel junctions (FTJs) has been attracting rapidly increasing attention owing to the emerging possibilities of non-volatile memory, logic and neuromorphic computing applications of these quantum nanostructures. Despite recent advances in experimental and theoretical studies of FTJs, many questions concerning their electrical behaviour still remain open. In particular, the role of ferroelectric/electrode interfaces and the separation of the ferroelectric-driven TER effect from electrochemical ('redox'-based) resistance-switching effects have to be clarified. Here we report the results of a comprehensive study of epitaxial junctions comprising BaTiO(3) barrier, La(0.7)Sr(0.3)MnO(3) bottom electrode and Au or Cu top electrodes. Our results demonstrate a giant electrode effect on the TER of these asymmetric FTJs. The revealed phenomena are attributed to the microscopic interfacial effect of ferroelectric origin, which is supported by the observation of redox-based resistance switching at much higher voltages.
RESUMO
Spin-polarized currents represent an efficient tool for manipulating ferromagnetic nanostructures but the critical current density necessary for the magnetization switching is usually too high for applications. Here we show theoretically that, in magnetic tunnel junctions having electric-field-dependent interfacial anisotropy, the critical density may reduce down to a very low level (~10(4) A cm(-2)) when the junction combines small conductance with the proximity of free layer to a size-driven spin reorientation transition. The theory explains easy magnetization switching recently discovered in CoFeB/MgO/CoFeB tunnel junctions, surprisingly showing that it happens when the spin-transfer torque is relatively small, and provides a recipe for the fabrication of magnetic tunnel junctions suitable for industrial memory applications.