Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nanoparticles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick et al. [Nano Lett. 4, 639 (2004)] and Wang et al. [Nano Lett. 4, 643 (2004)].     

Polarization potentials due to inelastic excitations

Dynamic polarization potentials generated by inelastic excitations of the projectile or target nuclei for three scattering systems are compared. Effects due to excitations to bound/unbound states as well as due to different multipolarities are studied. It is shown that the potentials exhibit similar radial dependence at large projectile-target separations; real parts change with the increased radius from repulsive to attractive while imaginary parts, close to zero or even emissive at the nuclear surface, are becoming absorptive at large distances. This general tendency is a consequence of the interplay between Coulomb and nuclear forces.     

Current-induced spin injection and surface torque in ferromagnetic metallic junctions

The joint influence of two current-induced effects, namely, longitudinal nonequilibrium spin injection and surface torque, on spin-valve-type ferromagnetic metallic junctions is considered theoretically. The current flows normally to layer boundaries. The analysis is based on solving a system of coupled equations of motion for mobile electron and lattice magnetizations. The boundary conditions for the equations of motion are derived from the continuity condition for the total magnetization flux in these subsystems. A dispersion relation is derived for spin wave fluctuations depending on the current through the junction. The fluctuations become unstable at currents exceeding some threshold value (usually, 10⁶?3 × 10⁷ A/cm²). The joint action of longitudinal spin injection and torque lowers the instability threshold. Current-induced spin injection decreases spin wave frequencies near the threshold and can strengthen magnetization pinning at the injecting contact.     

Anomalous surface enhanced molecular Raman scattering from inelastic tunneling spectroscopy junctions

Anomalously strong Raman spectra have been obtained from molecular monolayers adsorbed on the insulator in metal-insulator-metal tunnel junctions. We show unambiguously that Raman spectroscopy can readily detect molecular monolayers and consider the effects of surface roughness, the molecule-metal interface and the metal on our results.     

Anisotropic magnetoresistance and anisotropic tunneling magnetoresistance due to quantum interference in ferromagnetic metal break junctions

We measure the low-temperature resistance of permalloy break junctions as a function of contact size and the magnetic field angle in applied fields large enough to saturate the magnetization. For both nanometer-scale metallic contacts and tunneling devices we observe large changes in resistance with the angle, as large as 25% in the tunneling regime. The pattern of magnetoresistance is sensitive to changes in bias on a scale of a few mV. We interpret the effect as a consequence of conductance fluctuations due to quantum interference.     

Spin-polarized tunneling in ferromagnetic double barrier junctions

Spin-polarized tunneling in FMS/M/FMS double tunnel junctions where FMSs are ferromagnetic semiconductor layers and M is a metal spacer is studied theoretically within the single-site coherent potential approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated in the framework of the s-f model. The spin polarization in the FMS layers is observed to oscillates as a function of the number of atomic planes in the spacer layer. Amplitude of these oscillations decreases with increasing the exchange interaction in FMS layers. Received 9 June 2001 and Received in final form 20 August 2001     

Non local inelastic lifetime in micro-ohm tunneling junctions

By quenched condensation onto amorphous Sb ultrathin Cu films were prepared in the thickness range of a few atomic layers. They exhibit very short inelastic lifetimes. We investigated, whether their inelastic electronic lifetime is affected by coupling them to thick Cu films with much longer inelastic lifetimes. Our measurements show, that for sufficiently strong electronic coupling the inelastic lifetime of the ultra-thin Cu film is enhanced and finally becomes comparable to the inelastic lifetime of the thick Cu film. We also measured the Coulomb anomaly in the temperature dependence of the resistance. Our films confirm previous results, that the dynamical Coulomb interaction seems not to be long ranged, as expected.     

Dipole-exchange spin excitations in a thin ferromagnetic nanoshell

In this paper spin excitations in spherical ferromagnetic nanoshells are investigated. The magnetic dipoledipole interaction, the exchange interaction and the anisotropy effects are taken into consideration. For such spin excitations, an equation for the magnetic potential perturbation is obtained. For a nanoshell that is thin compared to its size, the dispersion relation for nonzero spin excitation modes and the only possible frequency for zero-mode spin excitations are found. Limitations on the mode numbers are derived.     

Simulations of inelastic electron tunneling spectroscopy of semifluorinated hexadecanethiol junctions

The inelastic electron tunneling spectroscopy (IETS) of semifluorinated hexadecanethiol junctions is theoretically studied. The numerical results show that the C-F vibration modes of semifluorinated alkanethiol series can not be detected, and the C-H stretching mode in IETS is related to the CH₂ vibration. It is demonstrated that the Raman modes are preferred over IR modes in IETS, which is in good agreement with the experimental measurements presented by Beebe et al. [Nano Lett., 2007, 7(5): 1364].      

Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements

Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.     

Micromagnetic phase transitions and spin wave excitations in a ferromagnetic stripe

Magnetic excitations of micrometer-wide ferromagnetic stripes subjected to a transverse applied field have been measured between 1 and 20 GHz. The complexity of the observed response is attributed to the spatially nonuniform equilibrium spin distribution. This one is modeled analytically and numerically, which allows one to distinguish two micromagnetic phases governing the ground state. The nucleation-related phase transitions are evidenced by soft modes, while the different observed resonances are attributed to spin wave modes localized in the two phases and at their interface.     

Solitary excitations in one-dimensional ferromagnetic spin chains with biquadratic exchange interaction

By using the traveling wave method, the solutions of the elliptic function wave and the solitary wave are obtained in a ferromagnetic spin chain with a biquadratic exchange interaction, a single ion anisotropic interaction and an anisotropic nearest neighbour interaction. The effects of the biquadratic exchange interaction and the single ion anisotropic interaction on the properties (width, peak and stability) of the soliton are investigated. It is also found that the effects vary with the strengths of these interactions.     

π-junction realization due to tunneling through a thin ferromagnetic layer

It is demonstrated that in superconductor-ferromagnet-superconductor (S/F/S) systems in the case of low interface transparency the transition into π-phase is not related with the oscillations of the superconducting order parameter in F-layer. Consequently, the π-phase may exist at very thin F-layer thickness. The crossover from π-to 0-phase results in the nonmonotonous temperature dependence of the critical current.     

Interplay of spin accumulation and Coulomb blockade in double ferromagnetic junctions

Single-electron tunneling in a double junction in which a nonmagnetic metallic island is separated from two ferromagnetic electrodes by tunnel barriers, is analysed theoretically in the sequential tunneling regime and for an arbitrary intrinsic spin relaxation time on the island. It is shown that nonequilibrium spin polarization of the island results in tunnel magnetoresistance due to rotation of the electrode magnetizations from antiparallel to parallel alignment. It is also shown that discrete charging of the island gives rise to a fine structure in the voltage dependence of the island spin polarization and tunnel magnetoresistance     

Near-field spectral effects due to electromagnetic surface excitations

We demonstrate theoretically that the spectra of electromagnetic emission of surface systems can display remarkable differences in the near and the far zones. The spectral changes occur due to the loss of evanescent modes and are especially pronounced for systems which support surface waves.     

Large spin Hall effect and tunneling magnetoresistance in iridium-based magnetic tunnel junctions

Magnetic tunnel junctions(MTJs) switched by spin-orbit torque(SOT) have attracted substantial interest owing to their advantages of ultrahigh speed and prolonged endurance. Both field-free magnetization switching and high tunneling magnetoresistance(TMR) are critical for the practical application of SOT magnetic random access memory(MRAM). In this work, we propose an MTJ structure based on an iridium(Ir) bottom layer. Ir metal is a desirable candidate for field-free SOT switching owing to its strong intrinsic spin Hall conductivity(SHC), which can be enhanced via doping. Herein, we study TMR in Ir-based MTJs with symmetric and asymmetric structures. Ir-based MTJs exhibit large TMR, which can be further enhanced by heavy metal symmetry owing to the resonant tunneling effect. Our comprehensive investigations illustrate that Ir-based MTJs are promising candidates for realizing SOT switching and high TMR.     

Quantum tunneling in ferromagnetic nanoparticles interacting with a spin thermostat: Effective Hamiltonian

An effective Hamiltonian is obtained which describes quantum tunneling in ferromagnetic nanoparticles in the presence of a hyperfine interaction of the electron spins in the nanoparticle with the microscopic spins in the environment (such as paramagnetic impurities or nuclear spins). A criterion of transition between ferromagnetic and antiferromagnetic tunneling regimes is found. The validity of the equations obtained is checked by the method of exact diagonalization. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 27–32 (10 January 1998)