Magnetoresistive effects are usually invariant on inversion of the magnetization direction. In non-centrosymmetric conductors, however, nonlinear resistive terms can give rise to a current dependence that is quadratic in the applied voltage and linear in the magnetization. Here we demonstrate that such conditions are realized in simple bilayer metal films where the spin-orbit interaction and spin-dependent scattering couple the current-induced spin accumulation to the electrical conductivity. We show that the longitudinal resistance of Ta|Co and Pt|Co bilayers changes when reversing the polarity of the current or the sign of the magnetization. This unidirectional magnetoresistance scales linearly with current density and has opposite sign in Ta and Pt, which we associate with the modification of the interface scattering potential induced by the spin Hall effect in these materials. Our results suggest a route to control the resistance and detect magnetization switching in spintronic devices using a two-terminal geometry, which applies also to heterostructures including topological insulators.T he effects of the magnetization on the electric conductivity of metals have been studied for a long time 1 , providing understanding of fundamental phenomena associated with electron transport and magnetism as well as a multitude of applications in sensor technology. The anisotropic magnetoresistance (AMR)-the change of the resistance of a material on rotation of the magnetization-is a prominent manifestation of spin-orbit coupling and spin-dependent conductivity in bulk ferromagnets 2,3 . In thin-film heterostructures, the additional possibility of orienting the magnetization of stacked ferromagnetic layers parallel or antiparallel to each other gives rise to the celebrated giant magnetoresistance (GMR) effect 4,5 , which has played a major role in all modern developments of spintronics. Together with the early spin-injection experiments 6,7 , the study of GMR revealed how non-equilibrium spin accumulation at the interface between ferromagnetic (FM) and normal metal (NM) conductors governs the propagation of spin currents 8-11 and, ultimately, the conductivity of multilayer systems 10,12 .Recently, it has been shown that significant spin accumulation at a FM/NM interface can be achieved using a current-in-plane (CIP) geometry owing to the spin Hall effect (SHE) in the NM (ref. 13). When the FM is a metal and NM is a heavy element such as Pt or Ta, the spin accumulation is strong enough to induce magnetization reversal of nanometre-thick FM layers at current densities of the order of j = 10 7 -10 8 A cm −2 (refs 14-16). When the FM is an insulator, such as yttrium iron garnet, the SHE causes an unusual magnetoresistance associated with the back-flow of a spin current into the NM when the spin accumulation μ s ∼ (j ×ẑ) is aligned with the magnetization of the FM, which increases the conductivity of the NM due to the inverse SHE (refs 17-20). This so-called spin Hall magnetoresistance (SMR) is characterized by R y < R z ≈ R x ...

We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field-dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers.We then study ferromagnet/heavy metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6nm)/Co(2.5nm) and negligible in Pt(6nm)/Co(2.5nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the field-like SOTs are about an order of magnitude smaller.

We report scanning tunneling spectroscopy measurements of the threshold energy for injecting electrons or holes into thin, conjugated polymer films deposited on Au(111) substrates. Combining these results with optical absorption measurements, we estimate an exciton binding energy of E b 0.36 6 0.10 eV for poly[(2-methoxy-5-dodecyloxy)-1,4-phenylenevinylene-co-1,4-phenylenevinylene] and E b 0.30 6 0.10 eV for poly (9,9'-dioctylfluorene). In addition, we determine the alignment of the electronic levels of the polymers relative to the substrate. [S0031-9007(98)06767-2]

The scanning force microscope is used to deposit charge carriers on insulating Si3N4 films and to monitor their recombination. The charge decay shows up as a discontinuous staircase, demonstrating singlecarrier resolution. The decay is found to be controlled by thermionic emission.PACS numbers: 73.25.+i, 61.16.Di, 73.40.Bf, 73.50.Gr The scanning force microscope (SFM) introduced by Binnig, Quate, and Gerber' is a remarkably useful instrument to map the surface topography of virtually any solid.Most noticeably, atomic resolution has been achieved on insulating crystals. Besides the repulsive contact force used in profilometry, attractive forces like the quantum-mechanical exchange, electrostatic, and magnetic dipolar forces were utilized to measure such quantities as metallic adhesion, electric surface potential variation, and magnetic domain structures.In two recent Letters, Stern et al. and Terris er al.have demonstrated that the SFM can be used to deposit and image charges on insulators, both by contact electrification (CE) as well as by corona discharge (CD).They speculated that single carriers could be observable.In this Letter the observation of single-charge recombination events is described. Ionized particles are deposited by a short CD (10 ms) onto silicon nitride (Si3N4) films prepared by plasma-enhanced chemical-vapor deposition (PECVD) onto degenerately doped (conducting) GaAs substrates. The detected charge signal following the CD was observed to decay within a few seconds in a staircase fashion, showing the charge quantization. The experiments described were performed in air at ambient conditions. The force is detected with a diA'erential interferometer described elsewhere. Figure 1 is a schematic of the tip apex, positioned at a distance d above an insulating film of thickness h. A voltage V applied to the tungsten tip (-) v" Insulator . :=. -, -'--==: Substrate~W~~~F IG. 1. Schematic explaining the detection of the excess charge q on an insulating film via the force Fl related to the image charge q;. results in the Coulomb force F=(V+&) G, where Gcontains the tip radius R, d, and h, as well as the relative dielectric constant s of the film. The term p is the contact potential between the tip and sample. Applying an ac voltage V= J2V", sin(cot) results in two measured oscillating force terms at m and 2', with the respective rms amplitudes F~=2V PG and F2 = V J2G from which G and p can be extracted. The tip-to-sample distance d is adjusted by a controller loop so that F2 and hence G remain constant. Since V and G are both constant, any contrast in F~is due to a varying contact potential p. A charge carrier q lying on the surface of the insulator induces image charges q; and q in the tip and the substrate, respectively (Fig. 1). Since charge conservation requires q;+q =q, the magnitude of q;/q is always less than unity, increasing when the tip-sample distance d is decreased with respect to the film thickness h.The value of F I is approximately given by F I =q; E" where E, is the rms electric field ...

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