We have discovered a positive unidirectional exchange anisotropy in antiferromagnetic (FeF 2) and ferromagnetic (Fe) bilayers cooled through the antiferromagnetic critical temperature T N in large magnetic fields. For low positive cooling fields, the ferromagnet's magnetization (M-H) loop center shifts to negative fields, as is normally observed in other systems. In contrast, large cooling fields can cause the shift to be positive. This can be explained if the FeF 2 surface spins couple to the external magnetic cooling field above T N and the FeF 2-Fe interaction is antiferromagnetic.

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF2. A device scale on-chip heater is deposited on a bilayer of MnF2 (110) (30 nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF2 (110) substrate. Using Pt as a spin detector layer it is possible to measure thermally generated spin current from MnF2 through the inverse spin Hall effect. The low temperature (2 -80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel the easy axis of the MnF2 thin film. When magnetic field is applied perpendicular to the easy axis, the spin flop transition is absent, as expected.

Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi 2 Se 3 revealed a strong dependence of the carrier relaxation time on the film thickness. For thicker films the relaxation dynamics are similar to those of bulk Bi 2 Se 3 , where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.Topological insulators (TIs) are novel electronic materials that have an insulator-type band gap in the bulk (for Bi 2 Se 3 E g ~ 0.3 eV) but have protected gapless conducting phase on their surface due to the combination of spin-orbit interactions and time-reversal symmetry. 1,2 The most effective experimental methods currently used to monitor metallic two-dimensional (2D) Dirac surface states (SS) of TIs are angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (TrARPES). 1-7 These techniques are equally sensitive to SS and the bulk atoms residing in the close proximity to the surface as a consequence of the extremely small penetration depth (a few nm) of incident energetic photons used for photoemission, combined with the limited escape depth of the electrons (also a few nm). Finite-size effects have also been studied for thin Bi 2 Se 3 films of only a few nm thick and a crossover of the three-dimensional (3D) TI Bi 2 Se 3 to the 2D limit (gapped SS) has been observed when the thickness is below six quintuple layers (~ 6 nm). 8 Reaching a similar sensitivity to SS using traditional optical pump-probe techniques (like transient reflectivity (TR)/transmission), which use less energetic photons in the visible/infrared range, seems problematic since the absorption length of the laser light normally used for these measurements (a few tens of nm) significantly exceeds the range where the effect of SS can actually be monitored. As a result, for bulk single crystals of Bi 2 Se 3 the transient optical response is dominated by the bulk contribution. To overcome the problem one can use SS/surface sensitive methods. An example of this approach has recently been demonstrated by illuminating Bi 2 Se 3 with circularly polarized near-infrared light. 9 The resulting photocurrent which reverses its direction with a reversal of the helicity of the light unambiguously proves the SS origin of the optical response. Another surface sensitive technique exploits the centrosymmetric nature of TI's, which governs exclusively the surface-related response which results in an optical second harmonic generation (SHG) process. 10,11 In this Letter we report on a new way to distinguish between the contributions from the TI (Bi 2 Se 3 ) bulk 3D states and the 2D gapless SS, which is based on differences in the carrier relaxation rates for the insulating and metallic phases. We demonstrate that the car...

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin pumping and inverse spin Hall effects (ISHEs), enable the reading and control of magnetic moments in spintronics. The direct observation of these effects remains elusive in antiferromagnetic-based devices. We report subterahertz spin pumping at the interface of the uniaxial insulating antiferromagnet manganese difluoride and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized subterahertz irradiation. Our results open the door to the controlled generation of coherent, pure spin currents at terahertz frequencies.

Transient reflectivity (TR) from thin films (6 -40 nm thick) of the topological insulator Bi 2 Se 3 reveal ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ~20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques.Thin films of topological insulators (TIs) are threedimensional (3D) materials that are insulating in the bulk (bandgap of Bi 2 Se 3 , for example, E g ~ 0.3 eV), but conductive at the surfaces due to two-dimensional (2D) Dirac surface states (SS) caused by the combination of strong spin-orbit interaction and time-reversal symmetry.

We have studied the effect of the interface structure on the exchange bias in the FeF 2-Fe system, for FeF 2 bulk single crystals or thin films. The exchange bias depends very strongly on the crystalline orientation of the antiferromagnet for both films and crystals. However, the interface roughness seems to have a strong effect mainly on the film systems. These results indicate that the exchange bias depends strongly on the spin structure at the interface, especially on the angle between the ferromagnetic and antiferromagnetic spins. We have also found a strong dependence of the hysteresis loops shape on the cooling field direction with respect to the antiferromagnetic anisotropy axis, induced by a rotation of the ferromagnetic easy axis as the sample is cooled through T N. For the single crystal systems the results imply the existence of a perpendicular coupling between the antiferromagnetic and ferromagnetic spins at low temperatures. ͓S0163-1829͑99͒02610-7͔

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