Tamalika Banerjee

Probing momentum distributions in magnetic tunnel junctions via hot-electron decay

The tunnel momentum distribution in a magnetic tunnel junction is probed by analyzing the decay of the hot electrons in the Co metal anode after tunneling, using a three-terminal transistor structure in which the hot-electron attenuation is sensitive to the tunnel momentum distribution. Solid state amorphous Al2O3 barriers and the vacuum barrier of a scanning tunneling microscope are compared. For the former the attenuation length in nominally the same Co is strikingly larger factor of 2, implying a more isotropic tunnel momentum distribution for Al2O3 barriers.

Epitaxial diodes of a half-metallic ferromagnet on an oxide semiconductor

We report on the fabrication and electrical characterization of epitaxial Schottky diodes of a half-metallic ferromagnet on an oxide semiconductor. La0.67Sr0.33MnO3 thin films are grown by pulsed laser deposition on niobium-doped SrTiO3 semiconductor substrates with two doping concentrations and a TiO2 surface termination. The current across the diodes is dominated by thermionic emission and shows high rectification and low reverse bias leakage. At room temperature, the Schottky barrier height is 0.95 eV (0.65 eV) and the ideality factor is 1.08 (1.18) for the diodes with a low (high) doped semiconductor. With decreasing temperature the Schottky barrier height decreases and the ideality factor increases.

Spin injection and perpendicular spin transport in graphite nanostructures

Organic- and carbon-based materials are attractive for spintronics because their small spin-orbit coupling and low hyperfine interaction is expected to give rise to large spin-relaxation times. However, the corresponding spin-relaxation length is not necessarily large when transport is via weakly interacting molecular orbitals. Here we use graphite as a model system and study spin transport in the direction perpendicular to the weakly bonded graphene sheets. We achieve injection of highly 75% spin-polarized electrons into graphite nanostructures of 300–500 nm across and up to 17 nm thick, and observe transport without any measurable loss of spin information. Direct visualization of local spin transport in graphite-based spin-valve sandwiches also shows spatially uniform and near-unity transmission for electrons at 1.8 eV above the Fermi level.

Temperature dependent transport characteristics of graphene/n-Si diodes

Realizing an optimal Schottky interface of graphene on Si is challenging, as the electrical transport strongly depends on the graphene quality and the fabrication processes. Such interfaces are of increasing research interest for integration in diverse electronic devices as they are thermally and chemically stable in all environments, unlike standard metal/semiconductor interfaces. We fabricate such interfaces with n-type Si at ambient conditions and find their electrical characteristics to be highly rectifying, with minimal reverse leakage current (<10−10 A) and rectification of more than 106. We extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83 eV for the CVD graphene devices at room temperature. The temperature dependent electrical characteristics suggest the influence of inhomogeneities at the graphene/n-Si interface. A quantitative analysis of the inhomogeneity in Schottky barrier heights is presented using the potential fluctuation model proposed by Werner and Guttler.

Electrical transport across Au/Nb : SrTiO3 Schottky interface with different Nb doping

We have investigated electron transport in Nb doped SrTiO3 single crystals for two doping densities. We find that the resistivity and mobility are temperature dependent in both whereas the carrier concentration is almost temperature invariant. We rationalize this using the hydrogenic theory for shallow donors. Further, we probe electrical transport across Schottky interfaces of Au on TiO2 terminated n-type SrTiO3. Quantitative analysis of macroscopic I-V measurements reveal thermionic emission dominated transport for the low doped substrate whereas it deviates from such behavior for the high doped substrate. This work is relevant for designing devices to study electronic transport using oxide-semiconductors.We have investigated electron transport in Nb doped SrTiO3 single crystals for two doping densities. We find that the resistivity and mobility are temperature dependent in both whereas the carrier concentration is almost temperature invariant. We rationalize this using the hydrogenic theory for shallow donors. Further, we probe electrical transport across Schottky interfaces of Au on TiO2 terminated n-type SrTiO3. Quantitative analysis of macroscopic I-V measurements reveal thermionic emission dominated transport for the low doped substrate whereas it deviates from such behavior for the high doped substrate. This work is relevant for designing devices to study electronic transport using oxide-semiconductors.

Effect of secondary electrons from latent tracks created in YBCO by swift heavy ion irradiation

Abstract Swift heavy ions (SHI) with electronic energy loss exceeding a value of 14.4 keV nm −1 create amorphized latent tracks in YBCO type superconductors. In the low fluence regime of an ion beam where tracks do not overlap, a decrease of the superconducting transition temperature as probed through resistivity studies, is not expected due to availability of percolating current paths. The present study however shows Tc decrease by about 1– 3 K in thin films of YBCO when irradiated by 250 MeV Ag ions at 79 K at a fluence of 5×1010– 1×10 12 ions cm −2 . The highest fluence used in the present study is three times less than the fluence where track overlapping becomes significant. The Tc tends to increase towards the preirradiation value on annealing the films at room temperature. To explain this unusual result, we consider the effect of ion irradiation in inducing materials modification not only through creation of amorphized latent tracks along the ion path, but also through creation of atomic disorder in the oxygen sublattice in the Cu–O chains of YBCO by the secondary electrons. These electrons are emitted radially from the tracks during the passage of the SHI. Considering the correlation between the charge state of copper and its oxygen coordination, we show in particular that the latter process is a consequence of the inelastic interaction of the SHI induced low-energy secondary electrons with the YBCO lattice, which result in chain oxygen disorder and Tc decrease.

Ballistic hole magnetic microscopy

A technique to study nanoscale spin transport of holes is presented: ballistic hole magnetic microscopy. The tip of a scanning tunneling microscope is used to inject hot electrons into a ferromagnetic heterostructure, where inelastic decay creates a distribution of electron-hole pairs. Spin-dependent transmission of the excited hot holes into an underlying p-type semiconductor collector induces a hole current in the valence band of the semiconductor, with magnetocurrent values up to 180%. The spin-filtering of holes is used to obtain local hysteresis loops and magnetic imaging with spatial resolution better than 30 nm.

Electric Field Control of Spin Lifetimes in Nb-SrTiO3 by Spin-Orbit Fields

We show electric field control of the spin accumulation at the interface of the oxide semiconductor Nb-SrTiO_{3} with Co/AlO_{x} spin injection contacts at room temperature. The in-plane spin lifetime τ_{∥}, as well as the ratio of the out-of-plane to in-plane spin lifetime τ_{⊥}/τ_{∥}, is manipulated by the built-in electric field at the semiconductor surface, without any additional gate contact. The origin of this manipulation is attributed to Rashba spin orbit fields (SOFs) at the Nb-SrTiO_{3} surface and shown to be consistent with theoretical model calculations based on SOF spin flip scattering. Additionally, the junction can be set in a high or low resistance state, leading to a nonvolatile control of τ_{⊥}/τ_{∥}, consistent with the manipulation of the Rashba SOF strength. Such room temperature electric field control over the spin state is essential for developing energy-efficient spintronic devices and shows promise for complex oxide based (spin) electronics.

Towards the understanding of the origin of charge-current-induced spin voltage signals in the topological insulator Bi2Se3

Topological insulators provide a new platform for spintronics due to the spin texture of the surface states that are topologically robust against elastic backscattering. Here we report on an investigation of the measured voltage obtained from efforts to electrically probe spin-momentum locking in the topological insulator Bi2Se3 using ferromagnetic contacts. Upon inverting the magnetization of the ferromagnetic contacts, we find a reversal of the measured voltage. Extensive analysis of the bias and temperature dependence of this voltage was done, considering the orientation of the magnetization relative to the current. Our findings indicate that the measured voltage can arise due to fringe-field-induced Hall voltages, different from current-induced spin polarization of the surface state charge carriers, as reported recently. Understanding the nontrivial origin of the measured voltage is important for realizing spintronic devices with topological insulators.

Probing electron transport across a LSMO/Nb:STO heterointerface at the nanoscale

We investigate electron transport across a complex oxide heterointerface of La0.67Sr0.33MnO3 (LSMO) on Nb:SrTiO3 (Nb:STO) at different temperatures. For this, we employ the conventional current-voltage method as well as the technique of ballistic electron emission microscopy (BEEM), which can probe lateral inhomogeneities in transport at the nanometer scale. From current-voltage measurements, we find that the Schottky barrier height (SBH) at the LSMO/Nb:STO interface decreases at low temperatures accompanied by a larger than unity ideality factor. This is ascribed to the tunneling dominated transport caused by the narrowing of the depletion width at the interface. However, BEEM studies of such unbiased interfaces do not exhibit SBH lowering at low temperatures, implying that this is triggered by the modification of the interface due to an applied bias and is not an intrinsic property of the interface. Interestingly, the SBH at the nanoscale, as extracted from BEEM studies, at different locations in the device is found to be spatially homogeneous and similar both at room temperature and at low temperatures. Our results highlight the application of BEEM in characterizing electron transport and its homogeneity at such unbiased complex oxide interfaces and yield insights into the origin of the temperature dependence of the SBH at biased interfaces. DOI: 10.1103/PhysRevB.87.085116