Tanmoy Pramanik

Structural and Electrical Properties of MoTe2 and MoSe2 Grown by Molecular Beam Epitaxy

We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.

Two-dimensional weak anti-localization in Bi2Te3 thin film grown on Si(111)-(7 × 7) surface by molecular beam epitaxy

We report on low temperature transport studies of Bi2Te3 topological insulator thin films grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. A sharp increase in the magnetoresistance with magnetic field at low temperature indicates the existence of weak anti-localization. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model, and the extracted phase coherence length shows a power-law dependence with temperature indicating the existence of a two-dimensional system. An insulating ground state has also been observed at low temperature showing a logarithmic divergence of the resistance that appears to be the influence of electron-electron interaction in a two-dimensional system.

Perpendicular magnetic anisotropy and spin glass-like behavior in molecular beam epitaxy grown chromium telluride thin films

Reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry, and other physical property measurements are used to investigate the structure, morphology, magnetic, and magnetotransport properties of (001)-oriented Cr2Te3 thin films grown on Al2O3(0001) and Si(111)-(7×7) surfaces by molecular beam epitaxy. Streaky RHEED patterns indicate flat smooth film growth on both substrates. STM studies show the hexagonal arrangements of surface atoms. Determination of the lattice parameter from the atomically resolved STM image is consistent with the bulk crystal structures. Magnetic measurements show the film is ferromagnetic, having a Curie temperature of about 180 K, and a spin glass-like behavior was observed below 35 K. Magnetotransport measurements show the metallic nature of the film with a perpendicular magnetic anisotropy along the c-axis.

Micromagnetic study of spin-transfer-torque switching of a ferromagnetic cross towards multi-state spin-transfer-torque based random access memory

We study spin-transfer-torque (STT) switching of a cross-shaped ferromagnet with unequal branches as the free layer in a magnetic tunnel junction using micromagnetic simulations. The free layer in the magnetic tunnel junction is thus designed to have four stable energy states using shape anisotropy. Switching shows distinct regions with increasing current density. Stability of the states against thermal fluctuations is considered, and the validity of the results for different dimensions and material parameters of the free layer ferromagnet is investigated. The results could be useful for a multi-bit STT-based memory.

Strong spin-orbit coupling and Zeeman spin splitting in angle dependent magnetoresistance of Bi2Te3

We have studied angle dependent magnetoresistance of Bi2Te3 thin film with field up to 9 T over 2–20 K temperatures. The perpendicular field magnetoresistance has been explained by the Hikami-Larkin-Nagaoka theory alone in a system with strong spin-orbit coupling, from which we have estimated the mean free path, the phase coherence length, and the spin-orbit relaxation time. We have obtained the out-of-plane spin-orbit relaxation time to be small and the in-plane spin-orbit relaxation time to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters are useful for spintronics applications. For parallel field magnetoresistance, we have confirmed the presence of Zeeman effect which is otherwise suppressed in perpendicular field magnetoresistance due to strong spin-orbit coupling. The parallel field data have been explained using both the contributions from the Maekawa-Fukuyama localization theory for non-interacting electrons and Lee-Ramakrishnan theory of electron-electron interactions. The estimated Zeeman g-factor and the strength of Coulomb screening parameter agree well with the theory. Finally, the anisotropy in magnetoresistance with respect to angle has been described by the Hikami-Larkin-Nagaoka theory. This anisotropy can be used in anisotropic magnetic sensor applications.

Magnetization switching of a metallic nanomagnet via current-induced surface spin-polarization of an underlying topological insulator

We consider a thermally stable, metallic nanoscale ferromagnet (FM) subject to spin-polarized current injection and exchange coupling from the spin-helically locked surface states of a topological insulator (TI) to evaluate possible non-volatile memory applications. We consider parallel transport in the TI and the metallic FM, and focus on the efficiency of magnetization switching as a function of transport between the TI and the FM. Transport is modeled as diffusive in the TI beneath the FM, consistent with the mobility in the TI at room temperature, and in the FM, which essentially serves as a constant potential region albeit spin-dependent except in the low conductivity, diffusive limit. Thus, it can be captured by drift-diffusion simulation, which allows for ready interpretation of the results. We calculate switching time and energy consumed per write operation using self-consistent transport, spin-transfer-torque (STT), and magnetization dynamics calculations. Calculated switching energies and times compare favorably to conventional spin-torque memory schemes for substantial interlayer conductivity. Nevertheless, we find that shunting of current from the TI to a metallic nanomagnet can substantially limit efficiency. Exacerbating the problem, STT from the TI effectively increases the TI resistivity. We show that for optimum performance, the sheet resistivity of the FM layer should be comparable to or larger than that of the TI surface layer. Thus, the effective conductivity of the FM layer becomes a critical design consideration for TI-based non-volatile memory.

Write Error Rate of Spin-Transfer-Torque Random Access Memory Including Micromagnetic Effects Using Rare Event Enhancement

Spin-transfer-torque random access memory (STT-RAM) is a promising candidate for the next generation of random access memory due to improved scalability, read-write speeds, and endurance. However, the write pulse duration must be long enough to ensure a low write error rate (WER), the probability that a bit will remain unswitched after the write pulse is turned OFF, in the presence of stochastic thermal effects. WERs on the scale of 10-9 or lower are desired. Within a macrospin approximation, WERs can be calculated analytically using the Fokker-Planck method to this point and beyond. However, dynamic micromagnetic effects within the bit can affect and lead to faster switching. Such micromagnetic effects can be addressed via numerical solution of the stochastic Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, determining WERs approaching 10-9 would require well over 109 such independent simulations, which is infeasible. In this paper, we explore the calculation of WER using rare event enhancement (REE), an approach that has been used for Monte Carlo simulation of other systems where rare events nevertheless remain important. Using a prototype REE approach tailored to the STT-RAM switching physics, we demonstrate reliable calculation of a WER to 10-9 with sets of only approximately 103 ongoing stochastic LLGS simulations, and the apparent ability to go further.

Proposal of a Multistate Memory Using Voltage Controlled Magnetic Anisotropy of a Cross-Shaped Ferromagnet

Voltage controlled magnetic anisotropy (VCMA)-induced precessional magnetization dynamics of a cross-shaped ferromagnet (FM) is studied by micromagnetic simulation. A cross-shaped FM, which has four minima in its energy landscape, can be used to store two bits. We show that, by selecting appropriate dimensions and taking into account interfacial perpendicular magnetic anisotropy, the cross-shaped FM can be switched from one state to the other states using VCMA, while the in-plane component of magnetization provides multistate functionality. VCMA-induced switching provides a low-power alternative to spin-transfer-torque switching of a similar cross-shaped FM studied previously. We estimate the thermal stability using the string method to capture the complex micromagnetic nature of the switching along the minimum energy path. The results could be useful toward development of a low-power multistate nonvolatile memory.

Angular dependence of magnetization reversal in epitaxial chromium telluride thin films with perpendicular magnetic anisotropy

Abstract We investigate magnetic anisotropy and magnetization reversal mechanism in chromium telluride thin films grown by molecular beam epitaxy. We report existence of strong perpendicular magnetic anisotropy in these thin films, along with a relatively strong second order anisotropy contribution. The angular variation of the switching field observed from the magnetoresistance measurement is explained quantitatively using a one-dimensional defect model. The model reveals the relative roles of nucleation and pinning in the magnetization reversal, depending on the applied field orientation. Micromagnetic simulations are performed to visualize the domain structure and switching process.

Intra-domain periodic defects in monolayer MoS2

We present an ultra-high vacuum scanning tunneling microscopy study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.