Showing 1–41 of 41 results for author: Bokor, J

Analysis of ultrafast magnetization switching dynamics in exchange-coupled ferromagnet-ferrimagnet heterostructures

Authors: Debanjan Polley, Jyotirmoy Chatterjee, Hyejin Jang, Jeffrey Bokor

Abstract: Magnetization switching in ferromagnets has so far been limited to the current-induced spin-orbit-torque effects. Recent observation of helicity-independent all-optical magnetization switching in exchange-coupled ferromagnet ferrimagnet heterostructures expanded the range and applicability of such ultrafast heat-driven magnetization switching. Here we report the element-resolved switching dynamics… ▽ More Magnetization switching in ferromagnets has so far been limited to the current-induced spin-orbit-torque effects. Recent observation of helicity-independent all-optical magnetization switching in exchange-coupled ferromagnet ferrimagnet heterostructures expanded the range and applicability of such ultrafast heat-driven magnetization switching. Here we report the element-resolved switching dynamics of such an exchange-coupled system, using a modified microscopic three-temperature model. We have studied the effect of i) the Curie temperature of the ferromagnet, ii) ferrimagnet composition, iii) the long-range RKKY exchange-coupling strength, and iv) the absorbed optical energy on the element-specific time-resolved magnetization dynamics. The phase-space of magnetization illustrates how the RKKY coupling strength and the absorbed optical energy influence the switching time. Our analysis demonstrates that the threshold switching energy depends on the composition of the ferrimagnet and the switching time depends on the Curie temperature of the ferromagnet as well as RKKY coupling strength. This simulation anticipates new insights into developing faster and more energy-efficient spintronics devices. △ Less

Submitted 28 March, 2023; originally announced March 2023.

Optical Switching in Tb/Co-Multilayer Based Nanoscale Magnetic Tunnel Junctions

Authors: Sucheta Mondal, Debanjan Polley, Akshay Pattabi, Jyotirmoy Chatterjee, David Salomoni, Luis Aviles-Felix, Aurélien Olivier, Miguel Rubio-Roy, Bernard Diény, Liliana Daniela Buda Prejbeanu, Ricardo Sousa, Ioan Lucian Prejbeanu, Jeffrey Bokor

Abstract: Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal by an ultrashort laser pulse is extremely important. We demonstrate optical switching of Tb/Comultilayer-based nanoscale MTJs by combining optical writing and electrical read-out methods. A 90 fs-long laser pulse switches the magneti… ▽ More Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal by an ultrashort laser pulse is extremely important. We demonstrate optical switching of Tb/Comultilayer-based nanoscale MTJs by combining optical writing and electrical read-out methods. A 90 fs-long laser pulse switches the magnetization of the storage layer (SL). The change in magnetoresistance between the SL and a reference layer (RL) is probed electrically across the tunnel barrier. Single-shot switching is demonstrated by varying the cell diameter from 300 nm to 20 nm. The anisotropy, magnetostatic coupling, and switching probability exhibit cell-size dependence. By suitable association of laser fluence and magnetic field, successive commutation between high-resistance and low-resistance states is achieved. The switching dynamics in a continuous film is probed with the magneto-optical Kerr effect technique. Our experimental findings provide strong support for the growing interest in ultrafast spintronic devices. △ Less

Submitted 18 December, 2022; originally announced December 2022.

Comments: total pages 22, Total figure 8

Picosecond Spin-Orbit Torque Induced Coherent Magnetization Switching in a Ferromagnet

Authors: Debanjan Polley, Akshay Pattabi, Ashwin Rastogi, Kaushalya Jhuria, Eva Diaz, Hanuman Singh, Aristide Lemaitre, Michel Hehn, Jon Gorchon, Jeffrey Bokor

Abstract: Electrically controllable non-volatile magnetic memories show great potential for the replacement of semiconductor-based technologies. Recently there has been strong interest in spin-orbit torque (SOT) induced magnetization reversal due to the device's increased lifetime and speed of operation. However, recent SOT switching studies reveal an incubation delay in the ~ns range due to stochasticity i… ▽ More Electrically controllable non-volatile magnetic memories show great potential for the replacement of semiconductor-based technologies. Recently there has been strong interest in spin-orbit torque (SOT) induced magnetization reversal due to the device's increased lifetime and speed of operation. However, recent SOT switching studies reveal an incubation delay in the ~ns range due to stochasticity in the nucleation of a magnetic domain during reversal. Here, we experimentally demonstrate ultrafast SOT-induced magnetization switching dynamics of a ferromagnet with no incubation delay by avoiding the nucleation process and driving the magnetization coherently. We employ an ultrafast photo-conducting switch and a co-planar strip line to generate and guide ~ps current pulses into the heavy metal/ferromagnet layer stack and induce ultrafast SOT. We use magneto-optical probing to investigate the magnetization switching dynamics with sub-picosecond time resolution. Depending on the relative current pulse and in-plane magnetic field polarities, we observe either an ultrafast demagnetization and subsequent recovery along with a SOT-induced precessional oscillation, or ultrafast SOT switching. The magnetization zero-crossing occurs in ~70 ps, which is approximately an order of magnitude faster than previous studies. Complete switching needs ~250 ps and is limited by the heat diffusion to the substrate. We use a macro-magnetic simulation coupled with an ultrafast heating model to analyze the effect of ultrafast thermal anisotropy torque and current-induced torque in the observed dynamics. Good agreement between our experimental results and the macro-spin model shows that the switching dynamics are coherent and present no noticeable incubation delay. Our work suggests a potential pathway toward dramatically increasing the writing speed of SOT magnetic random-access memory devices. △ Less

Submitted 15 November, 2022; originally announced November 2022.

Comments: 17 pages, 4 figures, Supplemental Material

Journal ref: Science Advances 9, edah5562 (2023)

Growth optimization and device integration of narrow-bandgap graphene nanoribbons

Authors: Gabriela Borin Barin, Qiang Sun, Marco Di Giovannantonio, Cheng-Zhuo Du, Xiao-Ye Wang, Juan Pablo Llinas, Zafer Mutlu, Yuxuan Lin, Jan Wilhelm, Jan Overbeck, Colin Daniels, Michael Lamparski, Hafeesudeen Sahabudeen, Mickael L. Perrin, José I. Urgel, Shantanu Mishra, Amogh Kinikar, Roland Widmer, Samuel Stolz, Max Bommert, Carlo Pignedoli, Xinliang Feng, Michel Calame, Klaus Müllen, Akimitsu Narita , et al. (4 additional authors not shown)

Abstract: The electronic, optical and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom-up fabrication based on molecular precursors. This approach offers a unique platform for all-carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for succe… ▽ More The electronic, optical and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom-up fabrication based on molecular precursors. This approach offers a unique platform for all-carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for successful device integration, with GNR length being the most critical parameter. In this work, we study the growth, characterization, and device integration of 5-atom wide armchair GNRs (5-AGNRs), which are expected to have an optimal band gap as active material in switching devices. 5-AGNRs are obtained via on-surface synthesis under ultra-high vacuum conditions from Br- and I-substituted precursors. We show that the use of I-substituted precursors and the optimization of the initial precursor coverage quintupled the average 5-AGNR length. This significant length increase allowed us to integrate 5-AGNRs into devices and to realize the first field-effect transistor based on narrow bandgap AGNRs that shows switching behavior at room temperature. Our study highlights that optimized growth protocols can successfully bridge between the sub-nanometer scale, where atomic precision is needed to control the electronic properties, and the scale of tens of nanometers relevant for successful device integration of GNRs. △ Less

Submitted 2 February, 2022; originally announced February 2022.

Scaling and Statistics of Bottom-Up Synthesized Armchair Graphene Nanoribbon Transistors

Authors: Yuxuan Lin, Zafer Mutlu, Gabriela Borin Barin, Jenny Hong, Juan Pablo Llinas, Akimitsu Narita, Hanuman Singh, Klaus Müllen, Pascal Ruffieux, Roman Fasel, Jeffrey Bokor

Abstract: Bottom-up assembled nanomaterials and nanostructures allow for the studies of rich and unprecedented quantum-related and mesoscopic transport phenomena. However, it can be difficult to quantify the correlations between the geometrical or structural parameters obtained from advanced microscopy and measured electrical characteristics when they are made into macroscopic devices. Here, we propose a st… ▽ More Bottom-up assembled nanomaterials and nanostructures allow for the studies of rich and unprecedented quantum-related and mesoscopic transport phenomena. However, it can be difficult to quantify the correlations between the geometrical or structural parameters obtained from advanced microscopy and measured electrical characteristics when they are made into macroscopic devices. Here, we propose a strategy to connect the nanomaterial morphologies and the device performance through a Monte Carlo device model and apply it to understand the scaling trends of bottom-up synthesized armchair graphene nanoribbon (GNR) transistors. A new nanofabrication process is developed for GNR transistors with channel length down to 7 nm. The impacts of the GNR spatial distributions and the device geometries on the device performance are investigated systematically through comparison of experimental data with the model. Through this study, challenges and opportunities of transistor technologies based on bottom-up synthesized GNRs are pinpointed, paving the way to the further improvement of the GNR device performance for future transistor technology nodes. △ Less

Submitted 23 January, 2022; originally announced January 2022.

Unifying femtosecond and picosecond single-pulse magnetic switching in GdFeCo

Authors: Florian Jakobs, Thomas Ostler, Charles-Henri Lambert, Yang Yang, Sayeef Salahuddin, Richard B. Wilson, Jon Gorchon, Jeffrey Bokor, Unai Atxitia

Abstract: Many questions are still open regarding the physical mechanisms behind the magnetic switching in GdFeCo alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in GdFeCo by using both several picosecond optical laser pulse a… ▽ More Many questions are still open regarding the physical mechanisms behind the magnetic switching in GdFeCo alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in GdFeCo by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femo- and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in GdFeCo alloys, for a wide range of system parameters, such as composition, laser power and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue, that switching with pulse durations as long as 15 picoseconds is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic GdFeCo alloys have been already demonstrated to show fast and energy-efficient processes, e.g. domain-wall motion in a track and spin-orbit torque switching in spintronics devices. △ Less

Submitted 30 April, 2020; originally announced April 2020.

Journal ref: Phys. Rev. B 103, 104422 (2021)

Picosecond Spin Orbit Torque Switching

Authors: Kaushalya Jhuria, Julius Hohlfeld, Akshay Pattabi, Elodie Martin, Aldo Ygnacio Arriola Córdova, Xinping Shi, Roberto Lo Conte, Sebastien Petit-Watelot, Juan Carlos Rojas-Sanchez, Gregory Malinowski, Stéphane Mangin, Aristide Lemaître, Michel Hehn, Jeffrey Bokor, Richard B. Wilson, Jon Gorchon

Abstract: Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An u… ▽ More Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications. △ Less

Submitted 23 August, 2020; v1 submitted 3 December, 2019; originally announced December 2019.

Comments: Includes article + supplementary information. Latest version uses full name of the first author. Nature Electronics (2020)

Role of Element-Specific Damping on the Ultrafast, Helicity-Independent All-Optical Switching Dynamics in Amorphous (Gd,Tb)Co Thin Films

Authors: Alejandro Ceballos, Akshay Pattabi, Amal El-Ghazaly, Sergiu Ruta, Christian P. Simon, Richard F. L. Evans, Thomas Ostler, Roy W. Chantrell, Ellis Kennedy, Mary Scott, Jeffrey Bokor, Frances Hellman

Abstract: Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal (\textit{a}-RE-TM) system… ▽ More Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal (\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic \textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, and elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing \textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that acounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS. △ Less

Submitted 15 May, 2020; v1 submitted 21 November, 2019; originally announced November 2019.

Journal ref: Phys. Rev. B 103, 024438 (2021)

The interfacial spin modulation of graphene on Fe(111)

Authors: J. Hong, H. -N. Hwang, A. T. NDiaye, J. Liang, G. Chen, Y. Park, L. T. Singh, Y. G. Jung, J. -H. Yang, J. -I. Jeong, A. K. Schmid, E. Arenholz, H. Yang, J. Bokor, C. -C. Hwang, L. You

Abstract: When Fe, which is a typical ferromagnet using d- or f-orbital states, is combined with 2D materials such as graphene, it offers many opportunities for spintronics. The origin of 2D magnetism is from magnetic insulating behaviors, which could result in magnetic excitations and also proximity effects. However, the phenomena were only observed at extremely low temperatures. Fe and graphene interfaces… ▽ More When Fe, which is a typical ferromagnet using d- or f-orbital states, is combined with 2D materials such as graphene, it offers many opportunities for spintronics. The origin of 2D magnetism is from magnetic insulating behaviors, which could result in magnetic excitations and also proximity effects. However, the phenomena were only observed at extremely low temperatures. Fe and graphene interfaces could control spin structures in which they show a unique atomic spin modulation and magnetic coupling through the interface. Another reason for covering graphene on Fe is to prevent oxidation under ambient conditions. We investigated the engineering of spin configurations by growing monolayer graphene on an Fe(111) single crystal surface and observed the presence of sharply branched, 3D tree-like domain structures. Magnetization by a sweeping magnetic field (m-H) revealed that the interface showed canted magnetization in the in-plane (IP) orientation. Moreover, graphene could completely prevent the oxidation of the Fe surface. The results indicate possible control of the spin structures at the atomic scale and the interface phenomena in the 2D structure. The study introduces a new approach for room temperature 2D magnetism. △ Less

Submitted 24 November, 2018; originally announced November 2018.

Comments: 22 pages, 4 figures

Negative Differential Resistance and Steep Switching in Chevron Graphene Nanoribbon Field Effect Transistors

Authors: Samuel Smith, Juan-Pablo Llinás, Jeffrey Bokor, Sayeef Salahuddin

Abstract: Ballistic quantum transport calculations based on the non-equilbrium Green's function formalism show that field-effect transistor devices made from chevron-type graphene nanoribbons (CGNRs) could exhibit negative differential resistance with peak-to-valley ratios in excess of 4800 at room temperature as well as steep-slope switching with 6 mV/decade subtheshold swing over five orders of magnitude… ▽ More Ballistic quantum transport calculations based on the non-equilbrium Green's function formalism show that field-effect transistor devices made from chevron-type graphene nanoribbons (CGNRs) could exhibit negative differential resistance with peak-to-valley ratios in excess of 4800 at room temperature as well as steep-slope switching with 6 mV/decade subtheshold swing over five orders of magnitude and ON-currents of 88$μ$A/$μ$m. This is enabled by the superlattice-like structure of these ribbons that have large periodic unit cells with regions of different effective bandgap, resulting in minibands and gaps in the density of states above the conduction band edge. The CGNR ribbon used in our proposed device has been previously fabricated with bottom-up chemical synthesis techniques and could be incorporated into an experimentally-realizable structure. △ Less

Submitted 16 March, 2017; originally announced March 2017.

Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers

Authors: Jon Gorchon, Charles-Henri Lambert, Yang Yang, Akshay Pattabi, Richard B. Wilson, Sayeef Salahuddin, Jeffrey Bokor

Abstract: In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has bee… ▽ More In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has been achieved without the need of any symmetry breaking magnetic field. This phenomenon is often referred to as all optical switching (AOS). However, so far, GdFeCo remains the only material system where such deterministic switching has been observed. When extended to ferromagnetic systems, which are of greater interest in many technological applications, only a partial effect can be achieved, which in turn requires repeated laser pulses for full switching. However, such repeated pulsing is not only energy hungry, it also negates the speed advantage of AOS. Motivated by this problem, we have developed a general method for single-shot, picosecond timescale, complete all optical switching of ferromagnetic materials. We demonstrate that in exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by a femtosecond laser pulse. We believe that this approach will greatly expand the range of materials and applications for ultrafast magnetic switching. △ Less

Submitted 29 July, 2017; v1 submitted 27 February, 2017; originally announced February 2017.

Comments: 11 pages, 3 figures, supplementary materials

Journal ref: Appl. Phys. Lett. 111, 042401 (2017)

Ultrafast Magnetization Reversal by Picosecond Electrical Pulses

Authors: Yang Yang, R. B. Wilson, Jon Gorchon, Charles-Henri Lambert, Sayeef Salahuddin, Jeffrey Bokor

Abstract: The field of spintronics involves the study of both spin and charge transport in solid state devices with a view toward increasing their functionality and efficiency. Alternatively, the field of ultrafast magnetism focuses on the use of femtosecond laser pulses to excite electrons in magnetic materials, which allows the magnetic order to be dramatically changed on unprecedented sub-picosecond time… ▽ More The field of spintronics involves the study of both spin and charge transport in solid state devices with a view toward increasing their functionality and efficiency. Alternatively, the field of ultrafast magnetism focuses on the use of femtosecond laser pulses to excite electrons in magnetic materials, which allows the magnetic order to be dramatically changed on unprecedented sub-picosecond time-scales. Here, we unite these two distinct research activities by using picosecond electrical pulses to rapidly excite electrons in a magnetic metal. We are able to deterministically and repetitively reverse the magnetization of a GdFeCo film with sub-10 picosecond electrical pulses. The magnetization reverses in ~10ps, which is more than an order of magnitude faster than any other electrically controlled magnetic switching. We attribute the deterministic switching of the magnetization to ultrafast excitation of the electrons, a fundamentally different mechanism from other current driven switching mechanisms such as spin-transfer-torque (STT) or spin-orbit-torque (SOT). The energy density required for switching is measured and the process is found to be efficient, projecting to only 4 fJ needed to switch a (20 nm)^3 cell, which is comparable to other state-of-the-art STT-MRAM memory devices. This discovery will launch a new field of research into picosecond spintronic phenomena and devices. △ Less

Submitted 12 October, 2016; v1 submitted 20 September, 2016; originally announced September 2016.

Journal ref: Sci. Adv. 3, e1603117 (2017)

Ultrafast Magnetic Switching of GdFeCo with Electronic Heat Currents

Authors: R. B. Wilson, Jon Gorchon, Yang Yang, Charles-Henri Lambert, Sayeef Salahuddin, Jeffrey Bokor

Abstract: We report the magnetic response of Au/GdFeCo bilayers to optical irradiation of the Au surface. For bilayers with Au thickness greater than 50 nm, the great majority of energy is absorbed by the Au electrons, creating an initial temperature differential of thousands of Kelvin between the Au and GdFeCo layers. The resulting electronic heat currents between the Au and GdFeCo layers last for several… ▽ More We report the magnetic response of Au/GdFeCo bilayers to optical irradiation of the Au surface. For bilayers with Au thickness greater than 50 nm, the great majority of energy is absorbed by the Au electrons, creating an initial temperature differential of thousands of Kelvin between the Au and GdFeCo layers. The resulting electronic heat currents between the Au and GdFeCo layers last for several picoseconds with energy flux in excess of 2 TW m-2, and provide sufficient heating to the GdFeCo electrons to induce deterministic reversal of the magnetic moment. △ Less

Submitted 17 July, 2017; v1 submitted 16 September, 2016; originally announced September 2016.

Comments: 17 pages, 5 figures

Journal ref: Phys. Rev. B 95, 180409 (2017)

Electric Current Induced Ultrafast Demagnetization

Authors: R. B. Wilson, Yang Yang, Jon Gorchon, Charles-Henri Lambert, Sayeef Salahuddin, Jeffrey Bokor

Abstract: We report the magnetic response of Co/Pt multilayers to picosecond electrical heating. Using photoconductive Auston switches, we generate electrical pulses with 5.5 picosecond duration and hundreds of pico-Joules to pass through Co/Pt multilayers. The electrical pulse heats the electrons in the Co/Pt multilayers and causes an ultrafast reduction in the magnetic moment. A comparison between optical… ▽ More We report the magnetic response of Co/Pt multilayers to picosecond electrical heating. Using photoconductive Auston switches, we generate electrical pulses with 5.5 picosecond duration and hundreds of pico-Joules to pass through Co/Pt multilayers. The electrical pulse heats the electrons in the Co/Pt multilayers and causes an ultrafast reduction in the magnetic moment. A comparison between optical and electrically induced demagnetization of the Co/Pt multilayers reveals significantly different dynamics for optical vs. electrical heating. We attribute the disparate dynamics to the dependence of the electron-phonon interaction on the average energy and total number of initially excited electrons. △ Less

Submitted 17 July, 2017; v1 submitted 2 September, 2016; originally announced September 2016.

Journal ref: Phys. Rev. B 96, 045105 (2017)

Electron-phonon interaction during optically induced ultrafast magnetization dynamics of Au/GdFeCo bilayers

Authors: Richard B Wilson, Charles-Henri Lambert, Jon Gorchon, Yang Yang, Sayeef Salahuddin, Jeffrey Bokor

Abstract: The temperature evolution of GdFeCo electrons following optical heating plays a key role in all optical switching of GdFeCo and is primarily governed by the strength of coupling between electrons and phonons. Typically, the strength of electron-phonon coupling in a metal is deduced by monitoring changes in reflectance following optical heating and then analyzing the transient reflectance with a si… ▽ More The temperature evolution of GdFeCo electrons following optical heating plays a key role in all optical switching of GdFeCo and is primarily governed by the strength of coupling between electrons and phonons. Typically, the strength of electron-phonon coupling in a metal is deduced by monitoring changes in reflectance following optical heating and then analyzing the transient reflectance with a simple two-temperature thermal model. In a magnetic metal, the change in reflectance cannot be assumed to depend only the electron and phonon temperatures because a metal's reflectance also depends on the magnetization. To deduce the electron-phonon coupling constant in GdFeCo, we analyze thermal transport in Au and GdFeCo bilayers following optical heating of the GdFeCo electrons. We use the reflectance of the Au layer to monitor the temperature evolution of the Au phonons. By interpreting the response of the bilayer to heating with a thermal model, we determine the electron-phonon coupling constant in GdFeCo to be 6 x 10^17 W/(m^3-K) corresponding to an electron-phonon relaxation time in GdFeCo of ~150 fs. △ Less

Submitted 2 September, 2016; originally announced September 2016.

Comments: 3 Figures

The role of electron and phonon temperatures in the helicity-independent all-optical switching of GdFeCo

Authors: Jon Gorchon, Richard B. Wilson, Yang Yang, Akshay Pattabi, Junyang Chen, Li He, Jianping Wang, Mo Li, Jeffrey Bokor

Abstract: Ultrafast optical heating of the electrons in ferrimagnetic metals can result in all-optical switching (AOS) of the magnetization. Here we report quantitative measurements of the temperature rise of GdFeCo thin films during helicity-independent AOS. Critical switching fluences are obtained as a function of the initial temperature of the sample and for laser pulse durations from 55 fs to 15 ps. We… ▽ More Ultrafast optical heating of the electrons in ferrimagnetic metals can result in all-optical switching (AOS) of the magnetization. Here we report quantitative measurements of the temperature rise of GdFeCo thin films during helicity-independent AOS. Critical switching fluences are obtained as a function of the initial temperature of the sample and for laser pulse durations from 55 fs to 15 ps. We conclude that non-equilibrium phenomena are necessary for helicity-independent AOS, although the peak electron temperature does not play a critical role. Pump-probe time-resolved experiments show that the switching time increases as the pulse duration increases, with 10 ps pulses resulting in switching times of ~sim 13 ps. These results raise new questions about the fundamental mechanism of helicity-independent AOS. △ Less

Submitted 10 September, 2016; v1 submitted 31 May, 2016; originally announced May 2016.

Comments: 18 pages, 6 figures and supplementary materials

Journal ref: Phys. Rev. B 94, 184406 (2016)

Short-Channel Field Effect Transistors with 9-Atom and 13-Atom wide Graphene Nanoribbons

Authors: Juan Pablo Llinas, Andrew Fairbrother, Gabriela Borin Barin, Wu Shi, Kyunghoon Lee, Shuang Wu, Byung Yong Choi, Rohit Braganza, Jordan Lear, Nicholas Kau, Wonwoo Choi, Chen Chen, Zahra Pedramrazi, Tim Dumslaff, Akimitsu Narita, Xinliang Feng, Klaus Müllen, Felix Fischer, Alex Zettl, Pascal Ruffieux, Eli Yablonovitch, Michael Crommie, Roman Fasel, Jeffrey Bokor

Abstract: Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) dev… ▽ More Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at room temperature. We find that the performance of these devices is limited by tunnelling through the Schottky barrier (SB) at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high performance short-channel FETs with bottom-up synthesized armchair GNRs. △ Less

Submitted 27 January, 2017; v1 submitted 21 May, 2016; originally announced May 2016.

Comments: v3: added local back gate data

Journal ref: Nature Communications 8, Article number: 633 (2017)

Model for multi-shot all-thermal all-optical switching in ferromagnets

Authors: Jon Gorchon, Yang Yang, Jeffrey Bokor

Abstract: All optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promis- ing technological applications. However, fundamental understanding of the underlying mechanisms remains elusive. Here we present a new model for multi-shot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of Magnetic Circular Dichrois… ▽ More All optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promis- ing technological applications. However, fundamental understanding of the underlying mechanisms remains elusive. Here we present a new model for multi-shot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of Magnetic Circular Dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots. Finally, we re- produce previous AOS results by simulating the sweeping of a laser beam on an FePtC granular ferromagnetic film. △ Less

Submitted 21 April, 2016; originally announced April 2016.

Comments: 10 pages, 4 figures, supplementary materials

Journal ref: Phys. Rev. B 94, 020409 (2016)

Experimental verification of Landauer's principle in erasure of nanomagnetic memory bits

Authors: J. Hong, B. Lambson, S. Dhuey, J. Bokor

Abstract: In 1961, R. Landauer proposed the principle that logical irreversibility is associated with physical irreversibility and further theorized that the erasure of information is fundamentally a dissipative process. Landauer posited that a fundamental energy cost is incurred by the erasure of information contained in the memory of a computation device. His theory states that to erase one binary bit of… ▽ More In 1961, R. Landauer proposed the principle that logical irreversibility is associated with physical irreversibility and further theorized that the erasure of information is fundamentally a dissipative process. Landauer posited that a fundamental energy cost is incurred by the erasure of information contained in the memory of a computation device. His theory states that to erase one binary bit of information from a physical memory element in contact with a heat bath at a given temperature, at least kT ln(2) of heat must be dissipated from the memory into the environment, where k is the Boltzmann constant and T is the temperature. Although this connection between information theory and thermodynamics has proven to be very useful for establishing boundary limits for physical processes, Landauer principle has been a subject of some debate. Despite the theoretical controversy and fundamental importance of Landauer erasure in information technology, this phenomenon has not been experimentally explored using any practical physical implementation for digital information. Here, we report an investigation of the thermodynamic limits of the memory erasure process using nanoscale magnetic memory bits, by far the most ubiquitous digital storage technology today. Through sensitive, temperature dependent magnetometry measurements, we observed that the amount of dissipated energy is consistent with the Landauer limit during an adiabatic erasure process in nanoscale, single domain magnetic thin film islands. This result confirms the connection between information thermodynamics and physical systems and also provides a foundation for the development of practical information processing technologies that approach the fundamental limit of energy dissipation. △ Less

Submitted 25 November, 2014; originally announced November 2014.

Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque without an External Magnetic Field by Engineering a Tilted Anisotropy

Authors: Long You, OukJae Lee, Debanjan Bhowmik, Dominic Labanowski, Jeongmin Hong, Jeffrey Bokor, Sayeef Salahuddin

Abstract: Spin orbit torque (SOT) provides an efficient way of generating spin current that promises to significantly reduce the current required for switching nanomagnets. However, an in-plane current generated SOT cannot deterministically switch a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-dens… ▽ More Spin orbit torque (SOT) provides an efficient way of generating spin current that promises to significantly reduce the current required for switching nanomagnets. However, an in-plane current generated SOT cannot deterministically switch a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-density data storage applications due to their significantly larger thermal stability in ultra-scaled dimensions. Here we show that it is possible switch a perpendicularly polarized magnet by SOT without needing an external magnetic field. This is accomplished by engineering an anisotropy in the magnets such that the magnetic easy axis slightly tilts away from the film-normal. Such a tilted anisotropy breaks the symmetry of the problem and makes it possible to switch the magnet deterministically. Using a simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of the magnetization by reversing the polarity of the applied current. This demonstration presents a new approach for controlling nanomagnets with spin orbit torque. △ Less

Submitted 31 May, 2015; v1 submitted 2 September, 2014; originally announced September 2014.

Journal ref: Proceedings of the National Academy of Sciences 112 (2015) 10310

Sub-nanosecond signal propagation in anisotropy engineered nanomagnetic logic chains

Authors: Zheng Gu, Mark E. Nowakowski, David B. Carlton, Ralph Storz, Mi-Young Im, Jeongmin Hong, Weilun Chao, Brian Lambson, Patrick Bennett, Mohmmad T. Alam, Matthew A. Marcus, Andrew Doran, Anthony Young, Andreas Scholl, Peter Fischer, Jeffrey Bokor

Abstract: Energy efficient nanomagnetic logic (NML) computing architectures propagate and process binary information by relying on dipolar field coupling to reorient closely-spaced nanoscale magnets. Signal propagation in nanomagnet chains of various sizes, shapes, and magnetic orientations has been previously characterized by static magnetic imaging experiments with low-speed adiabatic operation; however t… ▽ More Energy efficient nanomagnetic logic (NML) computing architectures propagate and process binary information by relying on dipolar field coupling to reorient closely-spaced nanoscale magnets. Signal propagation in nanomagnet chains of various sizes, shapes, and magnetic orientations has been previously characterized by static magnetic imaging experiments with low-speed adiabatic operation; however the mechanisms which determine the final state and their reproducibility over millions of cycles in high-speed operation (sub-ns time scale) have yet to be experimentally investigated. Monitoring NML operation at its ultimate intrinsic speed reveals features undetectable by conventional static imaging including individual nanomagnetic switching events and systematic error nucleation during signal propagation. Here, we present a new study of NML operation in a high speed regime at fast repetition rates. We perform direct imaging of digital signal propagation in permalloy nanomagnet chains with varying degrees of shape-engineered biaxial anisotropy using full-field magnetic soft x-ray transmission microscopy after applying single nanosecond magnetic field pulses. Further, we use time-resolved magnetic photo-emission electron microscopy to evaluate the sub-nanosecond dipolar coupling signal propagation dynamics in optimized chains with 100 ps time resolution as they are cycled with nanosecond field pulses at a rate of 3 MHz. An intrinsic switching time of 100 ps per magnet is observed. These experiments, and accompanying macro-spin and micromagnetic simulations, reveal the underlying physics of NML architectures repetitively operated on nanosecond timescales and identify relevant engineering parameters to optimize performance and reliability. △ Less

Submitted 13 August, 2014; originally announced August 2014.

Comments: Main article (22 pages, 4 figures), Supplementary info (11 pages, 5 sections)

Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque

Authors: Debanjan Bhowmik, Mark E. Nowakowski, Long You, OukJae Lee, David Keating, Mark Wong, Jeffrey Bokor, Sayeef Salahuddin

Abstract: Deterministic control of domain walls orthogonal to the direction of current flow is demonstrated by exploiting spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO multilayer in presence of an in-plane magnetic field. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an ext… ▽ More Deterministic control of domain walls orthogonal to the direction of current flow is demonstrated by exploiting spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO multilayer in presence of an in-plane magnetic field. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an external magnetic field. Reversing the polarity of either the current flow or the in-plane field is found to reverse the direction of the domain wall motion. From these measurements, which are unaffected by any conventional spin transfer torque by symmetry, we estimate the spin orbit torque efficiency of Ta to be 0.08. △ Less

Submitted 23 July, 2014; originally announced July 2014.

Comments: 10 pages of main manuscript, 4 figures, 20 pages of Supplementary Information

Speed and Reliability of Nanomagnetic Logic Technology

Authors: Zheng Gu, Mark E. Nowakowski, David B. Carlton, Ralph Storz, Jeongmin Hong, Weilun Chao, Brian Lambson, Patrick Bennett, Mohmmad T. Alam, Matthew A. Marcus, Andrew Doran, Anthony Young, Andreas Scholl, Jeffrey Bokor

Abstract: Nanomagnetic logic is an energy efficient computing architecture that relies on the dipole field coupling of neighboring magnets to transmit and process binary information. In this architecture, nanomagnet chains act as local interconnects. To assess the merits of this technology, the speed and reliability of magnetic signal transmission along these chains must be experimentally determined. In thi… ▽ More Nanomagnetic logic is an energy efficient computing architecture that relies on the dipole field coupling of neighboring magnets to transmit and process binary information. In this architecture, nanomagnet chains act as local interconnects. To assess the merits of this technology, the speed and reliability of magnetic signal transmission along these chains must be experimentally determined. In this work, time-resolved pump-probe x-ray photo-emission electron microscopy is used to observe magnetic signal transmission along a chain of nanomagnets. We resolve successive error-free switching events in a single nanomagnet chain at speeds on the order of 100 ps per nanomagnet, consistent with predictions based on micromagnetic modeling. Errors which disrupt transmission are also observed. We discuss the nature of these errors, and approaches for achieving reliable operation. △ Less

Submitted 25 March, 2014; originally announced March 2014.

Comments: 13 pages, 4 figures

Stark shift and field ionization of arsenic donors in $^{28}$Si-SOI structures

Authors: C. C. Lo, S. Simmons, R. Lo Nardo, C. D. Weis, A. M. Tyryshkin, J. Meijer, D. Rogalla, S. A. Lyon, J. Bokor, T. Schenkel, J. J. L. Morton

Abstract: We develop an efficient back gate for silicon-on-insulator (SOI) devices operating at cryogenic temperatures, and measure the quadratic hyperfine Stark shift parameter of arsenic donors in isotopically purified $^{28}$Si-SOI layers using such structures. The back gate is implemented using MeV ion implantation through the SOI layer forming a metallic electrode in the handle wafer, enabling large an… ▽ More We develop an efficient back gate for silicon-on-insulator (SOI) devices operating at cryogenic temperatures, and measure the quadratic hyperfine Stark shift parameter of arsenic donors in isotopically purified $^{28}$Si-SOI layers using such structures. The back gate is implemented using MeV ion implantation through the SOI layer forming a metallic electrode in the handle wafer, enabling large and uniform electric fields up to $\sim$ 2 V/$μ$m to be applied across the SOI layer. Utilizing this structure we measure the Stark shift parameters of arsenic donors embedded in the $^{28}$Si SOI layer and find a contact hyperfine Stark parameter of $η_a=-1.9\pm0.2\times10^{-3} μ$m$^2$/V$^2$. We also demonstrate electric-field driven dopant ionization in the SOI device layer, measured by electron spin resonance. △ Less

Submitted 27 January, 2014; originally announced January 2014.

Comments: 5 pages, 3 figures

Bottom-up Graphene Nanoribbon Field-Effect Transistors

Authors: Patrick B. Bennett, Zahra Pedramrazi, Ali Madani, Yen-Chia Chen, Dimas G. de Oteyza, Chen Chen, Felix R. Fischer, Michael F. Crommie, Jeffrey Bokor

Abstract: Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nanoscale chemically synthesized GNR field-effect trans… ▽ More Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nanoscale chemically synthesized GNR field-effect transistors, made possible by development of a new layer transfer process. We observe strong environmental sensitivity and unique transport behavior characteristic of sub-1nm width GNRs. △ Less

Submitted 1 October, 2013; originally announced October 2013.

Electrical activation and electron spin resonance measurements of implanted bismuth in isotopically enriched silicon-28

Authors: C. D. Weis, C. C. Lo, V. Lang, A. M. Tyryshkin, R. E. George, K. M. Yu, J. Bokor, S. A. Lyon, J. J. L. Morton, T. Schenkel

Abstract: We have performed continuous wave and pulsed electron spin resonance measurements of implanted bismuth donors in isotopically enriched silicon-28. Donors are electrically activated via thermal annealing with minimal diffusion. Damage from bismuth ion implantation is repaired during thermal annealing as evidenced by narrow spin resonance linewidths (B_pp=12uT and long spin coherence times T_2=0.7ms… ▽ More We have performed continuous wave and pulsed electron spin resonance measurements of implanted bismuth donors in isotopically enriched silicon-28. Donors are electrically activated via thermal annealing with minimal diffusion. Damage from bismuth ion implantation is repaired during thermal annealing as evidenced by narrow spin resonance linewidths (B_pp=12uT and long spin coherence times T_2=0.7ms, at temperature T=8K). The results qualify ion implanted bismuth as a promising candidate for spin qubit integration in silicon. △ Less

Submitted 26 February, 2012; v1 submitted 7 February, 2012; originally announced February 2012.

Comments: 4 pages, 4 figures

A spin quantum bit architecture with coupled donors and quantum dots in silicon

Authors: T. Schenkel, C. C. Lo, C. D. Weis, J. Bokor, A. M. Tyryshkin, S. A. Lyon

Abstract: Spins of donor electrons and nuclei in silicon are promising quantum bit (qubit) candidates which combine long coherence times with the fabrication finesse of the silicon nanotechnology industry. We outline a potentially scalable spin qubit architecture where donor nuclear and electron spins are coupled to spins of electrons in quantum dots and discuss requirements for donor placement aligned to q… ▽ More Spins of donor electrons and nuclei in silicon are promising quantum bit (qubit) candidates which combine long coherence times with the fabrication finesse of the silicon nanotechnology industry. We outline a potentially scalable spin qubit architecture where donor nuclear and electron spins are coupled to spins of electrons in quantum dots and discuss requirements for donor placement aligned to quantum dots by single ion implantation. △ Less

Submitted 10 October, 2011; originally announced October 2011.

Report number: in "Single Atom Electronics", E. Prati, T. Shinada (eds), Pan Stanford, 2013, pp. 255-279

Electrically detected magnetic resonance of neutral donors interacting with a two-dimensional electron gas

Authors: C. C. Lo, V. Lang, R. E. George, J. J. L. Morton, A. M. Tyryshkin, S. A. Lyon, J. Bokor, T. Schenkel

Abstract: We have measured the electrically detected magnetic resonance of channel-implanted donors in silicon field-effect transistors in resonant X- ($9.7\:$GHz) and W-band ($94\:$GHz) microwave cavities, with corresponding Zeeman fields of $0.35\:$T and $3.36\:$T, respectively. It is found that the conduction electron resonance signal increases by two orders of magnitude from X- to W-band, while the hype… ▽ More We have measured the electrically detected magnetic resonance of channel-implanted donors in silicon field-effect transistors in resonant X- ($9.7\:$GHz) and W-band ($94\:$GHz) microwave cavities, with corresponding Zeeman fields of $0.35\:$T and $3.36\:$T, respectively. It is found that the conduction electron resonance signal increases by two orders of magnitude from X- to W-band, while the hyperfine-split donor resonance signals are enhanced by over one order of magnitude. We rule out a bolometric origin of the resonance signals, and find that direct spin-dependent scattering between the two-dimensional electron gas and neutral donors is inconsistent with the experimental observations. We propose a new polarization transfer model from the donor to the conduction electrons as the main contributer to the spin resonance signals observed. △ Less

Submitted 17 December, 2010; originally announced December 2010.

Comments: 5 pages, 3 figures

Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

Authors: A. T. Zayak, Y. S. Hu, H. Choo, J. Bokor, S. Cabrini, P. J. Schuck, J. B. Neaton

Abstract: Using a combination of first-principles theory and experiments, we provide a quantitative explanation for chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces. With density functional theory calculations of the static Raman tensor, we demonstrate and quantify a strong mode-dependent modification of… ▽ More Using a combination of first-principles theory and experiments, we provide a quantitative explanation for chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces. With density functional theory calculations of the static Raman tensor, we demonstrate and quantify a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential, a well-defined property of each vibrational mode. A straightforward and general analysis is introduced that allows extraction of chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations. △ Less

Submitted 8 November, 2010; originally announced November 2010.

Comments: 5 pages, 4 figures and Supplementary material included as ancillary file

Device fabrication and transport measurements of FinFETs built with $^{28}$Si SOI wafers towards donor qubits in silicon

Authors: CC Lo, A Persaud, S Dhuey, D Olynick, F Borondics, MC Martin, HA Bechtel, J Bokor, T Schenkel

Abstract: We report fabrication of transistors in a FinFET geometry using isotopically purified silicon-28 -on-insulator (28-SOI) substrates. Donor electron spin coherence in natural silicon is limited by spectral diffusion due to the residual $^{29}$Si nuclear spin bath, making isotopically enriched nuclear spin-free $^{28}$Si substrates a promising candidate for forming spin quantum bit devices. The Fin… ▽ More We report fabrication of transistors in a FinFET geometry using isotopically purified silicon-28 -on-insulator (28-SOI) substrates. Donor electron spin coherence in natural silicon is limited by spectral diffusion due to the residual $^{29}$Si nuclear spin bath, making isotopically enriched nuclear spin-free $^{28}$Si substrates a promising candidate for forming spin quantum bit devices. The FinFET architecture is fully compatible with single-ion implant detection for donor-based qubits, and the donor spin-state readout through electrical detection of spin resonance. We describe device processing steps and discuss results on electrical transport measurements at 0.3 K. △ Less

Submitted 10 June, 2009; originally announced June 2009.

Comments: 10 pages, 6 figures

Critical issues in the formation of quantum computer test structures by ion implantation

Authors: T. Schenkel, C. C. Lo, C. D. Weis, A. Schuh, A. Persaud, J. Bokor

Abstract: The formation of quantum computer test structures in silicon by ion implantation enables the characterization of spin readout mechanisms with ensembles of dopant atoms and the development of single atom devices. We briefly review recent results in the characterization of spin dependent transport and single ion doping and then discuss the diffusion and segregation behaviour of phosphorus, antimon… ▽ More The formation of quantum computer test structures in silicon by ion implantation enables the characterization of spin readout mechanisms with ensembles of dopant atoms and the development of single atom devices. We briefly review recent results in the characterization of spin dependent transport and single ion doping and then discuss the diffusion and segregation behaviour of phosphorus, antimony and bismuth ions from low fluence, low energy implantations as characterized through depth profiling by secondary ion mass spectrometry (SIMS). Both phosphorus and bismuth are found to segregate to the SiO2/Si interface during activation anneals, while antimony diffusion is found to be minimal. An effect of the ion charge state on the range of antimony ions, 121Sb25+, in SiO2/Si is also discussed. △ Less

Submitted 29 April, 2009; originally announced April 2009.

Diameter-Dependent Electron Mobility of InAs Nanowires

Authors: Alexandra Ford, Johnny Ho, Yu-Lun Chueh, Yu-Chih Tseng, Zhiyong Fan, Jing Guo, Jeffrey Bokor, Ali Javey

Abstract: Temperature-dependent I-V and C-V spectroscopy of single InAs nanowire field-effect transistors were utilized to directly shed light on the intrinsic electron transport properties as a function of nanowire radius. From C-V characterizations, the densities of thermally-activated fixed charges and trap states on the surface of untreated (i.e., without any surface functionalization) nanowires are i… ▽ More Temperature-dependent I-V and C-V spectroscopy of single InAs nanowire field-effect transistors were utilized to directly shed light on the intrinsic electron transport properties as a function of nanowire radius. From C-V characterizations, the densities of thermally-activated fixed charges and trap states on the surface of untreated (i.e., without any surface functionalization) nanowires are investigated while enabling the accurate measurement of the gate oxide capacitance; therefore, leading to the direct assessment of the field-effect mobility for electrons. The field-effect mobility is found to monotonically decrease as the radius is reduced to sub-10 nm, with the low temperature transport data clearly highlighting the drastic impact of the surface roughness scattering on the mobility degradation for miniaturized nanowires. More generally, the approach presented here may serve as a versatile and powerful platform for in-depth characterization of nanoscale, electronic materials. △ Less

Submitted 3 December, 2008; originally announced December 2008.

Mapping of ion beam induced current changes in FinFETs

Authors: C. D. Weis, A. Schuh, A. Batra, A. Persaud, I. W. Rangelow, J. Bokor, C. C. Lo, S. Cabrini, D. Olynick, S. Duhey, T. Schenkel

Abstract: We report on progress in ion placement into silicon devices with scanning probe alignment. The device is imaged with a scanning force microscope (SFM) and an aligned argon beam (20 keV, 36 keV) is scanned over the transistor surface. Holes in the lever of the SFM tip collimate the argon beam to sizes of 1.6 um and 100 nm in diameter. Ion impacts upset the channel current due to formation of posi… ▽ More We report on progress in ion placement into silicon devices with scanning probe alignment. The device is imaged with a scanning force microscope (SFM) and an aligned argon beam (20 keV, 36 keV) is scanned over the transistor surface. Holes in the lever of the SFM tip collimate the argon beam to sizes of 1.6 um and 100 nm in diameter. Ion impacts upset the channel current due to formation of positive charges in the oxide areas. The induced changes in the source-drain current are recorded in dependence of the ion beam position in respect to the FinFET. Maps of local areas responding to the ion beam are obtained. △ Less

Submitted 12 September, 2008; v1 submitted 11 September, 2008; originally announced September 2008.

Comments: IBMM 2008 conference proceeding

Simulation Studies of Nanomagnet-Based Architecture

Authors: David Carlton, Nathan Emley, Eduard Tuchfeld, Jeffrey Bokor

Abstract: We report a simulation study on interacting ensembles of Co nanomagnets that can perform basic logic operations and propagate logic signals, where the state variable is the magnetization direction. Dipole field coupling between individual nanomagnets drives the logic functionality of the ensemble and coordinated arrangements of the nanomagnets allow for the logic signal to propagate in a predict… ▽ More We report a simulation study on interacting ensembles of Co nanomagnets that can perform basic logic operations and propagate logic signals, where the state variable is the magnetization direction. Dipole field coupling between individual nanomagnets drives the logic functionality of the ensemble and coordinated arrangements of the nanomagnets allow for the logic signal to propagate in a predictable way. Problems with the integrity of the logic signal arising from instabilities in the constituent magnetizations are solved by introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each nanomagnet. The enhanced stability allows for more complex components of a logic architecture capable of random combinatorial logic, including horizontal wires, vertical wires, junctions, fanout nodes, and a novel universal logic gate. Our simulations define the focus of scaling trends in nanomagnet-based logic and provide estimates of the energy dissipation and time per nanomagnet reversal. △ Less

Submitted 29 August, 2008; originally announced September 2008.

Spin-dependent scattering in a silicon transistor

Authors: Rogerio de Sousa, Cheuk Chi Lo, Jeffrey Bokor

Abstract: The scattering of conduction electrons off neutral donors depends sensitively on the relative orientation of their spin states. We present a theory of spin-dependent scattering in the two dimensional electron gas (2DEG) of field effect transistors. Our theory shows that the scattering mechanism is dominated by virtual transitions to negatively ionized donor levels. This effect translates into a… ▽ More The scattering of conduction electrons off neutral donors depends sensitively on the relative orientation of their spin states. We present a theory of spin-dependent scattering in the two dimensional electron gas (2DEG) of field effect transistors. Our theory shows that the scattering mechanism is dominated by virtual transitions to negatively ionized donor levels. This effect translates into a source-drain current that always gets reduced when donor spins are at resonance with a strong microwave field. We propose a model for donor impurities interacting with conduction electrons in a silicon transistor, and compare our explicit numerical calculations to electrically detected magnetic resonance (EDMR) experiments. Remarkably, we show that EDMR is optimal for donors placed into a sweet spot located at a narrow depth window quite far from the 2DEG interface. This allows significant optimization of spin signal intensity for the minimal number of donors placed into the sweet spot, enabling the development of single spin readout devices. Our theory reveals an interesting dependence on conduction electron spin polarization p_c. As p_c increases upon spin injection, the EDMR amplitude first increases as p_{c}^{2}, and then saturates when a polarization threshold p_T is reached. These results show that it is possible to use EDMR as an in-situ probe of carrier spin polarization in silicon and other materials with weak spin-orbit coupling. △ Less

Submitted 24 July, 2009; v1 submitted 27 June, 2008; originally announced June 2008.

Journal ref: Phys. Rev. B 80, 045320 (2009)

Single-atom doping for quantum device development in diamond and silicon

Authors: C. D. Weis, A. Schuh, A. Batra, A. Persaud, I. W. Rangelow, J. Bokor, C. C. Lo, S. Cabrini, E. Sideras-Haddad, G. D. Fuchs, R. Hanson, D. D. Awschalom, T. Schenkel

Abstract: The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. We describe a single atom injector, in which the im… ▽ More The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. We describe a single atom injector, in which the imaging and alignment capabilities of a scanning force microscope (SFM) are integrated with ion beams from a series of ion sources and with sensitive detection of current transients induced by incident ions. Ion beams are collimated by a small hole in the SFM tip and current changes induced by single ion impacts in transistor channels enable reliable detection of single ion hits. We discuss resolution limiting factors in ion placement and processing and paths to single atom (and color center) array formation for systematic testing of quantum computer architectures in silicon and diamond. △ Less

Submitted 12 June, 2008; originally announced June 2008.

Spin-Dependent Scattering off Neutral Antimony Donors in 28-Si Field-Effect Transistors

Authors: C. C. Lo, J. Bokor, T. Schenkel, A. M. Tyryshkin, S. A. Lyon

Abstract: We report measurements of spin-dependent scattering of conduction electrons by neutral donors in an accumulation-mode field-effect transistor formed in isotopically enriched silicon. Spin-dependent scattering was detected using electrically detected magnetic resonance where the spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. We di… ▽ More We report measurements of spin-dependent scattering of conduction electrons by neutral donors in an accumulation-mode field-effect transistor formed in isotopically enriched silicon. Spin-dependent scattering was detected using electrically detected magnetic resonance where the spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. We discuss the utilization of spin-dependent scattering as a mechanism for the readout of donor spin-states in silicon based quantum computers. △ Less

Submitted 6 November, 2007; v1 submitted 26 October, 2007; originally announced October 2007.

Comments: 14 pages, 3 figures. Correction made to figure3(b)

Detection of low energy single ion impacts in micron scale transistors at room temperature

Authors: A. Batra, C. D. Weis, J. Reijonen, A. Persaud, S. Cabrini, C. C. Lo, J. Bokor, T. Schenkel

Abstract: We report the detection of single ion impacts through monitoring of changes in the source-drain currents of field effect transistors (FET) at room temperature. Implant apertures are formed in the interlayer dielectrics and gate electrodes of planar, micro-scale FETs by electron beam assisted etching. FET currents increase due to the generation of positively charged defects in gate oxides when io… ▽ More We report the detection of single ion impacts through monitoring of changes in the source-drain currents of field effect transistors (FET) at room temperature. Implant apertures are formed in the interlayer dielectrics and gate electrodes of planar, micro-scale FETs by electron beam assisted etching. FET currents increase due to the generation of positively charged defects in gate oxides when ions (121Sb12+, 14+, Xe6+; 50 to 70 keV) impinge into channel regions. Implant damage is repaired by rapid thermal annealing, enabling iterative cycles of device doping and electrical characterization for development of single atom devices and studies of dopant fluctuation effects. △ Less

Submitted 27 September, 2007; v1 submitted 25 September, 2007; originally announced September 2007.

Stark Tuning of Donor Electron Spins in Silicon

Authors: Forrest R. Bradbury, Alexei M. Tyryshkin, Guillaume Sabouret, Jeff Bokor, Thomas Schenkel, Stephen A. Lyon

Abstract: We report Stark shift measurements for 121Sb donor electron spins in silicon using pulsed electron spin resonance. Interdigitated metal gates on top of a Sb-implanted 28Si epi-layer are used to apply electric fields. Two Stark effects are resolved: a decrease of the hyperfine coupling between electron and nuclear spins of the donor and a decrease in electron Zeeman g-factor. The hyperfine term p… ▽ More We report Stark shift measurements for 121Sb donor electron spins in silicon using pulsed electron spin resonance. Interdigitated metal gates on top of a Sb-implanted 28Si epi-layer are used to apply electric fields. Two Stark effects are resolved: a decrease of the hyperfine coupling between electron and nuclear spins of the donor and a decrease in electron Zeeman g-factor. The hyperfine term prevails at X-band magnetic fields of 0.35T, while the g-factor term is expected to dominate at higher magnetic fields. A significant linear Stark effect is also resolved presumably arising from strain. △ Less

Submitted 12 March, 2006; originally announced March 2006.

Comments: 10 pages, 4 figures, to be submitted to PRL

Journal ref: PRL 97, 176404 (2006)

Electrical activation and electron spin coherence of ultra low dose antimony implants in silicon

Authors: T. Schenkel, A. M. Tyryshkin, R. de Sousa, K. B. Whaley, J. Bokor, J. A. Liddle, A. Persaud, J. Shangkuan, I. Chakarov, S. A. Lyon

Abstract: We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically enriched 28Si and find high degrees of electrical activation and low levels of dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles peak… ▽ More We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically enriched 28Si and find high degrees of electrical activation and low levels of dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins. △ Less

Submitted 31 October, 2005; v1 submitted 13 July, 2005; originally announced July 2005.

Processing Issues in Top-Down Approaches to Quantum Computer Development in Silicon

Authors: S. -J. Park, A. Persaud, J. A. Liddle, J. Nilsson, J. Bokor, D. H. Schneider, I. Rangelow, T. Schenkel

Abstract: We describe critical processing issues in our development of single atom devices for solid-state quantum information processing. Integration of single 31P atoms with control gates and single electron transistor (SET) readout structures is addressed in a silicon-based approach. Results on electrical activation of low energy (15 keV) P implants in silicon show a strong dose effect on the electrica… ▽ More We describe critical processing issues in our development of single atom devices for solid-state quantum information processing. Integration of single 31P atoms with control gates and single electron transistor (SET) readout structures is addressed in a silicon-based approach. Results on electrical activation of low energy (15 keV) P implants in silicon show a strong dose effect on the electrical activation fractions. We identify dopant segregation to the SiO2/Si interface during rapid thermal annealing as a dopant loss channel and discuss measures of minimizing it. Silicon nanowire SET pairs with nanowire width of 10 to 20 nm are formed by electron beam lithography in SOI. We present first results from Coulomb blockade experiments and discuss issues of control gate integration for sub-40nm gate pitch levels. △ Less

Submitted 9 October, 2003; originally announced October 2003.