arXiv subjects

Pawan Tyagi

Publications and source records attributed to Pawan Tyagi.

16 recordsLinked to original sources

arXiv2026

Competing Effect of Biquadratic and Heisenberg Coupling on Magnetic Tunnel Junction Molecular Spintronics Devices

Heisenberg exchange coupling (HC) and biquadratic exchange coupling (BQC) are known to occur in magnetic tunnel junctions (MTJ) and nanoscale spintronics structures. MTJ-based molecular spintronics devices (MTJMSD) provide a platform to study these interactions and the correlated magnetic behavior they generate. Molecular spin channels formed along the exposed MTJ edge have been shown to produce strong exchange interactions that include HC and BQC, which can drive perpendicular alignment of spin vectors in adjacent ferromagnetic electrodes. Despite their importance, the competing roles of HC and BQC in MTJMSDs remain unclear. Monte Carlo simulations using a three-dimensional Heisenberg model were performed to systematically vary BQC strength under three conditions: no molecular HC, strong parallel HC, and strong antiparallel HC. The resulting magnetic and physical properties of the MTJMSDs were analyzed. Increasing BQC strength produced minimal changes in overall magnetization when strong HC was present, indicating that HC dominates device magnetization. Temporal evolution studies showed that devices with only BQC failed to reach magnetic stability, while devices with both HC and BQC achieved stable magnetic states due to the stabilizing influence of HC. These results show that BQC plays a secondary role in magnetization dynamics and cannot overcome the stronger stabilizing effect of HC. The presence of BQC offers a plausible explanation for experimentally observed magnetic phase orientations beyond simple parallel and antiparallel states in MTJMSDs.

cond-mat.stat-mech↗

arXiv2021

Study of Anisotropy on Ferromagnetic Electrodes of a Magnetic Tunnel Junction (MTJ)-Based Molecular Spintronics Device (MTJMSD)

Magnetic tunnel junction-based molecular spintronics devices (MTJMSDs) are designed by covalently connecting the paramagnetic molecules across two ferromagnets (FM) electrodes of a magnetic tunnel junction (MTJ). MTJMSD provides opportunities to connect FM electrodes of a vast range of anisotropy properties to a variety of molecules of length scale. Our prior studies showed that the paramagnetic molecules can produce strong antiferromagnetic coupling with FM electrodes. The device properties of MTJMSD depend upon various factors such as anisotropy, spin fluctuation, thermal energy, etc. In this paper, we report a theoretical Monte Carlo Simulation (MCS) study to explain the impact of anisotropy on the MTJMSD equilibrium properties. We studied the energy variation of the MTJMSD system with time as a function of FM electrode anisotropy. Experimentally designed FM electrodes of MTJMSD contain multi-layers of different ferromagnetic materials. These materials possess in-plane and out-of-plane magnetic anisotropy characteristics. To understand the competing effect of in-plane and out-of-plane anisotropy, we have computationally applied anisotropies on the left FM electrode. For the MCS study, the orientation of the device was kept along YZ plane. As a result, the applied anisotropy along the X-direction (\mathbit{A}_{\mathbit{Lx}}) and Y-direction (\mathbit{A}_{\mathbit{Ly}}) represent out-of-plane and in-plane anisotropy, respectively. We found that increasing anisotropy strength starts exhibiting diverse domain structures within an FM electrode. Increasing the magnitude of anisotropy was found to create stripe-shaped domains with opposite spins. These domains represent the different magnetic phases. However, the application of equal magnitude of in-plane and out-of-plane cancels the strip domain formation and lowers the magnetic moment of overall MTJMSD.

cond-mat.mtrl-sci↗

arXiv2021

Review of Sulphur Interaction based GaAs Surface Passivation and Its Potential Application in Magnetic Tunnel Junction (MTJ) based Molecular Spintronics Devices (MTJMSD)

GaAs surface is characterized by a high density of surface states, which preclude the utilization of this semiconducting material for the realization of several advanced devices. Sulfur-based passivation has been found significantly useful in reducing the effect of dangling bonds. In this article first, the problem associated with GaAs surface has been discussed in a tutorial form. Secondly, a brief introduction of a wide variety of surface passivation methods was introduced. Sulfur passivation, the most effective surface state quenching method, has been elaborated. Thirdly, current trends in the field of surface passivation of GaAs surface have been discussed. Our discussion also focusses on utilizing GaAs and alloys for the molecular electronics and molecular spintronics and based on our insights in the GaAs (P. Tyagi, MRS Advances 2 (51), 2915-2920 2017) and molecular spintronics field ((P. Tyagi, D. F. Li, S. M. Holmes and B. J. Hinds, J. Am. Chem. Soc. 129 (16), 4929, 2007) and P. Tyagi, C. Riso, U. Amir, C. Rojas-Dotti and J. Martínez-Lillo, RSC Advances 10 (22), 13006, 2020) ).

cond-mat.mtrl-sci↗

arXiv2021

Spin State of Single Molecular Magnet (SMM) Creating Long Range Ordering on Ferromagnetic Layers of Magnetic Tunnel Junction -A Monte Carlo Study

Single molecular magnet (SMM) like paramagnetic molecules interacting with the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) produce a new system that differs dramatically from the properties of isolated molecules and ferromagnets. However, it is unknown how far deep in the ferromagnetic electrode the impact of the paramagnetic molecule and ferromagnet interactions can travel for various levels of molecular spin states. Our prior experimental studies showed two types of paramagnetic SMMs, the hexanuclear Mn6 and octanuclear Fe-Ni molecular complexes, covalently bonded to ferromagnets produced unprecedented strong antiferromagnetic coupling between two ferromagnets at room temperature leading to a number of intriguing observations. In this paper, we report Monte Carlo Simulations (MCS) study focusing on the impact of the molecular spin state on cross junction shaped MTJ based molecular spintronics device (MTJMSD). Our MCS study focused on the Heisenberg model of MTJMSD and investigated the impact of various molecular coupling strengths, thermal energy, and molecular spin states. To gauge the impact of the molecular spin state on the region of ferromagnetic electrodes, we examined the spatial distribution of molecule-ferromagnet correlated phases. Our MCS study shows that under a strong coupling regime molecular spin state should be approximately 30 percent of the ferromagnetic electrode atomic spins to create long-range correlated phases.

cond-mat.mtrl-sci↗

arXiv2019

Large Resistance Change on Magnetic Tunnel Junction based Molecular Spintronics Devices

Molecular bridges covalently bonded to two ferromagnetic electrodes can transform ferromagnetic materials and produce intriguing spin transport characteristics. This paper discusses the impact of molecule induced strong coupling on spin transport. To study the molecular coupling effect organometallic molecular complex (OMC) was bridged between two ferromagnetic electrodes of a magnetic tunnel junction (Ta/Co/NiFe/AlOx/NiFe/Ta) along the exposed side edges. OMCs induced strong iter-ferromagnetic electrode coupling to yield drastic changes in transport properties of the magnetic tunnel junction testbed at the room temperature. These OMCs also transformed the magnetic properties of magnetic tunnel junctions. SQUID and ferromagnetic resonance studies provided insightful data to explain transport studies on the magnetic tunnel junction based molecular spintronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic Force Microscopy Revealing Molecule Impact on Magnetic Tunnel Junction Based Molecular Devices at Room Temperature

Commercially successful magnetic tunnel junction can harness the unmatched capabilities of molecular device elements by solving decade-old fabrication issues. Utilization of magnetic tunnel junction as a testbed for molecules also enables unprecedented magnetic studies of molecular spintronics devices. This paper utilizes magnetic force microscopy (MFM) to vividly show that organometallic molecules when bridged between two ferromagnetic electrodes along the magnetic tunnel junction edges, transformed the magnetic electrodes itself. Molecules impacted several hundred-micron areas of ferromagnetic electrodes at room temperature. Complementary, magnetic resonance and magnetometer studies supported the dramatic MFM results. Molecule induced changes in the magnetic electrodes impacted the transport of the magnetic tunnel junction and stabilized as much as six orders smaller current at room temperature. Magnetic tunnel junction based molecular devices can be a gateway to a vast range of commercially viable futuristic logic and memory devices that are controlled by the molecular quantum states near room temperature.

cond-mat.mtrl-sci↗

arXiv2019

Scope of Magnetic Tunnel Junction Based Molecular Electronics and Spintronics Devices

Dream of developing molecule-based logic and memory device is more than 70-year-old. Presently, molecule-based devices are also considered for quantum computation hardware. The recent studies have shown the molecule connected to metal leads can perform the qubit-based logic operation. This is an interesting question of why experimental progress is still very slow even when the scope of molecule-based devices may govern the advancement of next-generation logic and memory devices for the highest possible computer technologies. Molecules have the potential to be unmatched device elements as chemists can mass-produce an endless variety of molecules with novel optical, magnetic, and charge transport characteristics. However, the biggest challenge is to connect two metal leads to target molecules and develop a robust and versatile device fabrication technology that can be adopted for commercial-scale mass production. This paper discusses the distinct advantages of utilizing commercially successful magnetic tunnel junctions as a vehicle for developing molecular electronics and molecular spintronics devices. We describe the use of a tunnel junction with the exposed sides as a testbed for molecular devices. On the exposed sides of a tunnel, junction molecules are bridged across an insulator by chemically bonding with the two metal electrodes; sequential growth of metal-insulator-metal layers ensures that separation between two metal electrodes is controlled by the insulator thickness to the molecular device length scale. Here we also strongly emphasize a need for close collaboration between chemists and magnetic tunnel junction researchers. Such partnerships will have a strong potential to develop magnetic tunnel junction based molecular devices for the futuristic areas such as memory devices, magnetic metamaterials, and high sensitivity biosensors.

cond-mat.mtrl-sci↗

arXiv2019

Reduction of Spikes on the Sides of Patterned Thin Films for Magnetic Tunnel Junction Based Molecular Device Fabrication

Sputter thin film deposition after photolithography often produces unwanted spikes along the side edges. These spikes are a significant issue for the development of magnetic tunnel junction (MTJ)-based memory and molecular spintronics devices, microelectronics, and micro-electro-mechanical systems because they influence the properties of the other films deposited on the top. Our molecular spintronics devices that utilize MTJ as the testbed are almost short-lived and encountered high background current that masked the effect of molecular transport channels placed along the sides of MTJs. Therefore, tapered thin film edges are critically needed in devices. Here, we report a very cost-efficient and fast way of creating an optimum photoresist profile for the production of spike-free patterned films. This approach is based on performing a soaking in the photoresist developer after baking and before the UV exposure. However, the success of this method depends on multiple factors accounted for during photolithography - photoresist thickness (spin speed), baking temperature, soaking time and exposure time. Our recent experiments systematically studied the effect of these factors by following the L9 experimental scheme of the Taguchi Design of the experiment (TDOE). We discovered that baking temperature was the most influential parameter; presoak time and photoresist thickness were two other influential factors; exposure time was the least effective factor. We also found that 4000 rpm, 100 C soft baking, 60 s soaking, and 15 s UV exposure yielded the best results. Finally, the paper also discusses the interdependence of selected factors, and the impact of the individual levels of each factor. This study is expected to benefit MEMS and micro/nanoelectronics device researchers because it attempts at finding a cheaper and faster alternative to creating an optimum photoresist profile.

physics.app-ph↗

arXiv2019

Nanoscale Tantalum Layer Controlling the Magnetic Coupling between Two Ferromagnetic Electrodes via Insulator of a Magnetic Tunnel Junction

Ability to tailor the nature of the magnetic coupling between two ferromagnetic electrodes can enable the realization of new spintronics device systems. This paper discusses our finding that deposition of an ultrathin tantalum (Ta) on the NiFe top electrode reversed the nature of inter-ferromagnetic electrode coupling. We observed that the deposition of ~ 5 nm Ta on the top of a magnetic tunnel junction with Ta( 2 nm)/Co(5 nm)/NiFe (5 nm)/AlOx( 2 nm)/NiFe (10-15 nm) configuration changed the magnetic coupling between two ferromagnetic electrodes from antiferromagnetic to ferromagnetic. We investigated Ta effect using multiple magnetic characterizations like ferromagnetic resonance, magnetometry, and polarized neutron reflectometry. Ferromagnetic resonance characterization was very sensitive for detecting the changes in magnetic coupling via the insulating spacer. This simple approach of adding Ta film to alter the magnetic coupling can impact the other burgeoning areas like molecular spintronics. We found that preexisting magnetic coupling between two ferromagnetic electrodes impacted the resultant magnetic properties of magnetic tunnel junctions based molecular spintronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Photovoltaic Effect on Molecule Coupled Ferromagnetic Films of a Magnetic Tunnel Junction

Economical solar energy conversion to electricity can be boosted by the discovery of fundamentally new photovoltaic mechanism, and a suitable system to realize it with commonly available materials like iron (Fe) and nickel (Ni). This paper reports the observation of photovoltaic effect on a molecular spintronics device, composed of magnetic tunnel junction (MTJ) and organometallic molecular clusters (OMCs). A prefabricated MTJ with exposed side edges, after enabling the bridging of OMC channels between its two ferromagnetic films, exhibited following phenomenon (i) dramatic increase in exchange coupling, (ii) 3-6 orders current suppression and (iii) photovoltaic effect. This paper focuses on the photovoltaic effect. Control experiments on isolated ferromagnetic films suggested that OMCs neither affected the magnetic properties nor produced any photovoltaic effect; photovoltaic effect was only observed on the ferromagnetic films serving as magnetic electrodes in a MTJ. Present paper invites further investigation of the similar photovoltaic effect on other combinations of MTJs and promising magnetic molecules, like single molecular magnets, organometallic clusters and porphyrins. This research can lead to mass producible and economical spin photovoltaic devices.

cond-mat.mes-hall↗

arXiv2015

Addressing the Challenges of Using Ferromagnetic Electrodes in the Molecular Spintronics Devices

Ferromagnetic electrodes chemically bonded with thiol functionalized molecules can produce novel molecular spintronics devices. However, major challenges lie in developing Ferromagnetic electrodes based commercially viable device fabrication scheme that consider the susceptibility of ferromagnetic electrodes to oxidation, chemical etching, and stress induced deformations during fabrication and usage. This paper studies NiFe, an alloy used in the present day memory devices and high-temperature engineering applications, as a candidate FM electrode for the fabrication of MSDs. Our spectroscopic reflectance studies show that NiFe start oxidizing aggressively beyond 90 C. The NiFe surfaces, aged for several months or heated for several minutes below 90 C, exhibited remarkable electrochemical activity and were suitable for chemical bonding with the thiol functionalized molecular device elements. NiFe also demonstrated excellent etching resistance and minimized the mechanical stress induced deformities. This paper demonstrates the successful utilization of NiFe electrodes in the tunnel junction based molecular device fabrication approach. This paper is expected to fill the knowledge gap impeding the experimental development of ferromagnetic electrode based molecular spintronics devices for realizing novel logic and memory devices and observing a numerous theoretically predicted phenomenon.

cond-mat.mtrl-sci↗

arXiv2014

Monte Carlo Study of a wide range of Molecular Spintronics Devices

Molecular spintronics devices (MSDs) are highly promising candidates for enabling quantum computation and revolutionizing computer logic and memory. An advanced MSD will require the placement of magnetic molecules between the two ferromagnetic (FM) electrodes. Recent experimental studies showed that some magnetic molecules produced unprecedented strong exchange couplings between the two FM electrodes leading to intriguing magnetic and transport properties in a MSD. Future development of MSDs will critically depend on obtaining an in-depth understanding of the molecule induced exchange coupling, and its impact on switchability, functional temperature range, and stability. However, the large size of MSD systems and fragile device fabrication scheme continue to limit the theoretical and experimental studies of magnetic attributes produced by molecules in a MSD. This paper theoretically studies the MSD by performing Monte Carlo simulations (MCS). Our MCS encompasses the full range of MSDs that can be realized by establishing different kinds of magnetic interaction between magnetic molecules and FM electrodes. Our MSDs are represented by a 2D Ising model. We studied the effect of a wide range of molecule-FM electrode couplings on the basic properties of MSDs. This wide range covered (i) molecule possessing ferromagnetic coupling with both FM electrodes, (ii) molecule possessing antiferromagnetic coupling with both FM electrodes, and (iii) molecule possessing ferromagnetic coupling with one electrode and antiferromagnetic coupling with another FM electrode. Our MCS will enable the fundamental understanding and designing of a wide range of novel MSDs utilizing a variety of molecules and FM electrodes; these studies will also benefits nanomaterials based spintronics devices employing nanoclusters and quantum dots as the device elements.

cond-mat.mes-hall↗

arXiv2011

Room temperature current suppression on multilayer edge molecular spintronics device

Molecular conduction channels between two ferromagnetic electrodes can produce strong exchange coupling and dramatic effect on the spin transport, thus enabling the realization of novel logic and memory devices. However, fabrication of molecular spintronics devices is extremely challenging and inhibits the insightful experimental studies. Recently, we produced Multilayer Edge Molecular Spintronics Devices (MEMSDs) by bridging the organometallic molecular clusters (OMCs) across a ~2 nm thick insulator of a magnetic tunnel junction (MTJ), along its exposed side edges. These MEMSDs exhibited unprecedented increase in exchange coupling between ferromagnetic films and dramatic changes in the spin transport. This paper focuses on the dramatic current suppression phenomenon exhibited by MEMSDs at room temperature. In the event of current suppression, the effective MEMESDs' current reduced by as much as six orders in magnitude as compared to the leakage current level of a MTJ test bed. In the suppressed current state, MEMSD's transport could be affected by the temperature, light radiation, and magnetic field. In the suppressed current state MEMSD also showed photovoltaic effect. This study motivates the investigation of MEMSDs involving other combinations of MTJs and promising magnetic molecules like single molecular magnets and porphyrin. Observation of current suppression on similar systems will unequivocally establish the utility of MEMSD approach.

cond-mat.mes-hall↗

arXiv2011

Multilayer Edge Molecular Devices Based on Plasma Oxidation of Photolithographically Defined Bottom Metal Electrode

A multilayer edge molecular electronics device (MEMED), which utilize the two metal electrodes of a metal-insulator-metal tunnel junction as the two electrical leads to molecular channels, can overcome the long standing fabrication challenges for developing futuristic molecular devices. However, producing ultrathin insulator is the most challenging step in MEMED fabrication. A simplified molecular device approach was developed by avoiding the need of depositing a new materiel on the bottom electrode for growing ultrathin insulator. This paper discuss the approach for MEMED's insulator growth by one-step oxidation of a tantalum (Ta) bottom electrode, in the pholithographically defined region; i.e. ultrathin tantalum oxide (TaOx) insulator was grown by oxidizing bottom metal electrode itself. Organometallic molecular clusters (OMCs) were bridged across 1-3 nm TaOx along the perimeter of a tunnel junction to establish the highly efficient molecular conduction channels. OMC transformed the asymmetric transport profile of TaOx based tunnel junction into symmetric one. A TaOx based tunnel junction with top ferromagnetic (NiFe) electrode exhibited the transient current suppression by several orders. Further studies will be needed to strengthen the current suppression phenomenon, and to realize the full potential of TaOx based multilayer edge molecular spintronics devices.

cond-mat.mes-hall↗

arXiv2011

Fabrication of multilayer edge molecular electronics and spintronics devices

Advancement of molecular devices will critically depend on the approach to establish electrical connections to the functional molecule(s). We produced a molecular device strategy which is based on chemically attaching of molecules between the two magnetic/nonmagnetic metallic electrodes along the multilayer edge(s) of a prefabricated tunnel junction. Here, we present the fabrication methodology for producing these multilayer edge molecular electronics/spintronics devices (MEMEDs/MEMSDs) and details of the associated challenges and their solutions. The key highlight of our MEMED/MEMSD approach is the method of producing exposed side edge(s) of a tunnel junction for hosting molecular conduction channels by a simple liftoff method. The liftoff method ensured that along the tunnel junction edges, the minimum gap between the two metal electrodes equaled the thickness of the tunnel barrier. All of the tunnel junction test beds used a ~2 nm alumina (AlOx) tunnel barrier. We successfully bridged the magnetic organometallic molecular clusters and non-magnetic alkane molecules across the AlOx insulator along the exposed edges, to transform the prefabricated tunnel junction into the molecular electronics or spintronics devices. Tunnel junction test beds were fabricated with a variety of metal electrodes, such as NiFe, Co, Ni, Au, Ta, Cu and Si. Stability of ultrathin thin AlOx varied with the type of bottom metal electrodes used for making MEMED/MEMSD. Additionally, molecular solution used for bridging molecular channels in a MEMED was not compatible with all the metal electrodes; molecular solution resistant ferromagnetic electrodes were developed for the fabrication of MEMSDs. MEMSD approach offers an open platform to test virtually any combination of magnetic electrodes and magnetic molecules, including single molecular magnets.

cond-mat.mtrl-sci↗

arXiv2011

Molecule Induced Strong Exchange Coupling between Ferromagnetic Electrodes of a Magnetic Tunnel Junction

Multilayer edge molecular spintronics device (MEMSD) approach can produce novel logic and memory units for the computers. MEMSD are produced by bridging the molecular channels across the insulator, in the exposed edge region(s) of a magnetic tunnel junction (MTJ). The bridged molecular channels start serving as the dominant exchange coupling medium between the two ferromagnetic electrodes of a MTJ. Present study focus on the effect of molecule enhanced exchange coupling on the magnetic properties of the MTJ. This paper shows that organometallic molecular clusters (OMCs) strongly increased the magnetic coupling between the two ferromagnetic electrodes. SQUID magnetometer showed that OMCs transformed the typical hysteresis magnetization curve of a Co/NiFe/AlOx/NiFe MTJ into linear one. Ferromagnetic resonance studies showed that OMC bridges affected the two fundamental resonance peaks of the Co/NiFe/AlOx/NiFe MTJ. According to magnetic force microscopy, OMCs caused the disappearance of magnetic contrast from the Co/NiFe/AlOx/NiFe tunnel junction area. These three independent and complimentary experiments, suggested the development of extremely strong interlayer exchange coupling. This work delineated a practical route to control the exchange coupling between ferromagnetic electrodes. Ability to tailor magnetic coupling can lead to the development of molecule based quantum computation device architecture.

cond-mat.mtrl-sci↗