Starting New Frontiers with Magnetic Tunnel Junction Based Molecular Spintronics and Molecular Electronics

Significance 

Prof. Pawan Tyagi and his group from the University of the District of Columbia focus on a magnetic tunnel junction based molecular spintronic device (MTJMSD). The MTJMSD approach is an attempt to solve decade old device fabrication challenges and clear the pathways for realizing novel logic and memory devices for the futuristic computer [1]. In addition, the MTJMSD approach, depicted in Fig.1, turned out to be a new platform to allowing magnetic molecules to establish strong coupling between the magnetic electrodes resulting in current suppression like phenomenon currently that is published in the research journal, Organic Electronics [2].

Prof. Tyagi said “Concept of making molecule-based logic and memory devices begun around 1950, but it took around 40 years to start the experimental molecular device research and studies, around the same time when the field of spintronics was on rise. Overlapping interest in spintronics and molecular electronics gave birth to the field of molecular spintronics. Unfortunately, conventional experimental methods of making molecular spintronics devices are unable to make robust and mass producible devices to further propel the field into exotic applications such as molecule-based quantum computation and molecular quantum property dependent logic and memory devices. Also, new approach should allow molecules to exhibit their highest potential as a futuristic spintronics device elements by enabling the use of a variety of metallic electrodes (other than gold), control experiments, and much needed magnetic measurements. To serve these objectives the MTJMSD approach piggyback on the already commercialized spin valve technology utilizing magnetic tunnel junction (MTJ)”.

A MTJ is basically a component consisting of two ferromagnets separated by a thin insulator. Ideally, if the insulator is thin enough, electrons can tunnel from one ferromagnet into the other. Recent publications have highlighted that manipulation of the nature of a spacer between two ferromagnetic electrodes is an effective route to impact the magnetic properties of the overall magnetic films. As such, by connecting additional molecular spin pathways to the ferromagnetic electrodes of the MTJ, novel applications and research pathways could be established. Since last 17 years Prof. Tyagi has been focusing on the hypothesis that adding molecular channels between two ferromagnetic electrodes could bring unique advantages in terms of MTJ performance. Prof. Tyagi discovered that chemically stitching the molecular channels between two ferromagnetic films in fact produced very strong effect and transformed the magnetic [3], electrical[2], and optical [4] properties of the MTJ testbed. Consequently, his research at University of the District of Columbia aimed at understanding the impact of a variety of paramagnetic molecules on the magnetic properties of the MTJ testbed. In particular, Prof. Tyagi’s group conducted theoretical Monte Carlo studies to understand the experimentally observed molecule induced magnetic coupling strength that was of the order of ~50% of the inter-atomic bonding of the ferromagnetic electrode utilized in MTJMSDs [3]. It was hypothesized that molecule induced coupling impacting the ferromagnetic electrodes in MTJMSD should also produce significant changes in MTJ’s spin transport characteristics.

To experimentally study molecular coupling effect on transport properties the researchers attached paramagnetic molecules between two ferromagnetic electrodes of an MTJ along the exposed side edges (Fig.1). The MTJ testbed for MTJMSDs were fabricated on thermally oxidized silicon using the liftoff method[1, 5]. Various analysis and tests followed up as a means of validating their approach. It was observed that the strong molecule coupling between two ferromagnetic electrodes caused the drastic changes in transport properties of the MTJ testbed [2]. Additionally, the molecular transport channels along the tunnel junction edges decreased the MTJMSD current by >five orders of magnitude as compared to the leakage current of the bare tunnel junction at room temperature [2]. It must be noted that current suppression phenomenon was observed on the Co/NiFe/AlOx/NiFe MTJ configuration that also exhibited dramatic change in magnetic properties after the interaction with the same paramagnetic molecule [3]. However, this paramagnetic molecule produced typical current increase when gold metal electrodes were used. Hence, it was critical to utilize ferromagnetic metals to observe the current suppression discussed in the research journal, Organic Electronics [2]. MTJMSD’s suppressed current state could be perturbed rather in the early stage of MTJMSD fabrication. It was observed that forcing electric current through MTJMSD shifted it from the suppressed current state to the several order higher current state. However, MTJMSD quickly shifted back to suppressed state after a brief high current state. Hence, suppressed current state was the ground state for the MTJMSD discussed in the journal Organic Electronics [2].

Authors were unable to demonstrate the controllable switching from suppressed current to high current state, due to the strong molecule induced exchange coupling that could not be overcome by regular magnetic field created by the electromagnet. Prof. Tyagi’s group is currently focusing on the MTJMSD based spin valves that can exhibit controllable switching.

In summary, the study presented an in-depth assessment of the molecule induced dramatic changes in the magnetic and transport properties of the conventional magnetic tunnel junctions. Generally, the paramagnetic molecules used were seen to cause significant changes in the spin density of states due to the potential spin filtering effect. Also, the said molecules were also seen to produced antiferromagnetic coupling between the affected magnetic electrodes [3]. Three independent magnetic measurements conducted on an array of ~7000 MTJ to confirm the molecule’s effect. Altogether, MTJMSD can provide an advanced form of logic and memory devices, including a testbed for the molecule-based quantum computation devices.

The strength of our approach is that we used already commercially successful MTJ structure to transform into a molecular spintronics device that is termed as MTJ based Molecular spintronics device (MTJMSD). MTJMSD exhibited room temperature current suppression [2, 6]. MTJMSD approach brings unique opportunity to conduct myriad of magnetic study [3] to understand the mechanism behind current suppression and many other intriguing phenomenon, such as spin photovoltaic effect [4]. However, there is a need for further work and independent confirmation of the phenomenon by other groups to advance the field of MTJMSD”. Said Professor Pawan Tyagi in a statement to Advances in Engineering. He then added “To date it has been extremely difficult to conduct magnetic studies in conjunction with transport studies on the prior molecular devices. MTJMSD approach can be a mass producible testbed for evaluating a range of interesting molecules for novel memory devices to quantum computing devices”.

Acknowledgement: Pawan Tyagi thanks Dr. Bruce Hinds and Department of Chemical and Materials engineering at University of Kentucky for facilitating experimental work on MTJMSD during his PhD. He also thankfully acknowledges Dr. Stephen Holmes for producing molecules used in the reported work. Any opinions, findings, and conclusions expressed in this material are those of the author(s) and do not necessarily reflect the views of any funding agency and corresponding author’s past and present affiliations.

Starting New Frontiers with Magnetic Tunnel Junction Based Molecular Spintronics and Molecular Electronics  - Advances in Engineering
Fig. 1 MTJMSD formed by chemically bonding magnetic molecules along the exposed edges of magnetic tunnel junction.

About the author

Prof. Pawan Tyagi expertise is in the area of integrating nanomaterials into devices and sensors for advancing futuristic computer technology, biomedical devices, energy technology, and advanced manufacturing. He has made the seminal contribution in the area of tunnel junction based molecular spintronics devices. At University of the District of Columbia, he is serving as the founder and director of Nanotechnology Application Laboratory and leading several federally funded projects.

Prof. Tyagi has published more than 30 publications. Prof. Tyagi has 24 years of experience in materials science arising from his BS and MS in metallurgical and materials engineering at Indian Institute of Technology (IIT), industrial career, doctoral study at University of Kentucky, and postdoctoral research at Johns Hopkins University.

References

[1] P. Tyagi, “Multilayer Edge Molecular Electronics Devices: A Review,” J. Mater. Chem., vol. 21, pp. 4733-4742, 2011.

[2] P. Tyagi, C. Riso, and E. Friebe, “Magnetic Tunnel Junction Based Molecular Spintronics Devices Exhibiting Current Suppression At Room Temperature,” Organic Electronics, vol. 64, pp. 188-194, 2019.

[3] P. Tyagi, C. Baker, and C. D’Angelo, “Paramagnetic Molecule Induced Strong Antiferromagnetic Exchange Coupling on a Magnetic Tunnel Junction Based Molecular Spintronics Device,” Nanotechnology, vol. 26, p. 305602, 2015.

[4] P. Tyagi, “Spin Photovoltaic Effect on Molecule Coupled Ferromagnetic Films of a Magnetic Tunnel Junction,” ASME International Mechanical Engineering Congress and Exposition, vol. 6B: Energy, p. V06BT07A039, 2013.

[5] P. Tyagi, D. F. Li, S. M. Holmes, and B. J. Hinds, “Molecular Electrodes At The Exposed Edge Of Metal/Insulator/Metal Trilayer Structures,” J. Am. Chem. Soc., vol. 129, pp. 4929-4938, Apr 25 2007.

[6] P. Tyagi and E. Friebe, “Large Resistance Change on Magnetic Tunnel Junction based Molecular Spintronics Devices,” J. Mag. Mag. Mat., vol. 453, pp. 186-192, 2018.

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