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The single-molecule magnet (SMM) is demonstrated here to transform conventional magnetic tunnel junctions (MTJ), a memory device used in present-day computers, into solar cells. For the first time, we demonstrated an electronic spin-dependent solar cell effect on an SMM-transformed MTJ under illumination from unpolarized white light. We patterned cross-junction-shaped devices forming a CoFeB/MgO/CoFeB-based MTJ. The MgO barrier thickness at the intersection between the two exposed junction edges was less than the SMM extent, which enabled the SMM molecules to serve as channels to conduct spin-dependent transport. The SMM channels yielded a region of long-range magnetic ordering around these engineered molecular junctions. Our SMM possessed a hexanuclear [Mn6(μ3-O)2(H2N-sao)6(6-atha)2(EtOH)6] [H2N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate] complex and thiols end groups to form bonds with metal films. SMM-doped MTJs were shown to exhibit a solar cell effect and yielded ≈ 80 mV open-circuit voltage and ≈ 10 mA/cm2 saturation current density under illumination from one sun equivalent radiation dose. A room temperature Kelvin Probe AFM (KPAFM) study provided direct evidence that the SMM transformed the electronic properties of the MTJ's electrodes over a lateral area in excess of several thousand times larger in extent than the area spanned by the molecular junctions themselves. The decisive factor in observing this spin photovoltaic effect is the formation of SMM spin channels between the two different ferromagnetic electrodes, which in turn is able to catalyze the long-range transformation in each electrode around the junction area.more » « less
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Abstract Magnetic tunnel junction-based molecular spintronics device (MTJMSD) may enable novel magnetic metamaterials by chemically bonding magnetic molecules and ferromagnets (FM) with a vast range of magnetic anisotropy. MTJMSD have experimentally shown intriguing microscopic phenomenon such as the development of highly contrasting magnetic phases on a ferromagnetic electrode at room temperature. This paper focuses on Monte Carlo Simulations (MCS) on MTJMSD to understand the potential mechanism and explore fundamental knowledge about the impact of magnetic anisotropy. The selection of MCS is based on our prior study showing the potential of MCS in explaining experimental results (Tyagi et al. in Nanotechnology 26:305602, 2015). In this paper, MCS is carried out on the 3D Heisenberg model of cross-junction-shaped MTJMSDs. Our research represents the experimentally studied cross-junction-shaped MTJMSD where paramagnetic molecules are covalently bonded between two FM electrodes along the exposed side edges of the magnetic tunnel junction (MTJ). We have studied atomistic MTJMSDs properties by simulating a wide range of easy-axis anisotropy for the case of experimentally observed predominant molecule-induced strong antiferromagnetic coupling. Our study focused on understanding the effect of anisotropy of the FM electrodes on the overall MTJMSDs at various temperatures. This study shows that the multiple domains of opposite spins start to appear on an FM electrode as the easy-axis anisotropy increases. Interestingly, MCS results resembled the experimentally observed highly contrasted magnetic zones on the ferromagnetic electrodes of MTJMSD. The magnetic phases with starkly different spins were observed around the molecular junction on the FM electrode with high anisotropy.
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"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, various couplings, spin fluctuation, thermal energy, device size, etc. Here, we report a theoretical Monte Carlo Simulation (MCS) study to explain the impact of anisotropy on the MTJMSD equilibrium properties. We studied the magnetic properties of MTJMSDs when in-plane and out-of-plane anisotropies acted simultaneously and together on one of its ferromagnetic electrodes. In-plane anisotropy causes multiple magnetic phases of opposite spins. The multiple magnetic phases vanished at higher thermal energy. The device still maintained higher magnetic moment because of anisotropy. The out-of-plane anisotropy caused a dominant magnetic phase in an electrode rather than multiple magnetic phases. The simultaneous in-plane and out-of-plane anisotropies on the same electorate negated the anisotropy effect. The study of the competing effect of anisotropies opens the insight into experimental observations of MTJMSD studies."more » « less
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Spatial Impact Range of Single-Molecule Magnet (SMM) on Magnetic Tunnel Junction-Based Molecular Spintronic Devices (MTJMSDs) Marzieh Savadkoohi, Bishnu R Dahal, Eva Mutunga, Andrew Grizzle, Christopher D’Angelo, and Pawan Tyagi Magnetic Tunnel Junction-Based Molecular Spintronic Devices (MTJMSDs) are potential candidates for inventing highly correlated materials and devices. However, a knowledge gap exists about the impact of variation in length and thickness of ferromagnetic(FM) electrodes on molecular spintronics devices. This paper reports our experimental observations providing the dramatic impact of variation in ferromagnetic electrode length and thickness on paramagnetic molecule-based MTJMSD. Room temperature transport studies were performed to investigate the effect of FM electrode thickness. On the other hand, magnetic force microscopy measurements were conducted to understand the effect of FM electrode length extending beyond the molecular junction area, i.e., the site where paramagnetic molecules bridged between two FM. In the strong molecular coupling regime, transport study suggested thickness variation caused ~1000 to million-fold differences in junction conductivity. MFM study revealed near-zero magnetic contrast for pillar-shaped MTJMSD without any extended FM electrode. However, MFM images showed a multitude of microscopic magnetic phases on cross junction shaped MTJMSD where FM electrodes extended beyond the junction area. To understand the intriguing experimental results, we conducted an in-depth theoretical study using Monte Carlo Simulation (MCS) approach. MCS study utilized a Heisenberg atomic model of cross junction shaped MTJMSD to gain insights about room temperature transport and MFM experimental observations of microscopic MTJMSD. To make this study applicable for a wide variety of MTJMSDs, we systematically studied the effect of variation in molecular coupling strength between magnetic molecules and ferromagnetic (FM) electrodes of various dimensions.more » « less
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The intra-molecular coupling within multiple units of paramagnetic molecules can produce various effects on molecular spintronics devices (MSD). The effect of the nature of the strong magnetic coupling between a multi-segmented molecule with two ferromagnetic (FM) electrodes is unexplored. Such knowledge is of critical importance for magnetic tunnel junction-based molecular spintronics devices (MTJMSD). MTJMSD architecture experimentally allows very strong bonding between complex molecules and ferromagnetic electrodes. In our prior studies, we have extensively studied the atomic analog of the single molecular magnet. That means whole molecular geometry and internal features were approximated to appear as one atom representing that molecule. To advance the understanding of the impact of internal molecular structure on MTJMSD, we have focused on multi-segmented molecules. This research aims to fill the knowledge gap about the intramolecular coupling role in the magnetic properties of the MTJMSD. This study explored a double-segmented molecule containing two atomic sections, each with a net spin state and interacting via Heisenberg exchange coupling within molecules and with ferromagnetic electrodes. The effect of thermal energy was explored on the impact of intra-molecular coupling on the MTJMSD Heisenberg model. We performed Monte Carlo simulations(MCS) to study various possibilities in the strong molecule-ferromagnet coupling regime. This research provides insights into the influence of complex molecules on MSD that can be employed in futuristic computers and novel magnetic meta-materials.more » « less
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The hysteresis loop investigations of different size magnetic tunnel junction molecular spintronics devices (MTJMSD) have been done by Monte Carlo simulation (MCS). We employed a continuous MCS algorithm to investigate single-molecule magnet SMM’s spin state’s impact as a function of molecular exchange coupling strength. The applied magnetic fields were ramped at a variety of ranges of increments, unfolding physics behind the magnetization nature of each MTJMSD. The magnetic moment changes with applied magnetic fields exhibit the characteristics of devices being studied. The MTJMSDs were studied for ferromagnetic and antiferromagnetic exchange couplings. The magnetic moment saturation, retentivity, coercivity, and permeability are studied.more » « less