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.
- Award ID(s):
- 1914751
- NSF-PAR ID:
- 10333107
- Date Published:
- Journal Name:
- MRS 2022, Hawaii
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Paramagnetic single-molecule magnets (SMMs) interacting with the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) produce a new system. The properties and future scope of new systems differ 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 Mn 6 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 (P. Tyagi, et al. , Org. Electron. , 2019, 64 , 188–194. P. Tyagi, et al. , RSC Adv. , 2020, 10 , (22), 13006–13015). This paper reports a Monte Carlo Simulations (MCS) study focusing on the impact of the molecular spin state on a 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, the molecular spin state should be ∼30% of the ferromagnetic electrode's atomic spins to create long-range correlated phases.more » « less
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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 Nearly 70 years old dream of incorporating molecule as the device element is still challenged by competing defects in almost every experimentally tested molecular device approach. This paper focuses on the magnetic tunnel junction (MTJ) based molecular spintronics device (MTJMSD) method. An MTJMSD utilizes a tunnel barrier to ensure a robust and mass-producible physical gap between two ferromagnetic electrodes. MTJMSD approach may benefit from MTJ's industrial practices; however, the MTJMSD approach still needs to overcome additional challenges arising from the inclusion of magnetic molecules in conjunction with competing defects. Molecular device channels are covalently bonded between two ferromagnets across the insulating barrier. An insulating barrier may possess a variety of potential defects arising during the fabrication or operational phase. This paper describes an experimental and theoretical study of molecular coupling between ferromagnets in the presence of the competing coupling via an insulating tunnel barrier. We discuss the experimental observations of hillocks and pinhole-type defects producing inter-layer coupling that compete with molecular device elements. We performed theoretical simulations to encompass a wide range of competition between molecules and defects. Monte Carlo Simulation (MCS) was used for investigating the defect-induced inter-layer coupling on MTJMSD. Our research may help understand and design molecular spintronics devices utilizing various insulating spacers such as aluminum oxide (AlOx) and magnesium oxide (MgO) on a wide range of metal electrodes. This paper intends to provide practical insights for researchers intending to investigate the molecular device properties via the MTJMSD approach and do not have a background in magnetic tunnel junction fabrication.
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A device architecture utilizing a single-molecule magnet (SMM) as a device element between two ferromagnetic electrodes may open vast opportunities to create novel molecular spintronics devices. Here, we report a method of connecting an SMM to the ferromagnetic electrodes. We utilized a nickel (Ni)–AlO x –Ni magnetic tunnel junction (MTJ) with the exposed side edges as a test bed. In the present work, we utilized an SMM with a hexanuclear [Mn 6 (μ 3 -O) 2 (H 2 N-sao) 6 (6-atha) 2 (EtOH) 6 ] [H 2 N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate] complex that is attached to alkane tethers terminated with thiols. These Mn-based molecules were electrochemically bonded between the two Ni electrodes of an exposed-edge tunnel junction, which was produced by the lift-off method. The SMM-treated MTJ exhibited current enhancement and transitory current suppression at room temperature. Monte Carlo simulation was utilized to understand the transport properties of our molecular spintronics device.more » « less
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