Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W53: Molecular Magnetism: New Exciting DirectionsFocus Recordings Available
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Sponsoring Units: GMAG DMP Chair: Seunghwan Do, Oak Ridge National Laboratory Room: McCormick Place W-475B |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W53.00001: Molecular Magnetic Quantum Materials: Connecting DFT calculations to experiments Invited Speaker: Hai-Ping Cheng Magnetic molecules are unique among nano-magnets because of their monodispersity and stability. We investigate fundamental physical properties of single molecules, 1D chains, 2D sheets, and 3D crystals, as well as cluster crystallines and their interactions with substrates in heterogenous junctions with focus on physical processes critical to quantum information sciences and next generation electronics. The inherent large number of degrees of freedom in molecular systems brought by ligand fields and the complexity of 3d transition metal oxide cores expands greatly the tunability of the parameter space for magnetic anisotropy, inter-ion spin couplings, and strain induced magnetoelectric couplings. The relevant energy scales range from micro electron volts to electron volts. The need for accurate parameter estimation spanning six orders of magnitude presents a grand challenge in modeling for molecular magnetic quantum materials. In this talk, I give an overview on our effort bringing first-principles calculations based on density functional calculations of energetics such as binding energy, phonon energy, anisotropy energy into many-body and dynamic studies using the density matrix method, model Hamiltonians, and Monte-Carlo simulations. This approach connects first-principles theory to experiments across a broad range of problems, including spin coupling between qub(d)its, phases and phase diagrams in spin-crossover materials, manipulation and detection of entangled spin states, and the Dzyaloshinskii-Moriya interaction in magnetic molecules, and electron spin decoherence due to interaction with molecular environments. Finally, I will briefly discuss the role of computationally based theoretical prediction: the finding of Majorana zero energy state and the road map towards computational synthesis of magnetic molecules. |
Thursday, March 17, 2022 3:36PM - 3:48PM |
W53.00002: Towards Bimodal Electron Spin Resonance with a Low-Cost Flexible Spectrometer for Molecular Nanomagnet Experiments Charles Collett, Alexander K Gardner, Abigail Wesolek Molecular nanomagnets (MNMs) have shown much promise as potential qubits, harnessing their electrical engineerability and addressability with electron spin resonance (ESR). Many schemes for coupling MNMs to form multi-qubit systems, necessary for actual quantum computation, require two or more ESR frequencies to address the individual qubits. Our lab has developed a low-cost, flexible ESR spectrometer using a field-programmable gate array, custom loop-gap resonators (LGRs), and cheap off-the-shelf electronics. I will report recent work on refining its single-frequency performance, and progress towards integrating a bimodal LGR for simultaneous excitation of both resonant frequencies of a dimer of the MNM Cr7Mn. |
Thursday, March 17, 2022 3:48PM - 4:00PM |
W53.00003: Probing the atomic-scale magnetism of single molecules and engineered organometallic nanostructures with electron spin resonance STM Gregory Czap, Chris Lutz, Mark Sherwood Single atoms and molecules constitute the ultimate spatial limit for magnetic data storage, quantum information units, quantum sensors and spintronic devices. Recently, developments in Electron Spin Resonance spectroscopy using Scanning Tunneling Microscopes (STM-ESR) have enabled simultaneous atomic spatial resolution and ~ 100 neV energy resolution of adsorbate spin excitations. Previous STM-ESR studies have focused on magnetic 3d transition metal atoms, but the domain of organic spin radicals and complex organometallic structures remains unexplored. Here we report on efforts to extend the detection of STM-ESR to single magnetic molecules and spin radicals stabilized by a thin insulating layer grown on a metal substrate. The intensity of the ESR signal, Rabi rate and coherence times are compared to single metal atom benchmark systems, providing new insights into the mechanisms of STM-ESR. Further, we demonstrate novel organometallic compounds fabricated by tip manipulation with precise control over ligand binding site and orientation. These results present exciting new opportunities for exploring atomic scale magnetism through deliberate engineering of organometallic nanostructures. |
Thursday, March 17, 2022 4:00PM - 4:12PM |
W53.00004: Anisotropy effect on the overall properties of Magnetic Tunnel Junction-Based Molecular Spintronics Device Bishnu R Dahal, Marzieh Savadkoohi, Andrew Grizzle, Christopher D’Angelo, Pawan Tyagi Fabrication of robust and mass-producible molecular spintronics devices at the nanoscale is a major concern. To overcome this issue, we have designed a magnetic tunnel junction-based molecular spintronics device (MTJMSD). MTJMSD enables the utilization of ferromagnetic electrodes with a wide range of magnetic anisotropies. We used Monte Carlo Simulation to explain the impact of anisotropies on the MTJMSD equilibrium properties. We also investigated theoretically how the nature of magnetic M-H changes as we apply the in-plane and out-of-plane anisotropies. The application of anisotropy creates multiple magnetic phases of opposite magnetic spins on the same FM electrode. When in-plane and out-of-plane anisotropies are applied on an electrode simultaneously, the magnetic moment of the electrodes possessed its maximum value. The MTJMSD possessed its maximum value since the competing effect of anisotropies wiped out the multiple magnetic phases. we also investigated the effect of anisotropies on the nature of the M-H curves generated computationally. The effect of the in-plane anisotropy transformed the regular M-H of MTJMSD into a "wasp-waisted" -like M-H curve. The "wasp-waisted" -like M-H is hypothesized due to the presence of multiple magnetic phases present on the FM electrodes caused by strong magnetic anisotropy. The "wasp-waisted" nature of magnetic hysteresis was also observed experimentally below 200 K when the field is applied parallel to the plane of the MTJ. |
Thursday, March 17, 2022 4:12PM - 4:24PM |
W53.00005: Spin Flip Excitonic States in Molecular Magnets Karma Dema, Mark R Pederson, Zahra Hooshmand Recent work on Mn3, Mn(taa), and [MnIIIMnIII]3+ triplesalen provide substantial evidence that the presence of single ion S=1 states can be energetically more stable [1,2,3]. Based on these studies, we looked for the presence of S=1 centers, referred to here as Spin Flip Excitons (SFEs), in the Mn12O12(COOR)16[H2O]2 (Mn12-Ac) single-molecule magnet. Through DFT calculations, our results show that we can indeed get up to eight electronically stable low energy excitons states. The total spin of the structures reduces to S=6 and S=2 when four and eight Mn atoms carry a spin of S=1. There are significant changes in the moment of the outer Mn atoms and the bond length between Mn-OH2. The HUMO-LUMO gap narrows and the magnetic anisotropy energy decreases with the number of excitons. We have confirmed that some of the excitons carrying geometries are vibrationally stable. Progress on using calculated infrared frequencies and Raman frequencies for experimental confirmation will be reported. |
Thursday, March 17, 2022 4:24PM - 4:36PM |
W53.00006: 9.2 GHz Clock Transition in a Lu(II) Molecular Spin Qubit Arising from a Massive 3467 MHz Hyperfine Interaction Stephen Hill, Krishnendu Kundu, Jessica White, Samuel Moehring, Jason Yu, Joseph Ziller, Filipp Furche, William Evans Molecular spin qubits are attractive targets for next-generation quantum technologies [1] due to their intrinsic tunability via coordination chemistry techniques, enabling precise control of the relevant spin degrees of freedom and the possibility for massive scale-up via self-assembly. Here, we demonstrate chemical control of the degree of s-orbital mixing into the spin-bearing d-orbital associated with a series of spin-½ La(II) and Lu(II) molecules. Increased s-orbital character reduces spin-orbit coupling and enhances the electron-nuclear Fermi contact interaction. Both outcomes are beneficial for quantum applications: the former reduces spin-lattice relaxation, while the latter gives rise to a record molecular hyperfine interaction for Lu(II) that, in turn, generates a massive 9 GHz hyperfine clock transition and an order of magnitude increase in phase memory time [2]. These findings suggest new strategies for development of molecular quantum technologies, similar to trapped ion systems. |
Thursday, March 17, 2022 4:36PM - 4:48PM |
W53.00007: Voltage controlled transport properties of [Fe(Htrz)2(trz)](BF4) plus polyaniline composite. Esha Mishra, Thilini K Ekanayaka, Kayleigh McElveen, Rebecca Y Lai, Peter A Dowben Due to the molecular spin state switching, and the associated conductance change, the spin crossover system has potential in the development of nonvolatile memory devices but low 'on-state' resistance in spin crossover molecular films is essential for molecular memory applications [1]. The current-voltage I(V) and capacitance-voltage C(V) characteristics of the spin crossover molecule [Fe(Htrz)2(trz)](BF4) (where Htrz = 1H-1,2,4-triazole) plus polyaniline were studied to explore the voltage-dependent transport properties and the drift charge carrier lifetimes have been determined. The transport measurements for [Fe(Htrz)2(trz)](BF4) plus polyaniline composite were performed for both the high spin state and the low spin state and the conductance of the high spin state and the low spin state were compared. Transistor characteristics of the [Fe(Htrz)2(trz)](BF4) plus polyaniline composite were also studied to better understand the behavior of the composite as an organic field effect transistor (OFET). |
Thursday, March 17, 2022 4:48PM - 5:00PM |
W53.00008: Probing Magnetic Domains in Two-dimensional Magnets via Electric Transport Zhuangen Fu, Piumi I Samarawickrama, John Ackerman, Zhiqiang Mao, Wenyong Wang, TeYu Chien, Jinke Tang, Jifa Tian The recently discovered van der Waals (vdW) magnets provide new opportunities for studying magnetism at the 2D limit and designing novel spin devices. However, the role of magnetic domains in few-layer vdW magnets has been largely ignored. Here, we report the fabrication of high-quality magnetic tunneling devices based on 2D magnets and a systematic study of the tunneling magnetoresistances (TMR). In the TMR results, we not only observed the layer-dependent magnetization switching-induced large step-like resistance changes, but also identified a few mini step-like resistance changes. We further found that the corresponding coercivities and magnitudes of the mini step-like resistance changes are highly dependent on the cooling history of the device as well as the tunneling current. We attribute these observations to the appearance of magnetic domains in the few layer 2D magnets. Our results may shed some light on understanding the novel magnetism at low dimensions. |
Thursday, March 17, 2022 5:00PM - 5:12PM |
W53.00009: DFT investigation of surface Magnetism on CdSe Quantum Dot due to Dangling Bonds and Ligands George Kurian Surface magnetism of CdSe quantum dots (QD) is not well understood. Neeleashwar et al. [1] claim that surface dangling bonds are responsible for the observed magnetism and determined a dangling bond concentration of 2000 ppm for a 1.5 nm QD, that decreases as the size of QD increases. On the other hand, Meulenberg et al. [2] attribute the magnetism to ligands rather than dangling bonds. According to [2], the concentration of dangling bonds is higher [3], which should result in a larger Currie constant than that observed experimentally, therefore, cannot be the cause of the magnetism. We performed density function theory (DFT) calculations on CdSe QD to resolve the controversy over the experimentally observed results. Our results indicate that every surface atom with a coordination number less than four does not possess unpaired electrons. Only atoms with coordination numbers less than three possess unpaired electrons and, therefore, contribute to the observed magnetism. Our calculation of Cd14Se14 QD capped with OP(CH3)3 and N(CH3)3 indicates that the choice of ligands influences the density of states of the surface Cd atoms.
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