Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y53: Spintronics Devices Based on Molecular MagnetsFocus Recordings Available
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Sponsoring Units: GMAG DMP Chair: Jasminder Sidhu, University of Strathclyde Room: McCormick Place W-475B |
Friday, March 18, 2022 8:00AM - 8:36AM |
Y53.00001: Linking Intermolecular Geometry and Spin Coupling of Excitons in Organic Semiconductors Invited Speaker: Leah R Weiss The design and optimization of synthetic magnetic and optoelectronic materials requires precise chemical control of spin and electronic coupling. Here we focus on understandingthis structure-function relationship for light-harvesting applications of singlet fission: the production of two triplet excitons (each with spin S=1) following excitation of one singlet exciton (spin S=0). To quantitatively extract dipolar and exchange interactions between triplet excitons formed by singlet fission in a solid-state organic semiconductor, we have deployed broadband optically detected magnetic resonance [4], electron spin resonance [1,3], and magneto-optical spectroscopy [2]. Mapping the experimentally extracted spin parameters onto the molecular crystal structure provides a window into exciton localization and coupling in the molecular lattice. This mapping of spin properties to excited-state electronic structure is made possible by sustained excited-state spin polarization and coherence over microsecond timescales [1, 3]. These results --linking intermolecular geometry and spin interactions-- provide a key step toward chemically controlling intermolecular spin coupling for both optoelectronic applications and molecular quantum technologies. |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y53.00002: Supramolecular Spin Valve Effects in Graphene Quantum Dots Decorated with Single Molecule Magnets Amjad Alqahtani, Luke St. Marie, DaVonne Henry, Lubomir Havlicek, Jakub Hruby, Antonin Sojka, Jorge Navarro, Rachael L Myers-Ward, David K Gaskill, Ivan Nemec, Petr Neugebauer, Amy Y Liu, Paola Barbara Single-molecule magnets (SMMs) have promising applications in quantum computing and molecular spintronics. Previous work demonstrated spin valve effects in hybrid devices made of carbon nanotube or graphene quantum dots decorated with a few TbPc2 SMMs, yielding higher or lower electrical conductance through the dots depending on whether the magnetic moments of the molecules had parallel or antiparallel orientation. This electrical detection of the magnetization switching was demonstrated at temperatures below 0.6 K and by tuning the gate voltage of the quantum dot away from the Coulomb blockade region, in the conducting regime [1]. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y53.00003: Excitation and detection of spin waves in organic-based ferrimagnet V[TCNE]x (x ≈ 2) Jaspal S Bola Vanadium tetracyanoethylene, V[TCNE]x (x ≈ 2), is an organic-based ferrimagnet with an above room-temperature magnetic order. Quantized spin waves can be generated in V[TCNE]x which can be utilized for next-generation magnonic devices. Coherent spin-wave modes in V[TCNE]x can be excited with microwave irradiation and detected using inverse spin hall effect in adjacent Platinum layer [1]. A variety of magnons modes are shown to exhibit spin pumping capability [2]. Also, the ferromagnet resonance response of V[TCNE]x films exhibit Fano-type resonance with a continuum broadband absorption in the microwave range, which can be readily tuned by the microwave frequency [1]. |
Friday, March 18, 2022 9:00AM - 9:12AM |
Y53.00004: Spin-coupling induced anisotropy swings in ligated Co2 dimers Zahra Hooshmand, Jie-Xiang Yu, Hai-Ping Cheng, Mark R Pederson Single-molecule magnets with high magnetic anisotropy and easy-axis magnetization are considered to be potential building blocks of quantum computers. While much effort was initially focused on the class of molecules with high total ground-state spin, later studies showed that the molecules with an intrinsic high magnetic anisotropy are more likely to satisfy the requirements for applications as qubits. The theoretical and experimental studies usually focus on the overall magnetization of an SMM. However, in a recent study based on density functional theory, we have demonstrated that the local anisotropy of a ligated Co2 dimer provides the opportunity for manipulating each spin-carrying center separately [1]. Using the insight obtained from this result we found that both the magnitude and the orientation of magnetization is sensitive to local environment. Moreover, we found an unusual change in the magnetic anisotropy barrier when the spin coupling changes from ferromagnetic to antiferromagnetic. The swings in the magnitude of this barrier were also found to be sensitive to the local symmetry of each center. In this talk I will present the detailed results of this study and will discuss how such features enable quantum switching applications in this molecule. |
Friday, March 18, 2022 9:12AM - 9:24AM |
Y53.00005: Close-to-room-temperature magnetoelectric coupling in a molecule-based material Magdalena Owczarek, Minseong Lee, Wanyi Nie, Vivien Zapf Spin crossovers (SCOs) provide an attractive route to creating switchable materials properties. The spin state can be switched by magnetic and electric fields, light and pressure, and can in turn influence the electrical, optical and structural properties of a material. In the case of molecule-based SCO materials, strong distortion of the lattice accompanying the SCO can give rise to the appearance of electric dipoles that can couple to the magnetic field. Such achieved magnetoelectric coupling has been reported by now in several SCO systems with operating temperatures of 48–200 K and 333–338 K. Current efforts are focused on identifying materials with the coupling appearing at room temperature (RT) which would make them attractive for technological applications such as low power, tunable frequency devices, magnetic sensors, energy harvesting, computing, and data storage. Here we present an example of a polar Fe(II) material which close-to-RT SCO can be induced not only by temperature but also by a magnetic field as low as 5 T. Moreover, we found that changes in electrical properties (dielectric constant and electric polarization) related to the SCO can be induced also by applying low magnetic field near the SCO temperature. |
Friday, March 18, 2022 9:24AM - 9:36AM |
Y53.00006: Toward the Characterization of Atomic-Clock Transitions in the Nanomagnet [VO(TPP)] Brendan C Sheehan, Fabio Santanni, Guanchu Chen, Lorenzo Sorace, Roberta Sessoli, Jonathan R Friedman [VO(TPP)], a promising molecular nanomagnet-based qubit candidate, has been shown to have an impressive coherence time T2.1,2 We employ a homebuilt electron-spin resonance (ESR) apparatus to characterize the system's behavior across a variety of frequencies in the microwave regime, using both continuous wave (cw) and pulsed ESR. By taking advantage of the CPMG pulse sequence and diluting [VO(TPP)] to 2% in a nonmagnetic structural analogue, we have shown T2 to be greater than 8 μs at multiple frequencies. Previous work has indicated the possible presence of atomic clock transitions in [VO(TPP)] in the sub-GHz regime.1 Strong hyperfine coupling in this S = 1/2, I = 7/2 nanomagnet generates four unique clock transitions. Decoherence from dipolar interactions and external field fluctuations is minimized at the clock transitions; as a result, T2 has been shown to substantially increase at clock transitions in nanomagnets.3 Based on the behavior of other systems with clock transitions, we anticipate a substantially enhanced T2 at the clock transitions in [VO(TPP)]; we present progress toward testing that expectation. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y53.00007: The investigation of the magnetoelectric effect in a cobalt-dioxolene complex under 60 T pulsed magnetic fields James Wampler, Ping Wang, Michael Shatruk, Minseong Lee, Vivien Zapf Magnetic-field driven spin crossover (SCO) transitions provide an attractive route to realizing magnetoelectric (ME) coupling and reveal rich high-field phase diagrams with novel ME phases. Cobalt-dioxolene complexes have been shown to exhibit tautomeric SCO transitions, wherein a SCO occurs via electron transfer from the ligand to the central Co ion. This process creates a profound level of coupling between the spin state, the molecular electric dipole, and the chemical and lattice properties. These materials are promising candidates for a range of applications due to the range of mechanisms that can induce the SCO transition. Tautomeric transitions in cobalt-dioxolene complexes have been induced thermally, by visible light and X-Rays, by magnetic field and by pressure. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y53.00008: Spintronic THz Emission in Wide-bandgap Semiconductor Heterostructures Melike Biliroglu, Eric Vetter, Dovletgeldi Seyitliyev, Pramod Reddy, Ronny Kirste, Zlatko Sitar, Ramon Collazo, Kenan Gundogdu, Dali Sun The generation and detection of terahertz (THz) radiation have many potential applications ranging from high-speed communications to surveillance. Accessing a broad range of THz frequencies is a significant challenge for realizing THz applications. A spintronic method for THz generation in ferromagnetic/non-magnetic heterostructures is an emerging approach for producing THz fields. Here, the principle of operation is based on ultrafast demagnetization of the ferromagnetic layer in response to an incident, femtosecond laser pulse, followed by ultrafast decay of the resulting superdiffusive spin current into the non-magnetic layerthat leads to a transient THz pulse. Here we demonstrate spintronic THz generation in heterostructures comprising wide-bandgap semiconductors instead of heavy metals. These wide-bandgap semiconductors possess strong Rashba states due to their non-centrosymmetric crystal structures that are favorable for efficient spin-to-charge conversion for spintronic THz generation. In addition, their electrically tunable characteristics induced by chemical doping and gate control open new possibilities for potential gate-controlled spintronic THz devices. We find that the carrier concentration and interfacial electric field distribution in the semiconductor films strongly affect the resulting THz emission amplitude. Combining wide-bandgap semiconductor fabrications with spintronic THz devices will offer new solutions to future communication applications such as non-destructive characterization, wireless communication, and quantum computation. |
Friday, March 18, 2022 10:00AM - 10:12AM |
Y53.00009: Interface-generated orbital currents Timothy Mabe, Vivek P Amin, In Jun Park Spin and orbital currents can be used in spintronic devices to electrically control the magnetization of ferromagnets and antiferromagnets, providing an attractive write mechanism for magnetic memories. Discovering efficient ways to generate currents of angular momentum is thus a hallmark goal of spintronics. Theoretical and experimental evidence suggests that, under an applied, in plane electric field, out-of-plane flowing spin currents are generated at the interface between magnetic and nonmagnetic materials. However, symmetry arguments guarantee that interfaces must also generate currents of orbital angular momentum. Here we determine the strength and magnetization dependence of such interface-generated orbital currents using first principles transport calculations. Shedding light on new sources of angular momentum transfer within magnetic heterostructures will help pave the way for more energy efficient magnetic memories. |
Friday, March 18, 2022 10:12AM - 10:24AM |
Y53.00010: Electron Spin Resonance as a Direct Probe of Spinon Interactions in a Quantum Spin Chain Kirill Povarov, Timofei Soldatov, Ren-Bo Wang, Andrey Zheludev, Alexander Smirnov, Oleg A Starykh The strong, yet well-hidden backscattering interaction between fractionalized spinon excitations has been known as a part of the S=1/2 chain physics for a long time. However, its dramatic consequences for the dynamic properties, such as the appearance of the collective spin 1 mode at small momenta (analogue of the Fermi liquid's Silin mode), were realized only recently [1]. These fine details, occuring only in the magnetized state and only at nonzero momenta, are challenging for experimental observation. We have succeeded in experimental detection of these features using Electron Spin Resonance as the probe [2]. This was only possible due to the specific "momentum boost” present in the spectrum of a unique spin chain material K2CuSO4Br2 featuring the uniform Dzyaloshinskii-Moriya interaction pattern [3,4]. Description of the observed spectrum requires accounting for the backscattering interaction already on the qualitative level. Quantitative analysis allows us to estimate the backscattering constant to be as large as 2.38J, in units of the exchange interaction J. Yet the corresponding dimensionless interaction constant, that determines the Renormalization Group flow [5], is quite small, and allows for a successful direct comparison of our data with the RG theory predictions. |
Friday, March 18, 2022 10:24AM - 10:36AM |
Y53.00011: Analysis of simulation of vibronic coupling in a 4f molecular magnet using Far Infrared MagnetoSpectroscopy (FIRMS) and ab initio calculations. Jon G Kragskow, Nicholas F Chilton, Jonathan J Marbey, Joscha Nehrkorn, Stergios Piligkos, Stephen Hill, Mykhaylo Ozerov, Christian D Buch Vibronic coupling, the interaction between molecular vibrations and electronic states, is a fundamental effect that profoundly affects chemical processes. In the case of molecular magnetic materials, vibronic, or spin-phonon, coupling leads to magnetic relaxation, which equates to loss of magnetic memory and loss of phase coherence in molecular magnets and qubits, respectively. The study of vibronic coupling is challenging, and most experimental evidence is indirect. Here we employ far-infrared magnetospectroscopy to directly probe vibronic transitions in [Yb(trensal)] (where H3trensal = 2,2,2-tris(salicylideneimino)trimethylamine). We find intense signals near electronic states, which we show arise due to an “envelope effect” in the vibronic coupling Hamiltonian, which we calculate fully ab initio to simulate the spectra. We subsequently show that vibronic coupling is strongest for vibrational modes that simultaneously distort the first coordination sphere and break the C3 symmetry of the molecule. With this knowledge, vibrational modes could be identified and engineered to shift their energy towards or away from particular electronic states to alter their impact. Hence, these findings provide new insights towards developing general guidelines for the control of vibronic coupling in molecules. |
Friday, March 18, 2022 10:36AM - 10:48AM |
Y53.00012: Characterisation of magnetic relaxation on extremely long timescales William Blackmore, Nicholas F Chilton, Sophie Corner, Jack Emerson-King, Gemma Gransbury, David Mills, Peter Evans Recent advances in the field of single-molecule magnets have led to the development of very slow relaxing species. However, challenges remain in understanding the effect local environment has on the quantum tunnelling of magnetisation (QTM) and Raman processes in these materials. This is partly due to the extremely long relaxation times τ that are now being observed. For τ of the order of a week or longer, it is not feasible to measure DC decays all the way to equilibrium (Meq). Whilst we can calculate Meq, one cannot model how M(H) approaches Meq. This leads to an increasingly large asymmetry and width of the distribution of the rate, such that the measured τ* becomes meaningless. We investigate the distribution of the stretched exponential used to model DC decay measurements to develop a more realistic measure of τ. We also present low-temperature magnetometry measurements of the high-performance SMM [Dy(Cpttt)2][BArF] [1] secured in eicosane, and dissolved in dichloromethane and difluorobenzene. Using this, we explore the effect solvating samples has on the relaxation processes. |
Friday, March 18, 2022 10:48AM - 11:00AM |
Y53.00013: Vibronic effects on the quantum tunneling of magnetization in single-molecule magnets Andrea Mattioni, Nicholas F Chilton Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing at non-cryogenic temperatures [1]. Their magnetization dynamics are determined by an interplay between electronic and vibrational degrees of freedom, which can couple coherently leading to complex vibronic dynamics. While the relevance of the spin-phonon coupling for high-temperature magnetization dynamics is now well understood in terms of classical rate processes, its role in the low-temperature quantum tunneling regime is still debated and typically investigated with simplified models [2]. Here, we present a complete ab initio characterization of the magnetization dynamics of a dysprosocenium complex embedded in a realistic environment. Using a combination of perturbation theory and exact techniques, we quantify the contribution of vibronic interactions to the tunneling gap, and determine the conditions where vibronic contributions dominate. Our approach shows a direct connection between vibrational properties and quantum tunneling rates, and hence can point towards design guidelines for extending the fundamental limits to information storage in single-molecule magnets. |
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