Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C19: Magnetic Clusters and Molecular Magnets IFocus
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Sponsoring Units: GMAG DMP Chair: Jason Haraldsen, Univ of North Florida Room: LACC 308A |
Monday, March 5, 2018 2:30PM - 2:42PM |
C19.00001: Spin heptamer excitations in the pyrochlore antiferromagnets MgCr2O4 Jason Haraldsen, Oksana Zaharko In this study, we examine the spin heptamer excitations for the S = 3/2 systems of ACr2O4 (A=Mg, Zn). Using a Heisenberg spin-spin exchange Hamiltonian, we determined the exact representation for the energy eigenstates and inelastic neutron scattering structure factors for the S = 3/2 heptamer. The functional forms of inelastic neutron structure factors were calculated through an examination of the S = 1/2 heptamer. Since the individual structure factors are dependent on the excited sub-geometry of the cluster, not the total spin of the ions, the functional forms of the S = 3/2 excitations can be mapped on to similar S = 1/2 excitations. The mapping helps to reduce the fundamental basis set and allows one to calculate the closed form solutions. Therefore, we are able to clarify that the spin excitations of this pyrochlore antiferromagnet resemble discrete transitions of a spin heptamer. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C19.00002: Direct Spectroscopic Evidence for Berry-phase Interference in the Ni4 Single-molecule Magnet Robert Kwark, Eli Mansbach, Charles Collett, Jonathan Friedman, Rafael Allão Cassaro We present spectroscopic evidence for Berry-phase interference in the quantum tunneling of the spin in the Ni4 single-molecule magnet. The signature of destructive Berry-phase interference for tunnel-split states is that the tunnel splitting goes to zero (is "quenched") for certain values of field when applied along the hard axis. In fixed frequency ESR, this shows up as two transitions for the same pair of energy eigenstates, one at a field slightly lower than quench field and one slightly higher. We performed parallel-mode electron-spin resonance with f = 3.78 GHz to drive transitions between tunnel-split states in this molecule. We monitor the behavior of these transitions as a function of transverse magnetic field and the angle φ between the field and the hard axis of the molecule. We observe the predicted quench behavior for an excited-state tunnel splitting in Ni4 and can follow it as changing φ suppresses the interference effect. We find that the behavior is four-fold symmetric in φ, consistent with the expected symmetry of the system. Detailed simulations of all of the observed transitions reproduce the observations nicely. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C19.00003: Spin echo measurements at an avoided crossing in the molecular nanomagnet Cr7Mn Charles Collett, Kai-Isaak Ellers, Jonathan Friedman, Nicholas Russo, Kevin Kittilstved, Grigore Timco, Richard Winpenny We report spin echo measurements of spin relaxation times in dilute samples of the spin S = 1 molecular nanomagnet [(CH3)2NH2][C7MnF8((CH3)3CCOO)16] ("Cr7Mn") in the vicinity of an avoided crossing. Our samples are diluted by co-crystallizing Cr7Mn with Ga7Zn, a diamagnetic isostructural analogue. By coupling samples to a loop-gap resonator (LGR), we use a custom FPGA-based pulsed electron-spin resonance (ESR) spectrometer to perform parallel-mode spin echo experiments. With the resonant frequency of the LGR tuned to the tunnel splitting of Cr7Mn, ~4 GHz, we observe a clear spin echo signal near zero field, which decreases at higher fields. From this signal, we measure both T1 and T2, finding that T1 ~ 300 μs and T2 ~ 180 ns for a 0.5% sample at zero field. Surpisingly, our measured T2 values do not depend strongly on dilution. We will also present data on the field dependence of relaxation times. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C19.00004: EPR Studies of Lanthanide-Lanthanide Interactions in Triple-Decker Molecular Complexes Dorsa Komijani, Alberto Ghirri, Claudio Bonizzoni, Svetlana Klyatskaya, Eufemio Moreno-Pineda, Mario Ruben, Marco Affronte, Stephen Hill Lanthanide molecular nanomagnets, particularly sandwich-type complexes, have been studied extensively due to their potential application in information technologies [1]. The strong magnetic anisotropy associated with lanthanide ions originates from the large spin-orbit coupling in the 4f shell. However, shielding of the magnetic 4f electrons by the filled 5p and 6s orbitals poses challenges for synthesizing exchange coupled lanthanide compounds. Phthalocyanines (Pc) and porphyrins (Pi) are among the well-known macrocyclic molecules that have been widely used to form multilayered organometallic compounds with trivalent lanthanides [2]. In this work, we study f-f interactions between two lanthanide (Ln) ions in a series of symmetric and asymmetric triple-decker compounds, e.g., Pi-Ln-Pc-Ln-Pi, Pc-Ln-Pc-Ln-Pi, etc. We further explore the possibility of performing a CNOT quantum gate operation using an asymmetric Er-Er compound. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C19.00005: Direct Experimental Evidence of Toroidal Symmetry in the Lanthanum-based Molecular Magnet Dy8 Qing Zhang, Shiqi Li, Myriam Sarachik, Michael Baker, Theocharis Stamatatos, José Baldoví, Eugenio Coronado (Et4N)4[Dy8O(nd)8(NO3)10(H2O)2]2MeCN (Dy8) [1] is an intramolecular coupled lanthanide molecule with a Christmas star-like shape. Fits by an Ising type exchange Hamiltonian to low-temperature measurements of the angular dependence of the magnetization provide direct evidence of toroidal symmetry of the magnetic moments in this molecular nanomagnet. This experimental spin configuration result is consistent with the single ion easy axes, which are determined by an electrostatic crystal field point charge model. The short length scale for the decay of stray magnetic fields of toroidal qubits qualifies Dy8 as an excellent potential candidate for data storage and/or quantum computation. [1] D. I. Alexandropoulos, et al., Inorg. Chem. 53, 5420 (2014). |
Monday, March 5, 2018 3:30PM - 3:42PM |
C19.00006: Substitution Effects on Exchange Anisotropy in Heavy Atom Radicals Jonathan Marbey, Stephen Winter, Richard Oakley, Stephen Hill In the field of organic materials, bisdithiazolyl (and related) radicals have become attractive systems to study since their magnetic properties can be carefully tuned by varying and substituting their chalcanogen and halogen content. In this work, we focus specifically on heavy halide substitution in two families of molecule: IBSSEt (1) and IBBO (2). Structural differences result in a ferromagnetic ground state in 1, while 2 orders as a spin canted antiferromagnet. Herein, we report ferromagnetic resonance (FMR) measurements on 1 which show that substitution of I for Cl has no effect on the exchange anisotropy (EA). However, antiferromagnetic resonance (AFMR) measurements show that substituting I for F has a dramatic effect on the EA. Our analysis demonstrates that the sensitivity to the halide’s larger spin-orbit coupling is dictated by the molecular orbitals that dominate the exchange, in which the symmetry of the SOMO permits no sizeable spin density at the halide’s position, while the opposite is true for the LUMO. As shown by previous DFT studies, we are able to directly show that the lower energy LUMO hybridizes with the SOMO in 2, resulting in an augmentation of the EA upon heavy atom substitution. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C19.00007: Spectroscopic Investigation of the Metal-Metal Bonded Single-Molecule Magnet Fe6 Joscha Nehrkorn, Brian Malbrecht, Samuel Greer, Azar Aliabadi, Alexander Schnegg, Karsten Holldack, Theodore Betley, Carmen Herrmann, Stefan Stoll, Stephen Hill Polynuclear Single-Molecule Magnets (SMMs) comprise multiple exchange-coupled metal ions that can be collectively magnetized below a characteristic blocking temperature, TB. Up to now, low values of TB have hampered potential applications in data storage and quantum computing. When the exchange-coupled giant spin ground state is well separated from excited spin states, the SMM properties are dominated by magneto-crystalline anisotropy. However, in most cases, the constituent ions are coupled by relatively weak super-exchange interactions, leading to a separation between spin states of just a few kelvin. Recently, SMM properties were observed for a Fe6 molecule containing six iron ions strongly coupled via direct metal-metal bonds.[1] Here we present a spectroscopic investigation of this Fe6 molecule, where Frequency-Domain Fourier-Transform THz-EPR was combined with multi-frequency high-field EPR so that the magnetic anisotropy could be analyzed thoroughly. The experimental data suggest the ground state to be well separated, demonstrating the possibility of metal-metal bonded SMMs with drastically improved TB values. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C19.00008: Thermal stability of holmium single atom magnets in a magnetic field Fabian Natterer, Fabio Donati, François Patthey, Harald Brune The size of a stable magnetic bit has recently reached the atomic limit, which raised questions about the thermal stability and the means to control magnetic information. Using the example of holmium single atom magnets on magnesium oxide (MgO), we show magnetic bistability up to 35 K and reveal a coercive field of more than 8 T at that temperature. Our scanning tunneling microscopy study finds a spontaneous magnetization reversal at about 45 K and 8 T. We estimate the transverse magnetic anisotropy energy from magnetic field and bias voltage dependent switching of Ho atoms at 4.3 K and constrain the Ho/MgO ground state to either Jz = 7 or 8, both compatible with magnetic bistability at magnetic fields larger than 10 mT. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C19.00009: Magnetic quantum tunneling in Fe-doped Li3N: stable but manipulable states. Manuel Fix, James Atkinson, Paul Canfield, Enrique Del Barco, Anton Jesche Li2(Li1-xFex)N shows extremely large magnetic anisotropy and coercivity that are based on the presence of unquenched orbital moments [1,2]. For low Fe-concentrations the spin-relaxation becomes temperature-independent indicating a crossover from thermal excitations to the quantum tunneling regime at T = 10 K. The resonant character of this tunneling process is proven by a strong increase of the spin-flip probability in transverse magnetic fields. Longitudinal fields, on the other hand, lift the ground state degeneracy and destroy the tunneling condition. An increase of the relaxation time by four orders of magnitude in external fields of only a few millitesla reveals exceptionally sharp tunneling resonances. Accordingly the system can be set from stable to manipulable by tuning from off- to on-resonant using small applied fields. Li2(Li1-xFex)N represents a comparatively simple and clean model system that opens the possibility to study and utilize magnetic quantum tunneling at liquid helium temperatures. |
Monday, March 5, 2018 4:18PM - 4:54PM |
C19.00010: Fingerprinting molecular nanomagnets by four-dimensional inelastic neutron scattering. Invited Speaker: Paolo Santini Four-dimensional inelastic neutron scattering is an invaluable tool to study the spin dynamics of molecular nanomagnets. The power of the technique comes from the capability to measure the scattering cross-section over large portions of the energy-wavevector space, yielding a faithful portray of spin fluctuations on the space- and time-scales characterizing the internal dynamics of the magnetic core [1]. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C19.00011: Room-temperature superparamagnetism due to giant magnetic anisotropy in MoS defected single-layer MoS2 Mahtab Khan, Michael Leuenberger Room-temperature superparamagnetism due to a large magnetic anisotropy energy (MAE) of a single atom magnet has always been a prerequisite for nanoscale magnetic devices. Realization of two dimensional (2D) materials such as single-layer (SL) MoS2, has provided new platforms for exploring magnetic effects, which is important for both fundamental research and for industrial applications. Here, we use density functional theory (DFT) to show that the antisite defect (MoS) in SL MoS2 is magnetic in nature with a magnetic moment μ of ~ 2μB and, remarkably, exhibits an exceptionally large atomic scale MAE=ε∥-ε⊥ of ~500 meV. Our calculations reveal that this giant anisotropy is the joint effect of strong crystal field and significant spin-orbit coupling (SOC). In addition, the magnetic moment μ can be tuned between 1μB and 3μB by varying the Fermi energy εF, which can be achieved either by changing the gate voltage or by chemical doping. We also show that MAE can be raised to ~1 eV with n-type doping of the MoS2:MoS sample. Our systematic investigations deepen our understanding of spin-related phenomena in SL MoS2 and could provide a route to nanoscale spintronic devices. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C19.00012: Magnetic Properties of [Mnn]Cem Clusters Dianteng Chen, Xiangguo Li, Yun-Peng Wang, Xiaoguang Zhang, George Christou, Hai-Ping Cheng Manganese oxide single-molecule magnets [Mnn] (with n up to 84) have attracted much attention in the last few decades. More recently, [Mn]nCem compounds have been synthesized. Using first-principles quantum calculation, we studied several [Mn]nCem clusters including Mn3Ce2 and Mn5Ce3 with three different ligands. Magnetization in these clusters is mainly from the Mn ions. The magnetic interaction between the Mn ions can be either ferromagnetic or antiferromagnetic. We investigated energies of all spin configurations for each molecule. We fitted the total energies from first-principles calculations to Heisenberg model and extracted the exchange parameters, J. The J values from first-principles calculations and J’s from the experimental susceptibility curve were compared. Pathways of magnetic interaction were analyzed using the Wannier function method. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C19.00013: Absorption-site specific Kondo resonance observed in single molecule magnet TbPc2 on Ag(111) John Hellerstedt, Ales Cahlik, Martin Svec, Maria Moro, Pavel Jelinek The single molecule magnet bis(phthalocyaninato)terbium(III) (TbPc2) has attracted steady research attention as an exemplar system for realizing molecule based spin electronics. Lattices of these molecules interact through the π-electrons in their Pc ligands, which manifests itself experimentally as a Kondo resonance in spectroscopy measurements. Understanding these interactions is crucial for their subsequent use in quantum computing schema. We studied TbPc2 molecules evaporated in ultrahigh vacuum onto a single crystal Ag(111) surface, measured at 5K using combined scanning tunneling and non-contact atomic force microscopies (STM/ nc-AFM). Submolecular resolution achieved with a CO- functionalized tip offers unprecedented structural information, specifically regarding the two sub-lattices forming tight-packed TbPc2 islands. Kondo resonances are observed on molecules absorbed as a second layer on the larger TbPc2 islands: the presence or absence of the Kondo signature is exactly correlated with the sub-lattice absorption site. Spectroscopies taken with a CO- terminated tip showed the Kondo resonance shifted to the center of the ligand. The latest efforts to understand the structural origin of this site-specific Kondo resonance will be reported. |
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