Stephen Hill

Florida State University, Tallahassee, Florida, United States

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Publications (100)349.01 Total impact

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    ABSTRACT: [Mn3O(O2CMe)3(dpd)3/2)]2(I3)2 has been obtained from the reaction of 1,3-di(pyridin-2-yl)propane-1,3-dione dioxime (dpdH2) with triangular [Mn(III)3O(O2CMe)(py)3](ClO4). It comprises two [Mn(III)3O](7+) triangular units linked covalently by three dioximate ligands into a [Mn3]2 dimer. Solid state dc and ac magnetic susceptibility measurements reveal that each Mn3 subunit of the dimer is a separate single-molecule magnet (SMM) with an S = 6 ground state and that the two SMM units are very weakly ferromagnetically exchange coupled. High-frequency EPR spectroscopy on a single crystal displays signal splittings indicative of quantum superposition/entanglement of the two SMMs, and parallel studies on MeCN/toluene (1:1) frozen solutions reveal the same spectral features. The dimer thus retains its structure and inter-Mn3 coupling upon dissolution. This work establishes that covalently linked molecular oligomers of exchange-coupled SMMs can be prepared that retain their oligomeric nature and attendant inter-SMM quantum mechanical coupling in solution, providing a second phase for their study and demonstrating the feasibility of using solution methods for their deposition on surfaces and related substrates for study.
    Journal of the American Chemical Society 06/2015; 137(22). DOI:10.1021/jacs.5b02677 · 11.44 Impact Factor
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    ABSTRACT: A complex exhibiting valence delocalization was prepared from 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl (Py2TTA·), an inherently redox active pincer-type ligand, coordinated to iron (Fe(Py2TTA)Cl2 (1)). Complex 1 can be prepared via two routes, either from the reaction of the neutral radical with FeCl2 or by treatment of the anionic ligand (Py2TTA-) with FeCl3, demonstrating its unique redox behaviour. Electrochemical studies, solution absorption and solid-state diffuse reflectance measurements along with X-ray crystallography were carried out to elucidate the molecular and solid-state properties. Temperature- and field-dependent Mössbauer spectroscopy coupled with magnetic measurements revealed that 1 exhibits an isolated S = 5/2 ground spin state for which the low-temperature magnetic behaviour is dominated by exchange interactions between neighbouring molecules. This ground state is rationalized on the basis of DFT calculations that predict the presence of strong electronic interactions between the redox active ligand and metal. This interaction leads to the delocalization of β electron density over the two redox active centres and highlights the difficulty in assigning formal charges to 1.
    Dalton Transactions 05/2015; 44(22). DOI:10.1039/C5DT01374G · 4.20 Impact Factor
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    ABSTRACT: Molecular magnetism research incorporates fundamental concepts of coordination and organometallic chemistry and merges them with physics with one of the highest profile current topics being molecules that exhibit bistability, known as “Single Molecule Magnets” (SMMs). Recent strides in designing new generations of tiny magnets that retain their memory effect at higher temperatures provide promising evidence that they may be useful for new generations of nanoscale electronic devices and computers. The overarching goal of our research in this area is the use of discrete building units to control molecular architecture and the resulting magnetic properties. The implementation of this strategy is helpful for gleaning valuable information about how magnetic properties are affected by electronic as well as geometric structure as well as weak and strong chemical interactions. This talk will cover various topics including combining specific metals of the periodic table to enhance the magnetic properties. Experiments are being conducted to test the validity of several independent theoretical studies that predict that specific metal combinations will lead to higher temperature magnets with cyanide bridges. Experimental results are correlated with theory and augmented by the use of special techniques involving neutrons and high frequency and high-field spectroscopies.
    249th ACS National Meeting & Exposition, Denver, CO, USA; 03/2015
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    ABSTRACT: The development and study of molecular nanomagnets has witnessed tremendous progress in recent years, with potential applications on the horizon [1]. Of particular interest are so-called single-molecule magnets (SMMs) that display slow magnetic relaxation below a characteristic blocking temperature, TB, due to the combination of a large magnetic moment and appreciable spin-orbit (SO) anisotropy. Early efforts aimed at increasing TB focused primarily on maximizing the molecular spin ground state, S, associated with polynuclear clusters. However, this becomes challenging for large clusters whilst simultaneously maintaining the molecular anisotropy. Thus, a more direct route to increasing TB involves optimization of the magnetic anisotropy, albeit for simpler molecules in which one can exert considerable synthetic control over the ligand field (LF). This approach resulted in the first SMM comprising a single lanthanide ion in 2003 [2]. Since then, much effort has been directed towards increasing the magnetic anisotropy of mononuclear species. Here, we discuss two series of hexa-coordinate VIII complexes: one consisting of trans-dicyanide VIII building blocks with equatorial chelating ligands; the other involving a tridentate scorpionate ligand, with three halides completing the coordination sphere. Multi-high-field/frequency (up to 35 T and 450 GHz) EPR was employed to determine the magnetic anisotropy of these complexes. Members of the Jahn-Teller elongated cyanovanadate series can be described by an effective spin-only Hamiltonian with positive D parameters in the +3 to +6 cm‑1 range, and negligible rhombicity. On the other hand, one of the scorpionate complexes was found to have a very large easy-axis-type zero-field-splitting on the order of ‑40 cm‑1; this complex also possesses a non-zero, albeit small E value of 1.1 cm-1. The large magnetic anisotropy is thought to arise from the trigonal distortion imposed by the rigid scorpionate ligand, which also partially suppresses the Jahn-Teller physics, thereby giving rise to a considerable orbital contribution to the magnetic ground state. [1] Molecular Magnets - Physics and Applications, Springer Series on Nanoscience and Technology, eds J. Bartolomé, F. Luis, J. F. Fernández (Springer, 2014). [2] N. Ishikawa et al., J. Am. Chem. Soc. 125, 8694 (2003).
    249th ACS National Meeting & Exposition, Denver, CO; 03/2015
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    Stephen M Winter · Stephen Hill · Richard T Oakley
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    ABSTRACT: Recent developments in stable radical chemistry have afforded "heavy atom" radicals, neutral open-shell (S = 1/2) molecular species containing heavy p-block elements (S, Se), which display solid-state magnetic properties once considered exclusive to conventional metal-based magnets. These highly spin-delocalized radicals do not associate in the solid state and yet display extensive networks of close intermolecular interactions. Spin density on the heavy atoms allows for increased isotropic and spin-orbit mediated anisotropic exchange effects. Structural variations induced by chemical modification and physical pressure, coupled with ab-initio methods to estimate exchange energies, have facilitated the development of predictive structure/property relationships. These results, coupled with detailed theoretical analyses and magnetic resonance spectroscopic measurements, have provided insight into the magnetic structure of ferromagnetic and spin-canted antiferromagnetic ordered materials as well as an understanding of the importance of spin-orbit coupling contributions to magnetic hysteresis and anisotropy. Isotropic and anisotropic ferromagnetic exchange can also be enhanced indirectly by the incorporation of heavy atoms into nonspin-bearing sites, where they can contribute to multi-orbital spin-orbit coupling.
    Journal of the American Chemical Society 03/2015; 137(11). DOI:10.1021/jacs.5b00672 · 11.44 Impact Factor
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    ABSTRACT: A new "offset" analogue of the classical [Mn6O2]-core oxime-bridged single-molecule magnets is introduced with a modified stacking arrangement of the [Mn3O] units. Studies of the magnetic properties reveal antiferromagnetic exchange interactions, a spin S = 4 ground state and population of low-lying excited states. Slow relaxation of the magnetization can be detected, with a corresponding energy barrier of 35.8 K. Interpretation of these features is supported with high-frequency EPR studies, quantifying the easy-axis type magnetic anisotropy, leading to a biaxial system. Redox properties investigated by cyclic and differential pulse voltammetry reveal multiple irreversible redox processes.
    Inorganic Chemistry 01/2015; 54(4). DOI:10.1021/ic502787q · 4.79 Impact Factor
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    ABSTRACT: The combination of high-pressure ferromagnetic resonance (FMR) and an ab-initio scheme suitable for calculation of spin-orbit mediated anisotropic exchange interactions in molecular materials provides insights into the role of spin-orbit coupling (SOC) in a Se-based organic ferromagnet. FMR measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure (up to 2.2 GPa), in excellent agreement with calculations based on the known pressure dependence of the structure. The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of SOC in a wide range of organics where this effect is usually considered to be small.
    Physical Review B 01/2015; 91(1). DOI:10.1103/PhysRevB.91.014412 · 3.74 Impact Factor
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    ABSTRACT: We report a room temperature study on the electrical response of field-effect transistors (FETs) based on few-layered MoSe2, grown by a chemical vapor transport technique, mechanically exfoliated onto SiO2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current on to off ratios up to 106 for both hole and electron channels when applying a small excitation voltage. A rather small hysteresis is observed when sweeping the back-gate voltage between positive and negative values, indicating the near absence of charge “puddles”. For both channels the Hall-effect indicates Hall mobilities μ_H ≈ 250 cm^2/Vs which are comparable to the corresponding field-effect mobilities, i.e. μ_FE 150 to 200 cm2/Vs evaluated through the conventional two-terminal field-effect configuration. Therefore, our results suggest that MoSe2 could be a good candidate for p-n junctions composed of a single atomic layer and for low-power, complimentary logic applications.
    ACS Nano 07/2014; DOI:10.1021/nn501693d · 12.88 Impact Factor
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    ABSTRACT: The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below TN=13 K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.
    Physical Review B 06/2014; 89(21):214403. DOI:10.1103/PhysRevB.89.214403 · 3.74 Impact Factor
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    ABSTRACT: Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic coordination complexes. These complexes, [M(C2O4)3](3-) (M = Ru, Cr, Fe) and [M(CN)6](3-) (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (S = (1)/2, (3)/2, (5)/2) and spin-orbit coupling (ζ = 464, 880, 3100 cm(-1)) on quantum decoherence. Coherence times (T2) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin-orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru(C2O4)3](3-), was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, T2, and the presence of Rabi oscillations. The complex [Ru(C2O4)3](3-) exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru(C2O4)3](3-) as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence.
    Journal of the American Chemical Society 05/2014; 136(21). DOI:10.1021/ja5037397 · 11.44 Impact Factor
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    ABSTRACT: A family of axially distorted mononuclear vanadium (III) compounds of general formula, A[L3VX3] (3-9 ) (X = F, Cl or Br, A+ = Et4N+, nBu4N+ or PPN+ , L3 = Tp or Tp* (Tp = tris(-1-pyrazolyl)borohydride), Tp* = tris(3,5-dimethyl-1-pyrazolyl)borohydride)), were studied (Figure 1). Replacement of the Tp ligand in 3 with the stronger π-donor Tp* results in a near doubling of the magnitude of the axial zero-field splitting parameter Dz (Dz = -16.0 cm-1 in 3 , and -30.0 cm-1 in PPN[Tp*VCl3] 4 )as determined by magnetic measurements. Such findings support the notion that control of the axial crystal field distortion is an excellent way to control single-ion anisotropy. High Field-High Frequency EPR measurements on 4 revealed an even higher D value, -40.0 cm-1.
    247th ACS Natunal Meeting, Dallas, USA; 03/2014
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    ABSTRACT: This chapter provides a detailed overview of some of the primary spectroscopic methods that have contributed to the current understanding of molecular nanomagnets (MNs). These include: electron paramagnetic resonance (EPR); optical spectroscopy, including magnetic and X-ray magnetic circular dichroism (MCD/XMCD); inelastic neutron scattering (INS); and muon spin rotation (μ +SR). For each technique, a historical survey of the most important discoveries is provided, up to and including the most recent developments. Each section gives an introduction to the theoretical principles underpinning the techniques, as well as a description of experimental requirements and protocols. A common theme among the described spectroscopies is the fact that state-of-the-art measurements typically have to be performed at major research facilities such as synchrotrons (terahertz EPR and XMCD), high magnetic field laboratories (EPR), and accelerator facilities or reactors (INS and μ +SR). Details of such facilities are given where appropriate. Forefront issues that are addressed in the chapter include: the fundamental properties of both mono- and poly-nuclear single-molecule magnets (SMMs); the deployment of MNs in quantum information processing applications; the addressing of individual magnetic molecules on surfaces or in devices; the probing of spin dynamics in MNs using EPR, INS, and μ +SR; and studies of long-range magnetic ordering in MN crystals. An extensive list of references is provided. The chapter is intended for physicists, chemists, and materials scientists, particularly junior researchers who are just starting work in the field.
    Molecular Nanomagnets and Related Phenomena, 01/2014: pages 231-291;
  • Stephen Hill · Enrique del Barco
    Polyhedron 12/2013; 66:1-2. DOI:10.1016/j.poly.2013.06.032 · 2.05 Impact Factor
  • Junjie Liu · Stephen Hill
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    ABSTRACT: We examine the magnetization quantum tunneling (MQT) behavior expected for a single-molecule magnet (SMM) with improper rotational symmetry. The simplest possible realization is the [Ni-II(hmp)(dmb)Cl](4) cubane complex that crystallizes in the I41/a space group, resulting in S-4 molecular point-group symmetry. A mapping is performed of the energy-level diagram obtained via exact diagonalization of a multi-spin Hamiltonian onto that of a giant-spin model which assumes ferromagnetic coupling and a spin S = 4 ground state. The results are compared with a similar analysis for a C-3 symmetric Mn-3 SMM (S = 6 ground state). In the even rotational case (Ni-4), the time-reversal invariance associated with the spin-orbit interaction gives rise to a zero-field spin-Hamiltonian that possesses an additional mirror plane perpendicular to the S-4 axis, which is not a symmetry element of the molecular point-group. This conclusion applies quite generally to any molecule with improper rotational symmetry (S-q, with q even), including the more widely studied Mn-12 SMM. The combined Ni-4 and Mn-3 studies lead to some interesting predictions concerning MQT selection rules in molecules with even versus odd rotational symmetries. We conclude by considering a case with essentially no symmetry at all, by deliberately distorting the high-symmetry Ni-4 molecule. In this case, finite gaps are found at all intersections in the energy-level diagram, indicating a complete absence of MQT selection rules.
    Polyhedron 12/2013; 66:147-152. DOI:10.1016/j.poly.2013.03.018 · 2.05 Impact Factor
  • Stephen Hill
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    ABSTRACT: This review has been adapted from a talk given by the author at the Third Workshop on Current Trends in Molecular and Nanoscale Magnetism, organized by George Christou in Orlando, Florida, June 2010. A very basic introduction to the theory of resonant quantum tunneling in single-molecule magnets (SMMs) is first given, followed by a short discussion of the Mn-12-acetate single-molecule magnet (SMM) and the solvent disorder that is known to perturb the intrinsic magnetization tunneling behavior observed in this compound. The remainder of the review focuses on key developments resulting from the preparation of several high-symmetry, spin S = 10 Mn-12 clusters by the Christou group that do not suffer problems associated with disorder to the same extent as the original Mn-12-acetate. Studies of these newer/cleaner Mn-12's have thus contributed significantly to our current understanding of resonant magnetization tunneling in molecular nanomagnets. This review is intended for both chemists and physicists, particularly junior researchers who are just starting work in the field.
    Polyhedron 11/2013; 64:128-135. DOI:10.1016/j.poly.2013.03.005 · 2.05 Impact Factor
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    ABSTRACT: The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.
    Angewandte Chemie International Edition 10/2013; 52(43). DOI:10.1002/anie.201303005 · 11.26 Impact Factor
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    ABSTRACT: Low-energy excitations in the rare earth kagome spin system Pr3Ga5SiO14 (PGS) have been investigated using high-field electron spin resonance (ESR) techniques. Previous work has shown that PGS does not exhibit long-range magnetic order at temperatures down to 30 mK. The present low-temperature (1.3–20 K) field-scan measurements, made on a single crystal sample in applied fields up to 30 T and for several microwave frequencies, give a series of temperature-dependent absorption peaks that are very different to conventional ESR spectra. The observed response is interpreted in terms of discrete spin-wave excitations in antiferromagnetically correlated spin clusters. We present a theoretical model which, in the limit of small magnetic frustration effects, qualitatively describes the microwave-induced transitions.
    Physical Review B 09/2013; 88(9). DOI:10.1103/PhysRevB.88.094414 · 3.74 Impact Factor
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    ABSTRACT: Microwave-assisted synthesis has been used to obtain the family of dodecanuclear Ni(II) complexes [Ni12 (NO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ][ClO4 ]2 (1), [Ni12 (NO3 )(MeO)12 (BrC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ][ClO4 ]2 (2), [Ni12 (CO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ]2 [SO4 ] (3) and [Ni12 (NO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)8 (H2 O)7 ][NO3 ]2 (4). They contain three {Ni4 O4 } cubane units which template around a central μ6 anion, either NO3 (-) or CO3 (2-) . Their magnetic properties have been studied by superconducting quantum interference device (SQUID) magnetometry and high-field EPR measurements. The nanostructuration of the Ni12 species on mica surfaces is studied by AFM and grazing-incidence X-ray diffraction, which reveal the formation of polycrystalline thin layers.
    Chemistry - A European Journal 07/2013; 19(27). DOI:10.1002/chem.201204081 · 5.70 Impact Factor
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    ABSTRACT: We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T1e(-1) of other free radicals used in DNP at the same concentration, trityl OX063 has slower T1e(-1) than BDPA and 4-oxo-TEMPO. At T > 20 K, the T1e(-1)vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd(3+) doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T1e(-1) especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: gx = gy = 2.00319(3) and gz = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T1e(-1) is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.
    Physical Chemistry Chemical Physics 05/2013; 15. DOI:10.1039/c3cp50186h · 4.20 Impact Factor

Publication Stats

1k Citations
349.01 Total Impact Points

Institutions

  • 2009–2015
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 2010–2014
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2003–2012
    • University of Florida
      • • Department of Chemistry
      • • Department of Physics
      Gainesville, FL, United States
  • 2005
    • University of California, San Diego
      • Department of Chemistry and Biochemistry
      San Diego, CA, United States