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Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh3)2X2 (Co-X; X = Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and Phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S = 3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule is, the larger its ZFS magnitude is. The current work reveals dynamics of magnetic and phonons excitations in SMMs. Studies of such couplings in the future would help understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

The metal-metal-bonded molecule [Bu4N][(HL)2Fe6(dmf)2] (Fe6) was previously shown to possess a thermally isolated spin S = 19/2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc.2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe6. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn12-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

A pair of bimetallic compounds featuring Fe-Fe bonds, [Fe(iPrNPPh2)3FeR] (R = PMe3, ≡NtBu), have been investigated using High-Frequency Electron Paramagnetic Resonance (HFEPR) as well as field- and temperature-dependent 57Fe nuclear γ resonance (Mossbauer) spectroscopy. To gain insight into the local site electronic structure, we have concurrently studied a compound containing a single Fe(II) in a geometry analogous to that of one of the dimer sites. Our spectroscopic studies have allowed for the assessment of the electronic structure via the determination of the zero-field splitting and 57Fe hyperfine parameters for the entire series. We also report on our efforts to correlate structure with physical properties in metal-metal bonded systems using ligand field theory guided by quantum chemical calculations. Through the insight gained in this study, we discuss strategies for the design of single-molecule magnets based on polymetallic compounds linked via direct metal-metal bonds.

A series of trans-dicyanide vanadium(III) compounds based on acetylacetonate, (PPN)[VIII(acac)2(CN)2]·(PPN)Cl·2MeCN (1), and salen ligands, (Et4N)[VIII(salen)(CN)2] (2a), (PPN)[VIII(MeOsalen)(CN)2]·DMF·2MeCN (3), and (PPN)[VIII(salphen)(CN)2]·DMF (4) [salen = N,N'-ethylenebis(salicyl-imine), MeOsalen = N,N'-ethylenebis(methoxysalicylimine), salphen = N,N'-phenylenebis(salicyl-imine), and PPN = bis(triphenylphosphine)iminium], were prepared and structurally characterized. High-field EPR studies reveal that the complexes exhibit moderate magnetic anisotropy with positive D values of +5.70, +3.80, +4.05, and +3.99 cm-1 for 1-4, respectively.

Reactions involving reductive aggregation of MnO4- in methanol in the presence of CeIV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce3Mn5O8(OMe)(O2CBut)13(MeOH)] (1) and [Ce2Mn3O5(O2CPh)9(MeOH)3] (2), containing at least one {Mn2Ce2O4} cubane unit. The cores of both clusters contain Mnx units separated by three (1) or two (2) CeIV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = 17/2 and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mnx units that are ≥6 A apart and separated by four intervening bonds through diamagnetic CeIV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.

The nitroxyl radical 1-methyl-2-azaadamantane N-oxyl (Me-AZADO) exhibits magnetic bistability arising from a radical/dimer interconversion. The transition from the rotationally disordered paramagnetic plastic crystal, Me-AZADO, to the ordered diamagnetic crystalline phase, (Me-AZADO)2, has been conclusively demonstrated by crystal structure determination from high-resolution powder diffraction data and by solid-state NMR spectroscopy. The phase change is characterized by a wide thermal hysteresis with high sensitivity to even small applied pressures. The molecular dynamics of the phase transition from the plastic crystal to the conventional crystalline phase has been tracked by solid-state (1H and 13C) NMR and EPR spectroscopies.

Two tetranuclear compounds with a cubane-like structure were synthesized from a one-pot reaction between NiII and 2,2,6,6-tetramethyl-3,5-heptanedione (Hdpm) for 1 or 4,4,4-trifluoro-1-phenyl-1,3-b...

Spins in molecules are particularly attractive targets for next-generation quantum technologies, enabling chemically programmable qubits and potential for 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. These findings suggest new strategies for development of molecular quantum technologies, akin to trapped ion systems.

Materials with magnetic properties that can be modulated via optical excitation offer enticing opportunities for the development of next generation technologies. An exciting new class of photomagnetic materials based on bisdithiazolyl radicals has recently been developed. The dimorphic system in this study crystallizes in two phases, one composed of diamagnetic dimers and the other of paramagnetic radicals. Here we report on the use of high-field electron paramagnetic resonance spectroscopy to characterize both the thermally- and light-induced transitions in the dimer phase. During the course of this study we show that signals originating from residual radical defects in the dimer phase can be differentiated from those arising from the radical phase.

International audience; Silicon-mediated fluoride abstraction is demonstrated as a means of generating the first fluorido-cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans-[M IV F 4 (CN) 2 ] 2À (M = Re, Os), obtained from their homoleptic [M IV F 6 ] 2À parents. As shown by combined high-field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans-[ReF 4 (CN) 2 ] 2À as compared to [ReF 6 ] 2À , reflecting the severe departure from an ideal octahedral (O h point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building-blocks for the design of high-performance molecule-based magnetic materials.

Direct metal–metal bonding of two distinct first-row transition metals remains relatively unexplored compared to their second- and third-row heterobimetallic counterparts. Herein, a recently reported Fe–V triply bonded species, [V(iPrNPPh2)3FeI] (1; Kuppuswamy, S.; Powers, T. M.; Krogman, J. P.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Vanadium–iron complexes featuring metal–metal multiple bonds. Chem. Sci. 2013, 4, 3557–3565), is investigated using high-frequency electron paramagnetic resonance, field- and temperature-dependent 57Fe nuclear gamma resonance (Mossbauer) spectroscopy, and high-field electron-electron double resonance detected nuclear magnetic resonance. From the use of this suite of physical methods, we have assessed the electronic structure of 1. These studies allow us to establish the effective g tensors as well as the Fe/V electro-nuclear hyperfine interaction tensors of the spin S = 1/2 ground state. We have rationalized these tensors in the context of ligand field theory supporte...

A new family of binuclear complexes [MnIIILnIII(dpm)4(MeO)2(MeOH)2] is reported (where Ln = LaIII (1), PrIII (2), and EuIII(3)). These compounds were obtained from a one-pot reaction between 2,2,6,6-tetramethyl-3,5-heptanodione (Hdpm), MnII, and the respective LnIII salt in the presence of sodium methoxide. The derivative containing the diamagnetic ion LaIII has been synthesized in order to characterize the local anisotropy of the MnIII ion. High-field electron paramagnetic resonance (HFEPR) spectroscopy shows that the MnIII ion, with an elongated octahedral geometry in all compounds, has a significant axial zero-field splitting and a small rhombic anisotropy. Additionally, the HFEPR measurements indicate that there is almost no exchange between the spin carriers in these compounds, all of which exhibit field-induced slow relaxation of the magnetization.

The synthesis, X-ray crystal and electronic structures of [Fe3(μ3-O)(mpmae)2(OAc)2 Cl3], 1, where mpmae-H = 2-(N-methyl-N-((pyridine-2-yl)methyl)amino)ethanol, are described. This cluster comprises three high-spin ferric ions and exhibits a T-shaped site topology. Variable-frequency electron paramagnetic resonance measurements performed on single crystals of 1 demonstrate a total spin ST = 5/2 ground state, characterized by a small, negative, and nearly axial zero-field splitting tensor D = −0.49 cm–1, E/D ≈ 0.055. Analysis of magnetic susceptibility, magnetization, and magneto-structural correlations further corroborate the presence of a sextet ground-spin state. The observed ground state originates from the strong anti-ferromagnetic interaction of two iron(III) spins, with J = 115(5) cm–1, that, in turn, are only weakly coupled to the spin of the third site, with j = 7(1) cm–1. These exchange interactions lead to a ground state with magnetic properties that are essentially entirely determined by the wea...

The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2– upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2–: (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic ci...

Structural, magnetic and spectroscopic data of four complex salts, [Mn(napsal2323)]NTf2, 1,[Mn(napsal2323)]ClO4, 2, [Mn(napsal2323)]BF4, 3 and Mn(napsal2323)]NO3, 4, of the [Mn(napsal2323)]+ complex cation indicate that the Mn3+ ion is stabilized in the rare S = 1 spin triplet form in this ligand sphere. Zero-field splitting values of D = +19.6 cm-1 and |E| = 2.02 cm-1for complex 1 were obtained by High Field Electron Paramagnetic Resonance (HFEPR) measurements conducted over a range of frequencies and field strengths. Structural and magnetic data also indicate that co-crystallization of complexes 2 and 3 with 0.5 equivalents of ethanol yields the high spin S = 2 forms of the perchlorate and tetrafluoroborate solvates [Mn(napsal2323)]ClO4•0.5(C2H5OH), 2•0.5EtOH and [Mn(napsal2323)]BF4•0.5(C2H5OH), 3•0.5EtOH.

The family of reduced [Mn12O12(O2CR)16(H2O)4]n− (n = 1, 2) single-molecule magnets (SMMs) has been expanded with the synthesis of new members using I− as a stoichiometric one-electron reducing agent of the neutral n = 0 complexes. Electrochemical data, 1H NMR spectra of the [Mn12O12(O2CCH2Cl)16(H2O)3]0,−,2− salts, and the crystal structures of two complexes, (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] and (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3] have been obtained. For (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3], the data confirm an S = 10 ground state, and magnetization vs dc field scans on a pristine crystal exhibit hysteresis loops possessing steps due to quantum tunneling of magnetization (QTM). In addition, two forms of the cluster have been identified in the crystal with distinctly different relaxation barriers, with the faster-relaxing form being the majority one in pristine crystals from mother liquor, and the slower-relaxing one becoming the majority one in vacuum-dried samples. Similar behavior is seen for (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] with an S = 19/2 ground state, which shows three forms to be present in the hysteresis loops of a pristine crystal, all converting to a slow-relaxing form on vacuum-drying. High-frequency EPR spectra of the latter confirm a high axial anisotropy with D = −0.477 cm−1 and rationalize its effective relaxation barrier Ueff = 57 K. The combined work emphasizes a high sensitivity of such ultra-small nanomagnets to environmental influences, affecting their properties significantly, especially their quantum properties that are so important to many potential new 21st century applications.

Two new compounds [Ni(dca) 2 (dmdpy)] ( 1 ) and [Zn(dca) 2 (dmdpy)] ( 2 ), (dca = dicyanamide and dmdpy = 5,5′-dimethyl-2,2′-dipyridine) were synthesized and characterized by single crystal X-ray diffraction, FTIR and thermal analysis. In both compounds, the dmdpy ligand adopts a bidentate chelating coordination mode while dca anions act in a μ 1,5 bridging fashion. In 1 , the Ni II centers lie on a distorted octahedral geometry, whereas in 2 the Zn II geometry is distorted square pyramidal. In compounds 1 and 2 , dca ligands connect the metal sites giving rise to 2D and 1D coordination polymers, respectively. The magnetic properties of 1 were studied by magnetic susceptibility and high field/frequency electron paramagnetic resonance (HFEPR) measurements.

There is a body of literature that has given some empirical support to the notion that energy efficient buildings may be associated with significant financial premiums. However, such evidence is not entirely conclusive and may be confounded by an associated workplace quality factor. In light of the perceived lack of clarity within this body of literature to provide empirical support to the existence of financial premiums for building energy performance, this research aims to explore the potential financial value of property energy performance through the application of futures studies to income-based valuations. This is addressed through applying exploratory scenario descriptions of the UK energy system to the valuation of 24 commercial offices. In doing so, every opportunity is given to identify the significance of property energy performance to financial value. The results of this research demonstrate how, even when exploring an extreme set of diverse energy-system futures and accounting for the social costs of climate change, the potential of financial premiums for energy efficient property are not of the same order of magnitude as those observed in the literature. These differences may be explained by confounding associations between such labels and overall facility quality, or by short-term market sentiment.

The preparation, structures, and magnetic properties of two tricyanoferrates, [NEt4]2[(Tp*Me)FeII(CN)3]·MeCN (1) and [NEt4][(Tp*Me)FeIII(CN)3]·H2O (2), and three trinuclear derivatives, {[(Tp*Me)FeIII(CN)3]2[NiII(L)2]}·n(solvent) (L = bpy, 3; tren, 4; DETA, 5) are described. Magnetic measurements show that 2 is an S = 1/2 complex [g = 2.65(1)], while 3–5 display ST = 2 spin ground states, owing to cyanide-mediated ferromagnetic exchange between FeIIILS (S = 1/2) and NiII (S = 1) ions. The χT vs. T data for 3–5 were modelled using an isotropic Heisenberg model and give J/kB and gavg values of +11.1(1) K and 2.48(1) for 3, 9.1(1) K and 2.6(1) for 4, and +11.2(1) K and 2.41(5) for 5 (with zJ′ = –0.28 K). High-field/frequency EPR data indicates significant g-anisotropy in 2 (gz = 3.6, gy = 2.2, gx = 2.0) and [NEt4][(Tp*)FeIII(CN)3]·H2O (6) (gz = 3.5, gy = 2.0, gx = 1.3), while 5 shows D = –2.09 cm–1, E = 0.08 cm–1, and B40 = –2.3 × 10–3 cm–1 with gz = 2.4 and gx = gy = 1.95. Despite significant magnetic anisotropy in 3–5, frequency-dependent behavior is only seen for 5 above 1.8 K, suggesting that rapid quantum tunnelling of the magnetization is operative.

Coordinatively unsaturated Fe(III) metal sites were successfully incorporated into the iconic MOF-5 framework. This new structure, Fe(III) -iMOF-5, is the first example of an interpenetrated MOF linked through intercalated metal ions. Structural characterization was performed with single-crystal and powder XRD, followed by extensive analysis by spectroscopic methods and solid-state NMR, which reveals the paramagnetic ion through its interaction with the framework. EPR and Mössbauer spectroscopy confirmed that the intercalated ions were indeed Fe(III) , whereas DFT calculations were employed to ascertain the unique pentacoordinate architecture around the Fe(III) ion. Interestingly, this is also the first crystallographic evidence of pentacoordinate Zn(II) within the MOF-5 SBU. This new MOF structure displays the potential for metal-site addition as a framework connector, thus creating further opportunity for the innovative development of new MOF materials.

This work was performed at the National High Magnetic Field Laboratory (NHMFL), which is supported by the National Science Foundation (DMR-1157490) and the State of Florida. L.S. acknowledges support from the National Institutes of Health (AI091693) and the NHMFL User Collaboration Grants Program (Award No. 5080). G.E.F. acknowledges support from the National Science Foundation (MCB-1329467) and the National Institutes of Health (GM105409 and S10RR031603). S.H. acknowledges support from the National Science Foundation (DMR-1309463). J.M.E acknowledges support from the National Science Foundation (DGE-0802270). High-field, high-frequency electron paramagnetic resonance (EPR) spectroscopy at W- (∼95 GHz) and D-band (∼140 GHz) is important for investigating the conformational dynamics of flexible biological macromolecules because this frequency range has increased spectral sensitivity to nitroxide motion over the 100 ps to 2 ns regime. However, low concentration sensitivity remains a roadblock for studying aqueous samples at high magnetic fields. Here, we examine the sensitivity of a non-resonant thin-layer cylindrical sample holder, coupled to a quasi-optical induction-mode W-band EPR spectrometer (HiPER), for continuous wave (CW) EPR analyses of: (i) the aqueous nitroxide standard, TEMPO; (ii) the unstructured to α-helical transition of a model IDP protein; and (iii) the base-stacking transition in a kink-turn motif of a large 232 nt RNA. For sample volumes of ∼50 μL, concentration sensitivities of 2-20 μM were achieved, representing a ∼10-fold enhancement compared to a cylindrical TE011 resonator on a commercial Bruker W-band spectrometer. These results therefore highlight the sensitivity of the thin-layer sample holders employed in HiPER for spin-labeling studies of biological macromolecules at high fields, where applications can extend to other systems that are facilitated by the modest sample volumes and ease of sample loading and geometry. Postprint

The zero-field splitting parameters D and E in the iron(ii) complex [Fe(C3S5)2]2– are shown to be remarkably resistant to a twist of the inter-ligand dihedral angle (θd) from 90 to 70°., The relationship between electronic structure and zero-field splitting dictates key design parameters for magnetic molecules. In particular, to enable the directed synthesis of new electronic spin based qubits, developing complexes where zero-field splitting energies are invariant to structural changes is a critical challenge. Toward those ends, we report three salts of a new compound, a four-coordinate iron(ii) complex [Fe(C3S5)2]2– ([(18-crown-6)K]+ (1), Ph4P+ (2), Bu4N+ (3)) with a continuous structural variation in a single parameter, the dihedral angle (θd) between the two C3S52– ligands, as a function of counterion (θd = 89.98(4)° for 1 to 72.41(2)° for 3). Electron paramagnetic resonance data for 1–3 reveal zero-field splitting parameters that are unusually robust to the structural variation. Mössbauer spectroscopic measurements indicate that the structural variation in θd primarily affects the highest-energy 3d-orbitals (dxz and dyz) of the iron(ii) ion. These orbitals have the smallest impact on the zero-field splitting parameters, thus the distortion has a minor effect on D and E. These results represent the first part of a directed effort to understand how spin state energies may be fortified against structural distortions for future applications of qubits in non-crystalline environments.

Controlling and understanding transitions between molecular spin states allows selection of the most suitable ones for qubit encoding. Here we present a detailed investigation of single crystals of a polynuclear Cr8Zn molecular wheel using 241 GHz electron paramagnetic resonance (EPR) spectroscopy in high magnetic field. Continuous wave spectra are well reproduced by spin Hamiltonian calculations, which evidence that transitions in correspondence to a well-defined anticrossing involve mixed states with different total spin. We studied, by means of spin echo experiments, the temperature dependence of the dephasing time (T2) down to 1.35 K. These results are reproduced by considering both hyperfine and intermolecular dipolar interactions, evidencing that the dipolar contribution is completely suppressed at the lowest temperature. Overall, these results shed light on the effects of the decoherence mechanisms, whose understanding is crucial to exploit chemically engineered molecular states as a resource for quantum information processing.

A 3D entangled coordination polymer {[Co4(glu)4(bpp)2]}n 1 was synthesized from a solvothermal reaction between Co(II), glutaric acid (H2glu) and 1,3-bis(4-pyridyl)propane (bpp). The crystal structure was determined by single crystal X-ray diffraction and the topological description indicates a 3D → 3D interpenetration in the pcu-coordination polymer, pillared by bpp ligands. Crystal structure presents four crystallographically distinct Co(II) metal centers conferring different coordination environments: tetrahedral, octahedral and trigonal bipyramidal. Spectroscopic (FTIR and Raman), thermal analysis and magnetic studies were employed to chemical characterization. SQUID magnetometry analysis indicated weak overall antiferromagnetic interactions between the Co(II) ions. High-Field Electron Paramagnetic Resonance (HFEPR) spectra suggest that the Co(II) ions in compound 1 are high-spin (S = 3/2). The material showed good thermal stability decomposing above 300 °C. CO2 adsorption studies carried out in the pressure range of 0 to 40 bar at room temperature revealed a favorable solid-gas interaction at low pressure. The amount of CO2 adsorbed on compound 1 continuously increased as the pressure increased and no gas saturation is observed suggesting a multilayer phenomenon of CO2 uptake. Powder X-ray diffraction analysis of compound 1 after CO2 measurements at 40 bar revealed no structural change.

The field-dependent magnetization and magnetization blocking in single-crystalline samples of radical-bridged [DyIII4] cubane units are studied by ab initio calculations, micro-superconducting quantum interference device, and electron paramagnetic resonance. Because of the canted spin structure, a transverse method was applied to detect ferromagnetic and antiferromagnetic projections, for which remarkably different hysteresis magnetization loops were observed and investigated. Given 18 complexes in a unit cell of the crystal, a special ab initio-based approach has been developed to describe the magnetism in this material. The application of this ab initio methodology reveals a remarkable correlation between the evolution of the calculated magnetizations of each of the 18 molecules in the unit cell and the experimentally measured single-crystal hysteresis loop structure. This affords a level of insight into the microscopic magnetic properties that is unprecedented for a spin system of such complexity and h...

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 10(6) 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 μH ≃ 250 cm(2)/(V s), which are comparable to the corresponding field-effect mobilities, i.e., μFE ∼ 150 to 200 cm(2)/(V s) 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, complementary logic applications.

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3-3.5 K and 9.4-130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0° to 60°. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of [Formula: see text] in free-standing samples, indicating that the local basal crystal field is non-uniform.

An electron paramagnetic resonance (EPR) study of a single crystal of Co II -based single-chain magnets (SCM) is presented. Discrete resonant absorptions are associated to the presence of magnetic domains within the chains of finite lengths determined by a competition between intra-chain exchange interactions and thermally excited single spin fluctuations. The results are interpreted as a transition from single spin dynamics at high temperature ( T ∼20 K), associated to the Kramers doublet ground state of the individual Co II ions, to archetypical SCM dynamics at low temperatures, where intra-chain correlations form long magnetic domains, whose average length is imposed by the concentration of defects in the sample.

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.

High-field/frequency electron paramagnetic resonance measurements have been performed on a cyano-bridged trinuclear Fe2IIINiII complex and an analogous mononuclear FeIII building block. The mononuclear FeIII complex exhibits a low-spin configuration resulting from a strong-field coordination environment and can be modeled as an effective s = 1/2 Kramers ion that exhibits significant g-anisotropy: gz = 3.7, gy = 2.2 and gx = 1.92. The corresponding Fe2IIINiII complex possesses an S = 2 spin ground state as a result of ferromagnetic exchange that is mediated by the bridging cyano ligands. Simulations of frequency dependent EPR data establish that the Fe2IIINiII complex exhibits easy-axis type magnetoanisotropy, with best-fit simulation parameters of: S = 2, D = −2.09 cm−1, E = 0.08 cm−1, B40 = 2.3 × 10−3 cm−1, gz = 2.4, gy = gx = 1.95.

Background: Docosahexaenoic acid (DHA) is important for brain function, and its status is dependent on dietary intakes. Therefore, individuals who consume diets low in omega-3 (n23) polyunsaturated fatty acids may cognitively benefit from DHA supplementation. Sex and apolipoprotein E genotype (APOE) affect cognition and may modulate the response to DHA supplementation. Objectives: We investigated whether a DHA supplement improves cognitive performance in healthy young adults and whether sex and APOE modulate the response. Design: Healthy adults (n = 176; age range: 18‐45 y; nonsmoking and with a low intake of DHA) completed a 6-mo randomized, placebo-controlled, double-blind intervention in which they consumed 1.16 g DHA/d or a placebo. Cognitive performance was assessed by using a computerized cognitive test battery. For all tests, z scores were calculated and clustered into cognitive domains as follows: episodic and working memory, attention, reaction time (RT) of episodic and working memory, and attention and processing speed. ANCOVAwas conducted with sex and APOE as independent variables. Results: RTs of episodic and working memory improved with DHA

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn3 units are reported: dimeric [Mn6O2(O2CMe)8(CH3OH)2(pdpd)2] (3) and [Mn6O2(O2CMe)8(py)2(pdpd)2](ClO4)2 (4), and tetrameric [Mn12O4(O2CR)12(pdpd)6](ClO4)4 (R = Me (5), tBu (6)). They were all obtained employing 3-phenyl-1,5-di(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH2), either in direct synthesis reactions involving oxidation of MnII salts or in metathesis reactions with the preformed complex [Mn3O(O2CMe)6(py)3](ClO4) (1); complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two [MnIII2MnII(μ3-O)]6+ and [MnIII3(μ3-O)]7+ units, respectively, linked by two pdpd2− groups. Complexes 5 and 6 contain four [MnIII3(μ3-O)]7+ units linked by six pdpd2− groups into a rectangular tetramer [MnIII3]4. Solid-state dc magnetic susceptibility studies showed that the Mn3 subunits in 3 and 4 have a ground-state spin of S = 3/2 and S = 2, respectively, while the Mn3 subunits in 5 and 6 possess an S = 6 ground state. Complexes 5 and 6 exhibit frequency-dependent out-of-phase (χ′′M) ac susceptibility signals indicating 5 and 6 to be tetramers of Mn3 single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of 5·2CH2Cl2 provided precise spin Hamiltonian parameters of D = −0.33 cm−1, |E| = 0.03 cm−1, B04 = −8.0 × 10−5 cm−1, and g = 2.0. Magnetization vs. dc field sweeps on a single crystal of 5·xCH2Cl2 gave hysteresis loops below 1 K that exhibit exchange-biased quantum tunneling of magnetization (QTM) steps with a bias field of 0.19 T. Simulation of the loops determined that each Mn3 unit is exchange-coupled to the two neighbors linked to it by the pdpd2− linkers, with an antiferromagnetic inter-Mn3 exchange interaction of J/kB = −0.011 K (Ĥ = −2JŜi·Ŝj convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.

Continuous wave (cw) and pulsed high frequency electron paramagnetic resonance (HF-EPR) measurements were performed on an Fe-based magnetic cluster: Fe 7 O 4 (O 2 CPh) 11 (dmem) 2 , abbreviated Fe 7 . The cw EPR results show that two different molecular species exist in the crystal, with slightly different zero-field-splitting parameters. The spin decoherence time, T 2 , was measured at high magnetic fields and low temperatures, which makes it possible to obtain high spin polarization and to significantly reduce decoherence due to electron spin flip-flop processes. Theoretical fitting of T 2 versus temperature shows that, for crystalline samples of this molecule, spin flip-flop fluctuations represent the main source of spin decoherence at low temperatures, as reported also for the Fe 8 single-molecule magnet [Phys. Rev. Lett. 102 (2009) 087603]. Moreover, it is found that T 2 is position dependent within the EPR line, a model for which is given. We also note that this is the third example of an Fe-based cluster that exhibits a measurable decoherence time, and only the second involving a crystal.

High-frequency Electron Paramagnetic Resonance (EPR) and Quantum Tunneling of Magnetization (QTM) measurements have been carried out on single-crystal samples of a mixed-valent Mn 2 II Mn 2 III single-molecule magnet (SMM). Frequency, temperature and field-oriented studies reveal an S = 9 spin ground state with low-lying excited spin states. Numerical simulations were performed via exact diagonalization of a multi-spin Hamiltonian, in which the anisotropy of the molecule is based on the anisotropy of the individual ions and their couplings. We found that the EPR and QTM spectra may adequately be explained with two s = 5/2 MnII ions weakly coupled to a rigid s = 4 spin, formed by the two ferromagnetically coupled MnIII ions.

High-field, single-crystal EPR spectroscopy on a tetragonal bisdiselenazolyl ferromagnet has provided evidence for the presence of easy-axis magnetic anisotropy, with the crystallographic c axis as the easy axis and the ab plane as the hard plane. The observation of a zero-field gap in the resonance frequency is interpreted in terms of an anisotropy field several orders of magnitude larger than that observed in light-heteroatom, nonmetallic ferromagnets and comparable (on a per-site basis) to that observed in hexagonal close packed cobalt. The results indicate that large spin-orbit-induced magnetic anisotropies, typically associated with 3d-orbital-based ferromagnets, can also be found in heavy p-block radicals, suggesting that there may be major opportunities for the development of heavy p-block organic magnetic materials.

Two new members of the Mn(12) family of single-molecule magnets (SMMs), [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(Bu(t)OH)(H(2)O)(3)].2Bu(t)OH (3.2Bu(t)OH) and [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(C(5)H(11)OH)(4)] (4) (C(5)H(11)OH is 1-pentanol), are reported. They were synthesized from [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1) by carboxylate substitution and crystallization from the appropriate alcohol-containing solvent. Complexes 3 and 4 are new members of the recently established [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(solv)(4)] (solv = H(2)O, alcohols) family of SMMs. Only one bulky Bu(t)OH can be accommodated into 3, and even this causes significant distortion of the [Mn(12)O(12)] core. Variable-temperature, solid-state alternating current (AC) magnetization studies were carried out on complexes 3 and 4, and they established that both possess an S = 10 ground state spin and are SMMs. However, the magnetic behavior of the two compounds was found to be significantly different, with 4 showing out-of-phase AC peaks at higher temperatures than 3. High-frequency electron paramagnetic resonance (HFEPR) studies were carried out on single crystals of 3.2Bu(t)OH and 4, and these revealed that the axial zero-field splitting constant, D, is very different for the two compounds. Furthermore, it was established that 4 is the Mn(12) SMM with the highest kinetic barrier (U(eff)) to date. The results reveal alcohol substitution as an additional and convenient means to affect the magnetization relaxation barrier of the Mn(12) SMMs without major change to the ligation or oxidation state.

The aim of this study was to compare 7 weeks of soccer-specific small-sided game (SSG) and mixed generic fitness training, on selected physiological, perceptual and performance variables. Twenty-five elite youth players were randomly allocated to either a SSG (coach selected) or generic training group (GTG), in a randomised, parallel matched-group design. In addition to normal training, each group completed two fitness training sessions per week of equal duration. Players completed a V O (2 max) treadmill test, Multistage Fitness Test (MSFT), Yo-Yo Intermittent Recovery Test Level 1 (YYIRTL1), 12x20 m test of repeated-sprint ability (RSA) and 20-m sprint test pre and post training. Training heart rate, perceived training intensity and perceptual fatigue measures were recorded throughout the training period. There were no differences in training heart rate or perceptual well-being measures. However, the GTG did perceive their training to be more intense than SSG. There were no changes in either group for V O (2 max), MSFT, RSA or sprint performance. However, there were improvements in YYIRTL1 performance for both groups over time, but not between groups. The results show that both types of training are equally effective at improving pre-season YYIRTL1 performance, despite GTG being perceived to be more intense.

Very high-frequency (50–715 GHz) electron paramagnetic resonance (EPR) studies of the tetranuclear Co II complex [Co(hmp)(dmb)Cl] 4 ( 1 ), where dmb is 3,3-dimethyl-1-butanol and hmp − is the monoanion of 2-hydroxy-methylpyridine, reveal the presence of significant zero-field-splitting (ZFS) within the ground state spin multiplet. Meanwhile, low-temperature hysteresis measurements of 1 (and related Co II 4 complexes) provide evidence for slow magnetization relaxation, suggesting that it could be a single-molecule magnet (SMM). However, EPR studies of a Zn analog of 1 , doped with a small quantity of Co II , show the ground state of the Co II ions to be an effective spin S ′ = 1/2 Kramers doublet with a highly anisotropic g -tensor. The question then arises as to the origin of the ZFS within the ground state spin multiplet of 1 , as well as the slow magnetization relaxation. Here, we consider the effect of anisotropic exchange interactions between the effective spin S ′ = 1/2 Kramers ions within the tetranuclear complex. Such exchange anisotropy arises naturally when one treats the ground state of high-spin Co II as a Kramers doublet. Our model provides an explanation for the ZFS in the ground state observed via EPR, and can also account for qualitative features observed through magnetic measurements.

We report detailed numerical and spectroscopic studies of two complexes from a family of recently discovered Mn 6 III single-molecule magnets (SMMs) with large barriers to magnetization reversal. These complexes consist of a pair of Mn 3 III triangles with a ferromagnetic interaction between the triangles. Recent studies have shown that the exchange interactions within the triangular Mn 3 III units can be switched from antiferromagnetic to ferromagnetic, resulting in a switching of the spin from S = 4 to 12. This strategy to “increase S” has resulted in the highest magnetic energy barrier and blocking temperature for any known SMM to date. Extensive frequency, temperature and field-orientation dependent single-crystal high-frequency electron paramagnetic resonance measurements have been performed to determine the spin-Hamiltonian parameters associated with the lowest-lying spin multiplet for each complex. We compare the experimental findings with numerical calculations, where the total anisotropy for a complex is determined in terms of single-ion anisotropies using both projection operator techniques and exact matrix diagonalization methods. In particular, we find that the product of the molecular anisotropy, D, and spin, S, does not change significantly upon switching from S = 4 to 12, i.e. D goes down as S goes up. These studies provide important insights concerning strategies for designing SMMs with higher blocking temperatures, particularly for complexes containing manganese in its +3 oxidation state.

We review an extensive body of single-crystal high-frequency electron paramagnetic resonance (HFEPR) data in order to determine the transverse spin Hamiltonian parameters that control the tunneling of the direction of magnetization in a variety of integer and half-integer-spin single-molecule magnets (SMMs). The SMMs studied are members of the following families: S = 9/2 [Mn4O3Cl]6+; S = 5 [Mn3NiO4]6+; S = 6 [Mn3ZnO4]6+; and S = 4 [Ni4(OR)4]4+. HFEPR spectra for the half-integer S = 9/2 Mn4 complexes that have C3 symmetry do not provide measurable evidence for transverse spin Hamiltonian terms. This finding is consistent with the relatively large coercive field seen in the magnetization hysteresis loops for these complexes. On the other hand, a low symmetry S = 9/2 complex exhibits a much faster rate of ground-state magnetization tunneling, in agreement with HFEPR spectra for a powder sample that gives a rhombic zero-field splitting (ZFS) parameter of E = 0.140 cm−1. The S = 5 Mn3Ni systems exhibit magnetization tunneling that is much faster than seen for the high-symmetry S = 9/2 Mn4 complexes. This can be attributed to their integer-spin ground states. Like the C3 symmetry Mn4 SMMs, the HFEPR spectra for high-symmetry Mn3Ni complexes do not provide measurable evidence for transverse ZFS terms. However, the spectra exhibit broad peaks, suggesting distributions in the local molecular environments brought about by disordered solvate molecules. This disorder likely explains the fast tunneling in the high-symmetry S = 5 Mn3Ni systems, though one cannot rule out fourth- (and higher-) order interactions that cannot be detected by HFEPR due to the broad resonances. The one S = 6 Mn3Zn complex shows an even faster rate of tunneling compared to the isostructural S = 5 Mn3Ni complex. Finally, the S = 4 [Ni(hmp)(dmb)Cl]4 complex provides unique insights into the origin of fourth- (and higher-) order interactions found for many SMMs on the basis of analysis using a giant spin Hamiltonian (GSH) approximation. We conclude that the fourth-order anisotropy found for the S = 4 ground state of [Ni(hmp)(dmb)Cl]4 originates from the second-order ZFS interactions associated with the individual NiII ions, but only as a result of higher-order processes that occur via S-mixing between the ground state and higher-lying (S

Variable high-frequency electron paramagnetic resonance data were collected for a single crystal of [Zn(hmp)(dmb)Cl]4 (1) doped with a small quantity of high spin Co(II), where dmb is 3,3-dimethyl-1-butanol and hmp- is the monoanion of 2-hydroxy-methylpyridine. The lack of solvent in the lattice of complex 1 results in very little disorder. Consequently, the EPR spectra are extremely sharp, enabling precise comparisons with theoretical simulations. We find the ground state of the Co(II) ions to be an effective spin S′ = 1/2 Kramers’ doublet with a highly anisotropic g-tensor. The anisotropy is found to be of the easy-axis type, with the single-ion easy axis directions tilted away from the crystallographic c direction by 58°.

We present the results of a multi-high-frequency single-crystal EPR study of a recently discovered triangular trinuclear Mn complex, [Mn 3 O(O 2 CMe) 3 (mpko) 3 ](ClO 4 ) · 3CH 2 Cl 2 . The obtained data set confirm the findings of earlier magnetic measurements, which suggested that this complex is a single-molecule magnet with a spin ground state of S = 6. The zero-field splitting parameters obtained from the present EPR study are: D = −0.3 cm −1 , B 4 0 = - 3 × 10 - 5 cm - 1 and g = 2.00. We also find a significant transverse anisotropy which can be parameterized by a rhombic distortion with an E value of at least 0.015 cm −1 .

We report single crystal high-frequency electron paramagnetic resonance studies of two recently discovered half-integer Fe7 complexes: [Fe7O3(O2CBut)9(mda)3(H2O)3] (1) and [Fe7O4(O2CPh)11(dmem)2] (2). The obtained spectra confirm spin S = 5/2 ground states for both complexes. On the basis of detailed frequency and field orientation dependent studies, we find that complex 1 is a single-molecule magnet (uniaxial zero-field-splitting parameter, D 0). The EPR linewidth for complex 1 is considerably narrower in comparison to spectra obtained for other single-molecule magnets, suggesting the possibility of reduced decoherence from nuclear spins (56Fe has no nuclear moment).

We report high-resolution angle-dependent measurements of the Periodic Orbit Resonance (POR) effect in (TMTSF)2ClO4. We observe additional harmonic resonances that were not observed in previous studies. By measuring over a broad range of frequency, field, and field orientation, we find that all of the POR evolve from the Lebed magic angles observed in dc (ω=0) AMRO measurements. However, we find that the resonance angles vary with frequency in a manner that can easily be explained using a semiclassical model. This indicates that these angles are not ‘magic’ at frequencies on the order of the cyclotron frequency.

Recent electron paramagnetic resonance (EPR) and magnetic hysteresis experiments have demonstrated quantum entanglement between single-molecule magnets in a supramolecular [Mn4]2 dimer. Here we outline details of the computational methods used to calculate the EPR spectrum for the dimer system, with particular emphasis on the symmetries of the entangled eigenstates and the resultant EPR transition matrix elements.

High-field EPR and magnetic studies of a high-spin Ni(ii) trigonal bipyramidal complex reveal a giant axial magnetic anisotropy and a rare field-induced slow magnetic relaxation., Monometallic complexes based on 3d transition metal ions in certain axial coordination environments can exhibit appreciably enhanced magnetic anisotropy, important for memory applications, due to stabilisation of an unquenched orbital moment. For high-spin trigonal bipyramidal Ni(ii), if competing structural distortions can be minimised, this may result in an axial anisotropy that is at least an order of magnitude stronger than found for orbitally non-degenerate octahedral complexes. Broadband, high-field EPR studies of [Ni(MDABCO)2Cl3]ClO4 (1) confirm an unprecedented axial magnetic anisotropy, which pushes the limits of the familiar spin-only description. Crucially, compared to complexes with multidentate ligands that encapsulate the metal ion, we see only a very small degree of axial symmetry breaking. 1 displays field-induced slow magnetic relaxation, which is rare for monometallic Ni(ii) complexes due to efficient spin–lattice and quantum tunnelling relaxation pathways.

[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.

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.