Anne-Laure Barra

Approaching the dirac point in high-mobility multilayer epitaxial graphene.

Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

HIGH-FREQUENCY EPR SPECTRA OF A MOLECULAR NANOMAGNET : UNDERSTANDING QUANTUM TUNNELING OF THE MAGNETIZATION

EPR spectra have been recorded in very high field, up to 25T, and at high frequency, up to 525 GHz, on a polycristalline sample of Mn12ac (see paper for detailed formula), the first example of molecular cluster behaving like a nanomagnet. The simulation of the spectra has provided an accurate determination of the parameters of the spin hamiltonian (see paper for formula and values of the various parameters). The presence of the fourth order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to Quantum Tunneling between degenerate M states of the ground S=10 multiplet of the cluster. The term in (S_+^4 + S_-^4) is responsible of the transverse magnetic anisotropy and plays a crucial role in the mechanism of Quantum Tunneling. The HF-EPR spectra have for the first time evidenced its presence and quantified it.

Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a [{FeIIL}2{NbIV(CN)8}] Helical Chain Compound Designed with Heptacoordinate FeII

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear {Fe(II)Nb(IV)} compound with remarkable magnetic features when compared to other Fe(II)-based single chain magnets (SCM). Cyano-bridged heterometallic {3d-4d} and {3d-5d} chains are formed upon assembling Fe(II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, [M(CN)(8)](4-) (M = Nb(IV), Mo(IV), W(IV)). X-ray diffraction (single-crystal and powder) measurements reveal that the [{(H(2)O)Fe(L(1))}{M(CN)(8)}{Fe(L(1))}](infinity) architectures consist of isomorphous 1-D polymeric structures based on the alternation of {Fe(L(1))}(2+) and {M(CN)(8)}(4-) units (L(1) stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged {Fe-Nb} exchange interaction to be antiferromagnetic with J = -20 cm(-1) deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Delta/k(B) = 74 K and tau(0) = 4.6 x 10(-11) s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i.e., abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient {Fe-Nb} exchange interaction and significant anisotropy of the {Fe(L(1))} unit. High field EPR and magnetization experiments have revealed for the parent compound [Fe(L(1))(H(2)O)(2)]Cl(2) a negative zero field splitting parameter of D approximately = -17 cm(-1). The crystal structure, magnetic behavior, and Mossbauer data for [Fe(L(1))(H(2)O)(2)]Cl(2) are also reported.

How Perfect Can Graphene Be

We have identified the cyclotron resonance response of the purest graphene ever investigated, which can be found in nature on the surface of bulk graphite, in the form of decoupled layers from the substrate material. Probing such flakes with Landau level spectroscopy in the THz range at very low magnetic fields, we demonstrate a superior electronic quality of these ultralow density layers (n_{0} approximately 3 x 10;{9} cm;{-2}) expressed by the carrier mobility in excess of 10;{7} cm;{2}/(V * s) or scattering time of tau approximately 20 ps. These parameters set new and surprisingly high limits for intrinsic properties of graphene and represent an important challenge for further developments of current graphene technologies.

Magnetic anisotropy of the antiferromagnetic ring [Cr8F8Piv16].

A new tetragonal (P42(1)2) crystalline form of [Cr8F8Piv16] (HPiv = pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the CrIII ions is antiferromagnetic (J = 12 cm(-1)) which results in a S = 0 spin ground state. The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S = 1 and S = 2, respectively) are D1 = 1.59(3) cm(-1) or 1.63 cm(-1) (from CTM and HFEPR, respectively) and D2 = 0.37 cm(-1) (from HFEPR). The dipolar contributions to the ZFS of the S = 1 and S = 2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1) degrees with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic CrIII rings.

EPR spectroscopy at very high field

Abstract A very high field EPR spectrometer, which can operate at magnetic fields up to 19 T, has been constructed. The first EPR spectra of a nitroxide free radical in solution are reported. A discussion of the applications of high frequency EPR (corresponding to the far infrared) is presented.

High-Frequency EPR Spectra of [Fe8O2(OH)12(tacn)6]Br8: A Critical Appraisal of the Barrier for the Reorientation of the Magnetization in Single-Molecule Magnets

A detailed multifrequency high-field-high-frequency EPR (95-285 GHz) study has been performed on the single-molecule magnet of formula [Fe8O2(OH)12(tacn)6]Br8⋅9 H2O, in which tacn=1,4,7-triazacyclononane. Polycrystalline powder spectra have allowed the estimation of the zero-field splitting parameters up to fourth order terms. The single-crystal spectra have provided the principal directions of the magnetic anisotropy of the cluster. These results have been compared with an evaluation of the intra-cluster dipolar contribution to the magnetic anisotropy; this suggests that single-ion anisotropy is the main contributor to the magnetic anisotropy. The role of the transverse magnetic anisotropy in determining the height of the barrier for the reversal of the magnetization is also discussed.

Ising-type magnetic anisotropy and single molecule magnet behaviour in mononuclear trigonal bipyramidal Co(II) complexes

The magnetic anisotropy of two pentacoordinate trigonal bipyramidal (C3v symmetry) Co(II) complexes, [Co(Me6tren)Cl]ClO4 (1) and [Co(Me6tren)Br]Br (2), was investigated and analysed by magnetic studies, high field multifrequency electron paramagnetic resonance (EPR) and ab initio calculations. Negative D parameters expressing an Ising-type anisotropy (easy axis of magnetization) were found experimentally for both complexes. Calculations led to D values very close to the experimental ones, which allows a robust rationalisation of the magnetic anisotropy in these complexes. The wavefunctions of the ground and the first four excited states reveal that they are strongly multideterminantal i.e. linear combinations of several determinants. The most important contribution to the spin orbit coupling between the ground and lowest excited states stabilizes the largest MS = ±3/2 components of the S = 3/2 state and therefore brings a large negative contribution to D. The analysis of the difference between the magnitudes of the anisotropy of the two complexes led to the conclusion that a large Ising anisotropy is preferred when weak σ-donating ligands are in the equatorial plane and strong π-donating ones are in axial positions; thus providing an efficient tool to chemists to predict the magnetic anisotropy in these types of complexes. The investigation of the magnetic behaviour of a single crystal of 1 by micro-SQUID shows, as expected, the presence of an easy axis of magnetization. The magnetic behaviour is consistent with quantum tunnelling of the magnetization mediated by intermolecular three-dimensional antiferromagnetic exchange interactions. Upon dilution of the Co(II) molecules in the isostructural Zn(II) compound, a blocking of the magnetization below 2 K is demonstrated; it results in an opening of the magnetization hysteresis loop in zero applied magnetic field.

Origin of the Magnetic Anisotropy in Heptacoordinate Ni-II and Co-II Complexes

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2 H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

[ReF6]2−: A Robust Module for the Design of Molecule‐Based Magnetic Materials

A facile synthesis of the [ReF6 ](2-) ion and its use as a building block to synthesize magnetic systems are reported. Using dc and ac magnetic susceptibility measurements, INS and EPR spectroscopies, the magnetic properties of the isolated [ReF6 ](2-) unit in (PPh4 )2 [ReF6 ]⋅2 H2 O (1) have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the one-dimensional coordination polymer [Zn(viz)4 (ReF6 )]∞ (2, viz=1-vinylimidazole), demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy-plane-type anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [Ni(viz)4 (ReF6 )]∞ (3) analogue in which the ferromagnetic Ni(II) -Re(IV) interaction (+10.8 cm(-1) ) dwarfs the coupling present in related cyanide-bridged systems. These results reveal [ReF6 ](2-) to be an unique new module for the design of molecule-based magnetic materials.