Carley Paulsen

TWO-DIMENSIONAL NITROXIDE-BASED MOLECULAR MAGNETIC MATERIALS

Manganese(II) complexes of imidazolyl-substituted chelating nitronylnitroxides are characterized by a stereoselective arrangement of the metal centers and the ligands to give a honeycomblike layer structure (shown on the right). The magnetic properties at high temperature indicate the presence of a ferrimagnetic structure within the layers. At low temperatures, ferromagnetic ordering is observed.

The molecular approach to nanoscale magnetism

Abstract Molecular clusters of paramagnetic metal ions have been widely investigated as model for magnetism at the nanoscale, especially for quantum effects like the tunneling of the magnetic moment. We present here some recent results obtained on derivatives of the well-known Mn12 cluster, especially on the half-integer spin compounds. The role of the transverse anisotropy in the dynamics of the magnetization is here elucidated through the comparison of the tunneling rate of the magnetization in two Fe8 cluster compounds, which differ only in the transverse anisotropy. Local dipolar fields and nuclear hyperfine fields have also revealed to strongly affect the relaxation in the pure tunneling regime and recent experiment has allowed to determine the intrinsic linewidth of the tunneling resonance. The transverse field dependence of the relaxation rate of Fe8 has revealed oscillations that are analog to the topological constructive–destructive interference of the spin phase (Berry phase) and we review here some very recent results. The magnetic behavior of antiferromagnetic ring-shaped clusters is also discussed for their potential interest as models for antiferromagnetic particles. Some recent results obtained by other chemists in the synthesis of large spin clusters are also reviewed.

Studies of a nickel-based single molecule magnet: resonant quantum tunnelling in an S = 12 moleculeElectronic supplementary information (ESI) available: saturation magnetisation at 150 mK; 1/χ′ vs. T. See http://www.rsc.org/suppdata/cc/b1/b108894g/

A cyclic complex [Ni(12)(chp)(12)(O(2)CMe)(12)(thf)(6)(H(2)O)(6)] (1) has been synthesised and studied (chp=6-chloro-2-pyridonate). Complex 1 exhibits ferromagnetic exchange between the S=1 centres, giving an S=12 spin ground state. Detailed studies demonstrate that it is a single-molecule magnet with an energy barrier of approximately 10 K for reorientation of magnetisation. Resonant quantum tunnelling is also observed. The field between resonances allows accurate measurement of D, which is 0.067 K. Inelastic neutron scattering studies have allowed exchange parameters to be derived accurately, which was impossible from susceptibility data alone. Three exchange interactions are required: two ferromagnetic nearest neighbour interactions of approximately 11 and 2 cm(-1) and an anti-ferromagnetic next nearest neighbour interaction of -0.9 cm(-1).

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.

Studies of a Nickel-Based Single-Molecule Magnet

A cyclic complex [Ni12(chp)12(O2CMe)12(thf)6(H2O)6] (1) has been synthesised and studied (chp=6-chloro-2-pyridonate). Complex 1 exhibits ferromagnetic exchange between the S=1 centres, giving an S=12 spin ground state. Detailed studies demonstrate that it is a single-molecule magnet with an energy barrier of approximately 10 K for reorientation of magnetisation. Resonant quantum tunnelling is also observed. The field between resonances allows accurate measurement of D, which is 0.067 K. Inelastic neutron scattering studies have allowed exchange parameters to be derived accurately, which was impossible from susceptibility data alone. Three exchange interactions are required: two ferromagnetic nearest neighbour interactions of approximately 11 and 2 cm−1 and an anti-ferromagnetic next nearest neighbour interaction of −0.9 cm−1.

Observation of the Distribution of Molecular Spin States by Resonant Quantum Tunneling of the Magnetization

Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(\xi_H) of magnetic energy bias \xi_H

Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets

acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(\xi_H) (around the resonant condition \xi_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.

Control of the structural dimensionality in hydrogen-bonded self-assemblies of open-shell molecules. Extension of intermolecular ferromagnetic interactions in α-phenyl nitronyl nitroxide radicals into three dimensions

The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) centers (S(Re) = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)](2-) and [ReBr4(ox)](2-) centers, originating from a combination of spin-orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.

Low-temperature properties of the heavy-fermion compound CeRu2Si2 at the metamagnetic transition

The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different hydrogen-bonded molecular self-assemblies with distinct dimensionalities; the mono-ortho-substituted isomer is most remarkable since it shows a three-dimensional network of weak hydrogen bonds exhibiting a bulk ferromagnetic transition at 0.45 K.

Mesoscopic quantum tunneling of the magnetization

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu2Si2 are reported. The field variation of the linear term of the specific heat γ derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT → 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB →B*. Similar experiments performed on a single-crystalline Ce0.95La0.05Ru2Si2 sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu2Si2 approaches a magnetic instability atB* forT → 0.