Shang-Da Jiang

An organometallic single-ion magnet.

An organometallic single-ion magnet is synthesized with only 19 non-hydrogen atoms featuring an erbium ion sandwiched by two different aromatic ligands. This molecule displays a butterfly-shaped hysteresis loop at 1.8 K up to even 5 K. Alternating-current (ac) susceptibility measurement reveals the existence of two thermally activated magnetic relaxation processes with the energy barriers as high as 197 and 323 K, respectively.

A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior†

Single-molecule magnets (SMMs) have received much attention owing to their quantum tunneling and slow relaxation. These behaviors can be observed when the molecule has large ground spin state with a uniaxial magnetic anisotropy, namely negative zero-field splitting (ZFS) parameter D. Besides a large ground state, to increase the SMMs energy barrier and blocking temperature, it is of fundamental importance to control the magnetic anisotropy. It is also of great complexity to understand the conditions that determine the anisotropy or zero-field splitting properties for a cluster. As a result, SMMs containing only one spin carrier are of great interest because of the simplification in the analysis of local anisotropy and ZFS. Recently, some molecules with isolated 3d or 4f metal ions were observed to show a direct-current (dc) field-induced slow magnetic relaxation. Studies show that this kind of dc-field-dependent relaxation phenomenon is thermally activated, and the quench of fast quantum tunneling by the dc field could give rise to the slow relaxation. However, as commented by Benelli and Gatteschi, it is an unexpected and puzzling behavior, and the underlying mechanism is still unclear. Ishikawa et al. reported that the phthalocyanine (Pc) double-decker anion complexes [Pc2Ln] with a single Tb or Dy ion show slow relaxation without a dc field. A single Er ion in the polyoxometallate system [ErW10O36] 9 showed similar relaxation behavior in zero static magnetic field. The first single-actinide complex [U(Ph2BPz2)3] (containing the U III ion; Ph2BPz2 = diphenylbis(pyrazolylborate)) reported by Rinehart and Long shows a similar slow relaxation. Owing to the single-ion features, these complexes could be called “single-ion magnets”. In these complexes, ligand fields with a high-order single axis Cn (n> 2; n = 4 for Ref. [8, 9] and n = 3 for Ref. [10]) split the (2J + 1) degenerate ground states into new sublevels, which produces a uniaxial anisotropy, thus giving rise to a higher energy barrier for relaxation. The Dy ion, which possesses a Kramers ground state of H15/2, is an appealing paramagnetic source for the construction of SMMs in a suitable ligand-field symmetry and strength. Moreover, among reported SMMs, some of them are clusters containing Dy : Dy2, [11] Dy3, [12] Dy4, [13] Dy5, [14] Dy-containing chain, and tens of Dycontaining 3d–4f clusters. All three reported types of single-ion magnets are found with a high-order single axis defining the local symmetry. Pursuing this clue, we synthesized a series of mononuclear Ln compounds with a local symmetry close to D4d. Crystal analysis shows that the isomorphous complexes consist of a neutral mononuclear [Ln(acac)3(H2O)2] complex (Ln = Dy, Ho, Er, acac = acetylacetonate) together with an uncoordinated water molecule and an uncoordinated ethanol molecule (Figure 1a). In the Dy complex, Dy is coordinated by eight oxygen atoms with Dy O bonds of 2.311–2.434 , six of which come from the acetylacetonate ligand and two from coordinated water molecules. The eight oxygen atoms form an approximately square-antiprismatic coordination polyhedron, and the local symmetry of Dy is nearly D4d (Figure 1b). Actually, a similar lanthanide acac complex was firstly reported in 1968, but the uncoordinated water and

Series of Lanthanide Organometallic Single-Ion Magnets

The synthesis, structures, and magnetic properties of a series of lanthanide organometallic mixed sandwich molecules, (Cp*)Ln(COT), are investigated, where Cp* is the pentamethylcyclopentadiene anion and COT is the cyclooctatetraene dianion and Ln represents Tb(III), Dy(III), Ho(III), Er(III), and Tm(III). Among the five complexes, Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex. Both Dy and Ho complexes show obvious quantum tunneling magnetization relaxation in the absence of a static field. The diluted Ho complex behaves two sets of thermally activated relaxation as we reported previously in Er due to the COT ring static disorder. A stair-shaped hysteresis for the Er compound can be observed at 1.6 K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling decoherence relaxation rate increases from Er to Ho to Dy, which may be caused by the relative increase of transverse anisotropy coming from the larger tilting of the two aromatic rings within the molecule. The fine electronic structure is analyzed with ligand-field theory employing the effective Hamiltonian method. The zero-field splitting is determined to be Ising type, and the energy gap between the ground state and the first excited one is consistent with the barrier obtained by Arrhenius analysis.

Zero-field slow magnetic relaxation from single Co(II) ion: a transition metal single-molecule magnet with high anisotropy barrier

An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.

Direct measurement of dysprosium(III)···dysprosium(III) interactions in a single-molecule magnet.

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here we are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.

Room temperature quantum coherence in a potential molecular qubit

The successful development of a quantum computer would change the world, and current internet encryption methods would cease to function. However, no working quantum computer that even begins to rival conventional computers has been developed yet, which is due to the lack of suitable quantum bits. A key characteristic of a quantum bit is the coherence time. Transition metal complexes are very promising quantum bits, owing to their facile surface deposition and their chemical tunability. However, reported quantum coherence times have been unimpressive. Here we report very long quantum coherence times for a transition metal complex of 68 μs at low temperature (qubit figure of merit QM=3,400) and 1 μs at room temperature, much higher than previously reported values for such systems. We show that this achievement is because of the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.

Angular-Resolved Magnetometry Beyond Triclinic Crystals: Out-of-Equilibrium Studies of Cp*ErCOT Single-Molecule Magnet†

Angular-resolved single-crystal magnetometry is a key tool to characterise lanthanide-based materials with low symmetry, for which conjectures based on idealised geometries can be totally misleading. Unfortunately the technique is strictly successful only for triclinic structures, thus reducing significantly its application. By collecting out-of-equilibrium magnetisation data the technique was extended to the orthorhombic organometallic Cp*ErCOT single-molecule magnet (SMM), thus allowing for the first time the reconstruction of the molecular anisotropy tensor notwithstanding the two molecular orientations in the crystal lattice. The results, flanked by state-of-the-art ab initio calculations, confirmed the expected orientation of the molecular easy axis of magnetisation and thus validated the synthetic strategy based on organometallic complexes of a single lanthanide ion.

Spectroscopic determination of crystal field splittings in lanthanide double deckers

We have investigated the crystal field splitting in the archetypal lanthanide-based single-ion magnets and related complexes (NBu4)+[LnPc2]−·2dmf (Ln = Dy, Ho, Er; dmf = N,N-dimethylformamide) by means of far infrared and inelastic neutron scattering spectroscopies. In each case, we have found several features corresponding to direct crystal field transitions within the ground multiplet. The observation of three independent peaks in the holmium derivative enabled us to derive crystal field splitting parameters. In addition, we have carried out CASSCF calculations. We show that exploiting the interplay of CASSCF calculation (for the composition of the states) and advanced spectroscopic measurements (for accurate determination of the energies) is a very powerful approach to gain insight into the electronic structure of lanthanide-based single-molecule magnets.

Hydrothermal synthesis, structures and magnetic studies of transition metal sulfates containing hydrazine.

By employing hydrothermal method, six transition metal sulfates containing hydrazine (N(2)H(4)) have been obtained: [M(SO(4))(2)(N(2)H(5))(2)](n) (M = Mn(1), Co(2), Ni(3)) and [M(N(2)H(4))SO(4)](n) (M = Mn(4), Co(5), Ni(6)). Their crystal structures and magnetic properties have been investigated experimentally and theoretically. Compounds 1-3 consist of one-dimensional sulfate bridged homometallic chains with protonated hydrazine molecule as terminal ligand, and compounds 4-6 are hydrazing-sulfate mixed bridged homometallic three-dimensional frameworks. Compounds 1-6 exhibit antiferromagnetic coupling between M(2+) ions, but their magnetic properties differ at low temperatures because of the different single-ion anisotropy and crystal structures. The magnetostructural correlations and the magnetic coupling mechanism are analyzed by density functional theory calculations (DFT).