Y. Furukawa

Molecular Growth of a Core–Shell Polyoxometalate

The self-assembly of large metal oxide clusters usually proceeds via condensation steps thermodynamically driven by charge and nucleophilicity of the growing transient cluster fragments. Polyoxometalate (POM) chemistry has accumulated many strategies to interfere with these basic formation principles, aiming at both directed molecular growth and targeted functionalization by selective introduction of metal centers or organic moieties. POM structures integrating 3d or 4d transition-metal ions in particular attest to this approach, and they have led to a rich class of molecular materials ranging from molecular magnets to oxidation catalysts. In this context, polyoxotungstate clusters provide rigid and redox-stable scaffolds based on building-block-type fragments that are frequently derived from archetypal structures such as the Keggin, Dawson, or Lindqvist species. Additional heterometal cations coordinating to or interconnecting these nucleophilic structures are key to the reactivity and the electronic and magnetic characteristics of the resulting adducts. This situation is exemplified by the spherical {M72L30} Keplerate clusters that have been realized both as polymolybdate (M = Mo) and polytungstate (M = W) structures containing a variety of heterometal linkers (L = V, Cr, Fe, etc.). These clusters, comprising unique spin polytopes that represent molecular analogues of Kagom lattices, constitute structural platforms for subsequent reactions, ranging from redox reactions and partial heterometal exchange to condensations to oneand two-dimensional coordination networks—all of which alter the clusters magnetic characteristics while retaining the basic cluster structure. Moreover, recent development of POM-based single-molecule magnets raises the hope that magnetic POMs may find their way into areas such as molecular spintronics or quantum computing. However, the controlled growth of large metal oxide clusters remains elusive: precise prediction of the outcome is very difficult given the involvement of many, often labile, metal–oxygen bonds. The self-assembly mechanisms that underlie the formation of larger POMs in aqueous reaction solutions are barely established and are not compatible with the synthetic controls available in classical coordination chemistry, as exemplified by the use of molecular tectons, characterized by their specific connectivity constraints, in the rational production of supramolecular aggregates or porous metal–organic frameworks. We postulate that this roadblock in controlling the molecular growth steps in polyoxotungstate chemistry can be partially circumvented by kinetic control of competing reactions, as illustrated by the template-induced formation of a 4.3 nm manganese(III) polyoxotungstate cluster anion [Mn40P32W VI 224O888] 144 (1). The preparation of 1 starts from metastable [a-H2P2W12O48] 12 ({P2W12}), a hexavacant phosphotungstate derived from the plenary [a-P2W18O62] 6

Stabilization of an ambient-pressure collapsed tetragonal phase in CaFe[subscript 2]As[subscript 2] and tuning of the orthorhombic-antiferromagnetic transition temperature by over 70 K via control of nanoscale precipitates

We have found a remarkably large response of the transition temperature of CaFe2As2 single crystals grown from excess FeAs to annealing and quenching temperature. Whereas crystals that are annealed at 400 ◦ C exhibit a first-order phase transition from a high-temperature tetragonal to a low-temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960 ◦ C exhibit a transition from a high-temperature tetragonal phase to a low-temperature, nonmagnetic, collapsed tetragonal phase below 100 K. By use of temperature-dependent electrical resistivity, magnetic susceptibility, x-ray diffraction, M¨ ossbauer spectroscopy, and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing or quenching temperature from 400 ◦ to 850 ◦ C with the low-temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal, nonmagnetic for transition temperatures below 90 K. This suppression of the orthorhombic-antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal, nonmagnetic phase is similar to what has been observed for CaFe2As2 under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature-dependent width of formation of CaFe2As2 with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960 ◦ C there is a fine (of order 10 nm) semiuniform distribution of precipitate that can be associated with an average strain field, whereas for samples annealed at 400 ◦ C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe2As2 lattice.

Phase Transition from Antiferromagnetic Insulator to Ferromagnetic Metal in La2-xSrxCoO4 –Magnetization and NMR Studies–

Magnetic and zero-field nuclear magnetic resonance investigations have been made on the La 2- x Sr x CoO 4 system ( x =0–1.4), one of the isomorphous compounds of the superconducting cuprates. In the La-rich region ( x ≤0.5), the antiferromagnetic (AF) ordering persists and the internal magnetic field at 1.4 K remains almost constant ( H int ( Co)≃200 kOe), though the Neel temperature shows an initial strong decrease with a small amount of Sr substitution. When x increases further ( x ≥0.6), the AF state suddenly transforms to a ferromagnetic state and the Curie temperature shows a maximum ( T C =220 K) at x =0.9. Spontaneous magnetizations at T =4.2 K are, however, non-zero (2.2–1.6 µ B ) only in the range of x =0.7–0.9, and H int ( Co) strongly decreases from 190 kOe ( x =0.6) to below 40 kOe ( x =1.1). 59 Co spin-lattice relaxation time T 1 for x =0.7 and 0.9 follows the relation T 1 T =const., suggesting it is in a metallic state.

Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor

Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in most Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors, (π,0) and (0,π). Consequently, three different types of magnetic order are possible. Of these, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-doped and Co-doped CaKFe4As4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe4As4 structure. Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.Iron-based superconductors: making a hedgehog spin-vortex crystalThe magnetic texture of a new superconductor adopts a in-out spin, spin-vortex crystal motif, fulfilling theoretical predictions. Many iron-based superconductors have magnetic phases arising from combining two basic magnetic structures, but only two of three possible combinations had previously been observed. A team led by Paul Canfield of Iowa State University and Ames Laboratory have synthesised a material with the third type of magnetic structure called a hedgehog spin-vortex crystal. The authors began with a compound with spatial symmetry that could help stabilise the structure, but without magnetic order. By tuning the chemical composition they induced magnetism and successfully obtained the desired phase. The sensitivity of the magnetic state to the symmetry and composition indicates that different phases are energetically close, suggesting magnetic fluctuations may play a significant role in the physics of iron-based superconductors.

Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As75 NMR

We report

NMR study of ferromagnetism and superconductivity of RuSr2GdCu2O8

^{75}\mathrm{As}

31P-NMR study of low-energy spin excitations in spin ladder (VO)2P2O7 and spin dimer VO(HPO4)0.5H2O systems

nuclear magnetic resonance (NMR) studies on a new iron-based superconductor,

Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4spinel

{\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}

Magnetic exchange interactions in BaMn 2 As 2 : A case study of the J 1 - J 2 - J c Heisenberg model

, with

NMR study of antiferromagnetic spinel CoCo2O4 in paramagnetic and ordered state

{T}_{\mathrm{c}}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}35