Colette Boskovic, Roland Bircher, Philip L. W. Tregenna-Piggott, Hans U. Güdel, Carley Paulsen, Wolfgang Wernsdorfer, Anne-Laure Barra, Eugene Khatsko, Antonia Neels and Helen Stoeckli-Evans
Ferromagnetic and Antiferromagnetic Intermolecular Interactions in a New Family of Mn₄ Complexes with an Energy Barrier to Magnetization Reversal
J. Am. Chem. Soc. 125, 14046-14058 (2003)                                               

Abstract: A new family of tetranuclear Mn complexes [Mn₄X₄L₄] (H₂L = salicylidene-2-ethanolamine; X = Cl (1) or Br (2)) and [Mn₄Cl₄(L')₄] (H₂L' = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1-3 possess a square-shaped core with ferromagnetic exchange interactions between the four Mn^III centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range -0.10 to -0.20 cm^–1 for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of ~1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of "exchange-biased single-molecule magnets", where the exchange bias is controlled by chemical and structural modifications.

Mark Murrie, Simon J. Teat, Helen Stoeckli-Evans and Hans U. Güdel
Synthesis and Characterization of a Cobalt(II) Single-Molecule Magnet
Angew. Chem. 115, 4801-4804 (2003)                 
Angew. Chem. Int. Ed. 42, 4653-4656 (2003)                 

Abstract: (Article without Abstract)
Graphical Abstract: Small and attractive: The proligand citric acid has been used to assemble a new anionic hexanuclear cobalt(II) cluster (see picture). The complex displays single-molecule magnet (SMM) behavior, with an energy barrier to the reorientation of magnetization of 26 K, the highest value to have been recorded for a non-manganese based SMM.

Mikhail Sofin, Hans-Ulrich Güdel, Roland Bircher, Eva-Maria Peters and Martin Jansen
Na₁₀Co₄O₁₀, an Oligooxocobaltate(II, III) with Unusual Magnetic Properties
Angew. Chem. 115, 3651-3653 (2003)                 
Angew. Chem. Int. Ed. 42, 3527-2529 (2003)                 

Abstract: (Article without Abstract)
Graphical Abstract: The novel mixed-valent oligooxocobaltate can be synthesized through a direct reaction by the azide/nitrate route. The magnetic properties of the compound are characterized by the strongly distinct hierarchy in the magnetic exchange interactions within the complex anion (see picture).

Stephan Heer, Olaf Lehmann, Markus Haase and Hans-Ulrich Güdel
Blue, Green, and Red Upconversion Emission from Lanthanide-Doped LuPO₄ and YbPO₄ Nanocrystals in a Transparent Colloidal Solution
Angew. Chem. 115, 3288-3291 (2003)                 
Angew. Chem. Int. Ed. 42, 3179-3182 (2003)                 

Abstract: (Article without Abstract)
Graphical Abstract: Light converter: The emission of visible light (Vis) upon excitation in the nearinfrared (NIR) has been observed for the first time in a transparent solution. Excitation at 975 nm (marked red in the picture) of optically clear colloidal solutions of LuPO₄:Yb³⁺, Tm³⁺, and YbPO₄:Er³⁺ nanocrystals in chloroform leads to visible upconversion luminescence in the blue, green, or red spectral regions.

Ch. Rüegg, N. Cavadini, A. Furrer, H.-U. Güdel, K. Krämer, H. Mutka, A. Wildes, K. Habicht and P. Vorderwisch
Bose–Einstein condensation of the triplet states in the magnetic insulator TlCuCl₃
Nature 423, 62-65 (2003)                 

Abstract: Bose–Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl₃ and TlCuCl₃), two Cu²⁺ ions are antiferromagnetically coupled to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose–Einstein condensation becomes theoretically possible. At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose–Einstein condensation occurs. Here we report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl₃ single crystals. We verify unambiguously the theoretically predicted gapless Goldstone mode characteristic of the Bose–Einstein condensation of the triplet states.

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