High pressure studies in MnSi suggest the existence of a non-Fermi liquid state without quantum criticality. The observation of partial magnetic order in a small pocket of the pressure versus temperature phase diagram of MnSi has additionally inspired several proposals of complex spin textures in chiral magnets. We used neutron scattering to observe the formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortices, under applied magnetic fields in metallic and semiconducting B20 compounds. In strongly disordered systems the skyrmion lattice is hysteretic and extends over a large temperature range. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of spin order composed of topologically stable spin textures.

A series of homo- and heterotrinuclear complexes containing three face-sharing octahedra has been synthesized by using the pendent arm macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L0H3, and 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, LH3:  [{L0NiII}2NiII] (1) and [{LCoIII}2CoIII](PF6)3 (2); [{LCoIII}2Ni] n + (n = 2 (3), 3 (4), 4 (5)); [{LNi}2CoIII] n + (n = 1 (6), 2 (7), 3 (8)) and its linkage isomers [{LNi}Ni{CoIIIL}] n + (n = 1 (9), 2 (10), 3 (11)) and, finally, the complexes [{LNi}2Ni] n + (n = 0 (12), 1 (13), 2 (14), 3 (15)). In complex 1 three octahedral NiII ions form a linear array with two terminal [L0NiII]- moieties in a facial N3O3 donor set and a central NiII ion which is connected to the terminal ions via six phenolate bridging pendent arms of L0. In complexes 2−15 the three metal ions are always in the same ligand matrix yielding an N3M(μ-S)3M(μ-S)3MN3 first-coordination sphere regardless of the nature of the metal ions (nickel or cobalt) or their formal oxidation states. From temperature dependent magnetic susceptibility measurements it has been determined that 1 has an S = 3 ground state whereas in 12 it is S = 1. In order to understand this difference in exchange coupling (ferromagnetic in 1 and antiferromagnetic in 12) in two apparently very similar complexes the magnetic properties of 2−15 have been investigated. Complex 3 has an S = 1 and 4 an S = 1/2, and 5 is diamagnetic (S = 0) as is its isoelectronic counterpart 2. This indicates the availability of the oxidation states II, III, and IV of the central NiS6 unit. In the isostructural complexes 6, 7, and 8, two terminal nickel ions are bridged by a central diamagnetic CoIII. The exchange coupling between two terminal paramagnetic nickel ions was studied as a function of their formal oxidation state. In 6 the two NiII ions are ferromagnetically coupled (S = 2); the mixed-valent NiIINiIII species 7 has an S = 3/2 ground state and in 8 most probably two NiIII ions (d7 low spin) give rise to an S = 1 ground state. In contrast, in the series 9, 10, and 11 where two nickel ions are in a position adjacent to each other 9 has an S = 0 (antiferromagnetic coupling), but in the mixed-valent complex 10 an S = 3/2 ground state (ferromagnetic coupling) is observed. In 11 an S = 1 ground state prevails which may be achieved by ferromagnetic coupling between two NiIII ions. For the trinuclear nickel complexes 12−15 an S = 1 ground state has been determined for 12, an S = 3/2 for the mixed valent complex 13, and an S = 2 for 14, and 15 exhibits an S = 3/2 ground state. The Goodenough−Kanamori rules do not provide a consistent explanation for the observed ground states in all cases. The concept of double exchange, originally introduced by Zener in 1951, appears to provide a more appropriate description for the mixed-valent species 7, 10, 13, 14, and 15. This picture is corroborated by the electrochemistry and EPR spectroscopy of complexes.

A series of heterodinuclear complexes CunM, where M = Cr(III) (1), Mn(III) (2), Mn(II) (3), Fe(III) (4), Co(II) (5), Co(IlI) (6), Ni(II) (7), Cu(II) (8), and Zn(II) (9), containing the oximato dianion (Dopn* 12~) as bridging ligand and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as one of the two capping ligands have been synthesized by using the [Cu(DopnH)]+ cation (H2Dopn = 3,9-dimethyl-4,8-diazaundeca-3,8-diene-2,10-dione dioxime) as a ligand for the different ML"+ centers. The compounds have been characterized on the basis of IR, electronic, and EPR spectroscopy and variable-temperature (2-295 K) magnetic susceptibility measurements. The dinuclear complexes are quasi-isostructural with the copper(H) ion in a distorted square pyramidal environment, CuN40, and the M ion, except for that in 8, is six-coordinate with the MN3O3 or MN3O2CI coordination sphere. For M = Cu (8), the coordination geometry of M with the CuN302 chromophore is also square pyramidal. The crystal and molecular structures of the compounds [(Dopn)CuI1(OH2)CrIII(OCH3)L](C104)2,H20 (1) and [(Dopn)Cuu(u-CH3COO)-MnnIL] (C104)r2H20 (2) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system space group P2i/n, with cell constants a = 13.096(3) A, b= 17.933(4) A, c= 15.994(3) A, /3= 113.49(3)°, V= 3444.9(13) A3 4, and Z = 4. The structure consists of oximato-bridged CunCrIn dications and noncoordinated perchlorate anions, with a Cu-• -Cr distance of 3.86 A. The crystal data for 2 are as follows: orthorhombic, space group Z>212121, a = 12.275(4) A,b = 14.171(9) A, c = 19.780(3) A, V -3441(2) A3, Z = 4. The structure consists of a six-coordinate Mn(III) center, MnN3C>3, and the copper(II) center has an N40 donor set. An acetate group bridges the manganese and copper ions with a Cu-• -Mn separation of 3.54 A. A low-quality X-ray structure determination for the CunFein complex is also reported. Analysis of the susceptibility data yields a strong antiferromagnetic interaction (27 = -596 cm-1) between adjacent Cu(II) ions in 8, showing once again that bridging oximes are good mediators for exchange interactions. The strength of the effective antiferromagnetic interaction decreases with increasing number of unpaired electrons in this series, 8>7>5>4~3.Moderately strong ferromagnetic interactions have been observed for CunMnIU (2) (27 = +109 cm-1) and CunCrin (1) (27 = +37 cm-1). A qualitative rationale has been provided for the difference in magnetic behaviors. The X-band EPR spectra (3-77 K) have been measured to establish the ground states of the dinuclear complexes. Well-resolved S = 2 EPR spectra for different heterometal systems have been observed.

Two series of linear homo- and heterotrinuclear complexes, MnIIIMIIMnIII and MnIVMIIMnIV, where MII represents MnII (1 and 5), NiII (2 and 6), CuII (3 and 7), or ZnII (4 and 8), containing three dimethylglyoximato dianions (dmg2-) as bridging ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand for the terminal MnIII or MnIV ions, have been synthesized. Compounds 1−8 have been characterized on the basis of elemental analyses, IR, UV−vis, and EPR spectroscopy, and variable-temperature (2−295 K) magnetic susceptibility measurements. The trinuclear complexes are quasi-isostructural with the terminal manganese ions in a distorted octahedral environment, MnIII/IVN3O3, and the divalent metal ions M are six-coordinate with the MIIN6 chromophore. The molecular structures of the compounds [LMnIII{(μ-dmg)3MnII}MnIIIL](ClO4)2 (1) and [LMnIII{(μ-dmg)3CuII}MnIIIL](ClO4)2 (3) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system, space group C2/c, with cell constants a = 32.472(6) Å, b = 9.058(2) Å, c = 16.729(3) Å, β = 107.60(3)°, V = 4690.2(16) Å3, and Z = 4. The crystal data for 3 are as follows:  monoclinic, space group C2/c, a = 32.686(5) Å, b = 8.870(1) Å, c = 16.867(2) Å, β = 108.65(1), V = 4633(1) Å3, and Z = 4. Analyses of the susceptibility data indicate the presence of weak to moderate exchange interactions, both ferro- and antiferromagnetic, between the paramagnetic centers. It has been conclusively demonstrated that there are indeed two different coupling constants, J = J 12 = J 23 and J 13, operative in these linear trinuclear complexes. J 13 represents the exchange interaction between two terminal paramagnetic centers separated by a distance of ∼7 Å. The effect of J 13 on the energy-splitting pattern has been demonstrated by the variability of the ground states. A qualitative rationale has been provided for the difference in magnetic behaviors. The cyclic voltammograms of complexes 1−8 reveal two reversible and two quasireversible one-electron redox processes. The central divalent metal ion in these complexes is redox-inactive.

The ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane (H(3)L) has been synthesized and its coordination properties toward Cu(II), Ni(II), Co(II), and Mn(II) in the presence of air have been investigated. Copper(II) yields a mononuclear complex, [Cu(H(2)L)](ClO(4)) (1), cobalt(II) and manganese(II) ions yield mixed-valence Co(III)(2)Co(II) (2a) and Mn(II)(2)Mn(III) (4) complexes, whereas nickel(II) produces a tetranuclear [Ni(4)(HL)(3)](2+) (3) complex. The complexes have been structurally, magnetochemically, and spectroscopically characterized. Complex 3, a planar trigonal-shaped tetranuclear Ni(II) species, exhibits irregular spin-ladder. Variable-temperature (2-290 K) magnetic susceptibility analysis of 3 demonstrates antiferromagnetic exchange interactions (J = -13.4 cm(-1)) between the neighboring Ni(II) ions, which lead to the ground-state S(t) = 2.0 owing to the topology of the spin-carriers in 3. A bulk ferromaganetic interaction (J = +2 cm(-1)) is prevailing between the neighboring high-spin Mn(II) and high-spin Mn(III) ions leading to a ground state of S(t) = 7.0 for 4. The large ground-state spin value of S(t) = 7.0 has been confirmed by magnetization measurements at applied magnetic fields of 1, 4 and 7 T. A bridging monomethyl carbonato ligand formation occurs through an efficient CO(2) uptake from air in methanolic solutions containing a base in the case of complex 4.