Single-crystal magnetic susceptibility and specific heat studies of the one-dimensional copper complex [PM·Cu(NO3)2·(H2O)2]n (PM = pyrimidine) show that it behaves like a uniform S = 1/2 antiferromagnetic Heisenberg chain, characterized by the exchange parameter J/kB = 36 K. Specific heat measurements in the applied magnetic field, however, reveal the formation of a field-induced spin excitation gap, whose magnitude depends on the magnitude and direction of the field. This behaviour is inconsistent with the ideal S = 1/2 Heisenberg chain. In the low-temperature region, a contribution to the susceptibility, approximately proportional to 1/T, is observed which varies strongly with the varying direction of the magnetic field. The field-induced gap and the 1/T contribution are largest for the same field direction. Previous observations of a field-induced gap in the related compounds copper benzoate and Yb4As3 have been explained by the alternating g tensor and alternating Dzyaloshinkii-Moriya interaction, producing an effective staggered magnetic field at the Cu and Yb ions. We apply this model to [PM·Cu(NO3)2·(H2O)2]n and obtain a consistent quantitative explanation of the low-temperature susceptibility, the field-induced gap and their dependence on the magnetic-field direction.

Neutron diffraction, specific heat and magnetization measurements were applied to study the magnetic structures in Mn0.88Fe0.12WO4 using single crystal and powder samples. A commensurate antiferromagnetic structure with a propagation vector k = (1/4,1/2,1/2) below 13.5 K and an incommensurate structure with k = (0.23,0.5,0.508) between 13.5 K and 15.5 K were found to be the dominating ordered states. The temperature behaviour of the propagation vector above the IC-C phase transition shows a logarithmic behaviour. An FeWO4-type ordering of spins with k≈(1/2,0,0) is observed below 19 K and coexists between 15.5 K and 13.5 K with the MnWO4-type structure. Our experimental results indicate a cluster-like FeWO4-type magnetic ordering on a diluted 3D lattice, which interpenetrates the dominating MnWO4-type spin ordering. Below 13 K, with the majority of spins ordered in the commensurate MnWO4-type structure, reflections which can be indexed to a unit cell doubled along the crystallographic a-axis appear again; however, these intensities cannot purely be related to Bragg diffraction by ordered local magnetic moments at the (Mn, Fe) positions.

We have observed superconductivity in platinum powder compacts with grain sizes in the submicrometer range at transition temperatures and critical magnetic fields as high as 20 mK and 18 mT. These values are more than one order of magnitude larger compared to our previous results for samples with larger (micrometer) grain size. For a better understanding of the superconducting properties of the granular platinum samples, we have also investigated various properties in the normal state. The submicrometer samples exhibit strong lattice strain and significantly smaller Debye temperatures compared to bulk platinum. Moreover, the ferromagnetic spin fluctuations of the conduction electrons are partially quenched in these compacts. On the basis of these results, we discuss the origin of superconductivity in granular platinum.

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