We present a model compound for the $S$=1/2 ferromagnetic Heisenberg chain composed of the verdazyl-based complex $[$Zn(hfac)$_2]$$[$4-Cl-$o$-Py-V-(4-F)$_2]$. $Ab$ $initio$ MO calculations indicate a predominant ferromagnetic interaction forming an $
S$=1/2 ferromagnetic chain. The magnetic susceptibility and specific heat indicate a phase transition to an AF order owing to the finite interchain couplings. We explain the magnetic susceptibility and magnetization curve above the phase transition temperature based on the $S$=1/2 ferromagnetic Heisenberg chain. The magnetization curve in the ordered phase is described by a conventional AF two-sublattice model. Furthermore, the obtained magnetic specific heat reproduces the almost temperature-independent behavior of the $S$=1/2 ferromagnetic Heisenberg chain. In the low-temperature region, the magnetic specific heat exhibits $sqrt{T}$ dependence, which is attributed to the energy dispersion in the ferromagnetic chain.
We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain [Cu(pym)(H$_2$O)$_4$]SiF$_6cdot$H$_2$O (pym = pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of stag
gered $g$ tensors, a rich low-temperature excitation spectrum, a staggered susceptibility and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in non-chiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sine-Gordon model accounts well for the form of the temperature-dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of [Cu(pym)(H$_2$O)$_4$]SiF$_6cdot$H$_2$O, particularly a uniform Dzyaloshinskii-Moriya coupling and a four-fold periodic staggered field.
Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far, ac susceptibility measurements revealed multiple superconducting transitions occurring in the Sr3Ru2O7 region cut from Sr3Ru2O7-Sr2RuO4 eutectic crystals. Based on various e
xperimental results, some of us proposed the scenario in which Sr2RuO4 thin slabs with a few layers of the RuO2 plane are embedded in the Sr3Ru2O7 region as stacking faults and multiple superconducting transitions arise from the distribution of the slab thickness. To examine this scenario, we measured the resistivity along the ab plane (rho_ab) using a Sr3Ru2O7-region sample cut from the eutectic crystal, as well as along the c axis (rho_c) using the same crystal. As a result, we detected resistance drops associated with superconductivity only in rho_ab, but not in rho_c. These results support the Sr2RuO4 thin-slab scenario. In addition, we measured the resistivity of a single crystal of pure Sr3Ru2O7 with very high quality and found that pure Sr3Ru2O7 does not exhibit superconductivity down to 15 mK.
