Effects of Fe-substitution on the crystal structure and magnetic correlations of the geometrically frustrated antiferromagnets YBaCo4-xFexO7+{delta} (x = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) have been studied by neutron diffraction, Mossbauer spectroscopy
, and ac susceptibility. The compounds YBaCo4-xFexO7+{delta} have a special layered-type crystal structure with an alternating Kagome (6c site) and triangular (2a site) layers along the c axis. Fe3+ ions are found to be substituted at both the crystallographic 2a and 6c sites of Co ions. Mossbauer results show a high spin state of Fe3+ ions in a tetrahedral coordination. A reduction in the distortion of the Kagome lattice has been observed with the Fe-substitution. The correlation length of the short-range antiferromagnetic ordering decreases with the Fe-substitution. The sharpness of the magnetic transition also decreases with the Fe-substitution.
Field-induced magnetic ordering in the Haldane chain compound SrNi$_{2}$V$_{2}$O$_{8}$ and effect of anisotropy have been investigated using single crystals. Static susceptibility, inelastic neutron scattering, high-field magnetization, and low tempe
rature heat-capacity studies confirm a non-magnetic spin-singlet ground state and a gap between the singlet ground state and triplet excited states. The intra-chain exchange interaction is estimated to be $J sim 8.9{pm}$0.1 meV. Splitting of the dispersions into two modes with minimum energies 1.57 and 2.58 meV confirms the existence of single-ion anisotropy $D(S^z){^2}$. The value of {it D} is estimated to be $-0.51{pm}0.01$ meV and the easy axis is found to be along the crystallographic {it c}-axis. Field-induced magnetic ordering has been found with two critical fields [$mu_0H_c^{perp c} = 12.0{pm}$0.2 T and $mu_0H_c^{parallel c} = 20.8{pm}$0.5 T at 4.2 K]. Field-induced three-dimensional magnetic ordering above the critical fields is evident from the heat-capacity, susceptibility, and high-field magnetization study. The Phase diagram in the {it H-T} plane has been obtained from the high-field magnetization. The observed results are discussed in the light of theoretical predictions as well as earlier experimental reports on Haldane chain compounds.
