FERROMAGNETISM IN 1D AND 2D TRIANGULAR NICKEL(II)-BASED COMPOUNDS

We discuss in this paper the structure and magnetic properties of the one- and two-dimensional nickel(II) hydroxynitrates, Ni(OH)(NO3)H2O an d Ni-2(OH)(2+x)(NO3)(2-x) (x=0.64, 0.74, 1.14), respectively. The form er is described as a new double-chain system with two Heisenberg in-ch ain interactions between S=1 spins, and a zero-field splitting contrib ution arising from single-ion anisotropy. From numerical computations up to 10 spin units, the exchange interactions of both systems are sho wn to be ferromagnetic, i.e. J(1)=+26.9 K, and J(2)=+6.9 K, with g=2.2 2, and D=+1.76 K. At very low temperature, the magnetization and neutr on diffraction data indicate the occurrence of long-range antiferromag netic order with a metamagnetic transition for H-c=0.25 T. The non-sto ichiometric layered compounds Ni-2(OH)(2+x)(NO3)(2-x) show the signatu re of 2d ferromagnetism, as demonstrated for the hydroxide parent comp ound NI(OH),. From a comparison of these lamellar derivatives, it appe ars that the increase in the interlayer distance when (OH)(-) is subst ituted by (NO3)(-), associated with turbostratic disorder, strongly af fects the 3d long-range order. Indeed, whereas Ni(OH)(2) is a metamagn et with T-N=25.8 K, Ni-2(OH)(2+x)(NO3)(2-x) exhibits a spin-glass-like behavior below 10 K.