HIGH-FIELD ELECTRON SPIN RESONANCE STUDIES OF A NEW HALDANE SPIN

The investigation of low-dimensional quantum magnets in strong magnetic fields is currently a very attractive area of research. Among others, S=1 Heisenberg antiferromagnets are particularly interesting objects for investigation. In accordance with Haldane’s conjecture [1], the ground state of an isotropic S=1 chain is a spin singlet, separated from the first excited state by an energy gap ∆~0.4 J (where J is the intrachain spin interaction). Although the Haldane conjecture is widely accepted, there are many open questions regarding high-field properties of Haldane materials. Here we report on a synthesis and ESR characterization of a new S=1 spin chain material Ni(C2H8N2)2NO2Cl)4 (hereafter NENB). The compound was synthesized in similar fashion to NENP [2]. Ni(BF4)2·6H2O and 3 equivalents of 1,2-ethanediamine (en) were dissolved in water and the solutions combined. The resulting purple solution grew large crystals of [Ni(en)3](BF4)2 over the course of a few days. The product was isolated and air-dried to give a greater than 65 % yield of purple crystals. NENB and the Zn-doped material were then prepared as follows: 2 [Ni(en)3](BF4)2 + (1-x) Ni(BF4)2·6H2O + x Zn(BF4)2·6H2O + 3 NaNO2 [Ni1-yZny(en)2(NO2)](BF4) The reagents were combined in aqueous solution and the water allowed to evaporate slowly over the course of several weeks, resulting in the formation of large (>50 mg) single crystals of NENB. Crystals were harvested at low yields (typically 20-40%