High-field ESR measurements of a bond alternating Heisenberg chain system Ni2(μ-N3)3(dpt)2(ClO4)

High-field electron spin resonance has been measured on a Ni 2 (μ-N 3 ) 3 (dpt) 2 (ClO 4 ) single crystal, which is an S =1 Heisenberg antiferromagnet with bond alternation, in the frequency region from 50 to 370 GHz using pulsed magnetic fields up to 16 T. The g -values for H perpendicular and parallel to the single-crystal plane were determined to be 2.19 and 2.13, respectively. By analysing our results on frequency dependence at 20 K by the simple dimer model considering the singlet and triplet states, we were able to determine the single-ion anisotropy of the system as | D |=10.6 K and | E |=3.1 K for the first time. The spin gap was estimated to be 93 K from the analysis of the temperature dependence of the integrated intensity. These results will be discussed in connection with other S =1 Heisenberg antiferromagnets with bond alternation.