Observation of the S=1/2 Degrees of Freedom in an Undoped S=1 Linear-Chain Heisenberg Antiferromagnet

From electron spin resonance (ESR) and magnetization measurements on a single crystal sample of the S = 1 linear-chain Heisenberg antiferromagnet Ni(C2H8N2)2NO2ClO4 (NENP), we have found the presence of the S = 1/2 degrees of freedom. We observed six ESR lines in this sample, four of which are the same as those observed in NENP doped with the nonmagnetic atoms by Glarum et al. The temperature dependence of the intensity of the ESR signal is compared with the recent theory presented by Mitra et al.