Spin correlations at finite temperature in anS=1 one-dimensional antiferromagnet

Abstract
We have measured the temperature dependence of equal-time spin correlations in the quasi-one-dimensional S=1 antiferromagnet, Ni(C2 H8 N2 )2 NO2 ClO4 (NENP), by magnetic neutron scattering. The temperature dependence of the correlation length can be described as exponentially activated with an activation energy similar to the Haldane gaps in NENP.