Low-temperature magnetic measurements of anS=1 linear-chain Heisenberg antiferromagnet

The temperature-dependent magnetic susceptibility, χ(T), of two pure samples of the S=1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$), commonly known as NENP, has been measured from approximately 300 K to 300 μK. Our measurements of χ(T) are in agreement with existing results of other researchers who worked above 1.2 K. Below 1.2 K, χ(T) increases with decreasing temperature. The results suggest that the low-temperature increase of χ(T) is not a consequence of a single source of paramagnetic impurities in the samples but may arise from S=1/2 end-chain interactions.