Field-induced magnetic ordering in an alternating Heisenberg chain F ₅PNN

We have found the field-induced magnetic ordering of an S = 1/2 alternating linear Heisenberg antiferromagnet F₅PNN by measuring magnetic field dependence of the specific heat. A cusp-like anomaly is observed below 0.2 K between 3.5 and 6 T. From the extrapolation of H vs. T phase diagram to 0 K, the lower and the upper critical fields are expected to be about 3 and 7 T, respectively. The phase boundary is almost symmetric with respect to a horizontal line of H = 5 T. This behavior is consistent with the previously observed magnetization measurement in which magnetization began to increase at about 3 T and saturated at about 7 T.