First-Principles Bottom-Up Study of 1D to 3D Magnetic Transformation in the Copper Pyrazine Dinitrate S = 1/2 Antiferromagnetic Crystal

On the basis of magnetic susceptibility and heat capacity data, copper pyrazine dinitrate crystal [abbreviated CuPz(NO3)2] has long been considered a good prototype for S = 1/2 antiferromagnetic (AFM) Heisenberg chain behavior down to 0.05 K. However, a recent muon-spin rotation experiment indicated the presence of a previously unnoticed 1D to 3D magnetic transition below 0.107 K. Our aim in this work is to perform a rigorous quantitative study of the mechanism of this 1D−3D magnetic transformation, by doing a first-principles bottom-up study of the CuPz(NO3)2 crystal at 158 K, where the magnetic properties are clearly 1D, and at 2 K, at which the neutron structure (reported in this work) is considered nearly identical with that below 0.1 K (due to small thermal effects). A change in the magnetic topology is found between these two structures: at 158 K, there are isolated AFM spin chains (J intra = −5.23 cm−1), while at 2 K, the magnetic chains (J intra = −5.96 cm−1) weakly interact (the largest of the J inter parameters is −0.09 cm−1). This change is caused by thermal contraction upon cooling (no crystallographic phase transition is detected down to 2 K, and one will not likely occur below that temperature). The computed and experimental magnetic susceptibility χ(T) curves are nearly identical. The calculated heat capacity C p(T) curve has a maximum at 6.92 K, close to the 5.20 K maximum found in the experimental curve at zero external field. In spite of the 3D magnetic topology of the crystal at low temperature, the magnetic susceptibility and heat capacity curves behave as a pure 1D AFM chain in all regions because of the large J intra/J inter ratio (66.2 in absolute value) and the effect of including the J inter interactions will not be easily appreciated in any of these experiments. The impact of the presence of odd- and even-membered regular AFM finite chains in the CuPz(NO3)2 crystal has also been evaluated. Odd-membered interacting chains produce an increase in both χ(T) and C p(T) curves when the temperature is very close to zero, in agreement with the experimental observations, while even-membered chains produce a small shoulder in the C p(T) curve between 0.8 and 5 K. No changes are seen in the remaining regions. Concerning the spin gap, odd-membered chains present a quasi-zero gap but the finite even-membered chains still have a sizable one. Finally, the effect of increasing the magnitude of J inter was investigated by fixing the value of J int