Exchange constants in molecule-based magnets derived from density functional methods

Abstract

Cu(pyz)(NO3)2 is a quasi one-dimensional molecular antiferromagnet that exhibits three dimensional long-range magnetic order below TN = 110 mK due to the presence of weak inter-chain exchange couplings. Here we compare calculations of the three largest exchange coupling constants in this system using two techniques based on plane-wave basis-set density functional theory: (i) a dimer fragment approach and (ii) an approach using periodic boundary conditions. The calculated values of the large intrachain coupling constant are found to be consistent with experiment, showing the expected level of variation between different techniques and implementations. However, the interchain coupling constants are found to be smaller than the current limits on the resolution of the calculations. This is due to the computational limitations on convergence of absolute energy differences with respect to basis set, which are larger than the inter-chain couplings themselves. Our results imply that errors resulting from such limitations are inherent in the evaluation of small exchange constants in systems of this sort, and that many previously reported results should therefore be treated with caution.

Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnet

“…DFT indicates further that all of the subdominant exchange parameters are small. Although this places them below the resolution limits of our calculations [41], it also supports the experimental analysis above. We draw attention to the trend visible in DFT that compression of the axial bonds enhances J 3 strongly, from 60 mK at ambient pressure to 1.3 K at 20 kbar, without affecting J 1 or J 2 significantly.…”

Anomalous magnetic exchange in a dimerized quantum-magnet composed of unlike spin species

“…By considering the DFT energies corresponding to several collinear spin configurations, we calculated the exchange constants associated with each of the exchange pathways. The calculated exchange constants depend very strongly on the value used for the Hubbard U , as has previously been found for systems based on Cu [43] and V [44]. For U = 5 eV on both the Cu and V dorbitals, we obtained a value J 0 = 24.7(6) K for the intradimer exchange, which is broadly consistent with the experimental value.…”

Anomalous magnetic exchange in a dimerized quantum magnet composed of unlike spin species

“…By considering the DFT energies corresponding to several collinear spin configurations, we calculated the exchange constants associated with each of the exchange pathways. The calculated exchange constants depend very strongly on the value used for the Hubbard U , as has previously been found for systems based on Cu [43] and V [44]. For U = 5 eV on both the Cu and V d-orbitals, we obtained a value J 0 = 24.7(6) K for the intradimer exchange, which is broadly consistent with the experimental value.…”