The Mechanism of Magnetic Interaction in Spin‐Ladder Molecular Magnets: A First‐Principles, Bottom‐Up, Theoretical Study of the Magnetism in the Two‐Legged Spin‐Ladder Bis(2‐amino‐5‐nitropyridinium) Tetrabromocuprate Monohydrate

Abstract

AbstractThe magnetic properties of the isotropic (Jrail ≈ Jrung), two‐legged spin‐ladder bis(2‐amino‐5‐nitropyridinium) tetrabromocuprate monohydrate, [(5NAP)2CuBr4·H2O], have been studied using a first‐principles, bottom‐up approach, which allows computation of macroscopic magnetic properties (for instance, the magnetic susceptibility) of a crystal from only a knowledge of its crystal packing. Evaluation of the JAB parameters, using the 163 K X‐ray structure of [(5NAP)2CuBr4·H2O], indicates that the magnetic topology of this crystal is a two‐legged antiferromagnetic spin‐ladder, with values of Jrail and Jrung of –22.2 cm–1 and –19.7 cm–1, respectively. These values are very close to those that best fit the experimental magnetic susceptibility curve, which are–13.59 cm–1 and –14.16 cm–1, respectively. Very weak diagonal interactions within each ladder [J(d3) = –0.9 cm–1] and between nearby ladders [J(d4) = –0.3 cm–1] are also found. The computed magnetic susceptibility curve obtained using the two‐legged spin‐ladder properly reproduces the experimental magnetic curve (a quantitative agreement is obtained by applying a linear scaling factor of about 0.75 to the energies). The singlet–triplet spin‐gap of this spin‐ladder was computed to be 17 K, in close agreement with the experimental result (11 K). The change of the spin‐gap/Jrail with the size (2xL) of the magnetic spin‐ladder model space was found to converge towards 0.5, while that for a single‐legged (1xL) spin‐ladder converges towards zero, both in good agreement with the known trends for these systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)