Nature of the ferromagnetic ground state in the Mn4molecular magnet

Abstract

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn4 molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn 3+ and Mn 2+ . The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn 3+ ions (J bb ), but along two out of four exchange paths connecting Mn 3+ and Mn 2+ ions (J wb ). Within the perturbation theory we performed the microscopic analysis of different contributions to the exchange constants, which allows us to establish the mechanism for the largest ferromagnetic exchange. In the presence of the charge order the lowest in energy virtual excitations, contributing to the superexchange, will be not those across the Hubbard gap ∼ U , but will be those between the Mn 3+ and Mn 4+ ions, which cost much smaller energy V (≪ U ). Together with strong Hund's rule coupling and specific orbital order this leads to large ferromagnetic exchange interaction for two out of four Mn 2+ -Mn 3+pairs.

Experimental and theoretical study of the influence of peripheral environment on magnetic properties of tetranuclear manganese skeleton in new representatives of calix[4]arene-containing [MnII2 MnIII2] clusters

“…The exchange coupling constants for each of four pairs of two manganese ions in different oxidation states Mn IIIMn II (distances 3.295 Å and 3.298 Å after optimization, for each of two pairs of bonds Mn III -Mn II ) the values of the exchange coupling constants are positive: J 3 = 1.40 cm À1 and J 4 = 1.36 cm À1 , respectively. The difference in the values of the exchange interaction constants J 3 and J 4 is consistent with the results of work [25] .…”

Magnetic and electrical anisotropy with correlation and orbital effects in dimerized honeycomb ruthenate Li2RuO3

“…Honeycomb lattice ruthenate Li2RuO3, with four 4d electrons in the t2g manifold, has attracted significant attention owing to the presence of an orbital-selective Peierls transition (OSPT) leading to the strongest tendency toward dimerization among the known A2MO3 systems [8,9,10]. It undergoes a strong dimerization transition below a quite high transition temperature (~540 K) as reported in [8]; it also displays a large direct overlap between 4d orbitals of the transition metal ions compared to other Li2MO3 structures.…”

“…The short Ru-Ru bonds in Li2RuO3 are actually shorter than those in Ru metal. The local dimers of the short Ru-Ru bonds exhibit structural long-range order and form a valence bond solid (VBS) with a local spin singlet state [8,10]. In addition, at higher temperatures the dimers seem to survive locally [11], and the system exhibits a reduced local magnetic moment of S = 1/2, instead of the S = 1 [11] which would correspond to a typical 4d 4 electron configuration at the octahedral site.…”

Calculation of exchange constants of the Heisenberg model in plane-wave-based methods using the Green's function approach

“…In the meanfield approximation this gives Curie-Weiss temperature θ GGA = 87 K. This is again somewhat larger than experimental θ exp ∼ 50 − 60 K [54], but it agrees with what one may expect from the mean-field theory. An account of the on-site Coulomb repulsion in the GGA+U calculation leads to gradual growth of the FM component and results in total exchange J = −16 K (FM) for U = 4.5 eV and J H = 0.9 eV (as were used, e.g., in NaMn 7 O 12 [60] or in Mn 4 (hmp) 6 [61]), which agrees with Goodenough-Kanamori-Anderson rules [30,54], but is inconsistent with experiment [54].…”