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Giant exchange interaction in mixed lanthanides.

Vieru V, Iwahara N, Ungur L, Chibotaru LF - Sci Rep (2016)

Bottom Line: The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations.We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site.Contrary to general expectations the latter is not always favored by strong exchange interaction.

View Article: PubMed Central - PubMed

Affiliation: Theory of Nanomaterials Group, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium.

ABSTRACT
Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed (Ln = Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

No MeSH data available.


Related in: MedlinePlus

(a) Exchange core Ln3+--Ln3+ in the complex corresponding to D2h symmetry. (b) Magnetic orbitals in 1 obtained from DFT calculations. Only the f orbital involved in the kinetic exchange mechanism is shown.
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f2: (a) Exchange core Ln3+--Ln3+ in the complex corresponding to D2h symmetry. (b) Magnetic orbitals in 1 obtained from DFT calculations. Only the f orbital involved in the kinetic exchange mechanism is shown.

Mentions: To get insight into the mechanism responsible for the obtained huge value of , we projected a series of DFT calculations into the effective tight-binding and Hubbard models acting in the space of interacting magnetic orbitals of two Gd ions and the radical (see the Supplemental Material for details). Because of the D2h symmetry of the exchange core (Fig. 2a), the antibonding π* orbital accommodating the unpaired electron of radical overlaps with only one of the 4f orbitals on each Ln site, the xyz one (Fig. 2b). The corresponding transfer parameter t was derived for the Gd complex as t = 1407 cm−1. The value of t is obtained large because the radical’s magnetic orbital π* resides on nearest-neighbor atoms (nitrogens) to both lanthanides. Most important, this orbital is found to lie higher than the 4fxyz orbitals by as much as Δ = 5.2 × 104 cm−1 (Fig. 2b). Because of this huge energy gap, small electron promotion energy is expected for the electron transfer from the π* to the 4fxyz orbitals: the Coulomb repulsion energy between the transferred electron and the f electrons is cancelled at large extent by Δ. On the other hand, because of the same large gap Δ, the promotion energy of electron transfer from 4f to π* orbital is at least one order of magnitude larger. Therefore, the contribution of this process to the exchange coupling can be neglected. Indeed, our analysis using the Hubbard model gives the experimental for the Gd complex with (averaged) promotion energy of , a value many times smaller than typical “Hubbard U” in metal complexes28. Taking into account only the dominant virtual electron transfer, (4f)7 (π*)1 → (4f)8 (π*)0 → (4f)7 (π*)1, the kinetic contribution to the Gd3+-exchange parameter is written in a good approximation as 2930.


Giant exchange interaction in mixed lanthanides.

Vieru V, Iwahara N, Ungur L, Chibotaru LF - Sci Rep (2016)

(a) Exchange core Ln3+--Ln3+ in the complex corresponding to D2h symmetry. (b) Magnetic orbitals in 1 obtained from DFT calculations. Only the f orbital involved in the kinetic exchange mechanism is shown.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4834576&req=5

f2: (a) Exchange core Ln3+--Ln3+ in the complex corresponding to D2h symmetry. (b) Magnetic orbitals in 1 obtained from DFT calculations. Only the f orbital involved in the kinetic exchange mechanism is shown.
Mentions: To get insight into the mechanism responsible for the obtained huge value of , we projected a series of DFT calculations into the effective tight-binding and Hubbard models acting in the space of interacting magnetic orbitals of two Gd ions and the radical (see the Supplemental Material for details). Because of the D2h symmetry of the exchange core (Fig. 2a), the antibonding π* orbital accommodating the unpaired electron of radical overlaps with only one of the 4f orbitals on each Ln site, the xyz one (Fig. 2b). The corresponding transfer parameter t was derived for the Gd complex as t = 1407 cm−1. The value of t is obtained large because the radical’s magnetic orbital π* resides on nearest-neighbor atoms (nitrogens) to both lanthanides. Most important, this orbital is found to lie higher than the 4fxyz orbitals by as much as Δ = 5.2 × 104 cm−1 (Fig. 2b). Because of this huge energy gap, small electron promotion energy is expected for the electron transfer from the π* to the 4fxyz orbitals: the Coulomb repulsion energy between the transferred electron and the f electrons is cancelled at large extent by Δ. On the other hand, because of the same large gap Δ, the promotion energy of electron transfer from 4f to π* orbital is at least one order of magnitude larger. Therefore, the contribution of this process to the exchange coupling can be neglected. Indeed, our analysis using the Hubbard model gives the experimental for the Gd complex with (averaged) promotion energy of , a value many times smaller than typical “Hubbard U” in metal complexes28. Taking into account only the dominant virtual electron transfer, (4f)7 (π*)1 → (4f)8 (π*)0 → (4f)7 (π*)1, the kinetic contribution to the Gd3+-exchange parameter is written in a good approximation as 2930.

Bottom Line: The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations.We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site.Contrary to general expectations the latter is not always favored by strong exchange interaction.

View Article: PubMed Central - PubMed

Affiliation: Theory of Nanomaterials Group, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium.

ABSTRACT
Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed (Ln = Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

No MeSH data available.


Related in: MedlinePlus