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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

Molecular structure of Tb complex 2 and magnetic susceptibility in the series 1–5.(a) Colors’ legend for the balls: violet, Tb; blue, N; red, O; green, Si; grey, C. The hydrogen atoms are omitted for clarity. The violet dashed lines show the orientation of the main anisotropy axes of Tb ions in their ground doublet state, whereas the green dashed line shows the orientation of the main anisotropy axis in the ground exchange Kramers doublet. The violet arrows show the orientation of the local magnetic moments on Tb ions, and the blue arrow on the radical, in the ground exchange Kramers doublet. (b) Experimental (symbols) and ab initio calculated (lines) temperature-dependent powder magnetic susceptibility (χ) for 1–5. The experimental data were upscaled by 3, 3, 1% for 2, 3 and 5, respectively, and were downscaled by 2% for 4. The magnetic susceptibility curves were calculated following the way they have been measured1424, as M(H, T)/H at H = 1 T, averaged over all directions of magnetic field H relative to molecular frame. For the computational methodology of the magnetic axes and χT, see refs 32 and 33, respectively.
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f1: Molecular structure of Tb complex 2 and magnetic susceptibility in the series 1–5.(a) Colors’ legend for the balls: violet, Tb; blue, N; red, O; green, Si; grey, C. The hydrogen atoms are omitted for clarity. The violet dashed lines show the orientation of the main anisotropy axes of Tb ions in their ground doublet state, whereas the green dashed line shows the orientation of the main anisotropy axis in the ground exchange Kramers doublet. The violet arrows show the orientation of the local magnetic moments on Tb ions, and the blue arrow on the radical, in the ground exchange Kramers doublet. (b) Experimental (symbols) and ab initio calculated (lines) temperature-dependent powder magnetic susceptibility (χ) for 1–5. The experimental data were upscaled by 3, 3, 1% for 2, 3 and 5, respectively, and were downscaled by 2% for 4. The magnetic susceptibility curves were calculated following the way they have been measured1424, as M(H, T)/H at H = 1 T, averaged over all directions of magnetic field H relative to molecular frame. For the computational methodology of the magnetic axes and χT, see refs 32 and 33, respectively.

Mentions: This paradigm was recently challenged by a series of -radical bridged dilanthanide complexes [K(18 − crown − 6)]{[(Me3Si)2N] (THF)Ln}2 (μ − η2:η2 − N2) (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5), THF = tetrahydrofuran), shown in Fig. 1a1424. In some of these compounds the exchange interaction was found to be two orders of magnitude stronger than in any known lanthanide system. This is of the same order of magnitude as the crystal-field splitting of J-multiplets on the lanthanide sites, implying that the picture of exchange interaction involving individual crystal-field doublets, Eq. (1), is no longer valid for these compounds. Moreover, the terbium complex from this series exhibits a magnetic hysteresis at 14 K and a 100 s blocking time at 13.9 K (one of the highest blocking temperatures among existing SMMs24), suggesting a possible implication of the giant exchange interaction in this SMM behavior.


Giant exchange interaction in mixed lanthanides.

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

Molecular structure of Tb complex 2 and magnetic susceptibility in the series 1–5.(a) Colors’ legend for the balls: violet, Tb; blue, N; red, O; green, Si; grey, C. The hydrogen atoms are omitted for clarity. The violet dashed lines show the orientation of the main anisotropy axes of Tb ions in their ground doublet state, whereas the green dashed line shows the orientation of the main anisotropy axis in the ground exchange Kramers doublet. The violet arrows show the orientation of the local magnetic moments on Tb ions, and the blue arrow on the radical, in the ground exchange Kramers doublet. (b) Experimental (symbols) and ab initio calculated (lines) temperature-dependent powder magnetic susceptibility (χ) for 1–5. The experimental data were upscaled by 3, 3, 1% for 2, 3 and 5, respectively, and were downscaled by 2% for 4. The magnetic susceptibility curves were calculated following the way they have been measured1424, as M(H, T)/H at H = 1 T, averaged over all directions of magnetic field H relative to molecular frame. For the computational methodology of the magnetic axes and χT, see refs 32 and 33, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Molecular structure of Tb complex 2 and magnetic susceptibility in the series 1–5.(a) Colors’ legend for the balls: violet, Tb; blue, N; red, O; green, Si; grey, C. The hydrogen atoms are omitted for clarity. The violet dashed lines show the orientation of the main anisotropy axes of Tb ions in their ground doublet state, whereas the green dashed line shows the orientation of the main anisotropy axis in the ground exchange Kramers doublet. The violet arrows show the orientation of the local magnetic moments on Tb ions, and the blue arrow on the radical, in the ground exchange Kramers doublet. (b) Experimental (symbols) and ab initio calculated (lines) temperature-dependent powder magnetic susceptibility (χ) for 1–5. The experimental data were upscaled by 3, 3, 1% for 2, 3 and 5, respectively, and were downscaled by 2% for 4. The magnetic susceptibility curves were calculated following the way they have been measured1424, as M(H, T)/H at H = 1 T, averaged over all directions of magnetic field H relative to molecular frame. For the computational methodology of the magnetic axes and χT, see refs 32 and 33, respectively.
Mentions: This paradigm was recently challenged by a series of -radical bridged dilanthanide complexes [K(18 − crown − 6)]{[(Me3Si)2N] (THF)Ln}2 (μ − η2:η2 − N2) (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5), THF = tetrahydrofuran), shown in Fig. 1a1424. In some of these compounds the exchange interaction was found to be two orders of magnitude stronger than in any known lanthanide system. This is of the same order of magnitude as the crystal-field splitting of J-multiplets on the lanthanide sites, implying that the picture of exchange interaction involving individual crystal-field doublets, Eq. (1), is no longer valid for these compounds. Moreover, the terbium complex from this series exhibits a magnetic hysteresis at 14 K and a 100 s blocking time at 13.9 K (one of the highest blocking temperatures among existing SMMs24), suggesting a possible implication of the giant exchange interaction in this SMM behavior.

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