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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

The low-lying exchange spectrum and the magnetization blocking barrier in 2.(a) The violet bold lines show the CF levels on Tb ions, the green bold lines show the low-lying exchange levels. Each exchange level is placed according to the projection of its magnetic moment on the main magnetic axis of the ground exchange doublet (green dashed line in Fig. 1a). The exchange levels with the same number are two components of the corresponding KD. The thin dashed lines show the admixed CF states on Tb sites to the exchange states in percent (only admixtures >10% are shown). The number accompanying the blue line is the average magnetic moment matrix element (in μB) between the components of the lowest exchange KD; the rate of QTM in the ground exchange state is proportional to its square. The red arrows denote the relaxation path outlining the barrier of reversal of magnetization, with the same meaning of the corresponding numbers (see the text for more details). (b) The magnetization blocking barrier for 2 calculated in the absence of the admixture of excited CF states on Tb sites to the ground one via the exchange interaction.
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f3: The low-lying exchange spectrum and the magnetization blocking barrier in 2.(a) The violet bold lines show the CF levels on Tb ions, the green bold lines show the low-lying exchange levels. Each exchange level is placed according to the projection of its magnetic moment on the main magnetic axis of the ground exchange doublet (green dashed line in Fig. 1a). The exchange levels with the same number are two components of the corresponding KD. The thin dashed lines show the admixed CF states on Tb sites to the exchange states in percent (only admixtures >10% are shown). The number accompanying the blue line is the average magnetic moment matrix element (in μB) between the components of the lowest exchange KD; the rate of QTM in the ground exchange state is proportional to its square. The red arrows denote the relaxation path outlining the barrier of reversal of magnetization, with the same meaning of the corresponding numbers (see the text for more details). (b) The magnetization blocking barrier for 2 calculated in the absence of the admixture of excited CF states on Tb sites to the ground one via the exchange interaction.

Mentions: The low-lying exchange spectrum for the Tb complex is shown in Fig. 3a. The ground (1±) and the first two excited (2±, 3±) exchange Kramers doublets (KDs) mainly originate from the ground CF doublets on the Tb ions (94%, 87%, and 88%, respectively). However, the third and fourth excited exchange KDs (4±, 5±) represent almost equal mixtures of the ground and the first excited CF doublets on the Tb3+ sites. This is remarkable because the mixed CF states are separated by 166 cm−1 (Fig. 3a). Similar scenario is realized in 3 and 4, whereas in 5 the exchange interaction and the resulting mixing of CF states is relatively weak. The magnetic structure of the ground exchange KD is shown in Fig. 1a. The magnetic moments on Tb3+ sites are parallel due to inversion symmetry and almost coincide with the directions of the main magnetic axes in the ground local KDs (Fig. 1a). The magnetic moment of the radical, corresponding to isotropic S = 1/2, is rotated with respect to the magnetic moments on Tb sites by small angle θ (Table 1) due to the non-Heisenberg contributions to the exchange interaction23.


Giant exchange interaction in mixed lanthanides.

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

The low-lying exchange spectrum and the magnetization blocking barrier in 2.(a) The violet bold lines show the CF levels on Tb ions, the green bold lines show the low-lying exchange levels. Each exchange level is placed according to the projection of its magnetic moment on the main magnetic axis of the ground exchange doublet (green dashed line in Fig. 1a). The exchange levels with the same number are two components of the corresponding KD. The thin dashed lines show the admixed CF states on Tb sites to the exchange states in percent (only admixtures >10% are shown). The number accompanying the blue line is the average magnetic moment matrix element (in μB) between the components of the lowest exchange KD; the rate of QTM in the ground exchange state is proportional to its square. The red arrows denote the relaxation path outlining the barrier of reversal of magnetization, with the same meaning of the corresponding numbers (see the text for more details). (b) The magnetization blocking barrier for 2 calculated in the absence of the admixture of excited CF states on Tb sites to the ground one via the exchange interaction.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: The low-lying exchange spectrum and the magnetization blocking barrier in 2.(a) The violet bold lines show the CF levels on Tb ions, the green bold lines show the low-lying exchange levels. Each exchange level is placed according to the projection of its magnetic moment on the main magnetic axis of the ground exchange doublet (green dashed line in Fig. 1a). The exchange levels with the same number are two components of the corresponding KD. The thin dashed lines show the admixed CF states on Tb sites to the exchange states in percent (only admixtures >10% are shown). The number accompanying the blue line is the average magnetic moment matrix element (in μB) between the components of the lowest exchange KD; the rate of QTM in the ground exchange state is proportional to its square. The red arrows denote the relaxation path outlining the barrier of reversal of magnetization, with the same meaning of the corresponding numbers (see the text for more details). (b) The magnetization blocking barrier for 2 calculated in the absence of the admixture of excited CF states on Tb sites to the ground one via the exchange interaction.
Mentions: The low-lying exchange spectrum for the Tb complex is shown in Fig. 3a. The ground (1±) and the first two excited (2±, 3±) exchange Kramers doublets (KDs) mainly originate from the ground CF doublets on the Tb ions (94%, 87%, and 88%, respectively). However, the third and fourth excited exchange KDs (4±, 5±) represent almost equal mixtures of the ground and the first excited CF doublets on the Tb3+ sites. This is remarkable because the mixed CF states are separated by 166 cm−1 (Fig. 3a). Similar scenario is realized in 3 and 4, whereas in 5 the exchange interaction and the resulting mixing of CF states is relatively weak. The magnetic structure of the ground exchange KD is shown in Fig. 1a. The magnetic moments on Tb3+ sites are parallel due to inversion symmetry and almost coincide with the directions of the main magnetic axes in the ground local KDs (Fig. 1a). The magnetic moment of the radical, corresponding to isotropic S = 1/2, is rotated with respect to the magnetic moments on Tb sites by small angle θ (Table 1) due to the non-Heisenberg contributions to the exchange interaction23.

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