Giant exchange interaction in mixed lanthanides

TL;DR

This paper uncovers the microscopic mechanism behind the unusually strong exchange interactions in mixed lanthanide complexes, revealing a complex kinetic origin involving high-order angular momentum contributions, which also impacts magnetization blocking.

Contribution

It provides a detailed theoretical analysis combining modeling and ab initio calculations to explain the strong exchange interactions in mixed lanthanide complexes, a phenomenon previously not well understood.

Findings

Exchange splitting reaches hundreds of wavenumbers.

The mechanism is kinetic and involves high-order angular momentum terms.

Magnetization blocking is not always enhanced by strong exchange.

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$^{3+}$-N$_2^{3-}$-Ln$^{3+}$ (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 {\it 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.