Theoretical Understanding of Anisotropy in Molecular Nanomagnets

Abstract

The study of magnetic anisotropy in metal complexes is at the forefront of current molecular magnetism research because it represents the key property for potential application of molecular materials in spintronics, memory storage, and quantum computing. The anisotropy adds an order of complexity to magnetic properties of complexes, requiring more refined experimental techniques and the use of theoretical tools, especially, of new ab initio approaches for their description. In this review, we discuss the physical reasons for magnetic anisotropy and the mechanisms of its appearance in different metal complexes. Differences in the manifestation of magnetic anisotropy between complexes with weak and strong spin-orbit coupling as well as between single-ion and polynuclear compounds will be emphasized.

Absence of actual plateaus in zero-temperature magnetization curves of quantum spin clusters and chains

“…Dy 3+ typically has g ≈ 20) though this extraordinary large g-value usually correlates with a rather strong anisotropy in the exchange interaction. 52,56,57 Under the extreme situation, the XY-part of exchange coupling might be even of opposite sign than the Z-part (ferromagnetic versus antiferromagnetic) as it has been recently reported for the heterodinuclear Cr 3+ -Yb 3+ complex. 56 Last but not least, let us briefly comment on experimental implications for the quantum spin clusters studied in the present work.…”

Exchange interaction betweenJmultiplets

“…(29) it is easier (i) to obtain physical insight on the exchange interaction and (ii) to combine it with other terms such as crystal-field and Zeeman interaction included inĤ 0 . The latter can be treated at ab initio level [32,34].…”

“…This situation is encountered in mixed lanthanide-transition metal and lanthanide-radical complexes [1,32,47]. The exchange Hamiltonian between a J-multiplet and an isotropic spin is obtained in a similar way as Eq.…”

“…Both these questions can only be answered after a more complete derivation of exchange interaction between J multiplets on the basis of a reliable microscopic model. Besides, a microscopic description of J-J (J-S) exchange interaction is desirable due to a very large number of phenomenological parameters, in contrast to weakly-anisotropic spin systems containing only a few of them [31,32]. Given that many microscopic electronic parameters describing individual magnetic centers and their interaction can be accurately derived via density functional theory [33] or ab initio calculations [34], a microscopically derived Hamiltonian for J multiplets can become a powerful tool for the investigation of exchange interaction in materials containing lanthanides, actinides and transition metal ions with unquenched orbital momentum.…”

Redox Switches for Single‐Molecule Magnet Activity: An Ab Initio Insight

“…The two key quantities defining the efficiency of magnetization blocking are the rate of the quantum tunnelling of magnetization (QTM) and the height of the activation barrier for the reversal of magnetization . The first is basically determined by the axiality of the ground doublet of the molecule, while the second by the strength of the axial components of the crystal field (CF) on the metal sites or the exchange interaction between them, depending on the nature of the compounds . Thus in single‐ion complexes the barrier is determined by the CF splitting of the ground J ‐multiplet in the case of lanthanides, and zero‐field splitting (ZFS) of the ground S ‐term in the case of transition‐metal ions .…”