Breaking the record of magnetic anisotropy in transition-metal dimers
Magnetic anisotropy is one of the critical magnetic properties of magnetic nanostructures. In our Communications Physics paper "Large magnetic anisotropy in chemically engineered iridium dimer" we show the chemical way to boost the magnetic anisotropy energy in iridium dimer to enormously large value.
The main way to store information in the current information age is magnetic storage based on magnetic materials. Because of the accelerating miniaturization of memory devices, the size of magnetic unit is getting smaller and smaller and already reaches a few nanometers. In this realm, the superparamagnetic effect due to thermal fluctuations becomes the major obstacle to maintain the spin orientations of magnetic units. Large magnetic anisotropy energy (MAE) of magnetic unit is the only way to overcome the superparamagnetic effect, since it is the energy barrier for spin flip.
Small transition-metal (TM) clusters scaling in nanometer usually own large MAE and have attractive application prospects in high-density magnetic storage. As the smallest member of TM clusters, TM dimers possess special symmetry and molecular orbitals, which results in extraordinarily large MAEs compared to other magnetic nanostructures. In fact, it has been theoretically predicted that iridium dimer (Ir2) possesses the largest MAE (~70 meV) among free-standing homonuclear TM dimers.
It is an interesting question that whether we can boost the MAE of iridium dimer. Since the MAE of Ir2 is closely associated with the molecular orbitals, it is possible to manipulate the MAE through extra chemical bond, i.e. by attaching a light anion atom to Ir2. Our first-principles calculations demonstrate that a MAE up to 294 meV can be achieved in Ir2 functionalized with a halogen atom. In details, when a halogen atom (X) is attached to Ir2, they form a linear trimer Ir2X with X at one end of the Ir-Ir bond. The critical effect of Ir-X bonding is that the energy levels of the molecular orbitals ( and ) are modified, which are mainly responsible for the colossal MAE.
Our strategy can be generalized to design other magnetic molecules to obtain giant (MAE). The tactic of chemical functionalization introduces a new synthetic approach to chemically engineering the magnetic anisotropy of small magnetic nanostructures towards future-generation magnetic information storages using one to a few atoms per bit.
For more details, please go to https://www.nature.com/articles/s42005-018-0078-4.