Orbital degeneracy and magnetic properties of potassium clusters incorporated into nanoporous crystals of zeolite A

Abstract

Potassium nanoclusters in aluminosilicate zeolite A shows ferromagnetic properties, although bulk metal of K is nonmagnetic. K clusters are prepared by adsorbing guest K atoms into the supercages (α cages) of dehydrated zeolite A, where supercages with the inside diameter of ∼ 11 Å are arrayed in a simple cubic structure. By the loading of guest K atoms, s-electrons are provided for clusters, and successively occupy 1s- and 1p-like quantum states of cluster. The average number of guest K atoms in a cluster, n, is widely controlled up to 7.2. This material shows ferromagnetism at n > 2 at low temperatures, where s-electrons in the 1p state of clusters are responsible for the ferromagnetism. We have found a fine step at n=2 in magnetization and g value. The g values of ferromagnetic samples with 2 < n < 6 obviously decrease from 2 at low temperatures, indicating that an enhancement of the spin-orbit interaction (SOI) occurs due to the orbital degeneracy of the 1p state. An enhancement mechanism is proposed based on the Schmidt orthogonalized wave function. In the ferromagnetic sample, an anomalously large magnetization is observed at higher magnetic fields up to 52 T at low temperatures. The level crossing of LS multiplet terms in the degenerate 1p orbital of cluster is proposed as the possible origin of the anomalous increase in magnetization at higher fields. The ferromagnetism coexists with antiferromagnetic coupling between magnetic moments of clusters. The origin of the spontaneous magnetization of the ferromagnetism is interpreted in terms of spin-canting mechanism where the Dzyaloshinsky-Moriya interaction is strongly enhanced by the degenerate 1p orbital of K cluster. Possible structures of K cluster in the supercage are also discussed for the orbital degeneracy of 1p state