Ferromagnetism of Na–K Alloy Clusters Incorporated in Zeolite Low-Silica X

In a porous crystal of zeolite low-silica X (LSX), beta-cages and supercages (cavities) are arrayed in a diamond structure, respectively. We loaded potassium metal into zeolite LSX which has a chemical formula of Na7.3K4.7Al12Si12O48 per beta-cage (or supercage), and generated Na-K alloy clusters in beta-cages and/or supercages. We have investigated the magnetic properties, the optical ones and the electrical resistivity at various values of K-loading density n per beta-cage (or supercage) up to n = 9.7. Localized magnetic moments are observed at 8.2 < n < 9.7. Almost simultaneously, nearly pure ferromagnetism is observed at 8.4 < n < 9.7. The highest Curie temperature is approximate to 12K at n approximate to 9. Optical reflection spectra for 8 < n have a new band at 2.8 eV which is assigned to the optical excitation of s-electrons of clusters generated at beta-cages. The origin of the magnetic moments is assigned to these beta-cage clusters because of the coincidence between the growths of the 2.8 eV band and localized magnetic moments. The origin of magnetic ordering is explained by a ferromagnetic interaction between beta-cage clusters. All samples are found to be insulating from the temperature dependence of electrical resistivity. A direct magnetic interaction between beta-cage clusters is not expected because of the high electronic barrier between them. A ferromagnetic superexchange coupling between beta-cage clusters is newly proposed via the sp(3)-like closed-shell clusters at supercages. A thermal hysteresis is observed in the electrical resistivity at intermediate temperatures, and the origin is assigned to low-density carriers generated in the Na-K eutectic alloy structures in nanospace.