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Antiferromagnetic Phase Transition of K-Rb Alloy Nanoclusters Incorporated in Sodalite
Takehito Nakano,Yuko Ishida,Atsufumi Hanazawa,Yasuo Nozue 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.12
We prepared Rb-rich K-Rb alloy nanoclusters arrayed in the regular nanospace of aluminosilicatesodalite which has a bcc arrangement of cages. The average chemical formula of the cluster is(K1.5Rb2.5)3+, where one unpaired s-electron is shared by four alkali cations and is confined in acage. The magnetic susceptibility and the electron spin resonance clearly show an antiferromagneticphase transition at a N´eel temperature TN of approximately 90 - 100 K. The observed TNis higher than that in K3+4 (TN = 72 K) and (K3Rb)3+ clusters (TN = 80 K) in sodalites. Thisresult indicates a systematic enhancement of the antiferromagnetic exchange coupling between theadjacent nanoclusters by substituting Rb atoms for K ones. The size and the spatial distributionof the s-electron wave function in the nanocluster play a key role in the exchange coupling.
Takehito Nakano,Duong Thi Hanh,Akihiro Owaki,Yasuo Nozue,Nguyen Hoang Nam,Shingo Araki 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
Zeolite LSX (low-silica X) crystals have an aluminosilicate framework with regular supercagesand β-cages. They are arrayed in a double diamond structure. The loading density of guest Katoms per supercage (or β cage), n, can be controlled from 0 to ~ 9. At n < 2, samples are nearlynonmagnetic and insulating. The Curie constant has a clear peak at n = 3, and the electricalresistivity suddenly decreases simultaneously. The electrical resistivity suddenly decreases again atn = 6 and shows metallic phase at n > 6. These properties are explained by the polaron effectincluding the electron correlation. Ferrimagnetic properties are observed at n ~ 9. A remarkableincrease in the resistivity is observed at very low temperatures at n ~ 9, and is discussed in termsof the hypothesis of a Kondo insulator.
Exotic Magnetism of s-electron Cluster Arrays: Ferromagnetism, Ferrimagnetism and Antiferromagnetism
Takehito Nakano,Duong Thi Hanh,Yasuo Nozue,Nguyen Hoang Nam,Truong Cong Duan,Shingo Araki 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
Alkali metal nanoclusters can be stabilized in the regular cages of zeolite crystals by the loading ofguest alkali metals. Cages are connected by the sharing of windows of the framework, and arrayed insimple cubic, diamond and body centered cubic structures in zeolites A, X and sodalite, respectively. The s-electrons have the localized nature of nanoclusters with magnetic moments, and have mutualinteractions through the windows of cages. They show exotic magnetism depending on the structuretype of zeolites, the kind of alkali metals and the average loading density of alkali atoms per cage. In zeolite A, potassium clusters are formed in α-cages that have an inside diameter of 11 °A. Theyexhibit ferromagnetic properties explained by the canted antiferromagnetism of the Mott insulator,where the 1p-like degenerate orbitals of clusters play an essential role in the magnetic properties. Na-K alloy clusters generated at supercages and β-cages of low-silica X (LSX) zeolite exhibit N´eel’sN-type ferrimagnetism at specific loading densities of alkali metals. Alkali metal clusters in sodaliteshow the ideal Heisenberg antiferromagnetism of the Mott insulator.