Long-range three-dimensional magnetic structures of the spin S =1 hexamer cluster fedotovite-like A2 Cu3 O (SO4)3 (A2 = K2, NaK, Na2): A neutron diffraction study

V. Yu Pomjakushin, A. Podlesnyak, A. Furrer, E. V. Pomjakushina

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Abstract

The crystal and magnetic structures of the spin S=1 hexamer cluster fedotovite-like A2Cu3O(SO4)3 (A2=K2, NaK, Na2) were studied by neutron powder diffraction at temperatures 1.6-290 K. The crystal structures in all compounds are well refined in the monoclinic space group C2/c. The basic magnetic units of the compounds are copper hexamers, which are coupled by weak superexchange interactions giving rise to three-dimensional long-range magnetic order below 3.0<TN<4.7 K. We have found that for A2=K2 and NaK the propagation vector of the magnetic structure is k=[0,0,0], and the coupling of the Cu hexamers is ferromagnetic (FM) along the ab diagonal and antiferromagnetic along the bc diagonal. In contrast, for A=Na the propagation vector is k=[0,1,0], and the Cu hexamers are coupled antiferromagnetically (AFM) along the ab diagonal. The hexamers are formed by three Cu pairs arranged along the b axis. The calculated spin expectation values (s) for the simplest symmetric spin Hamiltonian (obtained from inelastic neutron spectroscopy) of the isolated hexamers in the mean field amounted to (s)=3/8 for the side spins Cu1 and Cu2 and (s)=1/4 for Cu3 in the middle. The Cu spins are FM coupled in pairs and AFM between neighboring pairs. The experimental magnetic moments of the Cu2+ ions turn out to be not completely collinear due to spin frustrations within the weak interhexamer interactions. The sizes of magnetic moments of Cu in the hexamers determined from the diffraction data are in fair agreement with the calculated values.

Original languageEnglish
Article number144409
JournalPhysical Review B
Volume109
Issue number14
DOIs
StatePublished - Apr 1 2024

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