NMR study of the paramagnetic state of low-dimensional magnets LiCu 2 O 2 and NaCu 2 O 2
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ISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
NMR Study of the Paramagnetic State of Low-Dimensional Magnets LiCu2O2 and NaCu2O2 A. F. Sadykova*, Yu. V. Piskunova, A. P. Gerashchenkoa,c, V. V. Ogloblicheva, A. G. Smol’nikova, S. V. Verkhovskiia, I. Yu. Arapovaa, Z. N. Volkovaa, K. N. Mikhaleva, and A. A. Bushb a Mikheev
Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620990 Russia b Moscow State Technical University of Radio Engineering, Electronics, and Automation, pr. Vernadskogo 78, Moscow, 119454 Russia c Ural Federal University named after the first President of Russia B.N. Eltsin, ul. Mira 19, Yekaterinburg, 620002 Russia *e-mail: [email protected] Received July 15, 2016
Abstract—A comprehensive NMR study of the magnetic properties of single crystal LiCu2O2 (LCO) and NaCu2O2 (NCO) is carried out in the paramagnetic region of the compounds for various orientations of single crystals in an external magnetic field. The values of the electric-field gradient (EFG) tensor, as well as the dipole and transferred hyperfine magnetic fields for 63,65Cu, 7Li, and 23Na nuclei are determined. The results are compared with the data obtained in previous NMR studies of the magnetically ordered state of LCO/NCO cuprates. DOI: 10.1134/S1063776117010071
1. INTRODUCTION LiCu2O2 and NaCu2O2 compounds belong to the family of low-dimensional cuprates containing edgeshared chains of Cu2+O4 squares with spin S = 1/2. Cu2+ ions carrying this spin are coupled to each other via both intrachain and interchain indirect superexchange interactions. Depending on the sign and the hierarchy of these interactions, quite diverse ground states can be realized in the above family, in particular, multiferroicity. The latter state in spiral spin-chain systems has been very intensively studied recently in the context of possible applications of these objects. LiCu2O2 and NaCu2O2 cuprates represent frustrated quasi-one-dimensional ferromagnets with Néel temperatures of TN = 23 K and TN = 12 K, respectively [1, 2]. Previous NMR studies of these objects have convincingly shown that both LCO and NCO have a helicoidal magnetic structure incommensurate with the crystal lattice [3–11]. However, in spite of the rather large number of experimental studies devoted to the magnetic properties of LiCu2O2 and NaCu2O2 [3– 18], the detailed magnetic structure of these compounds in the ground state still remains unclear. In particular, there is no unanimous opinion about the exact spatial orientation of spin spirals and the direction of their twisting in magnetically inequivalent spin chains of LCO/NCO. Moreover, the NaCu2O2 compound, in contrast to LiCu2O2, is not a multiferroic,
although it is isostructural to the latter. The cause of this fact is still unknown. The overwhelming majority of NMR studies of LCO and NCO have been focused on the magnetic structure of these objects; therefore, the measurements were carried out mostly at T < TN. However, the study of LCO and NCO in the paramagnetic state
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