High-field magnetic phase transitions and spin excitations in magnetoelectric LiNiPO4

Rasmus Toft-Petersen, Jens Jensen, Thomas Bagger Stibius Jensen, Niels Hessel Andersen, Niels Bech Christensen, Christof Niedermayer, Michel Kenzelmann, Markos Skoulatos, MD Le, Kim Lefmann, Sonja Rosenlund Hansen, Jiying Li, Jerel L. Zarestky, David Vaknin

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    Abstract

    The magnetically ordered phases and spin dynamics of magnetoelectric LiNiPO4 have been studied in fields up to 17.3 T along the c axis. Using neutron diffraction, we show that a previously proposed linearly polarized incommensurate (IC) structure exists only for temperatures just below the Neel temperature T-N. The ordered IC structure at the lowest temperatures is shown instead to be an elliptically polarized canted spiral for fields larger than 12 T. The transition between the two IC phases is of second order and takes place about 2 K below T-N. For mu H-0 > 16 T and temperatures below 10 K, the spiral structure is found to lock in to a period of five crystallographic unit cells along the b axis. Based on the neutron-diffraction data, combined with detailed magnetization measurements along all three crystallographic axes, we establish the magnetic phase diagrams for fields up to 17.3 T along c and for fields up to 16 T along a and b. The spin excitations in the high-field IC spiral phase have been studied in detail by inelastic neutron scattering. A mean-field analysis shows that the spin Hamiltonian derived previously from the low-temperature spin waves at zero field predicts the transition between the linear and elliptical polarization of the IC structure, and that a generalization of the spin-wave theory, assuming the random-phase approximation, accounts for the inelastic scattering data obtained in the commensurable uniform phase at fields below 12 T as well as those obtained in the high-field IC spiral phase.
    Original languageEnglish
    JournalPhysical Review B Condensed Matter
    Volume84
    Issue number5
    Pages (from-to)054408 (10 pages)
    ISSN0163-1829
    DOIs
    Publication statusPublished - 2011

    Keywords

    • Superconducting materials

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