We report on 6,7 Li nuclear magnetic resonance measurements of the spin-chain compound LiCu2O2 in the paramagnetic and magnetically ordered states. Below T ≈24 K the NMR lineshape presents a clear signature of incommensurate (IC) static modulation of the local magnetic field consistent with an IC spiral modulation of the magnetic moments.7 Li NMR reveals strong phason-like dynamical fluctuations extending well below 24 K. We hypothesize that a series of phase transitions at 24.2, 22.5, and 9 K reflects a "Devil's staircase" type behavior generic for IC systems. LDA based calculations of exchange integrals reveal a large in-chain frustration leading to a magnetical spiral.Despite extensive efforts during the last decades spin ordering in frustrated S=1/2 quantum spin chains still remains a matter of broad activities. [1,2,3,4,5, 6] Rich phase diagrams with commensurate (C) and incommensurate (IC) phases, with spin-and charge ordering, dimerization, or superconductivity have been predicted. Most studies have been focused on various cuprates with corner-or edge-shared CuO 4 plaquettes. Edge-sharing of CuO 4 plaquettes leads to CuO 2 chains with a nearly 90• Cu-O-Cu bond angle causing a reduced nearest neighbor (nn) transfer and a next-nearest neighbor (nnn) transfer of similar size allowing frustration effects. IC spiral states driven by ferromagnetic (FM) nn exchange and in-chain frustration have been predicted theoretically for CuO 2 chain compounds such as Ca 2 Y 2 Cu 5 O 10 but discarded experimentally. [5, 6] The observation of in-chain IC effects in undoped quasi-1D cuprates is so far restricted by a sharp magnetic field driven C-IC transition observed in the spin-Peierls system CuGeO 3 at high magnetic fields. [7] Here we report on 6,7 Li nuclear magnetic resonance measurements (NMR) and local density (LDA) based analysis of the electronic and magnetic structure of the chain compound LiCu 2 O 2 . We show that the observed spontaneous magnetic order can be described by a spiral modulation of the magnetic moments. Independently, LDA calculations and a subsequent Heisenberganalysis reveal strong in-chain frustration driving spiral ordering in accord with the NMR data.LiCu 2 O 2 is an insulating orthorhombic compound [1,8,9, 10] with bilayers of edge-shared Cu 2+ -O chains running parallel to the b-axis separated by Cu 1+ planes. It exhibits a high-temperature antiferromagnetic(AFM)-like Curie-Weiss susceptibility χ(T ). Low-temperature χ(T ) and specific heat stufies [1,11] point to a series of intrinsic phase transitions at T ≈24.2 K, T ≈22.5 K, and T ≈9 K pointing to a complex multi-stage rearrangement of the spin structure. Magnetization studies performed in external fields up to 5 T did not reveal any signatures of field-induced transitions. µSR data[11] point to a broad distribution of magnetic fields at the muon stopping sites. LSDA(+U) calculations point to an FM in-chain ordering.[10] However, a simple FM ordering is in conflict with the µSR data[11] and the AFM dimer liquid picture. [1,12] Thus, to...
Zero-field 57 Fe NMR spectrum has been observed in the 57 Fe-enriched ceramics of BiFeO3. The analysis of NMR lineshape confirms the existence in this ferroelectricantiferromagnetic compound of a long-period cycloidal magnetic structure and enables the first reconstruction of the actual spatial spin distribution along the cycloid which is found to be essentially anharmonic.
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