Spin wave resonances in antiferromagnets

Spin wave resonances with enormously large wave numbers corresponding to wave vectors 10^5-10^6 cm^{-1} are observed in thin plates of FeBO3. The study of spin wave resonances allows one to obtain information about the spin wave spectrum. The temperature dependence of a non-uniform exchange constant is determined for FeBO3. Considerable softening of the magnon spectrum resulting from the interaction of magnons, is observed at temperatures above 1/3 of the Neel temperature. The excitation level of spin wave resonances is found to depend significantly on the inhomogeneous elastic distortions artificially created in the sample. A theoretical model to describe the observed effects is proposed.Comment: 6 pages, 6 figure

Field and temperature dependence of the NMR relaxation rate in the magnetic quadrupolar liquid phase of spin-1/2 frustrated ferromagnetic chains

It is generally difficult to experimentally distinguish magnetic multipolar orders in spin systems. Recently, it was proposed that the temperature dependence of the nuclear magnetic resonance relaxation rate 1/T_1 can involve an indirect, but clear signature of the field-induced spin nematic or multipolar Tomonaga-Luttinger (TL) liquid phase [Phys. Rev. B79, 060406(R) (2009)]. In this paper, we evaluate accurately the field and temperature dependence of 1/T_1 in spin-1/2 frustrated J1-J2 chains combining field-theoretical techniques with numerical data. Our results demonstrate that isotherms of 1/T_1 as a function of magnetic field also exhibit distinctive non-monotonic behavior in spin nematic TL liquid, in contrast with the standard TL liquid in the spin-1/2 Heisenberg chain. The relevance of our results to quasi one-dimensional edge-sharing cuprate magnets, such as LiCuVO4, is discussed.Comment: 11 pages (2 column), 5 figures, published versio

Anisotropic Exchange in LiCuVO4_4 probed by ESR

We investigated the paramagnetic resonance in single crystals of LiCuVO$_4$ with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing CuO$_6$ octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, nicely agree with the parameters obtained from the angular dependence of the linewidth. The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish LiCuVO$_4$ as one-dimensional compound at high temperatures. PACS: 76.30.-v, 76.30.Fc, 75.30.EtComment: 18 pages, 6 figure

Magnetic phase diagram of the frustrated S=1/2 chain magnet LiCu_2O_2

We present the results of the magnetization and dielectric constant measurements on untwinned single crystal samples of the frustrated S=1/2 chain cuprate LiCu_2O_2. Novel magnetic phase transitions were observed. A spin flop transition of the spiral spin plane was observed for the field orientations H||a,b. The second magnetic transition was observed at H~15 T for all three principal field directions. This high field magnetic phase is discussed as a collinear spin-modulated phase which is expected for an S=1/2 nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic chain system