NMR IN MN-55(2-ONE-DIMENSIONAL ANTIFERROMAGNETIC CSMNBR3() NUCLEI IN THE QUASI)

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagn etic CsMnBr3, which has trigonal spin lattice, is investigated in deta il. The measurements were performed on a wide-band NMR decimeter micro wave-band spectrometer over a wide range of magnetic fields at tempera tures 1.3-4.2 K. All three branches of the NMR spectrum previously fou nd by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagne tic resonance spectrum. The experimental results in fields up to 40 kO e are described satisfactorily by an equation obtained by Zaliznyak ct al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work th at agree very well with experiment at all fields up to the ''collapse' ' field H-c of all sublattices. The unbiased NMR frequency in CsMnBr3 is determined to be nu(n0)=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn2+ spins, which a ccording to our data amounts to 28%, is determined more accurately. Th e field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, conf irming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (19 88)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices th at are perpendicular to the magnetic field; 2) the nonmonotonic temper ature dependence of the resonance field for the lower branch of the sp ectrum; 3) the presence of two branches of the NMR spectrum in large H -c fields, in which the CsMnBr3 must be a quasi-one-dimensional antife rromagnetic. (C) 1998 American Institute of Physics.