NMR in ⁵⁵Mn²⁺ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr₃

Abstract

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr₃, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that 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 v _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 according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. 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 that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 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 antiferromagnetic.