Optical investigation of the magnon spectra of pure and Ni2+-doped K2MnF4

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Abstract
Raman and far-infrared techniques have been used to study the magnon spectrum of two dimensional (2D) antiferromagnetic K2MnF4. Values for the exchange and anisotropy constants (J=(6.3+or-0.03) cm-1 and g mu BHA=(0.219+or-0.002) cm-1) are obtained by fitting the AFMR and the two magnon mode frequencies. In the course of the investigations of Ni2+-doped K2MnF4, fluorescence measurements have been included. Localised and pair modes of the s0,(s0+s0) and (s0+d) types respectively are observed. Line frequencies and shapes are in good agreement with theoretical predictions for 3D antiferromagnets adapted for 2D systems, resulting in J'=(21.05+or-0.08) cm-1. Data are presented also for the temperature dependence of the spectra up to about twice TN and compared with theory as far as available.