Optical absorption spectra in the quasi-two-dimensional antiferromagnets (NH3(CH2)nNH3)MnCl4(n=2, 3, 4, 5). I. Experimental

Abstract

Optical absorption spectra in the quasi-two-dimensional antiferromagnet (NH₃(CH₂)_nNH₃)MnCl₄ (n=2, 3, 4, 5) were investigated. In the transitions ⁶A_1g(S) to ⁴A_1g(G), ⁴E_g(G), ⁴T_2g(D), ⁴E_g(D) and ⁴T_1g(P), much fine structure caused by electric dipole transition appeared at low temperature. It was concluded that fine structure resulted from magnon-assisted transitions by taking into account the temperature dependence of the spectra. In these compounds, when an external magnetic field was applied along the spin easy axis, a drastic energy shift in the spectra was observed at the spin-flop transition (H₀=25-35 kOe). The authors reveal the origin of these spectral changes and classify the optical absorption spectra by their original excitons, taking into account these spectral changes. It is concluded that the analysis of the spectral shift at the spin-flop transition is one of the most powerful methods to assign the absorption spectra in these antiferromagnetic compounds. Spectral changes under uniaxial stress were also investigated.