High-frequency EPR study of the Unusual Multiferroic NH4CuCl3

Abstract We detail orientation and temperature dependent high-frequency 240 GHz EPR experiments undertaken on NH4CuCl3 from 11–290 K. At 290 K, the system is axially symmetric, producing a single Lorentzian peak with g ‖ = 2.2674 and g ⊥ = 2.0520 . The linewidth, Δ H pp varies according to a A + B cos 2 θ dependence, which is ascribed to the Dzyaloshinsky-Moriya antisymmetric interaction. Additionally, Δ H pp ∝ g - g e , a consequence of spin-orbit coupling. Temperature measurements along each principal g axis showed little temperature variation in g until around 50 K, at which point g ‖ began to increase and g ⊥ began to decrease with decreasing temperatures, with g remaining essentially constant. Δ H pp was found to significantly vary with respect to temperature for both principal g directions, with the line rapidly narrowing with decreasing temperatures from 290–45 K, while slowly beginning to broaden again until 11 K. We propose that an Orbach relaxation process is responsible for the linewidth behavior from 290–45 K, with Δ on the order of 370 ± 15 cm - 1 , and is related to the librational mode of the ammonium ion.