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 to 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, ΔHpp varies according to a A+Bcos2θ dependence, which is ascribed to the Dzyaloshinsky-Moriya antisymmetric interaction. Additionally, ΔHpp∝g-ge, 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. ΔHpp was found to significantly vary with respect to temperature for both principal g directions, with the line rapidly narrowing with decreasing temperatures from 290 to 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 to 45 K, with Δ on the order of 370±15cm-1, and is related to the librational mode of the ammonium ion.