Antiferromagnetism and magnetic frustration in the metalorganic compounds MCl2 -4SC( NH2)2, M = (Mn,Fe)

Abstract

We report magnetic and specific heat measurements at low temperatures ($>100$ mK) and high magnetic fields (up to 9 T) for the metalorganic compounds ${\mathrm{MnCl}}_{2}\text{\ensuremath{-}}4\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}$ (DTM) and ${\mathrm{FeCl}}_{2}\text{\ensuremath{-}}4\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}$ (DTF). Fits to the experimental data from the Curie-Weiss law and mean-field theory indicate the antiferromagnetic nature of both compounds. For temperatures down to $T=0.56$ K the compound DTM exhibits two successive transitions, which may be associated with different magnetic orderings. In this system the external magnetic field is a tuning parameter that allows us to access such magnetic phases separated by distinct quantum critical points. No indication of magnetic ordering is found in DTF down to 100 mK, making this compound a candidate for a strongly frustrated material with frustration parameter $f=|{\ensuremath{\theta}}_{\mathrm{CW}}|/{T}_{N}>100$. Finally, we discuss the absence of Bose-Einstein condensation of magnons and the possibility of a spin-orbital liquid state in DTF.