The influence of magnetic field on Cu(tn)Cl$_{2}$ - two-dimensional quantum magnet with N\'{e}el ground state.

AC susceptibility and neutron elastic scattering experiments were performed on Cu(tn)Cl$_{2}$ [1] to explore the origin of a field induced anomaly previously observed in specific heat below 1 K and in finite fields up to 7 T [2]. Isothermal AC susceptibility scans, made down to 40 mK and in fields up to 10 T, confirmed and extended the B vs. T phase diagram obtained from the specific heat data. Neutron spectra of a polycrystalline sample studied down to 0.5 K in B = 0 and 4 T did not reveal any magnetic peaks expected below 1 K and indicated a slight change of (200) nuclear peak intensity induced by the magnetic field. The possible reasons of the absence of a phase transition to ordered state will be discussed. [1] V. Zelenak \textit{et al.}, Inorg. Chem. \underline {45} (2006) 1774. [2] A. Orendacova \textit{et al.}, in preparation.