Thermodynamic properties and rare-earth spectroscopy of Cu₃Nd(SeO₃)₂O₂X (X = Cl, Br)

The kagome lattice of copper ions in francisite-type compounds is quite sensitive to external stimuli, being easily affected by temperature and magnetic field. The rare-earth ions inserted between buckled layers of transition metal add new dimension to magnetism in these systems. Neodymium francisites, Cu₃Nd(SeO₃)₂O₂X (X = Cl, Br), experience long range magnetic order at T_N = 34 K (Cl) or 35.2 K (Br) and spin reorientation transition at T_R = 4 K (Cl) or 4.6 K (Br) marked by sharp anomalies in magnetization M, specific heat C_p, and optical spectra. The spin reorientation of Cu²⁺ ions is due to d-f interaction of transition and rare-earth magnetic subsystems. Under action of modest external field B < 2 T both compounds exhibit metamagnetic phase transition, largely hysteretic in case of Cl compound at T < T_R.