Static and dynamic magnetic properties of two synthetic francisites Cu₃La(SeO₃)₂O₂X (X = Br and Cl)

The formation of long-range magnetic order at low temperatures was established in francisite—type compounds Cu₃La(SeO₃)₂O₂X (X = Br and Cl) through measurements of magnetic susceptibility, magnetization, specific heat and X-band electron spin resonance. The significantly enhanced critical index p = 1.0 ± 0.1 in Cu₃La(SeO₃)₂Br and p = 0.8 ± 0.1 in Cu₃La(SeO₃)₂Cl in the temperature dependence of the width of ESR signal evidence the reduced dimensionality of the kagome-type francisite’s magnetic subsystem. Under action of external magnetic field, the presumably non-collinear six-sublattices antiferromagnetic structure of these compounds experiences the first-order metamagnetic transformation. The B–T magnetic phase diagrams were established from the positions of singularities in temperature and field dependences of thermodynamic properties. Contrary to pristine mineral Cu₃Bi(SeO₃)₂Cl, no signature of structural phase transition was detected.