Francisites as new geometrically frustrated quasi-two-dimensional magnets

The synthesis of new geometrically frustrated layered systems has fueled experimental work and progress in building models of low-dimensional magnetism. Compounds with the structure of the francisite mineral, $\rm Cu_3Bi(SeO_3)_2O_2Cl$, are quasi-two-dimensional antiferromagnets with a kagome-type lattice. With the dominant ferromagnetic interaction in the layer and a weak interlayer antiferromagnetic bond, the main noncollinear state of francisite is easily destroyed by an external magnetic field, which opens the possibility of reversible switching between states with the minimum and maximum possible magnetization. In the region of metamagnetic transition, multiferroelectric effects and broadband absorption of electromagnetic waves are observed. The implantation of rare-earth ions $R$ into the Bi position is accompanied by spin-reorientation phase transitions in $\rm {Cu_3}$ $R$ $(\rm {SeO_3})_2\rm {O_2}$ $X$ compounds, where $X=\rm Cl$, $\rm Br$.