Impact of fluoride for hydroxide substitution on the magnetic properties of a Co-based single-ion magnet imbedded in the barium apatite crystal lattice

Cobalt-containing barium fluoro/hydroxy-apatites were prepared by a high-temperature solid state reaction in air. Cobalt ions are partially located in the trigonal channels of the apatite lattice, formally substituting the protons of hydroxide groups. The samples exhibit single-ion magnet features – slow relaxation of magnetization up to 40 K with parameters close to those in the parent cobalt-containing barium hydroxyapatite. A partial fluoride for hydroxide substitution (40%) facilitates the formation of the apatite phase, while the energy barrier for magnetization reversal retains its high value of 326 cm−1 and the irreversibility in low temperature magnetization is substantially increased. The 90% substitution results in the full disappearance of the high-energy single-ion magnetic centers, and the magnetic behavior suggests the presence of a high-spin cobalt ion in the +4 oxidation state.