Magnetic Properties Of The Triangular Lattice Magnets A(4)b ' B2o12 (a=ba, Sr, La; B '=co, Ni, Mn; B=w, Re)

The geometrically frustrated two-dimensional triangular lattice magnets A(4)B'B2O12 (A=Ba, Sr, La; B'=Co, Ni, Mn; B=W, Re) have been studied by x-ray diffraction, ac and dc susceptibilities, powder neutron diffraction, and specific-heat measurements. The results reveal the following: (i) The samples containing Co2+ (effective spin-1/2) and Ni2+ (spin-1) ions with small spin numbers exhibit ferromagnetic (FM) behavior and ordering, respectively, while the sample containing Mn2+ (spin-5/2) ions with a large spin number exhibits antiferromagnetic (AFM) ordering. We ascribe these spin-number-manipulated ground states to the competition between the AFM B'-O-O-B' and FM B'-O-B-O-B' superexchange interactions. (ii) The chemical pressure introduced into the Co-containing samples through the replacement of different-size ions on the A site finely tunes the FM behavior temperature of the system. This effect is not simply governed by the lattice parameters and requires more precise structural measurements to elucidate.