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The geometrically frustrated two dimensional triangular lattice magnets A4B'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 that (i) the samples containing Co 2+ (effective spin-1/2) and Ni 2+ (spin-1) ions with small spin numbers exhibit ferromagnetic (FM) ordering while the sample containing Mn 2+ (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 ordering temperature of the system. We attribute this effect to the modification of the FM interaction strength induced by the change of the O-B-O angle through chemical pressure.

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We successfully synthesized and characterized the triangular lattice anitferromagnet Ba8MnNb6O24, which comprises equilateral spin-5/2 Mn 2+ triangular layers separated by six non-magnetic Nb 5+ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin wave theory simulation on this system reveal that it has a 120 degree ordering ground state below TN = 1.45 K with in-plane nearest-neighbor exchange interaction ≈ 0.11 meV. While the large separation 18.9Å between magnetic layers makes the inter-layer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the km = (1/3, 1/3, 0) magnetic ordering. The magnetic properties of Ba8MnNb6O24, along with its classical excitation spectra, contrast with the related triple perovskite Ba3MnNb2O9, which shows easy-axis anisotropy, and the iso-structural compound Ba8CoNb6O24, in which the effective spin-1/2 Co 2+ spins do not order down to 60 mK and in which the spin dynamics shows sign of strong quantum effects.

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With the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba3CoSb2O9, a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba2.87Sr0.13CoSb2O9 with Sr doping on non-magnetic Ba2+ ion sites. The results show that Ba2.87Sr0.13CoSb2O9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120 degree spin structure at zero field with reduced ordered moment as 1.24 µB/Co; (iii) a series of spin state transitions for both H ∥ ab-plane and H ∥ c-axis. For H ∥ ab-plane, the magnetization plateau feature related to the up-up-down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba3CoSb2O9, which demonstrates that the non-magnetic ion site disorder (the Sr doping) plays a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.