Successive Phase Transitions and Multiferroicity in Deformed Triangular-Lattice Antiferromagnets Ca3MNb2O9 (M=Co, Ni) with Spatial Anisotropy

We constructed the magnetic field-temperature phase diagrams of new quasi-two-dimensional isosceles triangular lattice antiferromagnets (TLAF) Ca3MNb2O9 (M=Co, Ni) from dc and ac magnetic susceptibilities, specific heat, dielectric constant, and electric polarization measurements on single crystalline samples. Ca3CoNb2O9 with effective spin-1/2 Co2+ ions undergoes a two-step antiferromagnetic phase transition at T (N1) = 1.3 K and T (N2) = 1.5 K and enters a stripe ordered state at zero magnetic field. With increasing field, successive magnetic phase transitions, reminiscent of the up-up-down (uud) and the oblique phases, are observed. The dielectric constant of Ca3CoNb2O9 shows anomalies related to the magnetic phase transitions, but clear evidence of ferroelectricity is absent. Meanwhile, Ca3NiNb2O9 with spin-1 Ni2+ ions also shows a two-step antiferromagnetic transition at T (N1) = 3.8 K and T (N2) = 4.2 K at zero field. For Ca3NiNb2O9, the electric polarization in the magnetic ordered phases was clearly observed from the pyroelectric current measurements, which indicates its coexistence of magnetic ordering and ferroelectricity.