Nonmagnetic Impurity Effect on Magnetic Phase Transitions in an Isosceles Triangular Lattice Ising Chain Antiferromagnet CoNb₂O₆

We have investigated nonmagnetic impurity effect on the H||c–T magnetic phase diagram of an isosceles triangular lattice Ising antiferromagnet CoNb2O6, by means of neutron diffraction measurements using single crystals of Co1−xMgxNb2O6 with x = 0, 0.004, and 0.008. We have found that the commensurate antiferromagnetic (AF) ground state disappears by substituting only 0.8% of nonmagnetic Mg2+ ions for the magnetic Co2+ ions. On the other hand, the phase boundaries between the other phases, namely the field-induced ferrimagnetic phase, thermally-induced incommensurate (IC) magnetic phase and the paramagnetic phase, are hardly affected by the small amount of nonmagnetic substitution. We have also performed Monte Carlo simulations for the isosceles triangular lattice Ising model to understand the extremely high sensitivity to the nonmagnetic substitution. Consequently, we have revealed that the disappearance of the AF phase is not because the small amount of nonmagnetic impurities destabilize the AF phase, but because the phase transition from the IC phase to the AF phase is strongly suppressed by a pinning effect due to the impurities.