Nakajima, T.; Mitsuda, S.; Okano, H.; Inomoto, Y.; Kobayashi, S.; Prokes, K.; Gerischer, S.; Smeibidl, P.: Nonmagnetic Impurity Effect on Magnetic Phase Transitions in an Isosceles Triangular Lattice Ising Chain Antiferromagnet CoNb2O6. Journal of the Physical Society of Japan 83 (2014), p. 094723/1-7
10.7566/JPSJ.83.094723

Abstract:
We have investigated nonmagnetic impurity effect on the Hkc-T magnetic phase diagram of CoNb2O6, by means of neutron diffraction measurements using single-crystal samples of Co1−xMgxNb2O6 with x = 0; 0:004 and 0.008. This system consists of nearly isolated ferromagnetic Ising spin chains running along the c axis, which are arranged in an isosceles triangular lattice in the ab plane. Weak antiferromagnetic interactions between the chains lead to geometrical spin frustration in the basal plane, and induce a variety of magnetic orderings at low temperatures. 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 (FR) phase, thermally-induced incommensurate (IC) magnetic phase and the paramagnetic phase, are hardly affected by the small amount of nonmagnetic substitution. Comparing the present results with the previous study on CsCo1−xMgxCl3 [Mekata et al. J. Phys. Soc. Jpn. 56 4544 (1987)], we conclude that the disappearance of the AF phase is not because the nonmagnetic impurities reduce the mean field energy of the AF phase, but because the magnetic phase transition from the IC phase to the AF phase is suppressed by a pinning effect due to the impurities. By means of Monte Carlo simulations for the isosceles triangular lattice Ising model, we have also checked the stability of the AF phase against a few percent of nonmagnetic substitution. The similarity between Co1−xMgxNb2O6 and CsCo1−xMgxCl3 suggests that the existence of the 1D Ising chains with strong intrachain interactions and the magnetically disordered Co2+ sites in the IC phase are the keys to the extremely high sensitivity to the nonmagnetic impurities.