Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu₂O₂

A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet LiCu₂O₂ has been presented. Helical ordering transition temperature (T_h) of the original LiCu₂ O ₂ follows finite size scaling for less than ∼5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution ≥5.5% quenches the long-range helical ordering and introduces an intriguing Zn-level-dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for T_c (Zn>0.055,H). The magnetic coupling constants of nearest-neighbor J₁ and next-nearest-neighbor J₂ (α= J₂ / J₁) are extracted from high temperature series expansion fitting and N=16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin-driven multiferroic material.