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

TL;DR

This study investigates how nonmagnetic Zn impurities affect the magnetic phases and ordering in the quasi-two-dimensional helimagnet LiCu2O2, revealing impurity-induced phase transitions and the relationship between magnetic order and electric polarization.

Contribution

It provides a comprehensive phase diagram showing impurity effects on helimagnetic order and introduces new insights into the coupling between magnetic and electric properties in this material.

Findings

Helical ordering persists up to ~5.5% Zn substitution.

Higher Zn levels (>5.5%) suppress long-range order and induce phase transitions.

Magnetic coupling constants were quantitatively determined.

Abstract

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 LiCu2O2 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 (nn) J1 and next-nearest-neighbor (nnn) J2 (alpha=J2/J1) are extracted from high temperature series expansion (HTSE) 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.