Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnetLiCu2O2

Abstract

A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet LiCu 2 O 2 has been presented. Helical ordering transition temperature ͑T h ͒ of the original LiCu 2 O 2 follows finite size scaling for less than ϳ5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution Ն5.5% 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 ͑ZnϾ 0.055, H͒. The magnetic coupling constants of nearest-neighbor J 1 and next-nearest-neighbor J 2 ͑␣ = J 2 / J 1 ͒ 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 spindriven multiferroic material.

Study on the magnetic susceptibility of Li 1−x Na x Cu 2 O 2 single crystals

“…The temperature corresponding to the differential maximum T h is about 24 K, and does not change with Na doping. This temperature coincides with the phase transition point measured by specific heat measurements [1, 3,13], indicating the maximum probability of the completion of the spiral magnetic ordering transition. It can also be seen from Fig.…”

“…These results are different from those of the samples doped with Zn at the Cu sites. After Zn doping, the magnetic interactions J 1 and J 2 , are asynchronously reduced, so the frustration factor α decreases with the increasing doping content, resulting in the decrease of the temperature corresponding to the maximum probability of the magnetic transition [13]. Therefore, it should be the unchanged frustration factor α that causes the temperatures, corresponding to the maximum value of magnetic susceptibility and to the maximum probability of the magnetic transition, to be unchanged by Na doping.…”

“…In the low temperature region, there exists a transition from the paramagnetic state to the helical magnetic ordering state, and the effect of inter-chain super-exchange interaction J DC will be more significant [13] at lower temperatures. Therefore, the Eq.…”

“…It is an ideal material to study the magnetic properties of the quantum system with the competition between the classical incommensurability and the quantum commensurability. Quantum magnet LiCu 2 O 2 presents paramagnetic behaviors at high temperatures, and its magnetic structure can be well characterized by frustrated linear spin chain model with S ¼1/2 [1,2,13]. When temperature is larger than 60 K, the magnetic susceptibility can be well fitted with the high temperature zone formula of the spin S¼ 1/2 chain frustrated model (HTSE, High-Temperature Series Expansion) [1,14,15].…”

“…Recently, there were comparatively abundant researches on the magnetic structure, magnetic susceptibility, specific heat, and nuclear magnetic resonance (NMR) for the one dimensional quantum magnet LiCu 2 O 2 , and the investigations on X-ray diffraction, magnetic susceptibility and nuclear magnetic resonance were also performed on the samples with Zn [13] and V [7] doping at the Cu sites. However, the doping effects at the Li sites have not been fully investigated.…”