Various disordered ground states and 1/3 magnetization-plateau-like behavior in the S=1/2 Ti~(3+) kagome lattice antiferromagnets Rb_2NaTi_3F_(12), Cs_2NaTi_3F_(12), and Cs_2KTi_3F_(12)

摘要

We have investigated the crystal structure and magnetic properties of three kagome lattice antiferromagnets, Rb_2NaTi_3F_(12), Cs_2NaTi_3F_(12), and Cs_2KTi_3F_(12), using single crystals. These compounds represent a 5 = 1/2 kagome system consisting of magnetic Ti~(3+) ions, which is expected to have negligibly small Dzyaloshinsky-Moriya interaction. The structural analyses revealed that each of the three compounds has a slightly distorted kagome lattice. The distortion of the kagome lattice becomes small as the ionic radii of constituent alkali metals increase. All three compounds have nearly the same Weiss temperature of -45 K, and the ground states are disordered and strongly depend on the distortion. The ground states of Rb_2NaTi_3F_(12), Cs_2NaTi_3F_(12), and Cs_2KTi_3F_(12) are found to be a two-component state including approximately 1/3 nearly free spins, a gapless disordered state, and a gapped disordered state, respectively. Our experimental results suggest that the ground state of the ideal S = 1/2 Heisenberg kagome lattice antiferromagnet is gapped. In addition, the magnetization curves of Cs_2NaTi_3F_(12), and Cs_2KTi_3F_(12) show anomalies at approximately 1/3 of the full magnetic moment of Ti3+, which are a notable observation of signs of the theoretically proposed 1/3 magnetization plateau in S = 1/2 kagome antiferromagnets.