Study of Elementary Excitations and Microscopic Process of Phase Transitions in Triangular Antiferromagnets

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In this project we have studied mainly hexagonal ABX_3 compounds which behave as the one-dimensional spin systems in the paramagnetic state and as TAF at low-temperatures because of the antiferromagnetic exchange coupling between -BX_3- chains. Results obtained are summarized as follows :(1) We made an effort for sample preparation and obtained many good single crystals. With the crystals we collaborated with many people inside Nagoya University and out, so that many fruitful results were produced.(2) The ESR modes in hexagonal ABX_3 antiferromagnets which cannot be understood in terms of the conventional theory of antiferromagnetic resonance were derived within the framework of the mean field theory. The results were confirmed by ESR measurements on CsNiCl_3, CsNiBr_3, RbNiCl_3, CsMnI_3 CsMnBr_3 etc. We showed that a diagonal interchain exchange coupling is essential to understand the ESR modes without lacking the consistency with the spin structure and the spin wave dispersion measur … More ed by neutron scattering.(3) We investigated the phase transitions and the critical behavior of hexagonal ABX_3 antiferromagnets by means of susceptibility, torque, optical birefringence and neutron scattering measurements and obtained various results. For example, RbVBr_3 undergoes two successive phase transitions due to the inequality of exchange interactions in the c-plane. The spin ordering in the intermediate phase is the partially disordered (PD) structure. To the author's knowledge, RbVBr_3 is the first example having the PD phase in the Heisenberg spin system In Rb_<1-chi>K_<chi>NiCl_3 a multicritical point occurs on the diagram for temperature versus chi.(4) Spin wave excitations of hexagonal ABX_3 antiferromagnets were investigated precisely by neutron scattering. It was found that the linear spin wave theory gives good description of the despersion relation for AMnX_3 systems, while not for ANiX_3 and AVX_3 systems. The deviation arises from the quantum effect.(5) We have derived the ESR modes of CsCuCl_3 which has the 120ﾟ spin structure in the c-plane and a long period helical structure along the c-axis. Our results agrees well with experimental ones obtained by Palme et al. CsCuCl_3 undergoes a phase transition at H(〕SY.apprxeq.〔)12 T for H||c. Recently Shiba and Nikuni presented a theory that the transitionis caused by the quantum fluctuation of spins. Motokawa and his collaborators performed neutron scattering experiment with pulse magnet in KEK using a large single crystal prepared by us. The results obtained is consistent with the theory. The ESR modes in the high-field phase were also measured. The results can be interpreted in terms of the spin structure proposed by the theory. Less