Effects of geometrical spin frustration on triangular spin tubes formed in CsCrF₄ and α-KCrF₄

We conducted X-ray diffraction, magnetic susceptibility, heat capacity, high-field magnetization, and electron spin resonance experiments on high-quality polycrystalline S = 3/2 compounds consisting of equilateral and non-equilateral triangular spin tubes in CsCrF₄ and α-KCrF ₄, respectively. In both compounds, geometrical spin frustration in the triangular planes coexists with one-dimensionality along the tubes. In CsCrF₄, no magnetic long-range order appeared above 1.5K, which agrees with a prior report [H. Manaka et al.: J. Phys. Soc. Jpn. 78 (2009) 093701]. In a low temperature (T) regime, T-linear component of heat capacity appeared and magnetization curves showed finite initial slopes. As a result, CsCrF₄ consists of a gapless spin-liquid state encompassing resonating spin-singlet pairs not only in each equilateral triangle but also along the tubes. In contrast, for α-KCrF₄, successive magnetic phase transitions occurred at T_N1 = 2.5(1) K and at T_N2 = 4.0(1) K because superexchange interactions through three Cr-F-Cr paths in each non-equilateral triangle lost their equilibrium.