Effects of Geometrical Spin Frustration on Triangular Spin Tubes Formed in CsCrF4 and \alpha-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 \alpha-KCrF4, 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 1.5 K, 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 \alpha-KCrF4, successive magnetic phase transitions occurred at T_{\text{N1}} = 2.5(1) K and at T_{\text{N2}} = 4.0(1) K because superexchange interactions through three Cr--F--Cr paths in each non-equilateral triangle lost their equilibrium.