Studies of Crystal Structure and Spin State in Diluted Triangular Spin Tube KCr1-xAlxF4

We performed X-ray diffraction and magnetization measurements on diluted triangular spin tubes made of α-KCr1-xAlxF4. For α-KCrF4 (x = 0), successive antiferromagnetic phase transitions occurred at TN1 2.5(1) K and TN2 = 4.0(1) K, because superexchange interactions in each non-equilateral triangle lost their equilibrium below TN2. For 0 < x ≤ 0.10, the complex crystal structures are retained. When x increases, TN2 gradually decreases, but frustration factors indicating the magnitude of the geometrical spin frustration in each triangular plane increase. Because spin-flop transition fields dramatically decrease with increasing x, the substitution of Al3+ for Cr3+ creates isotropic magnetic states. We suspect that the spin structure is strongly influenced by the slight distortion of CrF6 octahedra caused by a magnetoelectric linear effect.