Substitution Effects on Magnetic Ground States with Geometrical Spin Frustration in Triangular Spin Tubes Formed in CsCrF4 and alpha-KCrF4

摘要

A chemical-impurity substitution in a geometrical spin-frustrated antiferromagnet often produces an unconventional antiferromagnetic state, because the ground states of the pure systems maintain a delicate balance among the spins. To investigate the changes introduced by randomness in the antiferromagnetic ground states of triangular spin tubes, we synthesized polycrystalline samples of CsCr(1-x)A(x)F(4) and alpha-KCr(1-x)A(x)F(4) (A = Al, Ga, or Fe). In CsCr1-xAlxF4 and CsCr1-xGaxF4, a glass-like phase transition appears at T-g similar to 5K. In CsCr1-xFexF4, the Cr3+-F--Fe3+ (high-spin state) superexchange interactions along the tubes turn ferromagnetic. As a result, an antiferromagnetic phase transition is clearly observed at 4.5(1) K for x = 0.06, compared with CsCrF4. The obvious phase transition is attributable to the ferromagnetic interactions, which strengthen the in-plane magnetic anisotropy that stabilizes the quasi-120 degrees spin structure. On the other hand, alpha-KCr(1-x)A(x)F(4) (A = Al, Ga, or Fe) exhibited an antiferromagnetic phase transition, similar to that in alpha-KCrF4. Contrary to CsCr1-xFexF4, alpha-KCr1-xFexF4 retained the antiferromagnetic Cr3+-F--Fe3+ (high-spin state) superexchange interactions.