Critical Behavior of CsVBr3and RbVBr3

Abstract

The magnetic phase diagrams of planar antiferromagnets CsVBr 3 and RbVBr 3 have been investigated by susceptibility measurements with the external field applied in the c -plane. CsVBr 3 with the CsNiCl 3 structure shows tetracritical behavior ( T N =20.3 K) as observed in CsMnBr 3 . The interval of the intermediate phase is only ∼ 1 K at H ≈8 T because of the strong intrachain exchange coupling. RbVBr 3 with a chemical unit cell enlarged to \(\sqrt{3}a\), \(\sqrt{3}a, c\) undergoes two transitions at T N1 =28.1 K and T N2 =21.0 K in zero field due to the inequality of exchange interactions in the c-plane. Both phase boundaries near the zero field transition points ( T 0 ) are described as H 2 ∝∣ T - T 0 ∣ φ with a single exponent φ≈0.8 for CsVBr 3 and φ≈1.0 for RbVBr 3 . The crossover exponent φ for CsVBr 3 is smaller than the value φ≈1.04 predicted for the n =2 chiral universality class.