Experimental Study of New Type Phase Transition in Triangular Lattice Antiferromagnet VCl2

Critical properties of a Heisenberg antiferromagnet on a triangular lattice are investigated experimentally using a quasi-two-dimensional antiferromagnet VCl 2 by means of neutron scattering. It is found that VCl 2 shows characteristic critical behaviors of two and three dimensional models. A little above T N , the line shape of χ( Q ) is found to be represented by the Ornstein-Zernike form (κ 2 + q 2 ) -1 . This suggests the existence of magnetic point defects which were predicted in the two dimensional model. The measurecl critical exponents are β=0.20±0.02, γ=1.05±0.03, and ν=0.62±0.05, in agreement with the theoretical work for three dimensional SO(3) systems. Successive phase transitions due to a small Ising anisotropy were found at T N1 =35.88±0.01 K and T N2 =35.80±0.01 K. The spin structure and the dispersion relation of the spin wave well below T N are also determined.