Effect of Nonmagnetic Impurities on the Low-Temperature Susceptibility of the Linear Antiferromagnet (CH₃)₄NMnCl₃ (TMMC)

The magnetic susceptibilities, χ_⁄⁄c and χ_⊥c, for the field parallel and perpendicular to the chain axis are calculated exactly of an anisotropic classical Heisenberg chain diluted randomly with nonmagnetic impurities. These impurities break up completely the host chain into independent segments (finite chains) with various lengths. It is shown that the combined effect of the finite chain and the dipolar anisotropy leads to the crossover in the relative values of χ_⁄⁄c and χ_⊥c from χ_⁄⁄c>_⊥c to χ_⊥c>χ_⁄⁄c at sufficiently low temperatures, which has recently been observed in (CH₃)₄NMn_1-xCd_xCl₃ (TMMC: Cd) and (CH₃)₄NMn_1-xCd_xCl₃ (TMMC: Cu). Numerical results for the temperature dependence of χ_⁄⁄c and χ_⊥c of these two systems are in good quantitative agreement with the experimental data.