Nonlinear Magnetic-Susceptibility of Two-Dimensional Magnets (C_nH_2n+1NH₃)₂CuCl₄withn=1, 2 and 3

Abstract

The two-dimensional (2-d) compounds (C n H 2 n +1 NH 3 ) 2 CuCl 4 with n =1, 2 and 3 have ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is ferromagnetic for n =1 and antiferromagnetic for n =2 and 3. The nonlinear magnetic-susceptibility χ 2 on these three magnets has been measured around the critical temperatures of the respective magnets using a mutual-inductance ac-magnetometer and a dc SQUID magnetometer. The curves of a log-log plot of χ 2 versus ε ≡ ( T - T c )/ T c for n =1 and 3 are discussed consistently with those of χ 0 versus ε , which indicates the crossover from a 2-d Ising to a 3-d Ising regime. The critical behavior of χ 2 ( T ) observed in n =2 reflects the successive crossovers, i.e., 2-d Heisenberg → 3-d Heisenberg → 3-d Ising as T approaches to T N .