Magnetic critical behavior and exponents of the quasi-two-dimensional ferromagnet [NH3(CH2)9NH3]CuBr4

The dc initial susceptibility, spontaneous magnetization, and critical isothermal magnetization measurements of powder [NH3(CH2)9NH3]CuBr4 (9DACuBr) are reported with emphasis on the anomalous behavior of the critical exponents. Because of the weak exchange interaction between the layers, it behaves as a nearly ideal two-dimensional Heisenberg ferromagnet at finite temperature. The transition temperature Tc=11.35 K was determined by the measurements of spontaneous magnetization and initial susceptibility. The critical exponents β=0.28, γ=1.38 (T>Tc), and γ′=0.45 (T<Tc), as well as an empirical relation δ′=δ+cHd for the effective critical exponent with δ=0.65, c=0.185, and d=0.175, were found to fit the data.