Magnetic properties and critical behavior of quasi-two-dimensional systems [C6H5

The magnetic properties and critical behavior of the quasi-two-dimensional ferromagnets [${\mathrm{C}}_{6}$${\mathrm{H}}_{5}$(${\mathrm{CH}}_{2}$${)}_{\mathit{n}}$${\mathrm{NH}}_{3}$${]}_{2}$${\mathrm{CuBr}}_{4}$, with n=1, 2, and 3, have been studied. These compounds are typical of layered magnets in that they assume distorted layer perovskite structures in which copper(II) bromide layers are sheathed by the bulk organic cations. Magnetization and susceptibility measurements show strong ferromagnetic intralayer exchange of the order of 20--25 K with weak interlayer exchange coupling of less than 1 K. The magnetic anisotropies are explored through EPR-linewidth analysis. The critical exponent \ensuremath{\gamma} and transition temperatures ${\mathit{T}}_{\mathit{c}}$ were determined by the measurements of ac initial susceptibility and isothermal magnetization. By employing a static scaling law to analyze the data of isothermal magnetizations near ${\mathit{T}}_{\mathit{c}}$, it was found that in a certain low-field and low-temperature region the powder magnetizations are suppressed so that the scaling hypothesis no longer holds. It is shown from the scaling analysis that the powder samples of these two-dimensional (2D) layer ferromagnets cannot be classified as simple ferromagnets whose critical behavior is described by the usual 2D critical theories.