DIMENSIONAL CROSSOVER IN THE MAGNETIC-PROPERTIES OF HIGHLY ANISOTROPIC ANTIFERROMAGNETS

Magnetic properties of highly anisotropic pervoskites based on oxide planes have commonly been interpreted in terms of the free-spin-wave approximation to a Heisenberg model. For this model, we calculate the Neel temperature, the magnetic moment, the spin-wave velocity, and the first-nearest-neighbor instantaneous spin correlator by varying the anisotropy ratio epsilon = J perpendicular-to / J parallel-to between the intraplane and interplane effective exchange couplings at three different levels of approximation: the ordinary free-spin-wave, the Tyablikov random-phase approximation (RPA), and a modified RPA obtained by adapting Callen original decoupling procedure. In all cases, we find that the crossover from three to two dimensions occurs at epsilon almost-equal-to 10(-3). By interpreting the available experimental data for La2CuO4 with our calculations, we also find that the value of epsilon verges on the two-dimensional side, although it can vary by a factor 10(2) depending on the approximation. Our results call for more accurate experimental determination and theoretical interpretation of the temperature-dependent magnetic excitations in the perovskite compounds. Their two-dimensional character favors, in fact, the existence of local magnetic excitations that outlive the disappearance of long-range order, as shown by the calculated behavior of the first-nearest-neighbor instantaneous spin correlator.