Magnetic transitions in one- and two-dimensional nanostructures

One-dimensional nanostructures based on single-layer carbon nanotubes (CNTs) with FeBr2 chains and two-dimensional nanostructures with Fe-O-Fe layers deposited on the surface of silica gel by molecular layering have been synthesized. An Mossbauer spectroscopy examination has shown that a magnetic structure with T C = 21 K and B in = 29.8 ± 0.5 T is formed in nanotubes with FeBr2 · 2H2O at T = 15 K. In the case of the Fe-O-Fe layers, the surface of silica gel has a nonmagnetic monolayer along with α-Fe2O3 nanoclusters with B in(1) = 48.5 ± 0.5 T and B in(2) = 46.4 ± 0.5 T at T = 15 K and the temperature T C = 210 K. The variation in magnetization with temperature is calculated using three models of phase transitions, namely, the model of critical indices, the two-dimensional Ising model (the Onsager formula), and the three-dimensional Heisenberg model. The experimental data are fitted best by the model of critical indices with the critical parameters α = 0.22 and 0.18 for 1D and 2D structures, respectively, demonstrating that these nanosystems approach two-dimensional structures (α = 0.15).