Hematite: Morin temperature of nanoparticles with different size

A spin-reorientation transition from a weakly ferromagnetic (WF) to an antiferromagnetic (AF) spin ordering in hematite (alpha-Fe2O3) during cooling occurs at Morin temperature (T-M similar to 264 K for bulk). The transition is strongly size dependent and T-M generally decreases with the decreasing volume of the particles. For particles smaller than approximately similar to 20 nm, the Morin transition may be even suppressed and disappears entirely as near-surface spins deviate strongly from the antiferromagnetic easy axis. We report an investigation on nanoparticles prepared by hydrothermal method and sol-gel technique (in silica) of pure alpha-Fe2O3 phase as confirmed by XRD (space group R-3c, lattice parameters a = 5.038(2) angstrom, c = 13.772(12) angstrom) differing in the median size derived by TEM: 5.6 nm, 26 nm, 42 nm and 103 nm. By means of Mossbauer spectra acquired between 4.2 and 300 K, we determined the relative concentrations of magnetic phases (WF and AF) within the Fe-57 enriched sample and searched for the best finite-scaling theoretical model (mean-field, 3D Heisenberg, Ising) describing the derived size dependence of Morin temperature of the nanoparticles with a log-normal size distribution. The comparison of relevant parameters derived from the fit of experimental data by theoretical model is consistent with the 3D Heisenberg model with scaling parameter lambda = 1.4, Morin temperature of bulk material T-M(infinity) = 265(1) K and correlation length xi(0) = 8.1(2) nm or Ising model with lambda = 1.6, T-M(infinity) = 265(1) K and xi(0) = 9.4(2) nm.