Abstract
A spin-reorientation transition from a weakly ferromagnetic (WF) to an antiferromagnetic (AF) spin ordering in hematite (α-Fe2O3) during cooling occurs at Morin temperature (TM~264 K for bulk). The transition is strongly size dependent and TM generally decreases with the decreasing volume of the particles.
For particles smaller than approximately ~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 α-Fe2O3 phase as confirmed by XRD (space group R-3c, lattice parameters a=5.038(2) Å, c=13.772(12) Å) differing in the median size derived by TEM: 5.6 nm, 26 nm, 42 nm and 103 nm.
By means of Mössbauer spectra acquired between 4.2 and 300 K, we determined the relative concentrations of magnetic phases (WF and AF) within the 57Fe 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 λ=1.4, Morin temperature of bulk material TM(oo)=265(1) K and correlation length ξ0=8.1(2) nm or Ising model with λ=1.6, TM(oo)=265(1) K and ξ0=9.4(2) nm.