Annealing control of hydrothermally grown hematite nanorods: Implication of structural changes and Cl concentration on weak ferromagnetism

In-situ X-ray diffraction coupled with thermogravimetry and differential scanning calorimetry were used to elucidate the crystal phase transition from akaganeite to hematite nanorods. The results indicate that though complete phase transformation occurs at similar to 663 K, residual chlorines from the hydrothermal synthesis conditions strongly affect spin moment. The high temperature (>1023 K) shock-annealing reduced the chlorine concentration and consequent suppress most of surface defects on the rods. High temperature annealed hematite nanorods were then studied in terms of magnetic properties under different applied hydrostatic pressures and exhibited a shift of T-M to higher values, from 261 to 282 K with initial rate (partial derivative T-M/partial derivative P coefficient) of 46 K GPa(-1). Such positive dependence of T-M with hydrostatic applied pressure is consistent with several works reported in the literature investigating different morphologies in bulk and at nanoscale. In fact, gradual reduction of Cl species increases the spin mobility allowing the appearance of weak-ferromagnetism at room temperature. The crossover results strongly support the hypothesis that Cl dominates spin mobility. (C) 2019 Elsevier B.V. All rights reserved.