E.A. Gómez Pineda

Entropy change linked to the magnetic field induced Morin transition in Hematite nanoparticles

The most stable form of iron oxide is Hematite (α-Fe2O3), which has interesting electronic, catalytic, and magnetic properties showing size dependent characteristics. At room temperature, Hematite is weakly ferromagnetic with a rhombohedral corundum structure. Upon cooling, the structure undergoes a first order spin reorientation, in which the net magnetic moment is lost. This transition is called the Morin transition. In this work, the first order Morin transition has been analyzed as a function of the temperature and applied magnetic field in Hematite nanoparticles. The magnetization was measured in the temperature range of the transformation at different applied magnetic fields to evaluate the entropy change linked to the Morin transition. The magnetic field promotes a shift of the transformation temperature. The change of entropy has been estimated on the basis of Clausius-Clapeyron type equation.

Morin transition in Hematite nanoparticles analyzed by neutron diffraction

Hematite (α-Fe2O3) undergoes a first order spin reorientation transition called the Morin transition: upon cooling, the moments align antiferromagnetically along the rhombohedral axis, and the net magnetic moment goes to zero. Morin transition temperature is around to 260K in bulk materials and depends on the mean particle size. In this work, the Morin transition has been studied by neutron diffraction as function of temperature and applied magnetic field in 47 nm nanoparticles. The Rietveld analysis of the diffraction spectra around the Morin transition shows a similar behavior to that found in bulk samples. On the other side, the magnetic field induced phase transformation has been analyzed.