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

Abstract

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

“…This angle is around 12.6 º at 10 K, which agrees with previously published values [20,21]. [22] and the (003) reflection intensity as a function of the applied magnetic field (Right). Finally, regarding the magnetostructural transition, an increase in magnetization upon heating across the Morin transition has been observed previously [22].…”

“…This direct observation evidences the magnetic field induced transition from anti to ferromagnetic behavior. Figure 3 (Right) shows a comparison between the magnetization measured by squid magnetometry [22] and the (003) reflection intensity as a function of the applied magnetic field.…”

“…[22] and the (003) reflection intensity as a function of the applied magnetic field (Right). Finally, regarding the magnetostructural transition, an increase in magnetization upon heating across the Morin transition has been observed previously [22]. The magnetization change during the transformation has been estimated as ∆M=0.3 emu/g for the maximum applied field of H=60 kOe.…”

Synthesis of phase pure iron oxide polymorphs thin films and their enhanced magnetic properties

“…Films made of iron oxide have attracted significant interest not only because of their fascinating physicochemical properties but also their potential use in scientific and technological applications [1][2][3][4][5]. It is well known that at room temperature the iron oxide exhibits three stoichiometric forms viz.…”

A Survey on the Mathematical Foundations of Axiomatic Entropy: Representability and Orderings

“…Thus, ∆S indicates a change in entropy as, say, ∆S = S 1 − S 2 , where S 1 and S 2 denote the entropies of two different equilibrium states. (This setting is still common nowadays in several contexts arising in Physics, see, e.g., [49]. )…”