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The Morin transition of α Fe2O3

The sample's magnetic properties after the pressure release were investigated by measuring the temperature dependence of its mass susceptibility, χ. Its χ profile contains two pronounced maxima at ~69 K (designated TN) and ~269 K (designated TM). Moving away from these temperatures, χ decreases, indicating a transition to an antiferromagnetic state. The profile of the maximum at ~269 K resembles that of the Morin transition of α-Fe2O3, i.e., the transition from a weakly ferromagnetic regime to an antiferromagnetic state accompanied by a 90° spin reorientation (from the α-Fe2O3 basal plane to the c-axis direction). The second maximum is associated with a sharp peak and indicates the Néel temperature of the ζ-Fe2O3 phase. The transition at ~69 K shows features characteristic of a second-order thermodynamic transition. Thus, TN can be regarded as the thermodynamic temperature of the passage from a magnetically disordered (paramagnetic) state to a magnetically ordered (antiferromagnetic) regime. The origin of the fall in the Néel temperature ζ-Fe2O3 with respect to β-Fe2O3 can be explained in terms of changes in the lattice volume. The lattice volume of ζ-Fe2O3 is larger than that of β-Fe2O3, resulting in a decrease in the strength of superexchange interactions. Following the equation reported by Bloch, ∂ln J = γ × ∂ln V (where J is the superexchange parameter (exchange integral), V is the lattice volume, and γ is the constant value given as –10/3), it turns out that the J value decreased as 93% by the phase transformation from β-Fe2O3 to ζ-Fe2O3. As TN depends on J, the decrease in J is then macroscopically manifested in the decrease in TN, as experimentally evidenced by the temperature behavior of χ. Here, it should be stressed that the decrease in TN does not result from strains or defects because the temperature differential of χ (i.e., ∂χ/∂T) at TN (69 K) is sharper than that at 269 K belonging to TM of α-Fe2O3. If the local disorder promoted the decrease in TN in β-Fe2O3, the peak at TN should be broadened. Therefore, TN at 69 K is considered the magnetic transition temperature of the ζ-Fe2O3 phase. If you are looking for high quality, high purity, and cost-effective Iron oxide, or if you require the latest price, please feel free to email contact mis-asia.

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