Neutron powder diffraction study of Win-bearing hematite, alpha-Fe__(2-x)Mn_xO_3, in the range 0 < = x <= 0.176

摘要

A detailed neutron powder diffraction and calorimetric study was conducted to determine the influence of increasing Mn-substitution on the crystal and the magnetic structures of hematite, a-Fe_2O_3. The study was initiated to determine, if Mn substitution may be responsible for unusual ferromagnetic properties of natural hematite samples from the Kalahari Mn field, South Africa. Natural as well as synthetic Mn-bearing hematite samples with the compositional range Fe_(2-x)Mn_xO_3 (x = 0 to 0.176) were examined. Calorimetric measurements were performed to determine the Neel T and the Morin Tm temperature transitions. All studied hematite samples, irrespective of chemical composition, display weak ferromagnetism at 295 K and coexistence of weak ferromagnetic and antiferromagnetic phases at 10 K. A slight decrease of the total magnetic moment and but a drastic decrease of Fn can be attributed to increasing Mn-substitution. The results illustrate that Mn substitution may contribute but cannot be the sole reason for the unusual magnetic properties of natural hematite samples.