Magnetic Phase Coexistence in Hematite Nanostructures below the Morin Transition: Structural Origin and Magnetic Ordering

Abstract

The coexistence of weak ferromagnetism and antiferromagnetism in hematite (α-Fe2O3) nanostructures below the Morin transition challenges the conventional picture of a homogeneous low-temperature antiferromagnetic state and points to the role of local structural heterogeneity in determining magnetic order. Here, we investigate the origin of this unusual phase coexistence using a complementary multitechnique approach combining synchrotron X-ray diffraction, pair distribution function (PDF) analysis, Mössbauer spectrometry, and neutron powder diffraction (NPD). Hematite nanostructures prepared by sol–gel autocombustion and by a commercial precipitation route were investigated down to 1.5 K. The combined results demonstrate that a weak-ferromagnetic fraction persists far below the Morin transition and coexists with the dominant antiferromagnetic phase. The residual weak-ferromagnetic fraction strongly depends on synthesis-dependent local structural features rather than on average crystallinity. The sol–gel-derived sample exhibits pronounced internal strain and local structural distortions, which stabilize a substantial WFM component at low temperature. In contrast, the more highly crystalline precipitated sample contains structural water and/or hydroxyl species, indicating that local chemical environments may provide an alternative pathway for perturbing the low-temperature magnetic state. These results reveal that nanoscale structural heterogeneity, including internal strain, local distortions, and surface-related chemical species, can influence the stability and spatial distribution of magnetic order across the Morin transition. Thus, the magnetic behavior of hematite nanostructures cannot be described by average crystallographic parameters alone but requires consideration of their local structural and chemical environment. This work highlights the central role of local structure in governing magnetic phase coexistence in nanoscale antiferromagnetic oxides.