Crack-induced magnetic properties in iron oxide nanoparticles: structure, morphology, phase transition and hysteresis behavior

Cracked magnetic nanoparticles provide a system for investigating how disrupted structural symmetry influences nanoscale magnetic behavior and enables the development of novel technological applications. In this study, cracked spherical hematite (α-Fe2O3) nanoparticles were synthesized by hydrolysis of aqueous FeCl3, for 2 h (S2h), 12 h (S12h) and 18 h (S18h). X-ray powder diffraction (XRPD) confirmed that all samples consist of phase-pure hematite. Transmission electron microscopy (TEM) revealed cracked-spherical hematite nanoparticles with diameters of 30 nm (S2h), 40 nm (S12h) and 70 nm (S18h). Magnetic characterization was performed using a superconducting quantum interference device (SQUID) magnetometer, which revealed several distinctive features of the magnetic behavior in nanoscale hematite. All samples exhibit a strongly suppressed Morin transition, associated with the spin reorientation from the antiferromagnetic to the weak-ferromagnetic state. The Morin transition temperatures are TM = 95.8 K for S2h and TM = 143.6 K for S12h. This pronounced reduction indicates a destabilized spin-reorientation process driven by symmetry breaking and the high surface-to-volume ratios of the cracked hematite nanoparticles. For sample S18h, the suppressed Morin transition temperature determined from the ZFC magnetization curve is TMZFC = 177.3 K, whereas the value obtained from the FC curve is TMFC = 149.5 K. This significant discrepancy is attributed to the presence of enhanced structural defects and surface-induced spin canting in the cracked hematite nanoparticles. Moreover, the surface spin ordering is observed at temperatures below 50 K, where the magnetization curves exhibit an upturn. Above TM, the S12h sample exhibits a highly reversible weak-ferromagnetic regime with negligible coercivity and remanence. This indicates that at room temperature, thermal fluctuations overcome the reduced anisotropy barriers that would otherwise stabilize its non-collinear spin states. At 5 K, S12h and S18h show complete overlap of the virgin and full hysteresis loops, confirming the absence of irreversible processes. Under high fields (|H|>2 kOe), the initial and remagnetization branches broaden significantly, whereas the demagnetization branch remains conventional, pointing to a gradual, field-driven reorganization of frustrated surface spins (spin flop). These findings demonstrate how crack-mediated structural disorder and finite-size effects govern the Morin transition and hysteresis response in hematite nanostructures, offering insights relevant for the design of disorder-engineered antiferromagnetic nanomaterials.