The inherent existence of multi phases in iron oxide nanostructures highlights the significance of them being investigated deliberately to understand and possibly control the phases. Here, the effects of annealing at 250 °C with a variable duration on the bulk magnetic and structural properties of high aspect ratio biphase iron oxide nanorods with ferrimagnetic Fe₃O₄ and antiferromagnetic α-Fe₂O₃ are explored. Increasing annealing time under a free flow of oxygen enhanced the α-Fe₂O₃ volume fraction and improved the crystallinity of the Fe₃O₄ phase, identified in changes in the magnetization as a function of annealing time. A critical annealing time of approximately 3 h maximized the presence of both phases, as observed via an enhancement in the magnetization and an interfacial pinning effect. This is attributed to disordered spins separating the magnetically distinct phases which tend to align with the application of a magnetic field at high temperatures. The increased antiferromagnetic phase can be distinguished due to the field-induced metamagnetic transitions observed in structures annealed for more than 3 h and was especially prominent in the 9 h annealed sample. Our controlled study in determining the changes in volume fractions with annealing time will enable precise control over phase tunability in iron oxide nanorods, allowing custom-made phase volume fractions in different applications ranging from spintronics to biomedical applications.

We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe₃O₄, FeO, and α-Fe₂O₃ phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe₃O₄ phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe₃O₄ phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in an α-Fe₂O₃ oxidized surface layer whose thickness scaled with particle size resulting in a shell made of α-Fe₂O₃ phase and a core containing Fe₃O₄ or a mixture of both Fe₃O₄ and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of the α-Fe₂O₃ phase) and Verwey (of the Fe₃O₄ phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe₃O₄ phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe₃O₄/α-Fe₂O₃ (FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.

We report on the magnetic properties of biphase iron oxide nanorods (NRs) consisting of ferrimagnetic Fe₃O₄ and antiferromagnetic α-Fe₂O₃ phases. Annealing as-prepared NRs at 250 °C for 5h, significantly improved the crystallinity of the Fe₃O₄ phase and enhanced the volume fraction of the α-Fe₂O₃ phase. Magnetometry data consistently reveal these two magnetically distinct phases, which are not in proximity to each other but separated by a region of disordered spins giving rise to enhanced magnetization at low temperatures when the sample was cooled down from 300 K in the presence of a 1T field to 10 K. This phenomenon which is also known as the pinning effect is much more pronounced in the annealed sample, resulting from the increased volume fraction of the α-Fe₂O₃ phase which could strengthen the interfacial spin frustration between these two phases and enhance the density of disordered spins at the interface.

We report on the effects of annealing temperatures ranging from 225 °C to 325 °C on the magnetic properties of high aspect ratio iron oxide nanorods consisting of a ferrimagnetic Fe₃O₄ phase and an antiferromagnetic α-Fe₂O₃ phase in an as-prepared state. Annealing at the aforementioned temperatures under a constant flow of O₂ for 3 h leads to an increment of the volume fraction of the antiferromagnetic α-Fe₂O₃ phase and concomitant enhancement of the crystallinity of the ferrimagnetic Fe₃O₄ phase. These opposing effects compete with each other, resulting in a decrease in global magnetization with increasing the annealing temperature. The desirable magnetic properties are achieved for the sample annealed at 250 °C. For all samples investigated, we observed an increase in low field magnetization at low temperatures after the sample is field cooled in the presence of a 1T magnetic field, which we attribute to the ordering of macro-spins of the weakly ordered antiferromagnetic α-Fe₂O₃ phase in the presence of the cooling field. Our study will pave the way for determining the optimal conditions to enhance the magnetic characteristics in iron oxide nanorods, which will enable its use in spintronics and biomedical applications.