Surface pinning effect and emergent magnetic properties in bi-phase iron oxide nanorods

Over the years, iron oxide (Fe$_{3}$O$_{4})$ nanorods (NRs) have been investigated for advanced magnetic hyperthermia, and spintronics applications. Here we propose a unique approach in creating a novel class of bi-phase (BP) iron oxide NRs. We demonstrate the formation of Fe$_{3}$O$_{4}+\alpha $-Fe$_{2}$O$_{3\, }$BP$_{\, }$NRs through a controlled annealing process. Hydrothermally grown Fe$_{3}$O$_{4}$ NRs were annealed at 250$^{0}$C for different periods (1-9h) to form Fe$_{3}$O$_{4}+\alpha $-Fe$_{2}$O$_{3\, }$BP structures. Magnetometry measurements indicate the sharpening of the Verwey transition with the increment of the annealing duration, leading to the improved crystallinity of the Fe$_{3}$O$_{4\, }$phase. Compared to the as-synthesized, the annealed NRs have a reduced saturation magnetization ($M_{S})$ owing to reduced volume fraction of Fe$_{3}$O$_{4\, }$and concomitant formation of the antiferromagnetic $\alpha $-Fe$_{2}$O$_{3}$ phase. With 5h of annealing a sharp drop in magnetization is observed due to Morin transition around 260K associated to $\alpha $-Fe$_{2}$O$_{3\, }$phase. The presence of canted/disordered spins at the phase boundary between the Fe$_{3}$O$_{4}$ and $\alpha $-Fe$_{2}$O$_{3}$ phases can be observed as the NRs are cooled down in 1T field from room temperature. With these observations the Fe$_{3}$O$_{4}+\alpha $-Fe$_{2}$O$_{3\, }$BP NRs would be an excellent model system for probing interfacial nanomagnetism.