Competing Magnetic Interactions and Field-Induced Metamagnetic Transition in Highly Crystalline Phase-Tunable Iron Oxide Nanorods

Abstract

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.

Keywords: Morin transition; Verwey transition; annealing; biphase iron oxide; magnetic hyperthermia; magnetic nanorods; metamagnetic transition; spintronics.

Figure 1

XRD patterns of the (a) as-prepared iron oxide nanorods (inset shows a TEM image of the as-prepared iron oxide nanorods), (b) 1 h annealed, (c) 3 h annealed, (d) 5 h annealed, (e) 7 h annealed, and (f) 9 h annealed. For comparison, the XRD data of (a) were taken from Ref. [33].

Figure 2

ZFC and FC M(T) curves in an applied field of 0.05 T for (a) as-prepared, (b) annealed for 1 h, (c) annealed for 3 h, (d) annealed for 5 h, (e) annealed for 7 h, and (f) annealed for 9 h iron oxide nanorods. For comparison, the M(T) data of (a) were taken from Ref. [33].

Figure 3

Magnetic hysteresis loops M(H) of (a) as-prepared, (b) annealed for 1 h, (c) annealed for 3 h, (d) annealed for 5 h, (e) annealed for 7 h, and (f) annealed for 9 h iron oxide nanorods at 300 K. For comparison, the M(H) data of (a) were taken from Ref. [33].

Figure 4

(a) Magnetic hysteresis loops M(H) between 250 K and 300 K, (b) magnetization curve and the change in magnetization with respect to field vs. field curve at 280 K, (c) Arrott plots between 250 K and 300 K, and (d) the transition field (H_Trans) vs. temperature of the 9 h annealed sample.

Figure 5

Magnetic hysteresis loops M(H) recorded with ZFC and FC protocols in an applied field of 1 T for (a) as-prepared, (b) annealed for 1 h, (c) annealed for 3 h, (d) annealed for 5 h, (e) annealed for 7 h, and (f) annealed for 9 h iron oxide nanorods at 10 K. For comparison, the M(H) data of (a) were taken from Ref. [33].

Figure 6

(a) Maximum magnetization at 1 T, (b) Coercivity at 10 K and 300 K, (c) Difference between the FC and ZFC magnetization at 10 K on 0.25 T and 1 T applied field of iron oxide nanorods, and (d) Percentage change in magnetization around VT and MT of 5, 7, and 9 h annealed iron oxide nanorod samples.

Figure 7

Temperature dependence of coercivity and coercivity difference between the 5 h annealed and 9 h annealed iron oxide nanorods.