A Strategy to Modulate the Bending Coupled Microwave Magnetism in Nanoscale Epitaxial Lithium Ferrite for Flexible Spintronic Devices

Abstract

With the development of flexible electronics, the mechanical flexibility of functional materials is becoming one of the most important factors that needs to be considered in materials selection. Recently, flexible epitaxial nanoscale magnetic materials have attracted increasing attention for flexible spintronics. However, the knowledge of the bending coupled dynamic magnetic properties is poor when integrating the materials in flexible devices, which calls for further quantitative analysis. Herein, a series of epitaxial LiFe₅O₈ (LFO) nanostructures are produced as research models, whose dynamic magnetic properties are characterized by ferromagnetic resonance (FMR) measurements. LFO films with different crystalline orientations are discussed to determine the influence from magnetocrystalline anisotropy. Moreover, LFO nanopillar arrays are grown on flexible substrates to reveal the contribution from the nanoscale morphology. It reveals that the bending tunability of the FMR spectra highly depends on the demagnetization field energy of the sample, which is decided by the magnetism and the shape factor in the nanostructure. Following this result, LFO film with high bending tunability of microwave magnetic properties, and LFO nanopillar arrays with stable properties under bending are obtained. This work shows guiding significances for the design of future flexible tunable/stable microwave magnetic devices.

Keywords: epitaxial oxide thin films; ferromagnetic resonance; flexible devices; magnetism.

Figure 1

XRD θ–2θ scans of A‐LFO and T‐LFO films with a) (001), b) (110), and c) (111) OOP orientations, respectively. Inset of (a)–(c): corresponding photographs of the T‐LFO films. FMR spectra of d) T‐LFO (001), e) T‐LFO (110), and f) T‐LFO (111) along IP (θ_H = 0°) and OOP (θ_H = 90°) orientations. g) Experimental (scatter) and fitting curves (dash line) of angular θ_H‐dependent H _r for T‐LFO films with different orientations. Upper left corner of (g): schematic illustration of FMR spectroscopy experimental set‐up for the film sample. h) M _s of T‐LFO films with different OOP orientations. Inset of (h): a table listed the 4πM _s value and the fitting parameters H _eff, H_{K 1}, and H_{K 2}.

Figure 2

a) Schematic illustration of FMR spectroscopy experimental set‐up for the sample under bending. Angular θ_H‐dependent FMR spectra for T‐LFO (001) film with b) unbent and c) bending status. d–h) Counter plot of the θ_H‐dependent‐integrated FMR spectra for the T‐LFO (001) film with different bending states.

Figure 3

a) Diagram of the simple model for explaining the bending tuned FMR spectra of the LFO film. b) Counter plot of the simulated θ_H‐dependent‐integrated FMR spectra for the T‐LFO (001) film with different bending status. Here, R _s = C/▵ϕ. c) Comparison of the experimental and simulated FMR spectra lines for T‐LFO films at θ_H = 75°, R = 7.6 mm. d–f) Experimental (scatter) and simulation curves (dash line) of angular θ_H‐dependent H _z for T‐LFO (001) films under different bending states. g) Angular θ_H‐dependent H_{z c} for T‐LFO (001) films under different bending states.

Figure 4

a) SEM image of the NP‐LFO (111) showing the surface morphology. b) The cross‐sectional TEM image showing the microstructure of the NP‐LFO (111). c) FMR spectra measured along IP and OOP orientations for the NP‐LFO (111). The scatter and the solid line are the experimental FMR data and the modified Dyson line, respectively. Inset of (c): corresponding M‐H loops of the NP‐LFO (111) measured at room temperature (300 K). d) Experimental (scatter) and fitting curves (dash line) of angular θ_H‐dependent H _r for the NP‐LFO and the M‐LFO samples. Inset of (d) is a table, in which the fitting parameters H _eff, H_{K 1}, and H_{K 2} are listed. e–h) Counter plot of the θ_H‐dependent‐integrated FMR spectra for the NP‐LFO (111) film with different bending states.

Figure 5

a) Diagram for the relationship among the 4πM _s, H _eff, MDR, and the promising application area of the LFO samples. b) The quantitative result of the bending radius R‐dependent H _zc for T‐LFO (001) and NP‐LFO (111) at θ_H = 90°.