Microscopic Origin of Magnetization Reversal in Nanoscale Exchange-Coupled Ferri/Ferromagnetic Bilayers: Implications for High Energy Density Permanent Magnets and Spintronic Devices

Abstract

Giant exchange bias shifts of several Tesla have been reported in ferrimagnetic/ferromagnetic bilayer systems, which could be highly beneficial for contemporary high energy density permanent magnets and spintronic devices. However, the lack of microscopic studies of the reversal owing to the difficulty of measuring few nanometer-wide magnetic structures in high fields precludes the assessment of the lateral size of the inhomogeneity in relation to the intended application. In this study, the magnetic reversal process of nanoscale exchange-coupled bilayer systems, consisting of a ferrimagnetic TbFeCo alloy layer and a ferromagnetic [Co/Ni/Pt] _N multilayer, was investigated. In particular, minor loop measurements, probing solely on the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend critically on the thickness of the ferromagnetic layer. For thick layers, irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the ferromagnetic layer. In contrast, thin ferromagnetic layers show a hysteresis-free reversal, which is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt multilayer and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer, as confirmed by micromagnetic simulations.

Figure 1

Schematic image illustrating the layer stacking of the two investigated FI/FM heterostructures.

Figure 2

M–H hysteresis loops of the FI/FM heterostructures obtained at 40 K. (a) Thicker FM layer (N = 9) switches irreversibly as apparent from the presence of a hysteresis in the minor loop (black). (b) In contrast, the thinner FM layer (N = 5) exhibits a fully reversible switching, as shown by the minor loop (black). The M–H hysteresis loops of the individual layers forming the heterostructures are displayed as well.

Figure 3

(a) Virgin M–H curve of the FI/FM heterostructure with N = 9 taken at 40 K starting from the demagnetized state. Five distinct field ranges are highlighted by different background colors. (b–i) In-field MFM images (2 μm × 2 μm) are displayed with an applied magnetic field ranging from 0 up to 2.80 T. (j) Schematics of the magnetic states typical for the five field ranges.

Figure 4

Schematic showing the underlying microscopic reversal mechanism for fully reversible switching via granular nucleation.

Figure 5

Model of the simulated FI/FM bilayer structure. The lateral dimension of the model and the thickness of the layers are given. The average grain diameter is 10 nm, and the discretization length is 2 nm. The color code shows the effective magnetic anisotropy, which is assumed to be normally distributed over the grains in the FM layer.

Figure 6

Simulated macroscopic minor loops of the FM layer coupled to the FI for two different FM thicknesses of 6 (N = 5, blue line and triangles) and 10.8 nm (N = 9, red lines and squares).

Figure 7

(a–j) Simulated MFM contrast of a thicker FM layer of 10.8 nm (N = 9) during its reversal for representative chosen external fields. The color code represents the quantity in eq 1. Panels (e–h) show the dynamic process of the domain wall motion during switching at 800 mT and not the relaxed magnetization state, as indicated by the simulation time. Panel (k) schematically shows the additional local field H_z,FI arising from the FI layer that hinders the lateral domain wall propagation in the FM (Figure 3d–g), in contrast to the simulation in panels (e–h).

Figure 8

(a–l) Simulated MFM contrast of a thinner FM layer of 6.0 nm (N = 5) during its reversal for representative chosen external fields. The color code represents the quantity of eq 1.