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.

Anisotropy in magnetic materials for sensors and actuators in soft robotic systems

“…This phenomenon occurs at the interface between an FM material and an adjacent antiFM (AFM) material, resulting in a unidirectional magnetization orientation in the FM material. 37,38 This interfacial coupling is not limited to FM/AFM bilayers but extends to other magnetic configurations, such as FM/AFM superlattices, 39 ferrimagnet (FI)/AFM, 40 FI/FM, 41 soft FM/hard FM, 42 and FM/spin glass systems. 43 Unidirectional pinning of the magnetization notably alters the hysteresis loop of the FM/AFM coupled layers when compared with that of a standalone FM layer, where the center of the hysteresis loop shifts from the zero magnetic field position (Fig.…”

Quantitative Magnetic Force Microscopy: Transfer-Function Method Revisited

“…Nevertheless, apart from these small deviations, a good agreement between simulated and measured data is obtained. Such an averaged TF can be used to optimize candidate magnetic moment distributions such that the simulated MFM data obtained from their stray field best matches the measured MFM data [16][17][18]. Note that the simulation of MFM images is performed by a convolution process, where the noise of the TF, which is particularly relevant at small spatial wavelengths [see yellow and blue lines in Fig.…”

“…In particular, MFM proves to be a reliable and adequate tool for imaging room-temperature magnetic skyrmions [11,12], which are of great interest due to their potential application in future-generation storage devices [13,14]. Following our early work on a quantification of the MFM tip response [15], and various applications thereof [16][17][18], there is growing interest in the development of improved MFM tip calibration and quantitative magnetic field measurement procedures [19][20][21][22][23][24].…”

“…This approach is used to specify the imaging properties of the probe: it calibrates the response of the MFM tip to different spatial wavelengths of the stray field and thus allows to access the quantitative information about the stray field emanating from the sample surface. This method has already been proven to be successful in various applications of quantitative MFM (QMFM) [16][17][18][30][31][32][33][34][35][36]. More recently, the TF method has gained renewed interest.…”

Applied Trends in Magnetic Rare Earth/Transition Metal Alloys and Multilayers

“…Another related aspect that could be worthy of study in the close future is the appearance of giant exchange bias shifts (of up to tens of kOe) in ferrimagnetic/ferromagnetic [ 48 , 49 , 50 ] and ferrimagnetic/ferrimagnetic [ 51 ] bilayers, which could be extremely useful for spin valves and other spintronic applications. Unfortunately, these exchange bias phenomena are usually observed at low temperatures, which decreases its potential for applications (usually requiring a working range of temperatures close to room temperature).…”