Controlling magnetic vortices through exchange bias

摘要

In recent years, intense research is being pursued studying patterned magnetic nanostructures, both for fundamental reasons and for their applications in recording media or in novel types of spintronic nanodevices. Here, the magnetic behavior of exchange coupled ferromagnetic (FM) - antiferromagnetic (AFM) disks, with composition Ta(5nm)/Py(12nm)/IrMn(5nm)/Pt(2nm), where Py, which denotes Permalloy (i.e., NiFe) is FM and IrMn is AFM, with micron and submicron diameters, is investigated [1,2]. These structures exhibit different magnetization reversal mechanisms depending on the direction of the applied magnetic field. Such behaviors are studied by means of magneto-optic Kerr effect, magnetic force microscopy imaging and micromagnetic simulations. As can be seen in Figure 1, shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields applied along the unidirectional coupling (i.e., exchange bias), which is defined by the field cooling direction from above the blocking temperature of the system (arbitrarily taken as 0°). However, beyond a critical angle from the field cooling direction, magnetization reversal occurs via coherent rotation. The value of this critical angle can be trigonometrically determined to be: θ{sub}C=arcsin[H{sub}(N,0)/H{sub}(E,0)], where H{sub}(E,0) and H{sub}(N,0) are the exchange bias field and the vortex nucleation field of the loop measured along 0°. Since both H{sub}(E,0) and H{sub}(N,0) depend on the disk diameter, the critical angle is different for different diameters (e.g., θ{sub}C=80° and 22° for disks with 400nm and 1μm diameter, respectively). Interestingly, for a fixed disk size, the critical angle can still be experimentally tailored by varying the magnitude of the exchange bias field. This can be achieved by subjecting the FM-AFM disks to field-cooling processes in fields of opposite polarity [2].