Micromagnetic simulation of magnetization reversal in Co/Ni multilayers

V.D. Tsiantos, T. Schrefl, D. Suess, W. Scholz, J. Fidler and J.M. Gonzales

Journal Physica B-Condensed Matter, (2000) submitted.

Magnetic sensors and magneto-electronic devices are based on magnetic nanostructures. Their application requires a well-defined switching behaviour which can be tailored changing either the microstructure or the intrinsic magnetic properties. The magnetization reversal process of Co/Ni/Co trilayers was studied using finite element micromagnetic simulations. A uniaxial anisotropy parallel to the film plane was assumed within the Co layer, whereas zero anisotropy was used to mimic the properties of Ni. This system is a perfect model to investigate the role of anisotropy and interphase exchange on the magnetization reversal process in exchange spring magnets. Indeed, the typical behaviour of a nanostructured, exchange coupled system is observed. Demagnetization curves were calculated for a multilayer with a Co layer thickness of 5 nm and a Ni layer thickness of 15 nm. Thus the net magnetization of a Co and a Ni layer are the same, since the spontaneous magnetization of Co is about three times larger than that of Ni. The computational region was a 200 nm x 200 nm with free boundary condition. Thus, in addition to the interplay between anisotropy and exchange, the demagnetization field from the edges significantly influences the reversal process. Figure 1 shows two distinct reversal processes depending on the strength of the interlayer exchange. To model the influence of interlayer exchange the exchange constant of Ni was reduced from A = 10-11 J/m to A=10-12 J/m in a 5 nm thick region next to the Co layer. Exchange hardening of the Ni layer provides a one-step reversal process if A>0.3*10-11 J/m. Both the Ni and the Co layer reverse at the same well-defined switching field. A two- step process occurs at interlayer exchange lower than one third of the bulk value. The Ni layer reverses its magnetization at a low opposite field and the magnetization of the Co keeps its initial direction. The already reversed Ni layer stabilizes the magnetization of the Co layer owing to magnetostatic interaction which leads to an increase of the coercive field as compared to the high exchange coupled systems. Figure 2 gives the magnetization pattern in the middle of the sample at Hext = -2 kA/m for weak interlayer exchange. The external field causes a nonuniform magnetic state within the Ni layer whereas the magnetization within the Co layer remains nearly parallel to the anisotropy axis. The transition from a one-step to a two-step reversal process occurs if the anisotropy of Co is considerably high (Ku = 450 kJ/m3). A completely different reversal mechanism occurs if the uniaxial anisotropy of the Co layer approaches 1 kJ/m3. Now demagnetizing effects override the uniaxial anisotropy. The Co layer reversed at lower opposing field, owing to its high magnetization. The remanence and the coercive field of the multilayer systems decrease with increasing interlayer exchange constant between the layers. In order to reduce the magnetostatic energy end domains are formed at zero applied field. The end domains become more pronounced with increased interlayer exchange which in turn facilitates magnetization reversal. In addition to the quasi-static behaviour, the numerical integration of the Landau-Lifshitz-Gilbert equation shows how the reversed domains are formed as a function of time.

This work was supported by the Austrian Science Fund (Y-132 PHY).

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