Magnetization Reversal in Micron-Sized Magnetic Thin Films

Abstract

We have measured and simulated the dynamics of magnetization reversal in 5 nm by 0.8 by 1.6 mm Ni 60 Fe 40 thin films. The films measured form the upper electrode of a spin-polarized tunnel junction so that the magnetization direction of the film can be probed by measuring the tunneling resistance of the junction. When a magnetic field pulse is applied, the time to switch the film magnetization changes from greater than 10 ns to less than 500 ps as the pulse amplitude is increased from the coercive field to 10 mT and beyond. We have simulated these transitions using micromagnetic modeling of the exact experimental conditions. The simulations agree well with the experimental measurements. [S0031-9007(98)07661-3] PACS numbers: 75.70.AkIn this paper we will investigate the dynamics of magnetization reversal in micron-sized magnets. This will be done both from measurement and modeling points of view. While the switching times of small ferromagnetic objects have been measured many times in the past [1], none of these earlier measurements were made in micronsized objects with picosecond time resolution. Recently several groups have made picosecond time resolution measurements using magneto-optic [3] or inductive probing [2] of the sample magnetization on larger samples. Also, while there has been widespread use of micromagnetic modeling to study the static properties of small structures, time domain simulations of how such micron-sized structures switch are rare [4]. As far as we know, this is the first paper where both measured and simulated dynamical properties are quantitatively compared on micronsized magnetic samples.In order to measure the magnetization reversal in micron-sized films, we chose to use spin-polarized tunneling as a probe of the magnetization direction. In this way, a simple-to-measure change in the tunneling resistance between a pinned and a free magnetic layer can serve to probe the magnetization dynamics of a micron-sized magnetic film. Using this method, we have measured the time required to reverse the magnetization direction in micron and submicron-sized thin films of a soft ferromagnet Ni 60 Fe 40 in the 100 ps to 100 ns range. In order to understand and visualize the dynamics of the magnetization reversal in such films we have developed time-domain micromagnetic models of the experimentally measured films that exactly simulate the experimental conditions. By comparing the results of the simulations with the measurements, the simulations can be vetted and some of the needed parameters can be determined.The sample structure consisted of two thin ferromagnetic films separated by an Al 2 O 3 tunnel junction [5]. The bottom film, the pinned layer, was an antiparallel (AP) pinned structure formed of two Co layers separated by a 0.6 nm Ru film. The top film, the free layer, was a 5 nm layer of Ni 60 Fe 40 . The structures reported here were all 0.8 by 1.6 mm in size, but both smaller and larger ones were also tested. There was a uniaxial anisotropy of approximately 800 J͞m 3 along the easy (long) a...

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