Simulation of current-driven switching

One of the most exciting new concepts for information storage is current-switched MRAM (Magnetic Read-Only Memory). Most magnetic storage (including conventional MRAM) involves switching the polarity of a magnetic element (a small piece of ferromagnetic material) by applying a magnetic field in the desired direction. In current-switched MRAM, the magnetic element is switched by injecting magnetized electrons, by passing an electric current into the element from a larger permanent magnet. A model for this “spin-injection torque” effect was introduced by Slonczewski in 1996. We have done micromagnetic simulations showing how this switching occurs. Fig. 1 shows a side view of a multilayer with lateral dimensions 60 x 20 nm.

The electrons flow from the bottom layer (7 planes of cells, magnetized to the right) through the copper layer to the top layer of Co, which is initially magnetized to the left, as seen from the top in Fig 2(a). In the simplest picture of the Slonczewski model, the electrons deliver their rightward magnetization to the top layer and cause it to switch (become rightward magnetized). In a thick film this would occur coherently, but in these very thin films we have found that thermal fluctuations are very important during the switching process, as seen in Fig. 2(b). This is the first simulation to account for fluctuations in the pinned layer and their magnetostatic influence on the switching layer.

A movie (6 MB) showing this switching process is available at http://bama.ua.edu/~visscher/mumag/CurrentSwitching.mov