In present MRAM devices magnetic nanostructures are switched by magnetic fields. Due to their non-local character, however, cross-talk between adjacent nanomagnets may occur. An elegant method to circumvent this problem is magnetization switching by spin-polarized currents, as observed in GMR [1] as well as in TMR [2] devices. However, the layered structures of these devices do not provide any insight into the details of the mechanism and the spatial distribution of the switching processes.
Spin-polarized scanning tunneling microscopy (SP-STM) is a well-established tool to reveal the magnetic structure of surfaces with spatial resolution down to the atomic scale. Besides, SP-STM takes advantage of a perfect TMR junction consisting of a vacuum barrier separating two magnetic electrodes, which are represented by the tip and the sample. This configuration prevents uncontrollable influences of layer intermixing and lattice imperfections which may play an important role in MBE-grown TMR junctions.
Our sample system consists of in-plane magnetized uniaxial Fe monolayer islands [3] which exhibit a superparamagnetic switching behavior at T~55 K. We will report on our SP-STM experiments aiming at switching the magnetization of single nanoislands by the injection of a spin-polarized current. While the mean lifetimes of each of the two possible magnetization directions are equal at low tunneling currents, a high, spin-polarized current of about 1000 nA leads to a distinct lifetime asymmetry. Furthermore, we will discuss heating and Oersted field effects induced by such high tunneling currents.
[1] J. A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000).
[2] Y. Liu et al., Appl. Phys. Lett. 82, 2871 (2003).
[3] M. Pratzer et al., Phys. Rev. Lett. 87, 127201 (2001). |