Time-resolved x-ray imaging of magnetization dynamics in spin-transfer torque devices

Time-resolved x-ray imaging techniques have recently demonstrated the capability to probe the magnetic switching of nanoscale devices. This technique has enabled, for example, the direct observation of the nonuniform intermediate states assumed by the magnetic free layer during reversal by a spin-polarized current. These experiments have shown an interesting size-dependent behavior associated with the motion of vortices to mediate the magnetization reversal which cannot be explained by the macrospin picture of spin-torque switching. In this paper we present both experimental and analytical results which show the origin of the complex switching behavior. We use time-resolved x-ray microscopy to further study the switching behavior of samples with $45°$ angle between the free and polarizing magnetic layers. A model is developed in terms of a linearized Landau-Lifshitz-Gilbert equation showing that the initial dynamics is dominated by the balance between the Oersted field and thermal fluctuations. The spin torque amplifies this dynamics, leading to a strong sensitivity to sample size, angle, and temperature. The model is in good agreement with current and previous experimental observations.

Figure 1

(Color online) Schematic representation of (a) VDS observed in $100\times 180\text{nm}$ samples, (b) CSF mechanism observed for $110\times 150\text{nm}$ and $85\times 135\text{nm}$ samples, and (c) switching according to the UR model. In all three cases, time proceeds downwards. In (b) two equivalent paths for the motion of the vortex core are possible which makes the switching inherently stochastic, as opposed to the VDS in (a). UR model is shown in (c) for comparing it to CSF in (b).

Figure 2

(Color online) Evolution of the magnetization during the positive pulse in a $125\times 200\text{nm}$ sample measured by way of time-resolved scanning transmission x-ray microscopy. The magnetization reversal proceeds via a vortex core moving through the sample. The vortex core stays in the sample as long as the current is on. The figure on the right below the schematic pulse sequence is a STXM image of the sample. The magnetization direction of the fixed layer is shown by the vertical arrow.

Figure 3

(Color online) Evolution of the magnetization during the negative pulse in a $125\times 200\text{nm}$ sample measured by way of time-resolved scanning transmission x-ray microscopy. The magnetization reversal proceeds via a vortex core moving through the sample. The vortex core stays in the sample as long as the current is on.

Figure 4

(Color online) Evolution of the magnetization during the positive pulse in a $125\times 150\text{nm}$ sample measured by way of time-resolved scanning transmission x-ray microscopy. The image on the right below the schematic pulse sequence is a STXM image of the sample. The magnetization direction of the fixed layer is shown by the vertical arrow in this image.

Figure 5

(Color online) Evolution of the magnetization during the negative pulse in a $125\times 150\text{nm}$ sample measured by way of time-resolved scanning transmission x-ray microscopy. The magnetization reversal proceeds via a vortex core moving through the sample, unlike reversal during the positive pulse.

Figure 6

(Color online) Schematic representation of the (a) C-state bending mode and (b) C-state rocking mode. Also shown are displacements of virtual vortex cores.