Time-resolved spin-torque switching in MgO-based perpendicularly magnetized tunnel junctions

T. Devolder, Joo-Von Kim, F. Garcia-Sanchez, J. Swerts, W. Kim, S. Couet, G. Kar, and A. Furnemont

Phys. Rev. B 93, 024420 – Published 25 January 2016

Abstract

We study nanosecond-scale spin-torque-induced switching in perpendicularly magnetized tunnel junctions. Although the switching voltages match with the macrospin instability threshold, the electrical signatures of the reversal indicate the presence of domain walls in junctions of various sizes. In the antiparallel (AP)-to-parallel (P) switching, a nucleation phase is followed by an irreversible flow of a wall through the sample at an average velocity of 40 m/s with back-and-forth oscillation movements indicating a Walker propagation regime. A model with a single wall locally responding to the spin torque reproduces the essential dynamical signatures of the reversal. The P-to-AP transition has a complex dynamics with dynamical back-hopping whose probability increases with voltage. We attribute this back-hopping to the instability of the nominally fixed layers.

Received 20 August 2015
Revised 7 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.024420

Physics Subject Headings (PhySH)

Domain walls Magnetic anisotropy Spintronics

Magnetic tunnel junctions

Magnetization measurements Microwave techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. Devolder^1,*, Joo-Von Kim¹, F. Garcia-Sanchez¹, J. Swerts², W. Kim², S. Couet², G. Kar², and A. Furnemont²

¹Institut d'Electronique Fondamentale, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
²IMEC, Kapeldreef 75, B-3001 Leuven, Belgium

^*Corresponding author: thibaut.devolder@u-psud.fr

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Issue

Vol. 93, Iss. 2 — 1 January 2016

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Images

Figure 1
Properties of the unpatterned MTJ. (a) Hard and easy axes loops; the easy axis recorded in decreasing field appears in a light gray color. (b) Spin-wave frequencies measured (symbols) and calculated (bold lines) during an easy axis loop upon increasing field. The dotted lines are the calculated modes of the other metastable configurations undergone during a decreasing field loop. Inset: sketch of the MTJ.
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Figure 2
Sixteen current-voltage loops of a $75 \times 150 {nm}^{2}$ MTJ when compensating the stray field of the fixed layers. Inset: Length cut of the in-plane $(B_{x})$ and out-of-plane $(B_{z})$ stray fields at the FL position. The shaded area denotes the DW width $π Δ$ .
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Figure 3
AP-to-P switching of a $75 \times 150 {nm}^{2}$ device. (a) Selected switching events representative of the two classes of switching events. The exponential decays ( $τ = 68$ ns) after the voltage steps are capacitive artefacts related to the experimental setup. The unambiguous labeling of the states (AP or P) is done thanks to a circuit derivation (see Supplementary Material [17]). (b) Zoom on eight single-step switching events, and average thereof for a triggering criterion at half of the switching signal. The events have been time-delayed for clarity. (c) DW positions as simulated in the 1D model for $σ j = 1.7$ GHz and for various field conditions mimicking the dipolar coupling with the fixed layers. The experimental (noisy) traces have been converted to DW positions assuming a single wall sweeping though the length of the MTJ. (d) Zoom on the onset of two-step switching events. (e) Sketch of the single-step reversal scenario simulated in panel (c). The dotted lines denote the wall width and the arrow the magnetization tilt $ϕ$ within the DW at position $q$ . (f) Sketch of the nucleation scenario at the onset of the reversal.
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Figure 4
Statistical analysis of the AP-to-P transitions of a $75 \times 150 {nm}^{2}$ device. (a) Examples of single-step switching events. The dashed arrows indicate the positions at which the domain wall velocity seems to vanish or transiently reverse. (b) Histogram of occurrence of a given wall position during a statistical set of single-step switching events. The red Gaussian distribution sketches an estimate of the experimental noise in the determination of the wall position as due to voltage noise. The dashed lines at the two maxima of the histogram indicate the domain wall positions at which the wall velocities are most likely to vanish.
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Figure 5
P-to-AP switching for a $100 \times 300 {nm}^{2}$ device. (a) Time evolution on the signal (noisy curves) for biases of $- 0.4$ V and $- 0.36$ V and sketch (bold line) of the time evolution of the resistance in the latter case. The exponential decays after the voltage steps are capacitive artefacts. The labeling of the states (near AP and P) is done thanks to a circuit derivation (see Supplemental Material [17]). (b) Zoom on the $- 0.36$ V trace, and (c) power spectrum thereof.
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