Experimental Observation of Single Skyrmion Signatures in a Magnetic Tunnel Junction

We have deterministically created a stable topological spin texture in magnetic tunnel junctions (MTJ) by using pulsed or microwave currents. The spin texture is characterized by a field-dependent intermediate resistance state and a new magnetic resonance. Micromagnetic simulations show that the observations are consistent with the nucleation of a single skyrmion, facilitated by a spatially nonuniform stray field. The unique resonance spectrum is identified as the skyrmion breathing mode and a skyrmion diameter of 75 nm is estimated. This work shows the possibility to create skyrmions in MTJs without the Dzyaloshinskii-Moriya interaction and could lead to noninvasive, on-chip skyrmion measurement.

Figure 1

(a) MTJ layer structure. Arrows denote average magnetization of each layer at zero applied field. The positive $z$ direction is taken to be parallel to the reference layer magnetization. (b) Out-of-plane hysteresis loop in the absence of pulsed or microwave current excitation, showing a completely perpendicular free layer magnetization. (c) Out-of-plane hysteresis loop in the presence of a repeated pulse of width 10 ns. A transition to an intermediate resistance state (IRS) appears when the pulse height exceeds $100\mu \mathrm{A}$. The $\mathrm{P}\to \mathrm{AP}$ switching field is reduced due to the presence of spin torques. (d) Circuit diagram for magnetoresistance measurements. (e) Magnetic field dependence of the IRS with no applied excitations following initialization at 24 mT (red line) and comparison to simulations of a single skyrmion (dotted black line). Right side: snapshots of the stable skyrmion configuration at 5 (upper) and 15 mT (lower).

Figure 2

(a) Inducing a transition to IRS in a 350 nm device by sweeping the magnetic field with the application of constant microwave current $I_{\mathrm{ac}}$. Field scan direction is from $-50$ to $+50\mathrm{mT}$. Colored lines are cuts shown in (b). (b) A cross-sectional view of (a) including the field sweep from negative to positive values to complete the hysteresis loop. (c) Microwave power dependence on $\mathrm{AP}\to \mathrm{IRS}$ transition at fixed frequency $f_{\text{drive}}=1.7\mathrm{GHz}$. (d) Stability of the IRS is demonstrated by applying a 1 sec $I_{\mathrm{ac}}$ burst (pulse applied in gray region) to transition from AP to IRS and allowing the free layer to reach static equilibrium. Static magnetic field was held at 20 mT and rf frequency $f_{\text{drive}}=1.6\mathrm{GHz}$.

Figure 3

Simultaneous measurement of dc resistance and magnetic resonance on the 250 nm MTJ when initialized in (a) the AP state and (b) the P state. The onset of the IRS at 17 mT in (a) is characterized by a new low-frequency mode at 1.2 GHz with a nonlinear dependence on field. In contrast, there is only a single discontinuity in the resonance spectrum representing the $\mathrm{P}\to \mathrm{AP}$ transition when initialized in P.

Figure 4

(a) Simulated stray field $z$ component (red) and $x$ component (dotted black) seen by the free layer, along a diametric cut. Inset: spatial profile of the $z$ component of the stray field. (b) Magnetic field dependence of the IRS (dotted red) and comparison with simulated skyrmion evolution with disorder (blue). The addition of magnetic disorder creates discontinuities in the magnetoresistance that match the observed behavior. (c) Snapshots of skyrmion formation via current pulse.

Figure 5

(a) The magnetic resonance landscape of the 250 nm device from Fig. 3 (colored symbols) and comparison to simulations (solid black lines). (b) Corresponding spatial profiles of ferromagnetic spin-wave mode amplitudes showing alternating radial (blue diamonds, yellow squares) and azimuthal (green circles) modes. Also shown is the skyrmion breathing mode (red triangles). Profiles for the FM (skyrmion) states were obtained from the Fourier transform of $m_x$ ($m_z$) dynamics for each cell in the simulation. (c) Frequency profile of the first FMR mode taken at 5 mT in the AP state. (d) Frequency profile of the skyrmion breathing mode taken at 20 mT. Insets are simulated profiles.