Anomalous low-frequency noise in synthetic antiferromagnets: Possible evidence of current-induced domain-wall motion

We investigate current-driven magnetization dynamics in synthetic ${\left[\text{Fe}/\text{Cr}\right]}_{10}$ multilayer antiferromagnets by using low-frequency voltage noise measurements. We observe suppression of the noise above a critical current density of about $2\times {10}^5\text{A}/{\text{cm}}^2$. Theoretical estimates suggest that this effect may be attributed to current-induced motion of domain walls in the antiferromagnet. The observed critical current density is about one order of magnitude smaller than for ferromagnetic systems. Our results are relevant for applications of antiferromagnetic metal spintronics in, e.g., magnetic memory storage technology.

Figure 1

(Color online) Main part: low-field dependence of the magnetic susceptibility in MML1 measured at $T=77\text{K}$. The inset shows a MFM image of MML1 obtained in zero magnetic field at $T=300\text{K}$ within $10\times 10\mu {\text{m}}^2$.

Figure 2

(Color online) (a) Comparison of giant magnetoresistance $\text{GMR}=\left[R\left(H\right)-R\left(60\text{kOe}\right)\right]/R\left(60\text{kOe}\right)\times 100$ measured at 10 K for MML1 and MML2: The arrows indicate the corresponding saturation fields $H_s$. (b) Magnetoresistance $\text{MR}\left(0\text{Oe}\right)=\left[R\left(H\right)-R\left(0\text{Oe}\right)\right]/R\left(0\text{Oe}\right)\times 100$ for MML1 and MML2 measured at 77 K. (c) Magnetoresistance $\text{MR}\left(400\text{Oe}\right)=\left[R\left(H\right)-R\left(400\text{Oe}\right)\right]/R\left(400\text{Oe}\right)\times 100$ for MML1 measured at $T=77\text{K}$ with different current densities that were also used for the noise measurements. The arrows indicate the DW nucleation $\left(H_n\right)$ and annihilation $\left(H_a\right)$ fields. (d) Low-field magnetoresistance for MML1 measured at $T=77\text{K}$ with very small current density and with current either parallel or perpendicular to the external magnetic field.

Figure 3

(Color online) (a) Dependence of the Hooge factor $\left(\alpha \right)$ on the magnetic field for MML1 at 77 K with different applied current densities. (b) Hooge factor plotted on a logarithmic scale vs magnetic field for MML1 at 77 K with two different applied current densities. The normalized noise vs field measurements are done with a current density of $j=3.3\times {10}^5\text{A}/{\text{cm}}^2$ and the results are shown both for increasing and decreasing magnetic field. The experiments have been carried out with dc current parallel to the external magnetic field and to the magnetic easy (001) axis.

Figure 4

(Color online) [(a) and (b)] Voltage noise (power spectral density) for MML1 for low and high biases, $j=6.6\times {10}^4\text{A}/{\text{cm}}^2$ and $j=3.3\times {10}^5\text{A}/{\text{cm}}^2$, respectively, for different applied fields inside $\left(H=+50\text{Oe}\right)$ or outside $\left(H=-125\text{Oe}\right)$ the interval $\left(H_n<H<H_a\right)$ where antiferromagnetic DWs nucleate and propagate at 77 K: The dotted lines represent $1/f$ variation in the noise power, while the black arrows relate the applied bias with the current density. Part (c) plots the averaged over the frequencies (1–25 Hz) product of the power spectral density multiplied by the frequency as a function of square of the applied bias voltage ${\text{V}}^2$. A nearly linear dependence $S_V\times f=\alpha \left(H\right)V^2$ is seen for the field $H=-125\text{Oe}$ outside the interval where antiferromagnetic DWs nucleate and propagate, whereas in the field range $\left(H_n<H<H_a\right)$ the parameter $S_V\times f$ exhibits a clearly different behavior discussed in the text.

Figure 5

(Color online) (a) Dependence of the Hooge factor $\left(\alpha \right)$ (on logarithmic scale) on the magnetic field for MML1 obtained from noise measurements at $T=10\text{K}$ with two different applied current densities. For comparison, the solid line shows the absolute values of the derivative $\partial R/\partial H$ measured with a current density of $2.3\times {10}^5\text{A}/{\text{cm}}^2$. The vertical arrows mark the regions of nucleation and annihilation of the domain walls determined from low-field magnetoresistance measurements. [(b) and (c)] Dependence of the Hooge factor on the current density measured in MML1 with and without domain walls for two different magnetic fields at temperatures of 77 K (part b) and of 10 K (part c).

Figure 6

(Color online) Hooge factor $\left(\alpha \right)$ vs magnetic field measured in the ferromagnetic Co line structure at 77 K with a current density of $3.3\times {10}^5\text{A}/{\text{cm}}^2$. The inset shows the field values of the maximum in the Hooge factor and the related maxima of the derivative $\partial R/\partial H$ as a function of current density.