Multistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructures

Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond $1.4\times {10}^7$ A/cm can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer.

Figure 1

Magnetic properties of the Au/Fe/MgO(001) Hall bars. (a) The Hall bar pattern with a sketch of the geometry for the magneto-transport measurements. (b) Planar Hall effect angular dependence. (c) Response of the transversal resistance $R_{xy}$ to external magnetic field scanned between $-60$ and 60 mT and applied at an angle ${\phi }_B=5^{\circ }$ to the longitudinal current $\vec{I}$. (d) Angular plot of the magnetic fields $B_{\mathrm{switch}}$ of the first and second jump in $R_{xy}$ derived from the positive $B_{\mathrm{ext}}$ scan of the planar Hall effect hysteresis loops. The cubic symmetry of the $B_{\mathrm{switch}}$ of the second magnetization jump reflects the fourfold magneto-crystalline anisotropy of the Fe(001) thin film.

Figure 2

Current-induced magnetization reorientation (a) Measurement of the PHE resistance $R_{xy}$ with increasing longitudinal current $I$ in Au/Fe(1.5 nm)/MgO(001) after saturation of the magnetization along the Fe[010] direction. (b) The numbers 1–7 mark the sequence of the measurements. First, a positive current $+I$ was scanned (1, black dots). Immediately afterwards, a negative current $-I$ was scanned (2–4, blue dots), followed directly by a positive current scan (5–7, red dots). A constant reference magnetic field $B_{\mathrm{ref}}\sim 1$ mT was applied in the Fe[$\overline{\text{1}}$10] direction in (a) and (b). (c) Sketch of the experiment. (d) The PHE resistance $R_{xy}$ measured using a small probing current $I=1$ mA (corresponding to $j=1.2\times {10}^6$ A/cm²) after applying “pulses” of $I=20$ mA (corresponding to $j=2.4\times {10}^7$ A/cm²) of either positive (in red) or negative (in blue) polarity for $\sim 1$ s to the Hall bar. A constant magnetic field $B_{\mathrm{ref}}\sim 1$ mT was applied along the Fe[$\overline{\text{1}}$10] direction. (e) The angular dependence of the PHE resistance $R_{xy}$ measured with a probing current $I=1$ mA and a saturating external magnetic field ($B_{\mathrm{ext}}=400$ mT).

Figure 3

The current-induced domain structure investigated by Kerr microscopy. A starting state of a homogeneous magnetization along the Hall bar saturated along the Fe[$\overline{\text{1}}$00] direction (bright domains) was set by applying a magnetic field. Afterwards, a sequence of Kerr images was acquired, each time after applying a 0.1-s-long current pulse along the Hall bar as marked in (e). Additionally, the PHE resistance of the induced magnetic state was measured using a 1 mA probing current between the contact pads marked in (a). (i) The transversal voltage $V_{xy}$ values read out after each of the current pulses. The current of the applied pulses was increased in steps of 1 mA as indicated by the arrow below the graph. $V_{xy}$ of the selected Kerr images (a–h) are marked in red.

Figure 4

(a) Kerr microscopy image of the Hall bar with current applied applied along Fe[110] and with external magnetic field $B_{\mathrm{ext}}=7$ mT along the Fe[1$\overline{\text{1}}$0] hard axis. The Kerr sensitivity was set parallel to the external magnetic field. The Kerr signal was extracted from the area marked in red. (b) The hard axis hysteresis loops were acquired for varying current density applied to the Hall bar. The inset shows the enlarged region of small magnetic fields where the shift (black arrow) of the hysteresis loops with respect to the zero external field and a decrease of the coercive field ($I=0$ mA in orange and $I=23$ mA in light blue) are visible for increasing current density. (c) Comparison of the experimentally observed hysteresis loop shift with the calculated values of the Oersted field for an infinite sheet (dashed line) and a strip line (dotted line). The current density was determined for the Hall bar width of 12 $\mu \mathrm{m}$ and total thickness of 5.5 nm. (d) The coercive field $B_c$ was extracted from the width $\mathrm{\Delta }B$ of the hysteresis loops as $B_c=\mathrm{\Delta }B/2$. $B_c$ stays constant for currents up to 10 mA and for higher currents begins to gradually decrease, which indicates the onset of increased sample heating. (e) Spatial variation of the magnitude and direction of the Oersted field calculated 1 nm above a strip-line antenna with a rectangular cross section carrying a current of 1 mA.

Figure 5

(a) The results of the AHE measurements of Au/Fe/MgO(001) Hall bars of different Fe thicknesses. (b) The anisotropy constants derived from the AHE hysteresis loops.

Figure 6

(a) Result of an exemplary PHE measurement of the Au/Fe(1.3 nm)/MgO(001) Hall bar with different external magnetic fields from 5 to 160 mT compared to $R_{0}sin\left(2{\phi }_B\right)$ with $R_0=9.6$ m$\mathrm{\Omega }$ (broken line). These results were obtained at $T=30$ K. The angle ${\phi }_B$ is measured between the applied current $I=15$ mA and the direction of the external magnetic field. (b) The difference between the magnetization direction (${\phi }_M$) and the external magnetic field direction (${\phi }_B$) calculated for different magnetic fields from the deviation of the measured $R_{xy}$ from the $R_{xy}\left({\phi }_B\right)\propto sin\left({\phi }_M\right)cos\left({\phi }_M\right)$ dependence expected at saturation.

Figure 7

(a) Results of the longitudinal MOKE measurements of the Au/Fe/MgO(001) Hall bars with different Fe thickness $t_{Fe}=1.3$ nm and $t_{Fe}=1.5$ nm along an easy magnetic axis. (b) Magnetic hysteresis measured using PHE in Au/Fe/MgO(001) Hall bars with different Fe thickness with magnetic field along a hard magnetic axis.

Figure 8

Results of the XAS measurements of Au/Fe/MgO(001) Hall bars at the Fe $L_{2,3}$ absorption edge. The measurement was performed for two Fe thicknesses $t_{Fe}=1.5$ nm and $t_{Fe}=1.3$ nm and through Au capping layers 2 and 4 nm thick: (a) $t_{Fe}=1.5$ nm and $t_{Au}=4$ nm, (b) $t_{Fe}=1.3$ nm and $t_{Au}=4$ nm, (c) $t_{Fe}=1.5$ nm and $t_{Au}=2$ nm, (d) $t_{Fe}=1.3$ nm and $t_{Au}=2$ nm.

Figure 9

Results of the current-induced magnetization switching experiment performed using Au (4 nm)/Fe(1.5 nm)/MgO(001) Hall bars. Different current ranges can be disgtinguished: (i) below the threshold for any switching (subthreshold), (ii) for intermediate switching into a magnetic domain state, and (iii) full switching into a saturated magnetization state. For details see the text.

Figure 10

Current-induced magnetic hysteresis loops measured using different speeds of the current sweep.