Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films

We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Néel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.

Figure 1

(a) Sketch of the studied Ta/Pt/Co/Ir$\left(t_{\mathrm{Ir}}\right)$/Pt layer stacks with a varying Ir thickness. (b) Polar Kerr hysteresis loops for samples with various Ir thicknesses. (c) Anisotropy field ${\mu }_0{H}_{\mathrm{K}}$ and areal magnetization $M_{\mathrm{s}}t$ as a function of $t_{\mathrm{Ir}}$.

Figure 2

(a) Experimental setup in the Kerr microscope for DMI measurement. The magnetic field is tilted by an angle $\delta$ with respect to the sample plane. (b) DW displacement in the case of $\delta ={90}^{\circ }$ after the application of a 1 s and ${\mu }_0{H}_z=7$ mT pulse. The initial DW position is indicated by the dashed line. (c) DW displacement in the case of $\delta =2.3^{\circ }$ after the application of a 1 s and ${\mu }_0{H}_x=+60$ mT pulse. (d) DW displacement after the application of a magnetic field pulse of the same length and ${\mu }_0{H}_x=-60$ mT. Black arrows indicate the equilibrium orientation of the magnetic moments within the DW.

Figure 3

Differential Kerr image of the DW displacement in the case of (a) Ir (0 Å), (b) Ir (2.3 Å), and (c) Ir (4.6 Å) after the application of a 1 s and 320 mT, 1 s and 130 mT, and 5 s and 60 mT field pulse, respectively. (d) DW velocity as a function of in-plane magnetic field $H_x$ in the case of Ir (0 Å), Ir (2.3 Å), and Ir (4.6 Å) for a DW creeping along the $x$ direction. The remanent field in $H_x$ is smaller than 1 mT. The dashed curves show the fits of the creep model described by Eq. (1) to the data.

Figure 4

DM field and $D$ as a function of Ir thickness. The region between two dashed lines depicts the range where the DW structure changes continuously from a Néel wall to a Bloch wall and to a Néel wall of opposite chirality. Below this line (blue area) the right-handed Néel wall is stable whereas above this line (red area) it is the left-handed Néel wall. The wall structures are depicted with sketches.

Figure 5

(a) High-angle annular dark field in a scanning transmission electron micrograph of the epitaxial Pt(3 nm)/Co(0.7 nm)/Pt(1 nm) trilayer. The darker Co layer is sandwiched between the two brighter Pt layers. (b) Differential Kerr image of the DW displacement in the epitaxial Pt/Co/Pt sample after the application of a 1 s long, ${\mu }_0{H}_x=100$ mT field pulse. (c) Comparison of DW velocities as a function of magnetic field in the polycrystalline and epitaxial films. The dashed curves show the fits of the creep model described by Eq. (1).