Tunable damping in the Heusler compound ${\mathrm{Co}}_{2-x}{\mathrm{Ir}}_x\mathrm{MnSi}$

Here we report on the realization of tuning the intrinsic damping in the half-metallic Heusler compound ${\mathrm{Co}}_2\mathrm{MnSi}$ by substituting Co by Ir. The work includes theoretical calculations and experimental measurements on bulk and thin films samples. Control of damping is to remove unwanted magnetization motion and suppress signal echoes through uncontrolled precession of the magnetization for future implementation of this material into, e.g., current perpendicular plane–giant-magnetoresistance sensors. Density functional calculations revealed stable magnetization and increasing damping parameter with Iridium concentration, whereas the half metallicity could be retained. The calculations are consistent with experimental results from bulk and thin film samples of this report and elucidate the linear dependence of the Gilbert damping parameter on the substituent concentration. This report again demonstrates the inherent tunability of Heusler compounds, which constitutes a pivotal feature of this material class.

Figure 1

The crystal structures of (a) ${\mathrm{Co}}_2\mathrm{MnSi}$ and (b) ${\mathrm{Co}}_{2-x}{\mathrm{Ir}}_x\mathrm{MnSi}$. A partial substitution of Co by Ir is shown. In contrast to a supercell approach, the substitution is realized through a mixed occupation of atomics sites.

Figure 2

SEM phase contrast images, numbers are at% composition Co-Ir-Mn-Si: (a) $x_{\mathrm{Ir}}=0.25$, single phase only; (b) $x_{\mathrm{Ir}}=0.5$, phase separation (green and blue) and intermediate phase (red).

Figure 3

XRD $2\theta /\omega$ scans for polycrystalline bulk samples for increasing substitution of Co by Ir.

Figure 4

The band structures of (a) ${\mathrm{Co}}_2\mathrm{MnSi}$ and (b) ${\mathrm{Co}}_{1.25}{\mathrm{Ir}}_{0.75}\mathrm{MnSi}$ are shown. Despite the substitution the half-metallic character is maintained: blue: majority states; red: minority states.

Figure 5

The temperature dependence of the calculated damping parameter $\alpha$ is shown for a set of substitution concentrations $x_{Ir}$.

Figure 6

(a) Dependence of $\alpha \left(x_{\mathrm{Ir}}=0.5\right)$ on the spin-orbit coupling parameter. (b) Dependence of $\alpha \left(x_{\mathrm{Ir}},T=277\text{K}\right)$ on the Ir concentration. (c) Variation of the DOS at ${\varepsilon }_F$ with increasing Ir content, $x_{\mathrm{Ir}}$.

Figure 7

Magnetic properties, AFM image and and XRD scans of the prepared films: (a)–(c) Magnetization loops (BHL) for three compositions with increasing Ir content, different RTA anneal steps. (d) AFM image of sample with $x_{\mathrm{Ir}}=0.23$. (e) $2\theta /\omega$ XRD scans of the same samples as in (a)–(c). The (0002) peak of the $20\AA$ Ru seed is broad and around $42.2^{\circ }$, adjacent to the (002) Heusler peak (substrate peak not shown).

Figure 8

Thickness-dependent measurements of $subs.$/30Ta/20Ru/xCMS/20Ru for (a) the magnetization reveal a 8 Å thick magnetically dead layer for CMS. (b) $\alpha$ scales inversely with the film thickness.

Figure 9

(a) IP FMR response, asymmetric Lorentzian fit and difference for selected Ir concentrations. The curves are shown with an offset for better visibility. (b) (zoomed in) Using results from (a) and fitting it using the solution to the LLG equation Eq. (2). (c) Fit for $\alpha$ and $\mathrm{\Gamma }$ with Eq. (4). For the samples with $x_{\mathrm{Ir}}=0.23$, 0.34, and 0.47 a linear fit without the 2M term was performed.

Figure 10

(a) Film/bulk lattice parameter obtained via XRD. (b) $M_{\mathrm{sat}}^{*}$ and $M_{\mathrm{eff}}$ (IP and OP FMR configuration), $H_{\mathrm{c}}$ and extrinsic contributions $\mathrm{\Delta }{H}_0$ (IP only). The red curve is the expected trend for a linearly increasing Mn depletion of 5 at% within this range. (c) Intrinsic damping ${\alpha }^{\parallel /\perp }\left(x_{\mathrm{Ir}}\right)$. (d) Product of ${\alpha }^{\parallel /\perp }$ with associated $M_{\mathrm{eff}}$.