Tunnel Magnetoresistance and Spin-Transfer-Torque Switching in Polycrystalline ${\mathrm{Co}}_2\mathrm{FeAl}$ Full-Heusler-Alloy Magnetic Tunnel Junctions on Amorphous $\mathrm{Si}/{\mathrm{SiO}}_2$ Substrates

We study polycrystalline $B2$-type ${\mathrm{Co}}_2\mathrm{FeAl}$ (CFA) full-Heusler-alloy-based magnetic tunnel junctions (MTJs) fabricated on a $\mathrm{Si}/{\mathrm{SiO}}_2$ amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high $B2$ ordering, and a flat surface are achieved by using a MgO buffer layer. A tunnel magnetoresistance ratio up to 175% is obtained for a MTJ with a $\mathrm{CFA}/\mathrm{MgO}/\mathrm{CoFe}$ structure on a 7.5-nm-thick MgO buffer. Spin-transfer-torque-induced magnetization switching is achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. By using a thermal activation model, the intrinsic critical current density ($J_{\mathit{c}0}$) is determined to be $8.2\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$, which is lower than $2.9\times {10}^7\mathrm{A}/{\mathrm{cm}}^2$, the value for epitaxial CFA MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We find that the Gilbert damping constant ($\alpha$) evaluated by using ferromagnetic resonance measurements for the polycrystalline CFA film is approximately 0.015 and is almost independent of the CFA thickness (2–18 nm). The low $J_{c0}$ for the polycrystalline MTJ is mainly attributed to the low $\alpha$ of the CFA layer compared with the value in the epitaxial one (approximately 0.04).

Figure 1

(a) Out-of-plane ($2\theta –\omega$ scan) XRD patterns for polycrystalline CFA full-Heusler-alloy films on MgO-buffered $\mathrm{Si}/{\mathrm{SiO}}_2$ amorphous substrates with varied MgO buffer thickness $t_{\mathrm{MgO}}$: 2.5, 5.0, 7.5, and 10.0 nm. (b) Normalized integrated intensity of the CFA(002) peak and (c) ratios of (002) to (004) peaks as a function of $t_{\mathrm{MgO}}$ for as-deposited (as-dep.) samples and those annealed at $400°\mathrm{C}$ and $480°\mathrm{C}$.

Figure 2

Surface morphology of polycrystalline CFA full-Heusler-alloy films on MgO-buffered $\mathrm{Si}/{\mathrm{SiO}}_2$ amorphous substrates with respect to the thickness of the MgO buffer. The CFA films are 30 nm thick and are deposited at RT and postannealed at $400°\mathrm{C}$ and $480°\mathrm{C}$, respectively. Inset: AFM image of the surface of the CFA film annealed at $400°\mathrm{C}$.

Figure 3

TMR ratios as a function of MgO buffer thickness $t_{\mathrm{MgO}}$ for polycrystalline $\mathrm{CFA}/\mathrm{MgO}/\mathrm{CoFe}$ MTJs with different thicknesses of the MgO barrier $t_{\text{barr}}$: 1.5, 1.8, and 2.0 nm. The 30-nm-thick CFA bottom electrodes are directly deposited on the MgO buffer layer and postannealed at $400°\mathrm{C}$ and $480°\mathrm{C}$ after deposition at RT. The MTJ stacks are annealed at $370°\mathrm{C}$ before the CIPT measurement.

Figure 4

The thickness of MgO barrier, $t_{\text{barr}}$, dependence of (a) TMR ratios and (b) $RA$ at RT for polycrystalline $\mathrm{CFA}/\mathrm{MgO}/\mathrm{CoFe}$ MTJs with as-deposited, $400°\mathrm{C}$ and $600°\mathrm{C}$ annealed $\mathrm{MgO}(7.5\mathrm{nm})/\mathrm{Cr}(40\mathrm{nm})$ buffer layers on ${\mathrm{SiO}}_2$ amorphous substrates, characterized by using CIPT measurement.

Figure 5

$RA$ dependence of TMR ratios for polycrystalline CFA MTJs with a 2-nm-thick CFA layer (“thin-CFA”) as a bottom electrode. The squared symbol indicates the TMR ratio for a polycrystalline CFA MTJ with a 30-nm-thick CFA layer (“thick-CFA”).

Figure 6

(a) Schematic illustration of the structure of a polycrystalline CFA MTJ nanopillar. (b) $R\text{−}H$ loops for a polycrystalline CFA MTJ nanopillar. Wide arrows show the magnetic configurations of bottom (free) and top (reference) electrodes of the MTJ, and narrow arrows indicate the sweep direction of the applied magnetic field. (c) $R\text{−}I$ loops for the MTJ at magnetic fields of 0, $-11$, and $-20\mathrm{Oe}$, respectively. Arrows indicate the sweep direction of the applied current. (d) Representative $R\text{−}I$ loops of the CFA MTJ at an applied magnetic field of $-11\mathrm{Oe}$. (e),(f) Switching probabilities for $I_{c,\mathrm{P}->\mathrm{AP}}$ and $I_{c,\mathrm{AP}->\mathrm{P}}$ obtained by repeating $R\text{−}I$ measurements for 300 times. Solid lines are fitting curves given by Eq. (3). All measurements are performed at RT.

Figure 7

The $H_{\text{ext}}$ dependence of (a) resonant frequency $f_0$, (b) demagnetization field $H_d$, and magnetic damping parameter ${\alpha }_H$ estimated by fitting the FMR spectrum at each magnetic field for a sample of an 18-nm-thick polycrystalline CFA film. The inset in (a) is typical FMR spectra at $H_{\text{ext}}$ of 500, 1000, and 1500 Oe. (c) CFA thickness $t$ dependence of saturation magnetization $M_s$ and damping constant ${\alpha }_0$ of the polycrystalline CFA film.