Impurity-induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated, using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high interfacial PMA while reducing the bulk magnetization and correlatively the easy-plane demagnetizing energy. As a result, the effective magnetic anisotropy tends to increase as a function of the Cr or V concentration resulting in an increase in the critical magnetic thickness at which the crossover from out-of-plane to in-plane anisotropy takes place. At the same time, the interfacial spin polarization is not affected by the magnetic layer bulk doping by Cr or V impurities and even enhanced in most situations thus favoring an increase of tunnel magnetoresistance (TMR) amplitude.

Figure 1

(a) Schematics of the calculated crystalline structure for a $\sqrt{2}\times \sqrt{2}$ unit cell of $\mathrm{Fe}/\mathrm{Fe}X/\mathrm{Fe}/\mathrm{MgO}$ with $X=\text{Cr}$, V. Fe,(Cr)V, Mg, and O are represented by red, magenta, green, and dark balls, respectively. (b) Variation of interfacial and effective anisotropy (top) as well as of interfacial spin polarization (bottom) as a function of $\mathrm{Fe}X$ [1ML] position within the Fe. MgO and total Fe thicknesses are fixed to 5 and 11 monolayers, respectively.

Figure 2

Atomic layer resolved contributions to the magnetic anisotropy for different $\mathrm{Fe}X$ positions within $\mathrm{Fe}/\mathrm{Fe}X/\mathrm{Fe}/\mathrm{MgO}$ (b)–(d) compared to the pure case (a). Red and blue bars represent $X=\text{V}$ and $X=\text{Cr}$ cases, respectively.

Figure 3

Dependencies of (a) interfacial anisotropy, (b) effective anisotropy, (c) interface polarization, and (d) TMR amplitude on relative $\mathrm{Fe}X$ thickness within the bulk of Fe for $\mathrm{Fe}/\mathrm{Fe}X/\mathrm{Fe}/\mathrm{MgO}$ ($X=\text{Cr}$, V). Inset shows the dependence of the calculated (closed points) saturation magnetization on the relative $\mathrm{Fe}X$ thickness as well as the corresponding experimental (open points) one on the V concentration in ${\mathrm{Fe}}_{(1-a)}X{}_a$/MgO extracted from Ref. [25]. $t_{\text{Fe}X}/t_{\mathrm{total}}$ corresponds exactly to the V concentration in Fe.