Abrupt Transition from Ferromagnetic to Antiferromagnetic of Interfacial Exchange in Perpendicularly Magnetized $L{1}_0$-MnGa/FeCo Tuned by Fermi Level Position

An abrupt transition of the interfacial exchange coupling from ferromagnetic to antiferromagnetic was observed in the interface of perpendicularly magnetized $L{1}_0$-$\mathrm{MnGa}/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ epitaxial bilayers when $x$ was around 25%. By considering the special band structure of the MnGa alloy, we present a model explaining this transition by the spin-polarization reversal of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ alloys due to the rise of the Fermi level as the Co content increases. The effect of interfacial exchange coupling on the coercive force ($H_c$) and the spin-dependent tunneling effect in perpendicular magnetic tunnel junctions (pMTJs) based on the coupled composite were also studied. Changes from the normal spin valve to inverted magnetoresistance loops corresponding to the coupling transition were observed in pMTJs with $\mathrm{MnGa}/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ as an electrode.

Figure 1

$M(H)$ loops of MnGa $(30\mathrm{nm})/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ (1.5 nm) with $x$ equal to 20% (a) and 60% (b). The gray line is the $M(H)$ loop of 30-nm MnGa single films included for comparison. (c) and (d) are the corresponding $\mathrm{MR}(H)$ loops of pMTJs tested in a perpendicular direction at room temperature. The solid lines in (c) and (d) are the fits based on the FM and AFM $J_{\mathrm{ex}}$ in $\mathrm{MnGa}/{\mathrm{Fe}}_{80}{\mathrm{Co}}_{20}$ and $\mathrm{MnGa}/{\mathrm{Fe}}_{40}{\mathrm{Co}}_{60}$ interfaces, respectively.

Figure 2

(a) ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ composition dependence of the ${\mathit{M}}_{\mathit{r}}/A$ (diamond) and $H_{\mathrm{c}}$ (circle) values of the MnGa $(30\mathrm{nm})/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ (1.5 nm) bilayer structure; a transition appears when $x$ is approximately 25%. (b) MR ratios at room temperature (solid square) and 5 K (open square) of pMTJs with $\mathrm{MnGa}/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ as bottom electrode. A transition from the normal PSV type to an inverted curve of perpendicular $\mathrm{MR}(H)$ occurs when $x$ is approximately 25%.

Figure 3

$J_{\mathrm{ex}}$ in the $\mathrm{MnG}\mathrm{a}/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ interface as a function of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ composition. A transition from FM to AFM coupling occurs when $x$ is approximately 25%.

Figure 4

(a) Illustration of the band structure of bcc ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ and $L{1}_0$ MnGa alloys. (b) Spin-up (black up triangle) and spin-down (gray down triangle) density of states at the Fermi level of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ alloy with bcc structure (from Ref. [35]).