Nontrivial spatial dependence of the spin torques in $L{1}_0$ FePt-based tunneling junctions

We present an ab initio study of the spin-transfer torque in Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider an FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultrathin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and $L{1}_0$ FePt, the torque is sharply attenuated, and it is transferred to FePt only for an Fe seed layer that is less than two atomic planes thick. We attribute these features to the different spatial profiles of the exchange and correlation field and the induced nonequilibrium spin accumulation. The calculated tunneling magnetoresistance of the Fe/MgO/FePt/Fe junctions studied is enhanced with respect to the one of Fe/MgO/Fe, while it is reduced with Fe intercalation. Our work shows that $L{1}_0$ FePt junctions can be promising candidates for current-operated magnetic devices and that the magnetic texture at the atomic scale has an important effect on the spin-transfer torque.

Figure 1

Setup for a quantum transport calculation of an Fe/MgO/Fe/FePt/Fe junction. The dashed rectangle delimits the scattering region from the leads. The green arrows indicate the different directions of the magnetizations of the magnetic layers on the left- and right-hand sides of the insulating barrier. The colored spheres represent atoms of different species: Fe atoms are in red, Pt atoms are in gray, O atoms are in light blue, Mg atoms are small red spheres.

Figure 2

Real-space profiles of the relevant components of (a) the exchange and correlation field $\mathbf{\Delta }$, (b) the nonequilibrium spin density $d\mathit{m}/dV$, and (c) the torkance $\mathbf{\tau }$ per unit ${\mu }_{\mathrm{B}}/e$ and area acting on the bcc Fe free layer. The colored background indicates the atomic species in the stack: red for Fe, blue for O, and green for Mg.

Figure 3

Study of the torkance in an FePt/Fe free layer. Left: the relevant components of (a) the exchange and correlation field $\mathbf{\Delta }$, (b) the nonequilibrium spin density $d\mathit{m}/dV$, and (c) the torkance per unit of ${\mu }_{\mathrm{B}}/e$ and area $\mathbf{\tau }$. Black squares and blue circles indicate results obtained without and with relativistic corrections, respectively. Right: comparison of the torkance per unit ${\mu }_{\mathrm{B}}/e$ and area of MTJs with (d) two, (e) four, and (f) six FePt unit cells. In both columns the colored background indicates the atomic species: red for Fe, gray for Pt, blue for O, and green for Mg.

Figure 4

Study of the torkance in an Fe/FePt/Fe free layer made of four FePt monolayers and a variable number of Fe monolayers inserted between MgO and FePt. Left: the relevant components of (a) the exchange and correlation field $\mathbf{\Delta }$, (b) the nonequilibrium spin density $d\mathit{m}/dV$, and (c) the torkance per unit of ${\mu }_{\mathrm{B}}/e$ and area $\mathbf{\tau }$. Black squares and blue circles indicate results obtained without and with relativistic corrections, respectively. Right: comparison of the torkance per unit ${\mu }_{\mathrm{B}}/e$ and area of MTJs with a seed layer comprising (d) one, (e) two, (f) three, and (g) four Fe monolayers. In all panels the colored background indicates the atomic species: red for Fe, gray for Pt, blue for O, and green for Mg.

Figure 5

(a) The total torkance per unit ${\mu }_{\mathrm{B}}/e$ and area acting on the free layer of Fe/MgO/Fe/FePt/Fe junctions with two, four, and six FePt monolayers and an Fe seed layer of $n_{\mathrm{SL}}=0,2,4$ atomic planes. (b) The calculated TMR in Fe/MgO/FePt(4)/Fe and Fe/MgO/Fe($n_{\mathrm{SL}}$)/FePt(4)/Fe junctions where the number in the parentheses indicates the number of unit cells in each layer with $n_{\mathrm{SL}}=2,4,8$ Fe unit cells. In both graphs the black dashed line represents the same quantity calculated for the Fe/MgO/Fe junction.

Figure 6

Torkance per unit ${\mu }_{\mathrm{B}}/e$ and area acting on the reference layer of (a) Fe/MgO/Fe, (b) Fe/MgO/FePt(4)/Fe, and (c) Fe/MgO/Fe(2)/FePt(4)/Fe MTJs. The colored background indicates the atomic species to which each point corresponds: red for Fe, blue for O, and green for Mg.

Figure 7

Study of the torkance in an Fe/MgO/Ni/FePt/Fe junction with different Ni seed layer thicknesses. Left: the relevant components of (a) the exchange and correlation field $\mathbf{\Delta }$, (b) the nonequilibrium spin density $d\mathit{m}/dV$, and (c) the torkance per unit of ${\mu }_{\mathrm{B}}/e$ and area $\mathbf{\tau }$. Right: comparison of the torkance per unit ${\mu }_{\mathrm{B}}/e$ and area of MTJs with a seed layer comprising (d) two, (e) four, and (f) six Ni monolayers. The colored background represents the different atomic species: red for Fe, gray for Pt, blue for O, green for Mg, and purple for Ni.