Giant Perpendicular Magnetic Anisotropy Enhancement in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>MgO</mml:mi></mml:math>-Based Magnetic Tunnel Junction by Using <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Co</mml:mi><mml:mo>/</mml:mo><mml:mi>Fe</mml:mi></mml:math> Composite Layer
Libor Vojáček, Fatima Ibrahim, Ali Hallal, B. Diény, Mairbek Chshiev
Abstract
Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT MRAM), which is nonvolatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density-functional-theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising $\mathrm{Fe}(n)\mathrm{Co}(m)\mathrm{Fe}(n)|\mathrm{Mg}\mathrm{O}$ storage layers [n and m denote the number of monolayers (ML)] with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m${}^{2}$, leveraging the interfacial perpendicular anisotropy of $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ along with a strain-induced bulk PMA discovered within bcc $\mathrm{Co}$. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the $\mathrm{Fe}(3\mathrm{ML})\mathrm{Co}(4\mathrm{ML})\mathrm{Fe}(3\mathrm{ML})$ as a storage layer for $\mathrm{MgO}$-based STT MRAM cells. The large PMA in strained bcc $\mathrm{Co}$ is explained in the framework of second-order perturbation theory by the $\mathrm{MgO}$-imposed strain and consequent changes in the energies of ${d}_{yz}$ and ${d}_{{z}^{2}}$ minority-spin bands.