Colossal electric field control of magnetic anisotropy at ferromagnetic interfaces induced by iridium overlayer

Abstract

Voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM) devices. Two major challenges for future MeRAM devices are to achieve large perpendicular magnetic anisotropy (PMA) and high voltage-controlled magnetic anisotropy (VCMA) coefficient of heavymetal/ferromagnet/insulator heterostructures (HM/FM/I). Employing ab initio electronic structure calculations we have investigated the effect of epitaxial strain and thickness of both FeCo and Ir layers as design parameters to optimize the PMA and VCMA of Ir/FeCo/MgO. We predict that the Ir cap layer can induce both large PMA and colossal VCMA efficiency which depend on the stain. More importantly, we predict that a single Ir cap monolayer gives rise to a VCMA efficiency one order of magnitude higher than that of thicker Ir layers. The underlying mechanism is the synergistic effects of the emergence of Ir local moments, the large Ir spin orbit coupling (SOC), and the large modulation of the magnetic anisotropy at the Ir/vacuum interface. These results provide useful guiding rules in the design of the next-generation of high performance MeRAM memory devices.

Electric field modulation of magnetism in ferrimagnetic Heusler heterostructures

“…The MCA per unit interfacial area is determined from E MCA = (E [100] − E [001] )/A, where E [100] and E [001] represent the total energy with in-plane and out-of-plane magnetization, respectively, and A is the in-plane area of the unit cell. The magnetic moment of ∼ 0.1 µ B of the interfacial Ir atom for the Ir(1)/Mn 3 Ga/MgO trilayer is much smaller than that of 0.7 µ B in the Ir(1ML)/FeCo/MgO, indicating a stronger hybridization effect in the Ir/Mn 3 Ga interface 43,44 . On the other hand, the emergence of magnetism on the Ir atoms in the Ir(3)/Mn 3 Ga(5)/MgO(5) heterostructures can be attributed to the proximity effect as the freestanding 3ML Ir is nonmag- netic.…”

“…The thin Ir overlayer was considered because it was shown to increase both the PMA and VCMA efficiency in FeCo-based heterostructures. [41][42][43][44][45] Density functional theory (DFT) calculations within the projector augmented-wave (PAW) method, 46 were carried out using the Vienna ab initio simulation package (VASP) 47 . We use the generalized gradient approximation (GGA) to describe the exchange-correlation energy as parametrized by Perdew et al 48 .…”

“…Furthermore, the latter heterostructure with a single Ir cap yields a colossal PMA of 24.75 erg/cm 2 which can be attributed to the large SOC constant of the magnetic Ir overlayer, similar to that of the Ir(1)/FeCo/MgO system. 43 In addition, we have also employed the force theorem where 20 [001] ] in the 2D Brillouin zone (BZ). Here, ε(n, k) [100]([001]) are the eigenvalues of the Hamiltonian for magnetization along the [100] ([001]) direction.…”

Origin of the large interfacial perpendicular magnetic anisotropy in MgO/Co2FeAl

“…where ξ is the SOC constant; o ↑ (u ↑ ) and o ↓ (u ↓ ) denote the occupied (unoccupied) spin-up and spin-down eigenstates, respectively; o ↑ (u ↑ ) and o ↓ (u ↓ ) represent eigenvalues of occupied (unoccupied) spin-up and spindown states, respectively; the L z (L x ) are the angular momentum operators. This theory has been used to successfully explain the K i distribution of interfacial Fe over Brillouin zone in Fe/MgO, 40,41 Fe/CuInSe 2 , 42 and Fe/MgAl 2 O 4 . 31 For a system with a large spin polarization like MgO/Co 2 FeAl, the coupling effects from the opposite spin channel can be neglected, and thus the MAE is mainly determined by the coupling between the occupied and unoccupied spin-down states near the Fermi level.…”

Strain driven phase transition and mechanism for Fe/Ir(111) films

“…For iron monolayers that are adsorbed on a vicinal iridium surface, the maximum local magnetic moment is located at the edge atom, whereas the least is at the kink atoms 34 . For Ir / FeCo bilayers on MgO, the Ir cap layer can induce both a large perpendicular magnetic anisotropy (PMA) and a colossally high voltage controlled magnetic anisotropy efficiency by the strain introduced at the interface 35 . The Fe monolayer tends to be antiferromagnetic for iron on Ir(100) with a relaxed geometry, while an unrelaxed Fe/Ir(001) sample exhibits a ferromagnetic ground state 36 .…”