Large magnetoresistance in symmetry-filtering scandium nitride junctions using first principles

Abstract

Magnetic tunnel junctions (MTJs) show great promise for implementation in high-performance spin transfer torque magnetic random-access memory (STT-MRAM) [1] and novel computing regimes such as magnetic domain wall logic [2] and neuromorphic computing [3]. MTJs consist of an insulating thin film tunneling barrier sandwiched between two ferromagnet electrodes. By altering the magnetization of the electrodes, the rate of tunneling across the barrier can be varied. This provides a mechanism to convert nonvolatile magnetic information to electrical information via modifying the resistivity of the device, achieving on/off tunnel magnetoresistance ratios (TMR) of over 600% at room temperature in state-of-the-art CoFeB/MgO/CoFeB MTJs [4]. Unfortunately, a handful of material setbacks stand in the way of the widespread adoption of even leading MgO MTJs over other emerging technologies, such as Resistive-RAM junctions, in next-generation architectures. MgO’s wide bandgap (7.8 eV) is associated with a high junction resistance-area (RA) product, greatly increasing power-draw and necessitating high operating voltages, which leads to device variability challenges and dielectric breakdown [5]. Additionally, MgO creates magnetic dead layers through the oxidation of interfacial Fe atoms and requires ultra-thin barrier layers that are difficult to fabricate and may contain pinholes [6]. These setbacks result in a reduction of the effective TMR and an increase in the RA-product, which motivates the search for new barrier materials. Here, we explore the properties of rock-salt structured Scandium Nitride (ScN) magnetoresistive junctions and find it a promising barrier material given its novel electron symmetry filtering properties, high TMR, and low RA-product. Fe/ScN/Fe MTJ supercells were constructed for 3,5,6,7 and 8 atomic layers of ScN, with the ScN lattice rotated by 45 degrees such that the N anions rest over the interfacial Fe, analogous to the Fe/MgO interface. Using the plane-wave basis density functional theory (DFT) package VASP [9], the MTJ supercells were converged and exported to Quantum Espresso [10] to calculate the complex bands, symmetry properties, and to perform spin-polarized electron transport calculations under the Landauer-Büttiker formalism as implemented in PWCOND [9]. A Hubbard +U correction of 4.5 eV was applied to Sc’s 3d orbital to correct the electronic band gaps using the PBE-GGA pseudopotentials. In this manner, the indirect Γ-X gap of the ScN was matched near to experimental values [7] at 1.31 eV. The conductance calculations find TMR ratios exceeding 1900% for sufficiently thick systems (> 6 atomic layers of ScN), with spin-dependent transport dominated by unique Δ2’ band filtering through Sc’s 3d orbital in the 6-layer system analysed [12]. This mechanism is different from the spin filtering in Fe/MgO/Fe MTJs, where MgO selects for the half-metallic Δ1 band present in bcc Fe or CoFe electrodes [8]. Instead, ScN selects for Fe’s Δ2’ electrons with large crystal momentum, which is only provided in the majority spin of the Fe band structure at the fermi energy. Even with the high TMR and symmetry filtering in ScN, the layer-dependent calculations predict that the devices remain highly conductive with respect to barrier thickness, unlike in MgO systems where the dominant Δ1 electron wavefunctions decay rapidly across the barrier [8]. Fabricated in a system with minimal symmetry mixing and atomic dislocations, magnetoresistive ScN junctions could operate with a fraction of the power of MgO MTJs, while allowing for thicker barriers with more lenient manufacturing tolerances and maintaining high TMRs characteristic of MgO MTJs. Partial funding from Sandia National Lab (SNL). SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525 **