Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions with an MgAl2O4 barrier

Abstract

Magnetocapacitance (MC) effect has been observed in systems where both symmetries of time-reversal and space-inversion are broken, for examples, in multiferroic materials and spintronic devices. The effect has received increasing attention due to its interesting physics and the prospect of applications. Recently, a large tunnel magnetocapacitance (TMC) of 332% at room temperature was reported using MgO-based (001)-textured magnetic tunnel junctions (MTJs). Here, we report further enhancement in TMC beyond 420% at room temperature using epitaxial MTJs with an MgAl2O4(001) barrier with a cation-disordered spinel structure. This large TMC is partially caused by the high effective tunneling spin polarization, resulted from the excellent lattice matching between the Fe electrodes and the MgAl2O4 barrier. The epitaxial nature of this MTJ system sports an enhanced spin-dependent coherent tunneling effect. Among other factors leading to the large TMC are the appearance of the spin capacitance, the large barrier height, and the suppression of spin flipping through the MgAl2O4 barrier. We explain the observed TMC by the Debye-Fröhlich modelled calculation incorporating Zhang-sigmoid formula, parabolic barrier approximation, and spin-dependent drift diffusion model. Furthermore, we predict a 1000% TMC in MTJs with a spin polarization of 0.8. These experimental and theoretical findings provide a deeper understanding on the intrinsic mechanism of the TMC effect. New applications based on large TMC may become possible in spintronics, such as multi-value memories, spin logic devices, magnetic sensors, and neuromorphic computing.

Large magnetocapacitance of up to 456% at room temperature in FeCo/MgAl2O4/FeCo(001) magnetic tunnel junctions

“…The TMC ratio for FeCo/MAO/FeCo MTJs is larger than that for previously reported Fe/MAO/Fe MTJs. 31) The TMC ratio remains above 400% even at 125 mV. In the negative region, the TMC ratio decreases as the bias voltage increases.…”

“…As a comparison, the largest TMC ratio reported previously for Fe/MAO/Fe MTJs was 227%. 31) As the frequency increases, the TMC ratio decreases to 22% at 1 MHz.…”

“…29) Additionally, a large TMC ratio at room temperature has been observed in Fe/MAO/Fe(001) MTJs prepared via the post-oxidation of MgAl. 31) Currently, the largest TMC ratio has been demonstrated using the Fe/MAO/Fe(001) MTJs: 227% under a low bias voltage (<1 mV) and 426% at a high bias of 325 mV. Similar to the TMR, the TMC is expected to increase with the effective spin polarization of the FM layers.…”

“…The fitting analysis reveals that the spin polarization P TMC is 0.73 and the P TMR is 0.72, which are higher than P TMC = 0.63 and P TMR = 0.69 of previously reported Fe/MAO/Fe MTJs. 31) Because the TMC indicates a peak at 140 Hz, the operating frequency is set to 140 Hz to investigate the bias-voltage dependence of the TMC.…”

Site-Specific Synthesis of Conductive Graphitic Nanomaterials on a NiFe Thin Film by Localized Laser Irradiation

“…In particular, NiFe (Permalloy) is a material with excellent properties such as high relative permeability, low coercivity, and near-zero magnetostriction, and it is widely used in fields such as magnetic shielding and magnetic sensors. In recent years, magnetic tunnel junctions (MTJs) and spin valves (SVs), stacks of magnetic layers with insulators, metal layers, or molecular materials, have attracted significant attention for application in highly sensitive magnetic sensors and magnetic memory (storage) devices that leverage their unique magnetoresistance and magnetocapacitance effects. − However, the formation of microelectrodes on these magnetic devices requires not only the precise fabrication of electrodes with nanoscale precision but also extra work to remove the oxide layer of a few nm formed on the NiFe surface by additional dry/wet etching or annealing processes . Therefore, for the formation of micro- and nano-sized spintronics devices, it is important to have a technology to easily fabricate microelectrodes that exhibit good conductivity at the desired position on the device with nanoscale precision.…”