Enhancement of voltage controlled magnetic anisotropy (VCMA) through electron depletion

Abstract

Recent advancement in the switching of perpendicular magnetic tunnel junctions with an electric field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current spin-transfer torque-magnetic tunnel junction and spin–orbit torque-magnetic tunnel junction devices, the typical linear fJ/V m range voltage controlled magnetic anisotropy (VCMA) needs to be significantly enhanced with approaches that include new materials or stack engineering. A possible bidirectional and 1.1 pJ/V m VCMA effect has been predicted by using heavily electron-depleted Fe/MgO interfaces. To improve upon existing VCMA technology, we have proposed inserting high work function materials underneath the magnetic layer. This will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime, where the pJ/V m and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the Ta/Pd(x)/Ta underlayer's work function. By varying the Pd thickness (x) from 0 to 10 nm, we have observed a tunable change in the Ta layer's work function from 4.32 to 4.90 eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the underlayer, we have performed DFT calculations on supercells of Ta/Pd(x)/Ta/CoFe/MgO, which demonstrate that electron depletion will not be fully screened at the CoFe/MgO interface. Gated pillar devices with Hall cross geometries were fabricated and tested to extract the anisotropy change as a function of applied gate voltage for samples with various Pd thicknesses. The electron-depleted Pd samples show three to six times VCMA improvement compared to the electron accumulated Ta control sample.

Selectively Controlled Ferromagnets by Electric Fields in van der Waals Ferromagnetic Heterojunctions

“…Controlling magnetic properties of FGT by charge transfer in our work is essentially different from the reported voltage-controlled magnetic anisotropy (VCMA) in the literature, − although the latter can also be achieved by modulation of carrier density , in addition to the piezoelectric strain effect − and the electrochemical effect. , There are pretty clear differences between that and our work: In conventional VCMA, the carrier density is usually modulated by gate voltages, where the carrier depletion and accumulation happen at the interface between insulator and ferromagnet. Thus, the modulation of carrier density in the conventional VCMA only applies for ultrathin ferromagnetic metals of a few nanometers due to the short screening length. , In contrast, the magnetic anisotropy for the relatively thick FGT electrodes (∼10–20 nm) is effectively tuned in our experiment due to the electric current directly passing through the FGT/CGT/FGT junction.…”

Strong Impact of Underlayers on the Voltage‐Controlled Magnetic Anisotropy in Interface Engineered Co/MgO Junctions with Heavy Metals

“…Since the work function and the electronic states of the underlayer affect the adjacent ferromagnetic layer, the VCMA effect is also expected to exhibit a variation by introducing different underlayer materials. [35] While experiments with different underlayers have already been carried out in CoFeB/MgO junctions, [36] it has been difficult to systematically discuss the effect of the underlayer on the VCMA effect in these systems: Since the thermal annealing is necessary to obtain enough PMA for the evaluation of the VCMA effect, it is unavoidable during annealing that a large diffusion of underlayer heavy metals occurs into the ferromagnetic layer. [36,37] Therefore, the correlation between the effect of underlayer and the additional VCMA effect caused by the insertion of heavy metal at the junction interface is still elusive.…”

Giant unilateral electric-field control of magnetic anisotropy in MgO/Rh2CoSb heterojunctions

“…1,23 Exploring large PMA magnetic heterostructures with large VCMA effect is one of the main goals in the development of this new MRAM device. [24][25][26][27][28][29][30][31] The VCMA effect can be evaluated by the VCMA coefficient. In the linear regime, the correlation between VCMA and electric field can be represented by an equation, 15,23,24 VCMA = bE I = bE ext /e > , where b is the VCMA coefficient, E ext is the external electric field, E I is the electric field inside the insulator, and e > is the out-of-plane dielectric constant of the insulator.…”

“…1,23 Exploring large PMA magnetic heterostructures with large VCMA effect is one of the main goals in the development of this new MRAM device. 24–31…”