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Register a new userTunnel magnetoresistance in scandium nitride magnetic tunnel junctions using first principles
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Authors
Suyogya Karki
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The magnetic tunnel junction is a cornerstone of spintronic devices and circuits, providing the main way to convert between magnetic and electrical information. In state-of-the-art magnetic tunnel junctions, magnesium oxide is used as the tunnel barrier between magnetic electrodes, providing a uniquely large tunnel magnetoresistance at room temperature. However, the wide bandgap and band alignment of magnesium oxide-iron systems increases the resistance-area product and causes challenges of device-to-device variability and tunnel barrier degradation under high current. Here, we study using first principles narrower-bandgap scandium nitride tunneling properties and transport in magnetic tunnel junctions in comparison to magnesium oxide. These simulations demonstrate a high tunnel magnetoresistance in Fe/ScN/Fe MTJs via {Delta}_1 and {Delta}_2 symmetry filtering with low wavefunction decay rates, allowing a low resistance-area product. The results show that scandium nitride could be a new tunnel barrier material for magnetic tunnel junction devices to overcome variability and current-injection challenges.
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While the effects of lattice mismatch-induced strain, mechanical strain, as well as the intrinsic strain of thin films are sometimes detrimental, resulting in mechanical deformation and failure, strain can also be usefully harnessed for applications such as data storage, transistors, solar cells, and strain gauges, among other things. Here, we demonstrate that quantum transport across magnetic tunnel junctions (MTJs) can be significantly affected by the introduction of controllable mechanical strain, achieving an enhancement factor of ~2 in the experimental tunneling magnetoresistance (TMR) ratio. We further correlate this strain-enhanced TMR with coherent spin tunneling through the MgO barrier. Moreover, the strain-enhanced TMR is analyzed using non-equilibrium Greens function (NEGF) quantum transport calculations. Our results help elucidate the TMR mechanism at the atomic level and can provide a new way to enhance, as well as tune, the quantum properties in nanoscale materials and devices.
Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA)in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-3d$ and O-2p orbitals at the interface between the metal and the insulator. On-site projected analysis show that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with $d_{yz(xz)}$ and $d_{z^2}$ character tend always to maintain the PMA while those with $d_{xy}$ and $d_{x^2-y^2}$ character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2ML and reaches a maximum value of 3.6 mJ/m$^2$.
We report a complementary metal oxide semiconductor (CMOS) technology compatible ferroelectric tunnel junction memristor grown directly on top of a Silicon substrate using a scandium doped aluminum nitride as the ferroelectric layer.
The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) tunneling regime when the interface is modified with layers of a different insulator and (ii) resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different sign at positive and negative bias suggesting possibilities of combining memory with logic functions.
The impact of 400 keV $Ar^+$ ion irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The irradiation-induced changes of the magnetic anisotropy, coupling energies and tunnel magnetoresistance (TMR) exhibited a correlated dependence on the ion fluence, which allowed us to distinguish between two irradiation regimes. In the low-fluence regime, ${Phi} < 10^{14} cm^{-2}$, the parameters required for having a functioning MTJ were preserved: the anisotropy of the FeCoB free layer (FL) was weakly modulated following a small decrease in the saturation magnetization $M_S$; the TMR decreased continuously; the interlayer exchange coupling (IEC) and the exchange bias (EB) decreased slightly. In the high-fluence regime, ${Phi} > 10^{14} cm^{-2}$, the MTJ was rendered inoperative: the modulation of the FL anisotropy was strong, caused by a strong decrease in $M_S$, ascribed to a high degree of interface intermixing between the FL and the Ta capping; the EB and IEC were also lost, likely due to intermixing of the layers composing the synthetic antiferromagnet; and the TMR vanished due to the irradiation-induced deterioration of the MgO barrier and MgO/FeCoB interfaces. We demonstrate that the layers surrounding the FL play a decisive role in determining the trend of the magnetic anisotropy evolution resulting from the irradiation, and that an ion-fluence window exists where such a modulation of magnetic anisotropy can occur, while not losing the TMR or the magnetic configuration of the MTJ.
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