Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

Abstract

Research on advanced materials such as multiferroic perovskites underscores promising applications, yet studies on these materials rarely address the impact of defects on the nominally expected materials property. Here, we revisit the comparatively simple oxide MgO as the model material system for spin-polarized solid-state tunnelling studies. We present a defect-mediated tunnelling potential landscape of localized states owing to explicitly identified defect species, against which we examine the bias and temperature dependence of magnetotransport. By mixing symmetry-resolved transport channels, a localized state may alter the effective barrier height for symmetry-resolved charge carriers, such that tunnelling magnetoresistance decreases most with increasing temperature when that state is addressed electrically. Thermal excitation promotes an occupancy switchover from the ground to the excited state of a defect, which impacts these magnetotransport characteristics. We thus resolve contradictions between experiment and theory in this otherwise canonical spintronics system, and propose a new perspective on defects in dielectrics.

Interfacial effects on the tunneling magnetoresistance inLa0.7Sr0.3MnO3/<

“…This dependence suggests the existence of impurity-assisted conduction channels through the barrier [27]. In fact, the presence of some defects levels in the band gap of MgO due to intrinsic microstructural defects (vacancies, interstitial and interfacial states), which can provide conducting channels in the MgO layer, leading to the degradation of the barrier performances, has been stated previously [15,28,29]. However, as shown in Ref.…”

“…S2 [16]); thus the positive TMR response observed in those samples cannot be attributed to a symmetry filtering effect at the MgO/Fe interface. The presence of an iron oxide layer would add more complexity to the electronic structure of the interface, enabling the suppression or even the sign reversal of the TMR response by temperature changes or by appropriate bias polarity [8,29,30]. In fact, recent reports have shown that an inversion of the spin polarization occurs at oxidized interfaces due to 3d-2p hybridization.…”

“…However, as shown in Ref. [29], hopping across localized states in the MgO/Fe system would not lead to an exponential increase of the junction resistance; thus other contributions, such as increase of resistance at the LSMO topmost layers due to oxygen migration, and 094428-3 structural disorder with the appearance of FeO X at the Fe/MgO interface, should also play a relevant role in this increase of the junction resistance. In principle a negative TMR response should be expected according to the negative and positive effective spin polarization of Fe/MgO interface and LSMO electrode, respectively.…”

“…The particular case of oxygen vacancies has been deeply analyzed in Ref. [29], showing that they can alter the effective barrier height for symmetry-resolved charge carriers, leading to a decrease of the TMR response as temperature increases.…”

“…It is worth mentioning that, in addition to Fe oxidation at the interface, the existence of oxygen vacancies within the MgO barrier itself [29,36], structural defects [37], and roughness [38] may also influence the interfacial electronic structure and therefore contribute to modify the TMR response. The particular case of oxygen vacancies has been deeply analyzed in Ref.…”