Δ 1 symmetry in the magnetic tunnel junction's (MTJ's) parallel orientation of electrode magnetization. Yet these high values of TMR are achieved across barriers with an energetic height as low as 0.4 eV. [7] This severe contradiction between experiment and theory, which enables spin-transfer torque (STT) experiments [2,8] thanks to a low resistance⋅area (RA) product, is suspected to reflect the impact on spintronics of structural defects in the MgO barrier [9] such as oxygen vacancies.The general consensus is that oxygen vacancies negatively impact tunneling spintronic performance, [10][11][12][13] leading to efforts to eliminate oxygen vacancies in MgO. [14] Recently, Schleicher et al. experimentally examined [15,16] how the ground and excited (denoted *) states of single oxygen vacancies, called F centers, decrease TMR. They also proposed, by cross-checking experimental [13,14] and theoretical [17,18] reports, that double oxygen vacancies, called M centers, might in fact enhance TMR. Yet a theory of spintronic tunneling mediated by M centers is lacking, as are magnetotransport experiments that explicitly identify this species within the MgO tunnel barrier. We confirm this counterintuitive yet very portent hypothetical proposal through ab initio calculations (performed within the generalized gradient approximation, or GGA; see the Experimental Section) and magnetotransport experiments, thereby bridging this knowledge gap.We present in Figure 1a the density of states (DOS) of bulk MgO without and with M centers (labeled M-MgO). The double oxygen vacancy results in the creation of two occupied energy levels below Fermi level (E F ), which we denote as the ground states M 1 and M 2 . Additionally, two new unoccupied energy states appear near the minimum of the conduction band, which we denote as the excited states M 1 * and M 2 *. Compared to the case of a F center, [10] the presence of pairs of ground and excited states reflects the bonding/antibonding interaction between the two F centers that form the M center (see insets to Figure 1a). Since the charge density onto the vacancy exhibits a mostly spherical distribution, we infer that the ground states of the M center are of s-like character. We note how the electron density plots also reveal a hybridization between the ground states and the first-nearest-neighbor oxygen ions. As we discuss Tunneling spintronic devices are foreseen to play an important role in emerging technologies, from data read-out and storage to processing, including neuromorphic computing. A counterintuitive suspicion is that double oxygen vacancies within the commonly used MgO barrier underscore the high spintronic performance. Here, how the peculiar electronic properties of these nanoscale objects experimentally enhance spintronic performance is demonstrated. The vacancy's ground state near the Fermi level theoretically promotes enhanced transmission across the barrier of electrons with the Δ 1 electronic symmetry that drives high spintronic performance. Annealing the MgO barrier experimen...
Ongoing research is exploring novel energy concepts ranging from classical to quantum thermodynamics. Ferromagnets carry substantial built-in energy due to ordered electron spins. Here, we propose to generate electrical power at room temperature by utilizing this magnetic energy to harvest thermal fluctuations on paramagnetic centers using spintronics. Our spin engine rectifies current fluctuations across the paramagnetic centers' spin states by utilizing so-called 'spinterfaces' with high spin polarization. Analytical and ab-initio theories suggest that experimental data at room temperature from a single MgO magnetic tunnel junction (MTJ) be linked to this spin engine. Device downscaling, other spintronic solutions to select a transport spin channel, and dual oxide/organic materials tracks to introduce paramagnetic centers into the tunnel barrier, widen opportunities for routine device reproduction. At present MgO MTJ densities in next-generation memories, this spin engine could lead to 'always-on' areal power densities that are highly competitive relative to other energy harvesting strategies.
Materials science and device studies have, when implemented jointly as "operando" studies, better revealed the causal link between the properties of the device's materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a device's operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junction's (MTJ's) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite FeO bonds parallel to the MTJ's interfaces. This explicit link between the device's spintronic performance and these FeO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.
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