Flowing an electrical current that is both of high areal density and large spin polarization across a magnetic tunnel junction (MTJ) can, through spin-transfer torque (STT), alter the relative magnetic orientation of the MTJ’s ferromagnetic electrodes. This effect has enabled key next-generation MTJ applications and commercialized products, from memories to artificial synapses and energy harvesters. As MTJs are now downscaled to 2 nm, basic experimental data challenge the accepted understanding of their operation. From transport spectroscopy, ferromagnetic resonance experiments and ab-initio calculations it is revealed that the high conductivity of STT-ready MTJs, and the STT effect therein, is mediated by oxygen vacancy complexes within the MgO barrier. Our work positions the oxygen vacancy at the core of MgO spintronics. This should disrupt the status-quo on STT-MRAM R&D, by generating defect-specific research and new ideas to confer additional functionality to these next-generation electronic devices, as a nanoelectronics platform to industrialize quantum physics.

Electrically manipulating the quantum properties of nano-objects, such as atoms or molecules, is typically done using scanning tunnelling microscopes and lateral junctions. The resulting nanotransport path is well established in these model devices. Societal applications require transposing this knowledge to nano-objects embedded within vertical solid-state junctions, which can advantageously harness spintronics to address these quantum properties thanks to ferromagnetic electrodes and high-quality interfaces. The challenge here is to ascertain the device's effective, buried nanotransport path, and to electrically involve these nano-objects in this path by shrinking the device area from the macro- to the nano-scale while maintaining high structural/chemical quality across the heterostructure. We've developed a low-tech, resist- and solvent-free technological process that can craft nanopillar devices from entire in-situ grown heterostructures, and use it to study magnetotransport between two Fe and Co ferromagnetic electrodes across a functional magnetic CoPc molecular layer. We observe how spin-flip transport across CoPc molecular spin chains promotes a specific magnetoresistance effect, and alters the nanojunction's magnetism through spintronic anisotropy. In the process, we identify three magnetic units along the effective nanotransport path thanks to a macrospin model of magnetotransport. Our work elegantly connects the until now loosely associated concepts of spin-flip spectroscopy, magnetic exchange bias and magnetotransport due to molecular spin chains, within a solid-state device. We notably measure a 5.9meV energy threshold for magnetic decoupling between the Fe layer's buried atoms and those in contact with the CoPc layer forming the so-called 'spinterface'. This provides a first insight into the experimental energetics of this promising low-power information encoding unit.

Experiments and theory are reexamining how the laws of thermodynamics are expressed in a quantum world. Most quantum thermodynamics research is performed at sub-Kelvin temperatures to prevent thermal fluctuations from smearing the quantum engine's discrete energy levels that mediate the asymmetric shuffling of electrons between the electrodes. Meanwhile, several groups report that building an electron-spin based implementation by placing the discrete spin states of paramagnetic centers between ferromagnetic electrodes can not only overcome this drawback, but also induce a net electrical power output despite an apparent thermal equilibrium. We illustrate this thermodynamics conundrum through measurements on several devices of large output power, which endures beyond room temperature. We've inserted the Co paramagnetic center in Co phthalocyanine molecules between electron spin-selecting Fe/C60 interfaces within vertical molecular nanojunctions. We observe output power as high as 450nW(24nW) at 40K(360K), which leapfrogs previous results, as well as classical spintronic energy harvesting strategies involving a thermal gradient. Our data links magnetic correlations between the fluctuating paramagnetic centers and output power. This device class also behaves as a spintronically controlled switch of current flow, and of its direction. We discuss the conceptual challenges raised by these measurements. Better understanding the phenomenon and further developing this technology could help accelerate the transition to clean energy. Abstract (150 words) Several experiments have suggested that building a quantum engine using the electron spin enables the harvesting of thermal fluctuations on paramagnetic centers even though the device is at thermal equilibrium. We illustrate this thermodynamics conundrum through measurements on several devices of large output power, which endures beyond room temperature. We've inserted the Co paramagnetic center in Co phthalocyanine molecules between electron spin-selecting Fe/C60 interfaces within vertical molecular nanojunctions. We observe output power as high as 450nW(24nW) at 40K(360K), which leapfrogs previous results, as well as classical spintronic energy harvesting strategies involving a thermal gradient. Our data links magnetic correlations between the fluctuating paramagnetic centers and output power. This device class also behaves as a spintronically controlled switch of current flow, and of its direction. We discuss the

Experiments and theory are reexamining how the laws of thermodynamics are expressed in a quantum world. Most quantum thermodynamics research is performed at sub-Kelvin temperatures to prevent thermal fluctuations from smearing the mesoscopic quantum engine's discrete energy levels that mediate the asymmetric shuffling of electrons between the electrodes. Meanwhile, several groups report that building an electron-spin based implementation by placing the discrete spin states of paramagnetic centers between ferromagnetic electrodes can not only overcome this drawback, but also induce a net electrical power output despite an apparent thermal equilibrium. We illustrate this apparent thermodynamics conundrum through measurements on several devices of large output power, which endures beyond room temperature. We've inserted the Co paramagnetic center in Co phthalocyanine molecules between electron spin-selecting Fe/C60 interfaces within vertical molecular nanojunctions. This device class behaves as a spintronically controlled switch of current flow, and of its direction. We observe dc current output over several hours, and output power as high as 450nW(24nW) at 40K(360K). This leapfrogs previous results, as well as other energy harvesting strategies involving a thermal gradient. Our data indicates that the output power is strongly altered when the partly fluctuating paramagnetic centers undergo a magnetic phase transition. This new conceptual ingredient in the spin engine can account for the device's operation beyond the boundaries of classical thermodynamics. Further clarifying the phenomenon and developing this technology could help accelerate the transition to clean energy.

On-going 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.

The field of tunnelling spintronics has flourished through the study of magnetic tunnel junctions (MTJs) with MgO barriers. The combination of high spintronic performance and low effective barrier heights has enabled new technologies, ranging from next-generation memories to bio-inspired computing. This combination is made possible by structural defects such as oxygen vacancies. So far, experiments have pegged an energy separation between these localized states and the Fermi level, while theory has predicted that these are in fact occupied states. To rationalize the defect-mediated potential tunnelling landscape, we have performed experiments in which we tune the MTJ's Fermi level by altering one electrode's work function. We find that switching the top electrode from FeCoB to FeB increases the amplitude of defect-mediated barrier heights. Ab initio theory attributes this increase to an increased energy separation between the localized states of single and double oxygen vacancies and the Fermi level. We thus extract a rationalized potential landscape of tunnelling across oxygen vacancies in MgO involving occupied states. In junctions with high R.A. product such as ours, this leads to a picture of hole tunnelling.

The organic spinterface describes the spin-polarized properties that develop, due to charge transfer, at the interface between a ferromagnetic metal (FM) and the molecules of an organic semiconductor. Yet, if the latter is also magnetic (e.g. molecular spin chains), the interfacial magnetic coupling can generate complexity within magnetotransport experiments. Also, assembling this interface may degrade the properties of its constituents (e.g. spin crossover or non-sublimable molecules). To circumvent these issues, one can separate the molecular and FM films using a less reactive nonmagnetic metal (NM). Spin-resolved photoemission spectroscopy measurements on the prototypical system Co(001)//Cu/Mnphthalocyanine (MnPc) reveal that the Cu/MnPc spinterface atop ferromagnetic Co is highly spin-polarized at room temperature, up to Cu spacer thicknesses of at least 10 monolayers. Ab-initio theory describes a spin polarization of the topmost Cu layer after molecular hybridization that can be accompanied by magnetic hardening effects. This spinterface's unexpected robustness paves the way for 1) integrating electronically fragile molecules within organic spinterfaces, and 2) manipulating molecular spin chains using the well-documented spin transfer torque properties of the FM/NM bilayer.

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 Δ1 electronic symmetry that drives high spintronic performance. Annealing the MgO barrier experimentally increases the ratio of double to single oxygen vacancies. This promotes a lower Δ1 barrier height, reduces the Δ5 transmission, and enhances spintronic performance, in agreement with theory. This novel nanoscale paradigm of tunneling spintronics should affect all research that utilizes this low barrier height (e.g., spin transfer torque), help establish an ultimate limit on laterally downscaling these devices, and promote new nanoscale quantum computing concepts.

The quantum mechanical tunnelling process conserves the quantum properties of the particle considered. As applied to solid-state tunnelling (SST), this physical law was verified, within the field of spintronics, regarding the electron spin in early experiments across Ge tunnel barriers, and in the 90s across Al2O3 barriers. The conservation of the quantum parameter of orbital occupancy, as grouped into electronic symmetries, was observed in the u002700s across MgO barriers, followed by SrTiO3 (STO). In the solid-state, an additional subtlety is the sign of the charge carrier: are holes or electrons involved in transport? We demonstrate that SST across MgO magnetic tunnel junctions (MTJs) involves holes by examining how shifting the MTJu0027s Fermi level alters the ensuing barrier heights defined by the barrieru0027s oxygen vacancies. In the process, we consolidate the description of tunnel barrier heights induced by specific oxygen-vacancy induced localized states. This should provide important insight into spin transfer torque physics across MgO.

The quantum mechanical tunnelling process conserves the quantum properties of the particle considered. As applied to solid-state tunnelling (SST), this physical law was verified, within the field of spintronics, regarding the electron spin in early experiments across Ge tunnel barriers, and in the 90s across Al2O3 barriers. The conservation of the quantum parameter of orbital occupancy, as grouped into electronic symmetries, was observed in the '00s across MgO barriers, followed by SrTiO3 (STO). Barrier defects, such as oxygen vacancies, partly conserve this electronic symmetry. In the solid-state, an additional subtlety is the sign of the charge carrier: are holes or electrons involved in transport? We demonstrate that SST across MgO magnetic tunnel junctions (MTJs) with a large resistance-area (RA) product involves holes by examining how shifting the MTJ's Fermi level alters the ensuing barrier heights defined by the barrier's oxygen vacancies. In the process, we consolidate the description of tunnel barrier heights induced by specific oxygen-vacancy induced localized states. Our work opens prospects to understand the concurrent observation of high TMR and spin transfer torque across MgO-based nanopillars.

Molecular spintronics is an effervescent field of research, which aims at combining spin physics and molecular nano-objects. In this article, we show that phthalocyanine molecules integrated in magnetic tunnel junctions (MTJs) can lead to magnetoresistance effects of different origins. We have investigated cobalt and manganese phthalocyanine molecule based magnetic tunnel junctions. CoPc MTJs exhibit both tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) effects of similar magnitude. However, for MnPc MTJs, a giant TAMR dominates with ratios up to ten thousands of percent. Strong features visible in the conductance suggest that spin-flip inelastic electron tunneling processes occur through the Mn atomic chain formed by the MnPc stacks. These results show that metallo-organic molecules could be used as a template to connect magnetic atomic chains or even a single magnetic atom in a solid-state device.

The conservation of an electron's spin and symmetry as it undergoes solid-state tunnelling within magnetic tunnel junctions (MTJs) is thought to be best understood using MgO-based MTJs(1). Yet the very large experimental values of tunnelling magnetoresistance (TMR) that justify this perception are often associated with tunnelling barrier heights well below those suggested by the MgO optical band gap. This combination of high TMR and low RA-product, while spawning spin-transfer/spin-orbit torque experiments and considerable industrial interest, cannot be explained by standard theory. Noting the impact of a tunnel barrier's altered stoichiometry on TMR 2, we reconcile this 10+ year-old contradiction between theory and experiment by considering the impact of the MgO barrier's structural defects 3-5. We find that the ground and excited states of oxygen vacancies can promote localized states within the band gap with differing electronic character. By setting symmetry-and temperature-dependent tunnelling barrier heights, they alter symmetry-polarized tunnelling and thus TMR. We will examine how annealing, depending on MgO growth conditions, can alter the nature of these localized states. This oxygen vacancy paradigm of inorganic tunnelling spintronics opens interesting perspectives into endowing the MTJ with additional functionalities, such as optically manipulating the MTJ's spintronic response.

Spin-polarized charge transfer between a ferromagnet and a molecule can promote molecular ferromagnetism 1, 2 and hybridized interfacial states3, 4. Observations of high spin-polarization of Fermi level states at room temperature5 designate such interfaces as a very promising candidate toward achieving a highly spin-polarized, nanoscale current source at room temperature, when compared to other solutions such as half-metallic systems and solid-state tunnelling over the past decades. We will discuss three aspects of this research. 1) Does the ferromagnet/molecule interface, also called an organic spinterface, exhibit this high spin-polarization as a generic feature? Spin-polarized photoemission experiments reveal that a high spin-polarization of electronics states at the Fermi level also exist at the simple interface between ferromagnetic cobalt and amorphous carbon6. Furthermore, this effect is general to an array of ferromagnetic and molecular candidates7. 2) Integrating molecules with intrinsic properties (e.g. spin crossover molecules) into a spinterface toward enhanced functionality requires lowering the charge transfer onto the molecule8 while magnetizing it1,2. We propose to achieve this by utilizing interlayer exchange coupling within a more advanced organic spinterface architecture. We present results at room temperature across the fcc Co(001)/Cu/manganese phthalocyanine (MnPc) system9. 3) Finally, we discuss how the Co/MnPc spinterface’s ferromagnetism stabilizes antiferromagnetic ordering at room temperature onto subsequent molecules away from the spinterface, which in turn can exchange bias the Co layer at low temperature10. Consequences include tunnelling anisotropic magnetoresistance across a CoPc tunnel barrier11. This augurs new possibilities to transmit spin information across organic semiconductors using spin flip excitations12.

The authors performed magnetotransport experiments to determine whether annealing alters the oxygen vacancy-mediated tunnelling potential landscape of the central portion of a MgO ultrathin film within sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions. Using the Îrel method reveals a temperature-dependent tunnelling barrier height for a non-annealed barrier that arises from single oxygen vacancies (F centres) and is qualitatively identical to that found for its partly and fully annealed counterparts. Thus these MTJs with F centres remain of F-type upon annealing. This explicitly confirms that the large tunnel-magnetoresistance (TMR) increase upon annealing results mainly from structural modifications of MgO and CoFeB and not from vacancy pairing within the barrier. Photoluminescence spectra performed on both annealed and non-annealed thin MgO films grown on CoFeB electrodes support this conclusion. This work should promote renewed scrutiny over the precise impact of annealing on tunnelling magnetotransport across MgO.

We deploy topographical and spectroscopic techniques to show that a strongly spin-polarized interface arises between ferromagnetic cobalt and an amorphous carbon layer. Scanning tunneling microscopy and spectroscopy show how a semiconducting carbon film with a low band gap of about 0.4 eV is formed atop the metallic interface. To understand how the cobalt/carbon interface is formed, we used X-ray photoemission spectroscopy to study the hybridization state of carbon. We find that the semiconducting layer consists mainly of sp2-bonded carbon atoms with a sp2-to-sp3 ratio between 1.4 and 1.8. The spin-polarized properties of the cobalt/carbon interface are studied by spin-resolved photoemission spectroscopy. We observe interface states close to the Fermi energy that are not exclusive to cobalt. These electronic states reveal a high degree of spin polarization at room temperature.

Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.

The research field of spintronics has sought, over the past 25 years and through several materials science tracks, a source of highly spin-polarized current at room temperature. Organic spinterfaces, which consist in an interface between a ferromagnetic metal and a molecule, represent the most promising track as demonstrated for a handful of interface candidates. How general is this effect? We deploy topographical and spectroscopic techniques to show that a strongly spin-polarized interface arises already between ferromagnetic cobalt and mere carbon atoms. Scanning tunneling microscopy and spectroscopy show how a dense semiconducting carbon film with a low band gap of about 0.4 eV is formed atop the metallic interface. Spin-resolved photoemission spectroscopy reveals a high degree of spin polarization at room temperature of carbon-induced interface states at the Fermi energy. From both our previous study of cobalt/phthalocyanine spinterfaces and present x-ray photoemission spectroscopy studies of the cobalt/carbon interface, we infer that these highly spin-polarized interface states arise mainly from sp2-bonded carbon atoms. We thus demonstrate the molecule-agnostic, generic nature of the spinterface formation.

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.