Consolidated picture of tunnelling spintronics across oxygen vacancy states in MgO

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

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceResearch 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

Magnetoresistance and spintronic anisotropy induced by spin excitations along molecular spin chains

Electrically manipulating the quantum properties of nano-objects, such as atoms or molecules, is typically done using scanning tunnelling microscopes 1-7 and lateral junctions 8-13. 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 14 to address these quantum properties thanks to ferromagnetic electrodes and high-quality interfaces 15-17. The challenge here is to ascertain the device's effective, buried nanotransport path 18 , and to electrically involve these nano-objects in this path by shrinking the device area from the macro-17,19-22 to the nano-scale 23-25 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 26,27. 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 28. 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 2,3 , magnetic exchange bias 29,30 and magnetotransport 24,25 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' 16. This provides a first insight into the experimental energetics of this promising low-power information encoding unit 31

Nat Mater

Molecular semiconductors may exhibit antiferromagnetic correlations well below room temperature. Although inorganic antiferromagnetic layers may exchange bias single-molecule magnets, the reciprocal effect of an antiferromagnetic molecular layer magnetically pinning an inorganic ferromagnetic layer through exchange bias has so far not been observed. We report on the magnetic interplay, extending beyond the interface, between a cobalt ferromagnetic layer and a paramagnetic organic manganese phthalocyanine (MnPc) layer. These ferromagnetic/organic interfaces are called spinterfaces because spin polarization arises on them. The robust magnetism of the Co/MnPc spinterface stabilizes antiferromagnetic ordering at room temperature within subsequent MnPc monolayers away from the interface. The inferred magnetic coupling strength is much larger than that found in similar bulk, thin or ultrathin systems. In addition, at lower temperature, the antiferromagnetic MnPc layer induces an exchange bias on the Co film, which is magnetically pinned. These findings create new routes towards designing organic spintronic devices.Comment in : Molecular spintronics: A warm exchange. [Nat Mater. 2015

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

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