Ordered Defects: A Path to High-Temperature Superconductivity and Magnetic Order

Pablo DE
2018 Research & Development in Material Science  
Introduction Defects in the atomic lattice of solids are sometimes desired. For example, atomic vacancies, single ones or more elaborated defective structures, can generate localized magnetic moments in a non-magnetic crystal lattice. Increasing their density to a few percent, magnetic order appears. Furthermore, certain interfaces can give rise to localized two-dimensional superconductivity with a broad range of critical temperatures. Old and new experimental facts emphasize the need to join e
more » ... the need to join e orts to start using systematically/ ordered defects" in solids to achieve room temperature superconductivity and magnetic order. Magnetic and superconducting orders at room temperature are highly desirable due to possibilities to apply these phenomena in devices at normal life conditions, apart from the huge basic research interest. Although magnetic order is found at 300K in a not so large list of materials, superconductivity at room temperature appeared to be much more difficult to end. Here, we would like to emphasize that ordered defects in some lattice structures can provide us a path to reach both phenomena at very high temperatures in materials that do not show them in their defect free state. We would like to pay attention here on two cases of lattice defects in solids. Namely, a single or a group of vacancies and two dimensional (2D) well dined interfaces in some specific atomic lattices. Vacancies can trigger a magnetic moment around its position in the atomic lattice. A large number of experimental and theoretical work has been done in this respect. For example, STM local measurements revealed that C-vacancies, produced by low energy ion irradiation at the surface of graphite, have a local magnetic moment [1]. Having a large enough density (5%) of hydrogen (or protons) or C-vacancies at certain positions [2,3], one can show experimentally that magnetic order at room temperature appears in graphite bulk samples. Several studies with techniques like element specific X-ray magnetic circular dichroism (XMCD) [4, 5] , NMR [6], magnetization and transport [7] indicate that the magnetic order triggered by defects is intrinsic and with Curie temperatures clearly above 300K. In the case of graphite [5] or ZnO [8, 9] , XMCD results indicate that the valence band is spin polarized in a relative large energy range, an apparently general feature in materials that show defect induced magnetism (DIM). Due to the rather simple way to trigger in non-magnetic materials, room temperature magnetic order by low energy ion irradiation, we may ask whether some kind of devices have been already proposed. Two recent examples are worth mentioning. The rest is the spin later that occurs at the interface between magnetic and non-magnetic regions of the same material, ZnO: Li in the reported case [10] . Whereas the magnetic path of the oxide micro-or nanostructure is produced by an inexpensive 300eV proton irradiation plasma chamber, the protected nonmagnetic semiconducting regions act as a potential well for the thermally activated conduction electrons. The interfaces between magnetic and non-magnetic regions do produce a giant positive magnetoresistance, in contrast to the small and negative magnetoresistance of the magnetic paths alone. This characteristic and other details of the homo-junctions open up a new and simple way to use the spin splitting created in the irradiated oxide for spintronic devices. Other unexpected result was obtained recently by lowenergy ion irradiation on TiO 2 lms. After a gentle ion irradiation hence, the originally non-magnetic lm becomes magnetic at room temperature due to Ti divacancies (which are stable at room temperature) and with the magnetization vector normal to the main area of the lm [11] . The rather large magnetic anisotropy is related apparently to the fact that the magnetic layer resides at the very near surface region. Further increase of the amount of defects by subsequent ion irradiation, vanishes the magnetic anisotropy. Recently obtained results [12] indicate that should be possible to produce nanostructured areas of TiO 2 with perpendicular magnetic anisotropy. Low energy ion irradiation and the existence of DIM in several oxides may open up a new method to reach perpendicular magnetic anisotropy by far simpler and economically advantageous than several others used nowadays [13] . Let us now discuss the other order phenomenon that appears at a specially ordered lattice defect, namely at certain 2D interfaces. Systematically done STM studies of graphene bilayers showed the existence of Van Hove singularities in the electronic density of states that shift to lower bias voltages the smaller the twist angle be-tween the graphene layers [14] . As emphasized recently Figure Res Dev Material Sci
doi:10.31031/rdms.2018.09.000705 fatcat:5kpisdcbojevtb47lwztmkhb3y