NiO Interface Research Articles

Electrode dependence of resistance switching in polycrystalline NiO films

We investigated resistance switching in top-electrode/NiO∕Pt structures where the top electrode was Au, Pt, Ti, or Al. For Pt∕NiO∕Pt and Au∕NiO∕Pt structures with ohmic contacts, the effective electric field inside the film was high enough to induce trapping or detrapping at defect states and thus resistance switching. For a Ti∕NiO∕Pt structure with well-defined Schottky contact at Ti∕NiO interface accompanied by an appreciable voltage drop, the effective electric field inside the NiO film was not enough to induce resistance switching. For an Al∕NiO∕Pt structure with a low Schottky barrier at the Al∕NiO interface, resistance switching could be induced at a higher voltage since the voltage drop at the Al∕NiO interface was not negligible but small.

Effect of impurities on the oxidation mechanism of nickel at 800°C

The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.

Studies of the electronic structure at the Fe3O4–NiO interface

The interfacial electronic structure between the metallic ferrimagnet Fe3O4 and the insulating antiferromagnet NiO is investigated in the lattice matched heteroepitaxial system Fe3O4 (100)–NiO (100) by growing ultrathin NiO films on single-crystal Fe3O4 (100) substrates. The Fe3O4 (√2×√2)R45° surface is characterized prior to growth by low-energy electron diffraction, reflection high-energy diffraction, scanning tunneling microscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy. UPS and XPS, which sample several monolayers in the substrate–overlayer structure, are used to monitor near-surface electronic properties versus NiO overlayer thickness. Comparison of experimental He II UPS spectra of the valence band electronic structure with a simple model of substrate–overlayer emission indicates that the electronic transition from Fe3O4 to NiO is nearly atomically sharp.

Oxide–Oxide Interfaces: Atomistic and Density Functional Study of Cubic‐ZrO2 (100) ‖ NiO (111)

The cubic‐ZrO2 (100) ‖ NiO (111) interface provides an opportunity for comparison between atomic‐scale measurements, atomistic simulations, and theoretical electronic structures. High‐resolution electron microscopy indicates that the oxides share a common oxygen layer and that the small lattice strain is largely taken up by NiO near the interface. Using simple Coulomb plus Buckingham‐type interatomic potentials, we are able to provide a more focused picture, revealing two types of boundary. The lowest energy interface is highly planar, almost ideal in structure; there is a second interface, of higher energy, that shows a rumpled structure with strain taken up by deformation of nickel chains. Depth profiling of atomic site energies permits calculation of interface versus bulk and surface energies, and it shows that the interface effects penetrate only two to three atomic layers. Embedded cluster density functional studies of bulk and interface‐region sites permit the characterization of perturbations of electronic density around the boundaries.

Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet.

Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin structure near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antiferromagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet.

Passivating films on nickel in alkaline solutions II. Ni(II) anodic film growth: quantitative treatment and the influence of the OH− concentration

Through the development of a general quantitative model for the understanding of the growth of continuous passivating films on metals it was possible to describe in detail the voltammetric growth of the NiO film on nickel in NaOH solutions. The transfer coefficient and the exchange current density at the Ni ∣ NiO interface are about l.5 and 4–8 μA cm−2, respectively, and vary with the NaOH concentration in the solution. The overpotential through the film always increases with the potential. The charge density of the film (proportional to its thickness) versus potential presents a sigmoidal behaviour. The electrical field inside the film tends first to a plateau followed by a new increase after the peak current density. In parallel, the ionic specific resistance inside the film passes through a minimum (at the peak potential) during the growth. As a whole it decreases with the increase of the growth velocity and passes through a minimum with the increase of NaOH concentration. All these facts are explained and in agreement with the proposed model.

Specular reflection in spin valves bounded by NiO layers

We have experimentally investigated the possibility of increased electron reflectivity induced by an insulating NiO layer that is used to exchange bias a metallic spin-valve. For this purpose Ni/sub 80/Fe/sub 20//Cu/Ni/sub 80/Fe/sub 20/ and Ni/sub 66/Fe/sub 16/Co/sub 18//Cu/Co/sub 90/Fe/sub 10/ spin-valves were grown and subsequently covered by insulating NiO or by metallic FeMn. In all cases the giant magnetoresistance of the NiO spin-valves is systematically larger upon a variation of the exchange-biased ferromagnetic layer thickness, also after correcting the data for the conductivity of FeMn which we have determined from a separately grown series of samples with variable FeMn thickness. The increased giant magnetoresistance ratios can be qualitatively understood on the basis of a semiclassical calculation in which (partial) specular reflectivity at the NiO interface has been included.

Co-γFe2O3/NiO Perpendicular Magnetic Recording Media

A study on Co-γFe2O3/NiO perpendicular magnetic anisotropic media was reviewed focusing on their fabrication process, crystallographic and magnetic properties, and recording characteristics. The media were prepared using reactive sputtering in Ar+O2 mixture gas and annealing in air at lower process temperature than 350°C. The NiO underlayer deposited in room temperature had NaCl-like structure with orientation. The Co-γFe2O3 layer prepared onto the NiO underlayer had also orientation. The media have large perpendicular anisotropy energy of 1-3×106 erg/cm3 which was originated from preferential crystalline orientation of axis of Co-γFe2O3and tensile stress at the Co-γFe2O3/NiO interface. The perpendicular coercivity of the media was widely changeable from 0.8 kOe to 6 kOe. The media was mechanically strong enough to use in sliding contact recording mode without protective overcoat layer. The media had excellent ultra-high-density recording performance, showing D50 over 150kFRPI and low media noise. It is concluded that the Co-γFe2O3/NiO medium is one of the promising candidates to achieve ultra-high-density recording.

Exchange coupling between NiO and NiFe thin films

NiO/NiFe bilayer thin films were prepared by rf reactive and dc magnetron sputtering, respectively. The exchange coupling strength between NiO and NiFe as a function of NiO texture and interface roughness was investigated by using different sputtering pressures, Au, and Cu buffer layers. The experimental results show that the exchange coupling field strongly depends on the NiO/NiFe interface roughness. In addition, we found the exchange coupling is largest for the (200) texture compared to the (111)-texture films. This is surprising since the bulk spin structure of NiO predicts the (200) plane to be compensated while the (111) plane is to be uncompensated.