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Zinc oxide (ZnO) is one of the most promising oxide possibilities for use in a number of industries due to its unique properties. Because of its broad direct bandgap (3.37 eV) and strong exciton binding energy (60 meV) at ambient temperature, ZnO not only conducts electricity well but also transmits visible light and emits UV light. Here, we investigated the effect of growth temperature on ZnO thin films by changing the growth temperatures from 400 oC to 450 oC. Radio-frequency (RF) magnetron sputtering was used to create ZnO thin films on Si(100) substrates. The atomic force microscopy (AFM) results show that the root-mean-square (RMS) roughness decreases from 6.1 ± 1.0 nm to 4.8 ± 0.6 nm as the growth temperatures increase. XRD patterns display the enhancement of ZnO’s structure when increasing the growth temperature. Our findings indicate that controlling growth temperature is the critical factor in producing high quality ZnO thin films.

Magnetite (Fe3O4) is a potential material for spintronic development due to its high Curie temperature (858 K) and half-metallic structure with only one spin polarization at Fermi level. The bulk properties of Fe3O4 make it a big challenge to grow perfectly stoichiometric thin films at a low temperature. Here, we report the structural and morphological evolution of the Fe3O4 thin films as a function of gas flow rate. Radio-frequency (RF) magnetron sputtering was used to fabricate Fe3O4 thin films on the MgO/Ta/SiO2 structure at room temperature. Atomic force microscopy (AFM) shows a spherical-like shape, the root-mean-square (RMS) roughness varies from 1.5 nm to 7.5 nm, and grain size increases from 30 nm to 74.3 nm. The structural properties of Fe3O4 films are dramatically enhanced by increasing the gas flow rate. Moreover, the resistivity (r) versus temperature (T) reveals the existence of a Verwey transition below 120 K, indicating the presence of Fe3O4.

We performed a comparative study on the Gaussian noise and memristance variation tolerance of three crossbar architectures, namely the complementary crossbar architecture, the twin crossbar architecture, and the single crossbar architecture, for neuromorphic image recognition and conducted an experiment to determine the performance of the single crossbar architecture for simple pattern recognition. Ten grayscale images with the size of 32 × 32 pixels were used for testing and comparing the recognition rates of the three architectures. The recognition rates of the three memristor crossbar architectures were compared to each other when the noise level of images was varied from −10 to 4 dB and the percentage of memristance variation was varied from 0% to 40%. The simulation results showed that the single crossbar architecture had the best Gaussian noise input and memristance variation tolerance in terms of recognition rate. At the signal-to-noise ratio of −10 dB, the single crossbar architecture produced a recognition rate of 91%, which was 2% and 87% higher than those of the twin crossbar architecture and the complementary crossbar architecture, respectively. When the memristance variation percentage reached 40%, the single crossbar architecture had a recognition rate as high as 67.8%, which was 1.8% and 9.8% higher than the recognition rates of the twin crossbar architecture and the complementary crossbar architecture, respectively. Finally, we carried out an experiment to determine the performance of the single crossbar architecture with a fabricated 3 × 3 memristor crossbar based on carbon fiber and aluminum film. The experiment proved successful implementation of pattern recognition with the single crossbar architecture.

We report on the interplay between surface morphology and the magnetic properties of Pt/Mn ultra-thin bilayers by measuring the temperature dependence of the resistivity of Hall bars cooled under application of an external magnetic field. The resistivity (ρ) versus temperature (T) curves measured under a magnetic field applied along the x-, y- and z- directions exhibit a clear difference between the zero-field-cooled (ZFC) and field-cooled (FC) traces. The disparity between the ZFC and FC traces allowed us to determine the direction of the magnetic easy-axis. The ρ vs. T characteristic of the Pt/Mn bilayers revealed the existence of antiferromagnetic (AF) order below 79 K, in agreement with the Neel temperature observed in temperature dependent susceptibility curves. We also investigated the morphological evolution of the Mn films as a function of Mn thickness by atomic force microscopy (AFM). We observed the formation of a Mn layered structure on a 2nm-thick Al2O3 buffer layer for films thicker than 2.5 nm. Our results suggest that bilayer structures consisting of non-magnetic noble metals on ultra-thin antiferromagnetic metals can be used as a probe to explore the complex magnetic properties of the AF layer.

Non-magnetic (NM) metals with strong spin-orbit coupling have been recently explored as a probe of interface magnetism on ferromagnetic insulators (FMI) by means of the spin Hall magnetoresistance (SMR) effect. In NM/FMI heterostructures, increasing the spin mixing conductance (SMC) at the interface comes as an important step towards devices with maximized SMR. Here we report on the study of SMR in Pt/Fe3O4 bilayers at cryogenic temperature, and identify a strong dependence of the determined real part of the complex SMC on the interface roughness. We tune the roughness of the Pt/Fe3O4 interface by controlling the growth conditions of the Fe3O4 films, namely by varying the thickness, growth technique, and post-annealing processes. Field-dependent and angular-dependent magnetoresistance measurements sustain the clear observation of SMR. The determined real part of the complex SMC of the Pt/Fe3O4 bilayers ranges from 4.96 × 1014 Ω−1 m−2 to 7.16 × 1014 Ω−1 m−2 and increases with the roughness of the Fe3O4 underlayer. We demonstrate experimentally that the interface morphology, acting as an effective interlayer potential, leads to an enhancement of the spin mixing conductance.

Ex-situ and in-situ morphological analyses were performed for Cu-phthalocyanine (CuPc) organic semiconductor films by using atomic force microscopy (AFM) and reflection high-energy electron diffraction (RHEED). The focus was the effects of post-annealing on the structural characteristics of CuPc films grown on MgO(001) layers by using an ultra-high-vacuum thermal evaporator. Sphere-to-nanofibril and 2-D to 3-D morphological transitions were observed with increasing CuPc thickness beyond 3 nm. The surface morphology and the crystallinity were drastically improved after an additional cooling of the post-annealed CuPc films thinner than 3 nm. Our results highlight that molecular orientation and structural ordering can be effectively controlled by using different temperature treatments and a proper combination of material, film thickness, and substrate.

Tuning the magnetoresistance behavior of heterostructures composed of nonmagnetic and ferromagnetic (FM) materials is crucial for improving their applicability in electronic and spintronic devices. In this study, we investigate whether the integration of organic layers to NiFe/Pt junctions can result in the modification of the magnetic moment of the FM layer using iron phthalocyanines (FePc) and copper phthalocyanines (CuPc) as the interfacial layers for controlling the spin-charge conversion. Relaxation of the out-of-plane magnetic hard axis of the NiFe/Pt junctions is observed, as a result of the modification of the interfacial magnetic structure. The transport measurements of the fabricated hybrid Hall bar junctions with NiFe/FePc/Pt and NiFe/CuPc/Pt reveal that although the intrinsic anisotropic magnetoresistance of the present Hall bar is maintained with the integration of interfacial metal phthalocyanine (MPc) layers, a change in the magnetic response along the axis perpendicular to the in-plane of Hall bars is observed, owing to the insertion of the interfacial MPc layers. The present method of interface engineering via integration of organic interfacial layers can act as a model system for controlling the spin-charge conversion behavior of magnetic heterojunction toward the development of multifunctional molecular-engineered spintronic devices.

Non-magnetic (NM) metals with strong spin-orbit coupling have been recently explored as a probe of interface magnetism on ferromagnetic insulators (FMI) by means of the spin Hall magnetoresistance (SMR) effect. In NM/FMI heterostructures, increasing the spin mixing conductance (SMC) at the interface comes as an important step towards devices with maximized SMR. Here we report on the study of SMR in Pt/Fe