Properties of Barium Ferrite Nanoparticles and Bacterial Cellulose-Barium Ferrite Nanocomposites Synthesized by a Hydrothermal Method

Barium ferrite (BFO) is a class of hard magnetic materials which is technologically important for many applications. Likewise, bacterial cellulose (BC) is a natural cellulose with a unique nanostructure and properties. Particularly, magnetic BC membrane, produced by incorporation of magnetic nanoparticles (NPs) in the BC structure, has recently been a research focus of many research groups. In this work, BFO NPs and BC/BFO nanocomposites were fabricated by hydrothermal synthesis. The BFO NPs could be fabricated only when the synthesis temperature reached 290 °C, with the faceted plate-like shape. Increasing the synthesis temperature gradually changed the magnetic properties from paramagnetic to superparamagnetic and ferromagnetic. Maximum Ms, Mr and Hc of 43 emu/g, 21 emu/g, and 1.6 kOe, respectively, were found. For BC/BFO nanocomposites, the hydrothermal synthesis conditions were limited by the stability of BC, i.e., 150 – 210 °C (for 1 h), or 1 – 7 h (at 190 °C). Using the higher temperature or time resulted in disintegration or decomposition of BC. It was found that very small NPs were coated on the BC nanofibers but the BFO phase was not observed by XRD. However, the magnetic measurement showed the hysteresis loops for the nanocomposites synthesized at 190 °C for 3 – 7 h. The observation of the hysteresis loops could be attributed to a small fraction of BFO in the nanocomposite that cannot be detected by XRD. The BC/BFO nanocomposite membranes were demonstrated for their magnetic attraction, flexibility, and lightness, which make them potential uses for flexible information storage or lightweight magnets

Nano-Morphological, Magnetic and Structural Properties of Ni Films Prepared by RF-Sputtering

Sputtered Ni Films with various deposited times (30, 60, 90,120 and 150 min.) were prepared on glass substrate by RF-sputtering in Argon gas to study effects of sputtering time on their morphological, magnetic and thermal properties. Surface morphological of Ni films were investigated by AFM. The AFM images show a small variation of surface roughness with sputtering time. Average surface roughness of Ni films over scan area of 1μm x 1μm, 5μm x 5μm and 10μm x 10μm are about 0.57, 1.64 and 2.49 nm, respectively. The AFM result infers that Ni films prepared by RF-sputtering exhibit surface roughness in order at nano-scale and have smoother surface than that prepared by DC-sputtering [1]. Structure of Ni films was characterized by XRD. The results display that Ni films exhibit a broad peak of Ni (FCC) phase in (111) plane with a hump at 2θ = 23o [1, 2]. Intensity of Ni (111) peak is increased with increasing sputtering time. Magnetic property of Ni films was study by VSM. The VSM results confirm that Ni films deposited for 90-150 min have a ferromagnetic phase and saturation magnetization is increased whereas coercive field is practically kept constant with increasing deposited time [3, 4]. The DTA result of Ni films show an exothermic peak at 850oC corresponding to decomposition of Ni atoms from the glass substrate. The results confirm that surface roughness, magnetic and structural properties Ni films prepared by RF sputtering can be improved by an appropriate deposited time

Magnetic graphene oxide nanocomposites for selective miRNA separation and recovery

In this study, we developed magnetic graphene oxide composites by chemically attaching Fe3O4 nanoparticles to graphene oxide nanosheets. Characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM), confirmed the successful synthesis of Fe3O4@GO composites with desirable properties. The resulting composites exhibited superparamagnetic behavior, solubility, and compatibility for efficient miRNA separation. Using miR-29a as a model, we demonstrated the effective binding of miR-29a to the magnetic graphene oxide (GO) composites at an optimal concentration of 1.5 mg/mL, followed by a simple separation using magnetic forces. Additionally, the addition of 5.0 M urea enhanced the miRNA recovery. These findings highlight the potential use of our magnetic graphene oxide composites for the efficient separation and recovery of miR-29a, suggesting their broad applicability in various miRNA-based studies. Further exploration can focus on investigating endogenous miRNAs with aberrant expression patterns, contributing to the advancements in precision medicine

3D atom probe studies of thin film magnetic materials

A combination of TEM and 3D atom probe has been used to study thin film magnetic materials for read head applications. Three different types of magnetic thin films have been investigated: (1) Co-based films for bias magnets, (2) magnetic tunnel junction (MTJ) with . AIOx barrier and (3) MTJ with MgO barrier. Unlike in longitudinal recording media, the Co-based films showed no Cr segregation to grain boundaries. This difference is likely to be due to the addition of Pt, which reduces the miscibility gap of the Co-Cr system, and film deposition at room temperature which reduces . the kinetics of segregation. However, some compositional variations were observed within grains. Increasing the angle of film deposition resulted in lower stress in the film, presumably due to better lattice matching between the magnetic layer and the seed layer. The observed coercivity enhancement with the larger deposition angle is attributed to changes in the resultant microstructure, specifically a smaller average grain size and more in-plane orientation of the magnetic easy axis. In the as-deposited MTJ. with an AIOx barrier, the barrier was found to be a discontinuous layer which was wavy and non-uniform in thickness. The AIOx layer was seen to form an interconnecting network of oxide islands, consistent with the islands forming at the grain boundaries of the underlayer. After annealing, the AIOx layer had become thicker, smoother, more uniform in thickness and contained fewer pinholes. The AI:O ratio changed from approximately 4:3 before annealing to close to the stoichiometry of 2:3 after annealing. Three energy contributions were considered as possible driving force for the transformation of the separated islands into a continuous layer: pinhole geometry, metal/oxide interface energy and strain energy. The improved TMR with annealing was mainly attributed to the improvement in the quality of the barrier. For the MTJ with an MgO barrier, the MgO deposited on a flat amorphous surface showed a [001] texture. The amorphous CoFeB partially crystallised after annealing and formed the (001)[110]CoFeB II (001)[100]MgO II (001)[110]CoFeB orientation relationship in some areas. Variations were observed in the chemistry of the barrier, but this was not as extreme as in the case of AIOx• The Mg:O ratio within the oxide was measured to be approximately 1:1 and was not changed after annealing. B was found to diffuse from the CoFeB layers: this must have occurred during deposition, as its distribution was the same before and after annealing. In the region of high MgO concentration, B was found mostly at the CoFeB/MgO interface and the CoFeB/Ru interface whereas in the region of low MgO concentration, B was found throughout layers. Mn in the AF layer was seen to have intermixed with the CoFe synthetic AF layer. The MgO layer topography was anti-correlated to the Ru layer but the effect decreased during annealing.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Investigation of the spin Seebeck effect and anomalous Nernst effect in a bulk carbon material

Since the discovery of the spin Seebeck effect (SSE) in 2008, it has become one of the most active topics in the spin caloritronics research field. It opened up a new way to create the spin current by a combination of magnetic fields and heat. The SSE was observed in many kinds of materials including metallic, semiconductor, or insulating magnets, as well as non-magnetic materials. On the other hand, carbon-based materials have become one of the most exciting research areas recently due to its low cost, abundance and some exceptional functionalities. In this work, we have investigated the possibility of the SSE in bulk carbon materials for the first time. Thin platinum film (Pt), coated on the smoothened surface of the bulk carbon, was used as the spin detector via the inverse spin Hall effect (ISHE). The experiment for observing longitudinal SSE in the bulk carbon was set up by applying a magnetic field up to 30 kOe to the sample with the direction perpendicular to the applied temperature gradient. The induced voltage from the SSE was extracted. However, for conductive materials, e.g. carbon, the voltage signal under this set up could be a combination of the SSE and the anomalous Nernst effect (ANE). Therefore, two measurement configurations were carried out, i.e. the in-plane magnetization (IM), and the perpendicular-to-plane magnetization (PM). For the IM configuration, the SSE + ANE signals were detected where as the only ANE signal existed in the PM configuration. The results showed that there were the differences between the voltage signals from the IM and PM configurations implying the possibility of the SSE in the bulk carbon material. Moreover, it was found that the difference in the IM and PM signals was a function of the magnetic field strength, temperature difference, and measurement temperature. Although the magnitude of the possible SSE voltage in this experiment was rather low (less than 0.5 μV at 50 K), this research showed that potential of using low cost and abundant bulk carbon as spin current supplier or thermoelectric power generators. Keywords: Spin Seebeck effect, Anomalous Nernst effect, Inverse spin Hall effect, Bulk carbo

Size-Controllable Melt-Electrospun Polycaprolactone (PCL) Fibers with a Sodium Chloride Additive

Melt-electrospun polycaprolactone (PCL) fibers were fabricated by using NaCl as an additive. The size and morphology of the PCL fibers could be controlled by varying the concentration of the additive. The smallest size of the fibers (2.67 ± 0.57) µm was found in the sample with 8 wt% NaCl, which was an order of magnitude smaller than the PCL fibers without the additive. The melt-electrospun fibers were characterized using the differential scanning calorimeter (DSC), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) techniques. Interestingly, a trace of NaCl was not found in any melt-electrospun fiber. The remaining PCL after melt-electrospinning was evaporated by annealing, and the NaCl residual was found in the glass syringe. The result confirmed that the NaCl additive was not ejected from the glass syringe in the melt-electrospinning process. Instead, the NaCl additive changed the viscosity and the polarization of the molten polymer. Two parameters are crucial in determining the size and morphology of the electrospun fibers. The higher NaCl concentration could lead to higher polarization of the polymer melt and thus a stronger electrostatic force, but it could also result in an exceedingly high viscosity for melt-electrospinning. In addition, the absence of NaCl in the melt-electrospun PCL fibers is advantageous. The fibers need not be cleaned to remove additives and can be directly exploited in applications, such as tissue engineering or wound dressing