Magnetic and Electrical Characterizations of Half‐Metallic Fe3O4 Nanowires

Abstract

Half-metallic materials, such as CrO2, La0.7Sr0.3MnO3 (LSMO), and Fe3O4 are highly attractive for spintronics applications because of their high spin polarization. Among these materials, magnetite (Fe3O4) is superior to others because of its high Curie temperature (Tc) of 858 K, which is crucial for thermal stability in device applications. In addition, magnetite has proven to be a ferromagnetic material with a high spin polarization (ca. 100 %) at the Fermi level, which results in a metallic minority spin channel and a semiconductor majority spin channel. Besides the utilization of spin electronics, magnetite can also be used as catalyst and in tunneling magnetoresistance (TMR) and giant magnetoresistive (GMR) devices. Previously, various studies in electron transport and magnetoresistance (MR) of magnetite have mainly focused on 2D structures, such as epitaxial thin films, polycrystalline films, and nanoclusters. Recently, the electronic characteristics of 1D magnetite nanostructures have received much attention because of their unique electron-transport behaviors, which may be different from those of the bulk. In addition, low-dimensional Fe3O4 nanoparticles are particularly promising in biomedical applications, such as drug transport/delivery, cell separation and imaging, and therapeutic in vivo technologies. In this study, a simple vapor–solid growth method was applied to grow a-Fe2O3 NWs in an oxygen-deficient environment; magnetite NWs were then formed by converting the vertically aligned a-Fe2O3 NWs template in a reductive atmosphere. An extensive investigation on the mechanism of transforming a-Fe2O3 NWs to Fe3O4 NWs has been published elsewhere. Electrical measurements were performed by fabricating nanodevices in which the NWs were laid on top of the designed Si chips. The Verwey temperature-transition phenomenon was observed in low-temperature measurements of the nanodevices. The magnetic behavior of the NWs was investigated by superconducting quantum interference device (SQUID) measurements. In addition, a magnetic flux map was acquired by electron holography, which revealed the magnetic microstructure of the 1D magnetite nanowires. Figure 1A and B shows a top-view morphology image of a-Fe2O3 and Fe3O4 NWs, respectively. After the reduction process, the morphology of the Fe3O4 NWs was very similar to that of the a-Fe2O3 template. Figure 1C shows a transmission electron microscopy (TEM) image of the magnetite NWs; the high-resolution TEM (HRTEM) image of the modulated a-Fe2O3 NW due to the oxygen deficiency and the magnetite diffraction pattern (DP) are shown in the insets of the image. The HRTEM image in Figure 1D reveals the single-crystalline structure of the NWs, without linear or planar defects. The two d-spacings of 0.29 nm were identified as Fe3O4 {022} planes. The diffraction pattern, shown in the inset in Figure 1D, also illustrates the single-crystal nature of the NWs at the [111] zone axis. Figure 2A shows a scanning electron microscopy (SEM) image of the nanodevices; an enlarged image of one of the devices in Figure 2A is shown in Figure 2B. The two-point I–V measurements were performed at room temperature in a LabView controlled measurement system under ambient conditions. The linear I–V curves (shown in Fig. 2C) indicate that the characteristics fit well to Ohm’s law. The zero-field resistivities of the nanodevices were estimated by the following equation: R = qL/A (R: resistance, q: resistivity, A: cross-section area, L: NW length). The diameter and length of the measured NWs were 25 nm and 0.7526 lm, respectively. Assuming that the Fe3O4 NWs were of a circular cross-section, the obtained resistivity was 10.30 X cm; approximately three orders of magnitude larger than that of bulk magnetite crystal (19 000 lX cm). The large measured difference between the NWs and the single crystal may be due to contact resistance and surface scattering, resulting from the high surface ratio. However, the surface-scattering mechanism, based on the Fuchs–Sonderheimer (FS) theory, indicates that the aspect ratio is an important factor to the total resistance of nanostructures. According to FS theory, when surface scattering is the dominant mechanism the resistivity of a nanowire deC O M M U N IC A TI O N