**1. Introduction**

In TMR, electrical current flows across a barrier of nanometric thin insulator layer between two ferromagnetic metal electrodes when an external magnetic field is applied parallel to the trilayer surface. TMR is one of the few examples of macroscopic quantum mechanical phenomena that have no classical explanation. TMR has a wide array of applications including magnetic random access memory for futuristic quantum computer and ultrasensitive sensors [1]. TMR is the basic building block of magnetic tunnel junctions (MTJs); specifically Fe/MgO/Fe TMR having one of the highest magnetoresistance (MR) ratio is of current interest as evidenced by

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

several studies of its magnetic properties [2–7]. Theory predicts several thousand percent MR ratio for Fe/MgO/Fe TMR trilayers when an interface is modeled to possess an abrupt change from Fe to MgO without any diffusion at the interface [3]. When diffusion at the interface is introduced in the theoretical modeling by way of interface oxidation, the MR ratio dropped to 1000% [2]. First-principle modeling showed that diffusion at the interface plays a major role in decreasing the calculated value of the MR ratio to the extent when only 16% of Fe is replaced by Mg and vice versa; the MR ratio agreed with experimental values for higher values of the insulator (MgO) thickness [4]. The noncollinear nature of the atomic moments with the bulk magnetization at the interface of Fe and MgO will perhaps affect the net anisotropy, or it may be a spin scattering locality [5]. Experimental measurements showed that single-crystal Fe/ MgO/Fe has an MR ratio of 180% at room temperature and 247% at 20 K [6]. It was also shown experimentally that for crystalline MgO (001), the barrier between the electrodes resulted in a 220% MR ratio at room temperature [7]. The symmetry of electronic wave functions plays a paramount role in coherent spin-polarized tunneling, which gives rise to enormous TMR effects. Shape anisotropy plays an important role in coherent spin-polarized tunneling of electrons across the insulator barrier; thus, switching the geometry of TMR from planar films to nanowires will increase the coherence thus increasing the MR ratio [8, 9]. The importance of shape anisotropy in ferromagnet/insulator/ferromagnet trilayer structures in particular for Fe/ MgO/Fe and the effect of encapsulation of a TMR by carbon tubes are discussed in this chapter based mainly on magnetic measurement results and subsequent interpretations.

symmetrical with respect to the barrier-normal axis are connected to the electronic states in the barrier region and have significant tunneling probability. The MR ratio is defined as follows:

Nanowires of Fe/MgO/Fe Encapsulated in Carbon Nanotubes

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where *R*p and *R*ap are the tunnel resistance when the magnetizations of the two electrodes are

A TMR system reveals unique properties with attractive effects for technological applications. Besides giant and tunneling magnetoresistance, it presents other remarkable effects such as antiferromagnetic exchange coupling, oscillatory behavior of exchange coupling and biquadratic exchange couplings. Nanometric ultrathin films exhibit an out-of-plane uniaxial surface

**Figure 2.** Schematic of planar TMR and array of TMR structures. Both sets of samples were grown using the same magnetron sputtering deposition tool. The planar films will establish initial growth conditions for arrays of nanocolumns.

MR ratio = (*R*ap − *R*p)/*R*p,

aligned in parallel and antiparallel, respectively (**Figure 1**).

TMR-based magnetic tunnel junctions are theoretically expected to exhibit an extremely high MR ratio due to coherent tunneling [2]. When the coherency of electron wave functions is conserved during tunneling, only conduction electrons whose wave functions are totally

**Figure 1.** Magnetron sputtering tool with 3 DC/1 RF sources and a load-lock.

symmetrical with respect to the barrier-normal axis are connected to the electronic states in the barrier region and have significant tunneling probability. The MR ratio is defined as follows:

several studies of its magnetic properties [2–7]. Theory predicts several thousand percent MR ratio for Fe/MgO/Fe TMR trilayers when an interface is modeled to possess an abrupt change from Fe to MgO without any diffusion at the interface [3]. When diffusion at the interface is introduced in the theoretical modeling by way of interface oxidation, the MR ratio dropped to 1000% [2]. First-principle modeling showed that diffusion at the interface plays a major role in decreasing the calculated value of the MR ratio to the extent when only 16% of Fe is replaced by Mg and vice versa; the MR ratio agreed with experimental values for higher values of the insulator (MgO) thickness [4]. The noncollinear nature of the atomic moments with the bulk magnetization at the interface of Fe and MgO will perhaps affect the net anisotropy, or it may be a spin scattering locality [5]. Experimental measurements showed that single-crystal Fe/ MgO/Fe has an MR ratio of 180% at room temperature and 247% at 20 K [6]. It was also shown experimentally that for crystalline MgO (001), the barrier between the electrodes resulted in a 220% MR ratio at room temperature [7]. The symmetry of electronic wave functions plays a paramount role in coherent spin-polarized tunneling, which gives rise to enormous TMR effects. Shape anisotropy plays an important role in coherent spin-polarized tunneling of electrons across the insulator barrier; thus, switching the geometry of TMR from planar films to nanowires will increase the coherence thus increasing the MR ratio [8, 9]. The importance of shape anisotropy in ferromagnet/insulator/ferromagnet trilayer structures in particular for Fe/ MgO/Fe and the effect of encapsulation of a TMR by carbon tubes are discussed in this chapter

4 Nanowires - Synthesis, Properties and Applications

based mainly on magnetic measurement results and subsequent interpretations.

**Figure 1.** Magnetron sputtering tool with 3 DC/1 RF sources and a load-lock.

TMR-based magnetic tunnel junctions are theoretically expected to exhibit an extremely high MR ratio due to coherent tunneling [2]. When the coherency of electron wave functions is conserved during tunneling, only conduction electrons whose wave functions are totally

$$\text{MR ratio = } (R\_{\text{ap}} - R\_{\text{p}}) / R\_{\text{p}}$$

where *R*p and *R*ap are the tunnel resistance when the magnetizations of the two electrodes are aligned in parallel and antiparallel, respectively (**Figure 1**).

A TMR system reveals unique properties with attractive effects for technological applications. Besides giant and tunneling magnetoresistance, it presents other remarkable effects such as antiferromagnetic exchange coupling, oscillatory behavior of exchange coupling and biquadratic exchange couplings. Nanometric ultrathin films exhibit an out-of-plane uniaxial surface

**Figure 2.** Schematic of planar TMR and array of TMR structures. Both sets of samples were grown using the same magnetron sputtering deposition tool. The planar films will establish initial growth conditions for arrays of nanocolumns.

anisotropy sufficient to overcome the demagnetizing field. This feature makes it important for high-density magnetic media, highly sensitive sensors and parts for a quantum computer. This system in addition to its enormous potential for technological applications is an attractive research object in nanomagnetism.

Samples were prepared in the form of nanometric thin films and nanocolumns as shown in **Figure 2**. Nanocolumn arrays were fabricated with different shapes and in-plane orientations by glancing angle deposition (GLAD). This will provide shape anisotropy, which will compete with surface and volume anisotropy. Many unique and fascinating properties have already been demonstrated by nanocolumns synthesized using GLAD, such as superior mechanical toughness, higher luminescence efficiency, enhancement of thermoelectric figure of merit and lowered lasing threshold. Homogeneous nanowires and nanowire networks have been previously used as chemical sensors, field-effect transistors and inverters, photodetectors, light-emitting diodes, lasers and logic gates. Very recently, by altering the compositions of the nanostructures during fabrication, super-lattice nanowire has been demonstrated, which can greatly increase the versatility and application of these building blocks in nanoscale electronic, photonic, and biological applications. Fe nanocolumns were used to synthesize metal-assisted protein crystallization [10]. Possible applications, including thermoelectrics, nanobarcodes, injection lasers and one-dimensional waveguides, could be implemented through these super-lattice nanostructure building blocks. One very important issue associated with these studies is how to assemble one-dimensional nanostructures in an effective and controllable way. GLAD produces columnar structures through the effect of shadowing during film growth, while the substrate rotation controls the shape of the columns.

Nanowires were synthesized by magnetron DC and RF sputtering in the nanometric interior

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Nanometric thin films were epitaxially grown on the MgO (100) substrate of dimensions 5 mm × 5 mm × 0.5 mm using magnetron DC and RF sputtering at several temperatures. All substrates were degassed at 350°C in vacuum of 0.1 μTorr for 1800 s, and samples were pre- and postannealed at a preselected deposition temperature for 1800 s in vacuum. The source substrate distance was kept fixed at 30 cm, and the substrate surface normal was kept at 45° with a line connecting the center of the sample to the center of the target, while being rotated at a constant rate of 20 rpm for uniform deposition. Under these conditions, epitaxial Fe grows on MgO (100) due to a good lattice match of MgO and Fe, and weak interface interaction [13, 14] free standing Fe is formed. The deposition rate for Fe was 0.17 nm/s as calibrated by the deposition time versus thickness measurements for Fe films several hundred nm thick. In my previous research on thin films of Fe/MgO/Fe, several planar samples were synthesized at several substrate tem-

peratures [12]. The film synthesized at 100°C has the highest saturation magnetization.

eters remained the same as in the synthesis of thin film (**Figure 3**).

**3. Structural characterization**

depicts a uniform composition.

Nanocolumns of Fe/MgO/Fe were synthesized at a glancing angle of 70°, and other param-

All sets of samples were grown using the same magnetron sputtering deposition tool. The planar films established initial growth conditions for the arrays of nanowires and nanocolumns.

Thin film samples of Fe/MgO/Fe on MgO (100) substrates were characterized by XRD (miniflex Rigaku X-ray diffraction of 40 kV/40 mA) using CuKα radiation in θ–2θ geometry (**Figure 4**). Surface morphologies of nanowires of Fe/MgO/Fe grown in the interior cylindrical space of CNTs were characterized by SEM/STEM in a previous study [12]. The SEM/STEM scan

X-ray absorption spectroscopy (XAS) measurements on thin films and nanowires of Fe/ MgO/Fe carried out at beamline 4UB at the National Synchrotron Light Source (NSLS) in

cylindrical volume of carbon nanotubes at a substrate temperature of 100°C.

**Figure 3.** Schematic of planar TMR and array of TMR structure.
