**4. Results from magnetic measurements**

In a previous work, it was shown that magnetic force microscopy (MFM) of nanowires of Fe/MgO/Fe revealed the presence of stripe domains [12]; stripe domain was totally absent in films, and it was slightly present in nanocolumns. This shows that the moments are completely aligned in plane in case of thin films. In nanowires, the magnetic moment has a significant out-of-plane component.

X-ray magnetic circular dichroism (XMCD) measurements of nanowires of Fe/MgO/Fe were carried out at beamline 4UB at NSLS in BNL [12]. Background-corrected XMCD signal shows that the nanowires' XMCD signal at the Fe L<sup>3</sup> and L<sup>2</sup> edges is larger [12]. Moreover, a switching between the two edges occurs, and at around 712 eV of photon energy, the intensity of the nanowires' XMCD is smaller [12].

Vibrating sample magnetometer (VSM) measurements were carried out using Vector Magnetometer Model 10 VSM system from MicroSense equipped with 3 T electromagnet (**Figures 5–7**). In these figures, the coercive field, saturation and remanent magnetization values of thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at the substrate temperature of 100°C versus the angle between the applied magnetic field and the surface normal are shown.

In **Figure 5**, the coercive field (Hc) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying between 0 and 360° is depicted. The coercive field value of nanowires is higher than both nanocolumns and thin films. At an angle of 270°, there is an outlier value for thin films that is higher than both nanowires and nanocolumns value of Hc at 270°. The Hc values of nanowires

**Figure 6.** Saturation magnetization (Ms) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at

several angles between applied field and the sample surface varied between 0 and 360°.

**Figure 5.** Coercive field (Hc) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles

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between applied field and the sample surface varied between 0 and 360°.

Nanowires of Fe/MgO/Fe Encapsulated in Carbon Nanotubes http://dx.doi.org/10.5772/intechopen.79819 9

**Figure 5.** Coercive field (Hc) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between applied field and the sample surface varied between 0 and 360°.

Brookhaven National Laboratory (BNL) by the author revealed the existence of a missing

**Figure 4.** XRD of Fe/MgO/Fe/MgO (100) synthesized at several substrate temperatures and of pristine MgO (100).

peaks at 723.5 eV are of equal height for the nanowires and a leading peak in the film's spectra.

In a previous work, it was shown that magnetic force microscopy (MFM) of nanowires of Fe/MgO/Fe revealed the presence of stripe domains [12]; stripe domain was totally absent in films, and it was slightly present in nanocolumns. This shows that the moments are completely aligned in plane in case of thin films. In nanowires, the magnetic moment has a signifi-

X-ray magnetic circular dichroism (XMCD) measurements of nanowires of Fe/MgO/Fe were carried out at beamline 4UB at NSLS in BNL [12]. Background-corrected XMCD signal shows

ing between the two edges occurs, and at around 712 eV of photon energy, the intensity of the

Vibrating sample magnetometer (VSM) measurements were carried out using Vector Magnetometer Model 10 VSM system from MicroSense equipped with 3 T electromagnet (**Figures 5–7**). In these figures, the coercive field, saturation and remanent magnetization values of thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at the substrate temperature of 100°C versus the angle between the applied magnetic field and the surface

In **Figure 5**, the coercive field (Hc) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying

and L<sup>2</sup>

edge in the nanowire spectrum at 717 eV. At the Fe L<sup>2</sup>

edge, the double

edges is larger [12]. Moreover, a switch-

shoulder at the Fe L<sup>3</sup>

8 Nanowires - Synthesis, Properties and Applications

cant out-of-plane component.

nanowires' XMCD is smaller [12].

normal are shown.

**4. Results from magnetic measurements**

that the nanowires' XMCD signal at the Fe L<sup>3</sup>

**Figure 6.** Saturation magnetization (Ms) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between applied field and the sample surface varied between 0 and 360°.

between 0 and 360° is depicted. The coercive field value of nanowires is higher than both nanocolumns and thin films. At an angle of 270°, there is an outlier value for thin films that is higher than both nanowires and nanocolumns value of Hc at 270°. The Hc values of nanowires

**Figure 7.** Remanant magnetization (Mr) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between applied field and the sample surface varied between 0 and 360°.

oscillate with a period of 90°, whereas those of thin films oscillate with a period of 180°. For nanocolumns, the coercive field values are constant at all angles. The in-plane value of the coercive field of nanowires is higher by 754% than planar thin films.

Saturation magnetization (Ms) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying between 0 and 360° is depicted in **Figure 6**. The saturation magnetization of all three oscillates with a period of 180°. The saturation magnetization of nanowires is slightly higher than nanocolumns and thin films at all angles.

Remanent magnetization (Mr) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying between 0 and 360° is shown in **Figure 7**. The remanent magnetization of all three oscillates with a period of 180°. The remanent magnetization of thin films is higher than nanowires and nanocolumns at all angles except at 90 and 270°.

shown to have pronounced twofold symmetry in the rotational hysteresis loop that appeared for an applied field of 5000 Oe and higher [12]. Here, it is shown for the first time that nanocolumns also show pronounced twofold symmetry in the rotational hysteresis loop for an applied field of 5000 Oe and higher (**Figure 8)**. Torque on the film is 20 times higher than on nanowires for the same applied field [12]. **Figure 8** shows that TMM measurements for nanocolumns in the polar plot along equivalent crystallographic direction yields torque on

**Figure 8.** (a) TMM measurements of nanocolumns of Fe/MgO/Fe at several fields while the angle between applied field

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**Figure 9** depicts Mr/Ms ratio measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C. External

the nanocolumns are 10 times higher than on nanowires for the same applied field.

and the sample surface rotate between 0 and 360°. (b) The same figure in part a in polar plot.

Magnetic torque measurements for nanocolumns of Fe/MgO/Fe synthesized at 100°C for several applied fields were carried out using the EV7 torque magnetometer (TMM) system equipped with a 2 T electromagnet (**Figure 8**). Torque magnetometer measurements of thin films and nanowires results from the previous study show a similar trend such as a twofold symmetry [12]. TMM measurements of nanocolumns of Fe/MgO/Fe synthesized at 100°C for several applied fields are shown in **Figure 8(a)** and in the polar plot in **Figure 8(b)**. TMM measurements for planar film and nanowires of Fe/MgO/Fe both synthesized at 100°C were

oscillate with a period of 90°, whereas those of thin films oscillate with a period of 180°. For nanocolumns, the coercive field values are constant at all angles. The in-plane value of the

**Figure 7.** Remanant magnetization (Mr) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at

Saturation magnetization (Ms) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying between 0 and 360° is depicted in **Figure 6**. The saturation magnetization of all three oscillates with a period of 180°. The saturation magnetization of nanowires is slightly higher than

Remanent magnetization (Mr) of nanocolumns (NC), nanowires (NW) and thin films (TF) of Fe/MgO/Fe at several angles between the applied field and the sample surface varying between 0 and 360° is shown in **Figure 7**. The remanent magnetization of all three oscillates with a period of 180°. The remanent magnetization of thin films is higher than nanowires and

Magnetic torque measurements for nanocolumns of Fe/MgO/Fe synthesized at 100°C for several applied fields were carried out using the EV7 torque magnetometer (TMM) system equipped with a 2 T electromagnet (**Figure 8**). Torque magnetometer measurements of thin films and nanowires results from the previous study show a similar trend such as a twofold symmetry [12]. TMM measurements of nanocolumns of Fe/MgO/Fe synthesized at 100°C for several applied fields are shown in **Figure 8(a)** and in the polar plot in **Figure 8(b)**. TMM measurements for planar film and nanowires of Fe/MgO/Fe both synthesized at 100°C were

coercive field of nanowires is higher by 754% than planar thin films.

several angles between applied field and the sample surface varied between 0 and 360°.

nanocolumns and thin films at all angles.

10 Nanowires - Synthesis, Properties and Applications

nanocolumns at all angles except at 90 and 270°.

**Figure 8.** (a) TMM measurements of nanocolumns of Fe/MgO/Fe at several fields while the angle between applied field and the sample surface rotate between 0 and 360°. (b) The same figure in part a in polar plot.

shown to have pronounced twofold symmetry in the rotational hysteresis loop that appeared for an applied field of 5000 Oe and higher [12]. Here, it is shown for the first time that nanocolumns also show pronounced twofold symmetry in the rotational hysteresis loop for an applied field of 5000 Oe and higher (**Figure 8)**. Torque on the film is 20 times higher than on nanowires for the same applied field [12]. **Figure 8** shows that TMM measurements for nanocolumns in the polar plot along equivalent crystallographic direction yields torque on the nanocolumns are 10 times higher than on nanowires for the same applied field.

**Figure 9** depicts Mr/Ms ratio measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C. External

**Figure 9.** (a) Mr/Ms measurements of thin films Fe/MgO/Fe at several fields. (b) Mr/Ms measurements of nanowires of Fe/MgO/Fe at several fields. (c) Mr/Ms measurements of nanocolumns of Fe/MgO/Fe at several fields.

magnetic field applied along equivalent crystallographic directions did not produce equivalent hysteresis loops. This is obvious when comparing Mr/Ms ratio obtained for symmetric orientations, every 15° from 0 to 360° as shown in **Figure 9**. Mr/Ms ratio extracted from VSM measurements at every 15° between 0 and 360° for planar film, nanowires and nanocolumns of Fe/MgO/ Fe synthesized at 100°C is shown in **Figure 9**. In all three cases, a twofold symmetry is observed.

value of Hc as shown in **Figure 11(a)** is the sample synthesized at 100°C. Low-temperature VSM measurements of thin films of Fe/MgO/Fe synthesized at 100°C are shown in **Figure 11**; as expected, all three magnetization parameters Hc, Mr, and Ms increased as temperature is lowered; more pronounced increase was observed in Hc. As shown in **Figure 11(a)**, the value of Hc increased by 61%, Mr increased by 16% and Ms increased by 11% at 80 K compared to their room temperature values, respectively. **Figure 11(b)** depicts hysteresis loops of the Fe/

**Figure 11.** (a) Low-temperature VSM measurements of Hc, Ms and Mr measured at several temperatures for thin films of Fe/MgO/Fe synthesized at 100°C. The lines are drawn connecting the points as a guide. (b) Hysteresis loops of Fe/MgO/

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Fe/MgO (100) thin film synthesized at 100°C measured at several low-temperature values.

As shown in **Figure 5**, the in-plane coercive field (Hc) of nanowires is higher than thin films' Hc by 754%, and nanocolumns' Hc is higher than thin films' Hc by 403%. These higher values of Hc for nanowires are due to magneto crystalline shape anisotropy since both nanowires' and nanocolumns' Hc values are higher than Hc of planar films Magneto crystalline anisotropy according to density functional theory is due to change in the relative occupancy of the 3d orbitals of Fe atoms at the interface of Fe/MgO [15]. The reason nanowires' Hc is higher than nanocolumns' Hc is encapsulation by carbon nanotubes and subsequent interaction of C atoms with Fe atoms. This interaction between C and Fe atoms is between the π-electronic states of carbon and 3d bands of the Fe surface. First-principle calculation predicts weak interaction between the Fe layer and the MgO substrate making the Fe film to act as a free-standing Fe monolayer (3.10 μB), with enhanced magnetic moment [13]. Mössbauer measurement has also shown higher hyperfine field attributed to the interface region between the epitaxial Fe and MgO layers [16]. This study on the hybrid interface between carbonbased organic molecules and ferromagnetic surfaces is very important in the development of wearable spintronics and environmentally friendly sensors based on organic spintronics [17]. Nanowires of α-Fe synthesized inside alumina templates and single nanowires inside dense nickel nanowire arrays [18, 19] depict a B-H loop that narrows when the field is perpendicular

MgO/Fe planar film at low temperatures.

**5. Discussion**

**Figure 10** depicts a coercive field measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C. **Figure 10** depicts a coercive field measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C.A strong dependence on an angle was observed in the films than in the nanowires. Coercive field (Hc) value extracted from VSM measurements at every 15° between 0 and 360° for planar films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C is shown in **Figure 10**. In all three cases, different symmetries are observed. Nanowires depict a fourfold symmetry, whereas a well-defined twofold symmetry is observed in nanocolumns. The conspicuous symmetry difference in nanowires could be due to shape anisotropy and also due to the hybridization that occurs between the π-electronic states of carbon and 3d bands of the Fe surface.

**Figure 11** depicts temperature-dependent magnetization measurements of planar nanometric thin films of Fe/MgO/Fe/MgO (100) synthesized at 100°C. The planar thin film with the highest

**Figure 10.** (a) Hc measurements of thin films of Fe/MgO/Fe at several fields. (b) Hc measurements of nanowires of Fe/ MgO/Fe at several fields. (c) Hc measurements of nanocolumns of Fe/MgO/Fe at several fields.

**Figure 11.** (a) Low-temperature VSM measurements of Hc, Ms and Mr measured at several temperatures for thin films of Fe/MgO/Fe synthesized at 100°C. The lines are drawn connecting the points as a guide. (b) Hysteresis loops of Fe/MgO/ Fe/MgO (100) thin film synthesized at 100°C measured at several low-temperature values.

value of Hc as shown in **Figure 11(a)** is the sample synthesized at 100°C. Low-temperature VSM measurements of thin films of Fe/MgO/Fe synthesized at 100°C are shown in **Figure 11**; as expected, all three magnetization parameters Hc, Mr, and Ms increased as temperature is lowered; more pronounced increase was observed in Hc. As shown in **Figure 11(a)**, the value of Hc increased by 61%, Mr increased by 16% and Ms increased by 11% at 80 K compared to their room temperature values, respectively. **Figure 11(b)** depicts hysteresis loops of the Fe/ MgO/Fe planar film at low temperatures.

## **5. Discussion**

magnetic field applied along equivalent crystallographic directions did not produce equivalent hysteresis loops. This is obvious when comparing Mr/Ms ratio obtained for symmetric orientations, every 15° from 0 to 360° as shown in **Figure 9**. Mr/Ms ratio extracted from VSM measurements at every 15° between 0 and 360° for planar film, nanowires and nanocolumns of Fe/MgO/ Fe synthesized at 100°C is shown in **Figure 9**. In all three cases, a twofold symmetry is observed. **Figure 10** depicts a coercive field measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C. **Figure 10** depicts a coercive field measured using VSM at several angles with respect to an applied field for thin films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C.A strong dependence on an angle was observed in the films than in the nanowires. Coercive field (Hc) value extracted from VSM measurements at every 15° between 0 and 360° for planar films, nanowires and nanocolumns of Fe/MgO/Fe synthesized at 100°C is shown in **Figure 10**. In all three cases, different symmetries are observed. Nanowires depict a fourfold symmetry, whereas a well-defined twofold symmetry is observed in nanocolumns. The conspicuous symmetry difference in nanowires could be due to shape anisotropy and also due to the hybridization that occurs between the π-electronic states of carbon and 3d bands of the Fe surface.

**Figure 9.** (a) Mr/Ms measurements of thin films Fe/MgO/Fe at several fields. (b) Mr/Ms measurements of nanowires of

Fe/MgO/Fe at several fields. (c) Mr/Ms measurements of nanocolumns of Fe/MgO/Fe at several fields.

12 Nanowires - Synthesis, Properties and Applications

**Figure 11** depicts temperature-dependent magnetization measurements of planar nanometric thin films of Fe/MgO/Fe/MgO (100) synthesized at 100°C. The planar thin film with the highest

**Figure 10.** (a) Hc measurements of thin films of Fe/MgO/Fe at several fields. (b) Hc measurements of nanowires of Fe/

MgO/Fe at several fields. (c) Hc measurements of nanocolumns of Fe/MgO/Fe at several fields.

As shown in **Figure 5**, the in-plane coercive field (Hc) of nanowires is higher than thin films' Hc by 754%, and nanocolumns' Hc is higher than thin films' Hc by 403%. These higher values of Hc for nanowires are due to magneto crystalline shape anisotropy since both nanowires' and nanocolumns' Hc values are higher than Hc of planar films Magneto crystalline anisotropy according to density functional theory is due to change in the relative occupancy of the 3d orbitals of Fe atoms at the interface of Fe/MgO [15]. The reason nanowires' Hc is higher than nanocolumns' Hc is encapsulation by carbon nanotubes and subsequent interaction of C atoms with Fe atoms. This interaction between C and Fe atoms is between the π-electronic states of carbon and 3d bands of the Fe surface. First-principle calculation predicts weak interaction between the Fe layer and the MgO substrate making the Fe film to act as a free-standing Fe monolayer (3.10 μB), with enhanced magnetic moment [13]. Mössbauer measurement has also shown higher hyperfine field attributed to the interface region between the epitaxial Fe and MgO layers [16]. This study on the hybrid interface between carbonbased organic molecules and ferromagnetic surfaces is very important in the development of wearable spintronics and environmentally friendly sensors based on organic spintronics [17]. Nanowires of α-Fe synthesized inside alumina templates and single nanowires inside dense nickel nanowire arrays [18, 19] depict a B-H loop that narrows when the field is perpendicular to the nanowire axis; this deformation in the B-H loop indicates small dipole interactions. The small dipole interactions in these systems are due to the large spacing between the nanowires. In this chapter, nanowires of Fe/MgO/Fe for all orientations of the applied field depict B-H loops that have the same width without narrowing deformation in the B-H loops indicating high dipole interactions. The reason for this high dipolar interactions is the densely grown nanowires with close proximity to each other.

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