**2.3 BaM thin film grown on (0001)** *c***-plane Al2O3 substrate**

In microwave device applications, BaM films usually have a thickness of several microns. For spintronic devices, the thickness is reduced to tens of nanometers. **Figure 3** shows the structure and magnetic properties of nanometer-thick BaM thin films grown on a *c*-axis Al2O3 substrate. The atomic force microscopy (AFM) image in **Figure 3a** shows a uniform and smooth surface, and the analysis of the AFM data yielded an RMS surface roughness of 0.19 0.03 nm. These results, together with other AFM data not shown, indicate that the BaM film has a reasonably good surface, which is critical for the realization of high-quality BaM thin films. The roughness value here is an average over the measurements of nine different 1 1 μm areas, and the uncertainty is the corresponding standard deviation.

**Figure 3b** shows a 2*θ*/*ω x*-ray diffraction (XRD) scan, with the XRD intensity on a log scale. The *x*-ray *θ* rotation gave a scattered beam that matched the specular reflection from the surface. The detected (001) diffraction peaks all come from *c*-plane scattering of the BaM film. The (006) sapphire substrate peak was also detected. The hysteresis loops in **Figure 3c** were measured by a vibrating sample magnetometer with different field orientations, as indicated. The loops clearly show that the BaM film has perpendicular anisotropy, which confirms the *c*-axis orientation of the film. Analysis of the hysteresis data yielded an effective perpendicular anisotropy field around *H*ani = 20 kOe, which is larger than the bulk value (17 kOe). The normalized saturation magnetization 4*πM*<sup>s</sup> = 4.16 kG, which is lower than the bulk value of BaM (4.70 kG). **Figure 3d** presents a ferromagnetic resonance (FMR)

#### **Figure 3.**

*Structure and magnetic properties of BaM thin films. (a) Atomic force microscope of 5 nm BaM thin film. (b)* x*-ray diffraction of 5 nm BaM thin film. (c) Hysteresis loops of 5 nm BaM thin film. Blue circles, H along out-of-plane direction. Red circles, H along in-plane direction. (d) Ferromagnetic resonance of 20 nm BaM thin film with H along out-of-plane direction. a, b, and c are adapted from [10].*

curve obtained with a 20-nm-thick BaM film at *ω* = 66 GHz. Because of the strong perpendicular anistropy field, the ferromagnetic resonance of BaM film appears between 50 GHz and 75 GHz. In the graph, the blue circles show an FMR profile measured at 66 GHz. The Lorentzian function (red curve) fits the data points better than the Gaussian fit shown as the olive curve, indicating that the film has a uniform quality. The fitting yielded a peak-to-peak linewidth *ΔH* = 26.59 � 0.60 Oe and an FMR resonance field *H*res = 10.24 kOe. Similar measurements can be carried out to FMR profiles at a variety of frequencies, and a Kittel equation can fit the curve with a magnetic field applied out-of-plane:

$$
\omega = 2\pi |\chi| (H\_{\rm res} + H\_{\rm ani} - 4\pi M\_{\rm s}) \tag{1}
$$

of 1.2 μm. Thus, the AFM data show a relatively rough surface with an RMS surface roughness of 14.3 � 1.6 nm. **Figure 4b** presents an XRD spectrum. The spectrum consists of a strong peak from the sapphire substrate and the two other peaks for the

*m*-planes of the BaM film, indicating the in-plane orientation of the *c*-axis. **Figure 4c** presents the two hysteresis loops of the film measured by a vibrating sample magnetometer. One of the loops was measured with the magnetic field applied along the *c*-axis, while the other was measured with the field also in the film plane but perpendicular to the *c*-axis. The dashed lines indicate the extrapolations used to determine the effective anisotropy field *H*ani. The dotted line indicates the determination of the saturation induction 4*πM*s. These data show that the film has a well-defined in-plane uniaxial anisotropy with the easy axis along the *c*-axis. From the hysteresis loops, it can be concluded that *H*ani = 16.5 kOe and 4*πM*<sup>s</sup> = 3.87 kG. Moreover, the film has a large remanent magnetization-to-saturation magnetization ratio of about 0.89 along the *c*-axis. This large ratio results from the strong in-plane uniaxial anisotropy. This means that the film is self-biased to a large degree at zero

*Perpendicular Magnetic Insulator Films for Spintronics DOI: http://dx.doi.org/10.5772/intechopen.92277*

fields, which allows for the use of BaM in self-bias spintronic experiments. **Figure 4d** shows the FMR spectrum of the *c*-axis in-plane BaM film shown by the blue, open circles measured at 64 GHz. The curve is fitted to the derivatives of a trial Lorentzian function and a Gaussian function. Similar to the FMR curve in Section 2.3, it fits better with the Lorentzian function, indicating a uniform film. The Lorentzian fitting yields *H*res = 4.55 kOe and *ΔH* = 318 Oe. The dependence of *H*ani and the microwave frequency *f* can be fitted by a Kittel equation for field

*<sup>ω</sup>* <sup>¼</sup> <sup>2</sup>*π*∣*γ*<sup>∣</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

In the following sections, we introduce recent spintronic experiments using MIs with strong anisotropy fields. Devices that incorporate the unique properties of MIs are an excellent potential solution for the power consumption and heat dissipation problems of conventional electronics, as they would consume much less energy and generate significantly less heat. We introduce the use of different techniques in generating pure spin currents, using bilayer heterostructures of a normal metal (NM)/ferromagnetic material. There are a variety of normal metal choices such as platinum (Pt) and Gold (Au). Both have been explored and tested in spintronics

In the first two sections, we will explore the generation of pure spin currents using the spin Seebeck effect (SSE) and the photo-spin-voltaic effect (PSVE). Both techniques take advantage of a NM coupled with a MI. In SSE, a temperature gradient in the MI is the main factor that induces the MI to inject pure spin currents into the NM layer. In PSVE however, the light of certain wavelengths reaching the atomic layers of the NM, exciting the NM electrons near the NM/MI interface, is what generates the pure spin currents. SSE and PSVE Experimentation results will also be explored and discussed. Then, in the last two sections, we will demonstrate how pure spin currents can be used practically to enhance magnetic switching in MIs in a significant and meaningful way. NM/MI bilayers will not be the only type of heterostructure discussed here, we will also explore topological insulator/MI structures and demonstrate the significance of topological insulators in spintronics.

**3. Spintronic applications with magnetic insulators**

ð Þ *H*res þ *H*ani ð Þ *H*res þ *H*ani þ 4*πMs*

p (3)

applied in-plane:

**43**

**3.1 Introduction to spintronics**

related studies and experiments [18–33].

Such fitting yielded a gyromagnetic ratio *γ* = 2.80 � 0.01 MHz/Oe and *H*ani = 19.12 � 0.04 kOe. The linewidth *ΔH* vs. frequency *ω* data can be fitted with the following equation:

$$
\Delta H = \frac{2a}{\sqrt{3}|\mathbf{y}|} \frac{a\mathbf{o}}{2\pi} + \Delta H\_0 \tag{2}
$$

where *α* is the damping constant and *H*<sup>0</sup> is the inhomogeneity line broadening, which is a parameter associated with *α* describing the damping of the material. The FMR measurements yielded a damping constant *<sup>α</sup>* = (9.7 � 1.1)�10�<sup>4</sup> .
