**2.1 Atomic structure of BaM thin films**

makes use of the spin-orbit coupling-produced SHE in the HM film to convert an in-plane charge current to a pure spin current that flows across the HM thickness. This produces spin accumulation at the HM/FM interface and therefore exerts a SOT on FM. In this case, each electron in the applied current can undergo multiple spin-flip scattering at the interface, therefore enabling more efficient switching

The ferromagnetic films used in most of the SOT studies were all conductive. A direct consequence is the severe shunting current in the ferromagnet layer, which not only limits the switching efficiency but also causes parasitic effects. For example, previous works have shown that interfacing a TI with a conductive FM film can result in a significant modification or even complete suppression of the topological surface states (TSSs) in the TI layer. In a TI/FM heterostructure, the TSSs may have been largely spoiled by the FM electrons. This means that many large spin-orbit torques observed in TI/FM structures may not be due to TSS. In this context, the use of MIs in an HM/MI heterostructure can effectively avoid the shunting current. Moreover, the TSSs in a TI/MI structure can be preserved except for the opening of a small gap at the Dirac point when strong coupling exists at the interface. This will

Magnetic insulators include a large class of materials, including spinels, garnets,

and ferrites. They have a general chemical formula of M(Fe*x*O*y*), where M is representing non-iron metallic elements. MIs have several advantages over magnetic metals for SOT device applications. First, in a heavy metal/MI heterostructure, the charge current only flows in the HM layer but not in the MI layer. In contrast, in an HM/magnetic metal structure, the charge current also flows in the FM, resulting in certain parasitic effects. When the HM layer is replaced by a topological insulator

with high resistivity, the advantage of zero shunting currents in the MI film becomes particularly important. Moreover, interfacing a topological insulator (TI) with a conductive FM can result in a significant modification or even complete suppression of the topological surface states (TSSs) in the TI layer. The use of a magnetic insulator can effectively avoid the shunting current; TSSs in a TI/magnetic

In the ferrite family, hexagonal ferrites have strong magnetocrystalline anisotropy. For example, M-type barium ferrite (BaFe12O19, noted as BaM) has an anisotropy field of 17 kOe. The perpendicular anisotropy in MI films originates from bulk intrinsic anisotropy rather than interfacial anisotropy [4]. This means that, when being used for actual devices, the BaM film has no constrains on the thickness. This is in strong contrast with the ferromagnetic metal counterpart (e.g., CoFeB/MgO) that often has to be very thin to realize interfacial perpendicular anisotropy. In addition, the magnetic damping is usually significantly lower in MIs than in FMs. For example, the intrinsic Gilbert damping constant in BaM materials is 7 <sup>10</sup><sup>4</sup>

which is at least 10 times smaller than the value in permalloy [5]. This advantage is significant for spin-torque oscillator applications, where the current threshold for self-oscillations decreases with the damping, as well as for logic device applications

This chapter reviews the main advances made in spintronic experiments with BaM over the past several years. Section 2 gives a brief introduction to BaM and discusses its crystalline structure, magnetic properties, and thin film growth techniques. This section serves to provide a background for the discussions in the following sections. Section 3 reviews the advances of spintronic experiments with BaM. Section 3.1 provides an overview of the related spintronic experiments. Section 3.2 discusses the generation of pure spin currents through the spin Seebeck effect and photo-spin-voltaic effect in the Pt/BaM structure. Section 3.3 discusses the spin-orbit torque-assisted switching in BaM. Section 3.4 discusses the use of

,

than in the conventional spin-transfer torque case.

*Magnetic Materials and Magnetic Levitation*

enable the magnetization switching due to *bona fide* TSSs.

insulator (MI) structure can also be well preserved.

that require low-damping, insulating spin channels.

**38**

BaM is a hexagonal ferrite, which consists of close-packed layers of oxygen ions. **Figure 1** shows a unit cell of BaM. The Ba2+ ion is large, as is the O<sup>2</sup> ion, and the barium always replaces oxygen somewhere in the oxygen lattice. The close-packed layers form six fundamental blocks, namely, S, S\* , R, R\* , T, and T\* [5–7]. The S block consists of close-packed oxygen layers stacking in an ABCABC … sequence. It has a cubic spinel arrangement with the <111 > axis along the vertical direction. There are two units of Fe3O4 without any barium ions in each S block. The R block comprises close-packed oxygen layers stacking in an ABAB … sequence. It has a hexagonal closest packed structure along the vertical axis. Each R block has a unit formula of BaFe6O11. The T block is made of four oxygen layers, with a barium ion replacing an oxygen ion in the middle two layers, which gives a unit formula of Ba2Fe8O14. The S\* , R\* , and T\* blocks are 180° rotations around the c-axis from the S, R, and T blocks. BaM is built from the stacking of S, R, S\* , and R\* blocks.

Trivalent Fe3+ ions occupy tetrahedral and octahedral sites as well as one trigonal bipyramidal site. Different sites account for different spin orientations and Bohr magnetons (*μB*). For example, a tetrahedral site contributes 2*μB*, while an octahedral site contributes 4*μ<sup>B</sup>* with opposite spin orientations in the S block. In the end, S, S\* , R, and R\* blocks contribute 2*μ<sup>B</sup>* each, leading to a moment of 40*μ<sup>B</sup>* for each unit cell. This gives a saturation magnetization of 4700 G in bulk BaM. BaM has a strong

**Figure 1.** *Crystalline structure of M-type barium ferrite. Blue ball, Ba2+. Yellow ball, Fe3+. Red ball, O2.*

anisotropy field of 17 kOe, which is along the *c* axis. This comes from the trigonal bipyramidal site Fe3+ ions, as well as breaking crystal symmetry in the R/R\* blocks. This is the most distinguished property of BaM, because the perpendicular anisotropy field originates from bulk intrinsic anisotropy. BaM has a large *c* constant of 23.2 Å and an *a* constant of 5.89 Å. The *x*-ray density is about 5.29 g/cm<sup>3</sup> . The Curie temperature of bulk BaM is 725 K, which is much higher than the room temperature. The exchange constant is 6.4 <sup>10</sup><sup>7</sup> erg/cm [7].

deposition, the substrate is cooled down at a rate of 2°C/min in 400 Torr oxygen. The sample is then annealed at 850°C for 4 h in a standalone tube furnace, with a

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)

*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].*

heating rate of 10°C/min and a cooling rate of 2°C/min.

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

**Figure 3.**

**41**

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