Perpendicular Magnetic Insulator Films for Spintronics

*Laith Alahmed and Peng Li*

## **Abstract**

The recent progress in spintronics opens up new directions for novel device concepts and fundamental understandings. This is possible because of magnetic insulators (MIs), which have paved the way toward pure spin current-based spintronics. MIs with perpendicular anisotropy expand the horizon further, enabling new functionalities such as low-power spin-orbit torque switching, highspeed domain-wall motion, high-frequency spin-orbit torque oscillation, etc. In this chapter, we review recent progress in spintronic experiments using barium hexagonal ferrite BaFe12O19—a magnetic insulator with perpendicular anisotropy. These results lay the foundation for using MIs with perpendicular anisotropy as a medium to develop new energy-efficient pure spin current-based electronics.

**Keywords:** magnetic insulator, perpendicular anisotropy, spin current, spin-orbit torque, spintronics

### **1. Introduction**

Spintronics, also known as spin electronics, is a newly emerging field of research that focuses on the spin degree of freedom of electrons rather than their charge. Charge current is a flow of electrons from one point to another under the influence of an electric field. In spintronics, spin current can propagate within the material. A pure spin current can be generated through effects such as the spin Hall effect (SHE), spin pumping, spin-wave propagation, etc. The pure spin currents consume much less energy than charge currents. This is because of the absence of charge flow that eliminates the power consumption needed for the electric field required to drive charge flow [1–3].

In spintronics, magnetization switching is of both fundamental interest and technological significance. One way to switch the magnetization of a ferromagnetic film is through the spin filtering effect. In this case, a spin-polarized electrical current will be generated. As the polarized electrons flow through the ferromagnetic film, they transfer angular momentum to the film and produce a spin-transfer torque to switch the film. This torque is called spin-transfer torque (STT). Magnetic random-access memory based on STT has already been commercialized in recent years.

The above-mentioned spin-torque switching, however, has a limit. The angular momentum transferred per unit charge in the applied current usually cannot exceed a quantum of spin (ℏ/2). Recent work demonstrates that one can exceed this limit by the use of spin-orbit torque (SOT). The demonstration generally takes a nonmagnetic heavy metal (HM)/ferromagnetic metal (FM) bilayered structure and 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 than in the conventional spin-transfer torque case.

topological insulator/BaM heterostructure for magnetization switching. Finally, Section 3.5 provides an outlook in the field of BaM materials and devices.

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

, R\*

, R, R\*

, and T\* blocks are 180°

, T, and T\*

,

close-packed layers form six fundamental blocks, namely, S, S\*

[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,

rotations around the c-axis from the S, R, and T blocks. BaM is built from the

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

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

**2. Properties of barium ferrite thin films**

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

which gives a unit formula of Ba2Fe8O14. The S\*

, and R\* blocks.

stacking of S, R, S\*

**Figure 1.**

**39**

**2.1 Atomic structure of BaM thin films**

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

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 insulator (MI) structure can also be well preserved.

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 that require low-damping, insulating spin channels.

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

topological insulator/BaM heterostructure for magnetization switching. Finally, Section 3.5 provides an outlook in the field of BaM materials and devices.
