UC Berkeley

In the field of spintronics, the transport of spin and charge is explored. Within the last decade, significant progress has been made towards the understanding of spin-charge coupling in metallic systems. Further, novel spintronic device applications have been realized. In this regard, spin-orbit torque is of particular interest, as it enables the manipulation of magnetic ordering and has been proven to be useful for memory and logic applications. Here, spin-orbit torque describes the generation of a pure spin current via the spin-Hall effect in a heavy metal or the Rashba-Edelstein effect in a heavy metal\slash ferromagnet interface. When the spin-current diffuses into an adjacent ferromagnet, the magnet generally experiences a torque and thus magnetization dynamics is excited.

In this thesis, we investigate spin-orbit torque in transition metal-rare earth ferrimagnets. While these ferrimagnets provide technological advantages compared to ferromagnets, such as magnetization tunability and bulk perpendicular magnetic anisotropy, the angular momentum transfer process has not been well understood. We show that the spin-orbit torque driven angular momentum transfer follows the sign of the total magnetization, while transport-based effects such as the anomalous Hall effect are only sensitive to the transition metal magnetization. Further, we demonstrate the spin-orbit torque switching of ultra-thick GdFeCo films and show that GdFeCo is a promising material for magnetic memory devices due to a high spin-orbit torque switching efficiency j_c/\Delta.

In the second part of this thesis, we study spin-orbit torques in topological insulator\slash ferromagnet heterostructures. To this end, harmonic Hall measurements are performed in BiSb\Co heterostructures. We find a large second harmonic voltage response stemming from the ordinary Nernst effect, while signatures of spin-orbit torque are absent. This is in contrast to a recent report in literature reporting spin-Hall angles significantly larger than unity. We conclude that the ordinary Nernst effect can be a spurious signal in harmonic Hall measurements where the in-plane magnetic field is rotated in the film plane.

The third part of this thesis discusses the influence of mechanical stress on the performance of magnetic tunnel junctions for spin-transfer torque magnetic random access memory. To this end, a 4-point bending setup that allows the application of constant stress over a large substrate area is developed. We find that the performance of the magnetic tunnel junctions is very robust to external stress with changes in tunnel magneto-resistance (TMR), switching current I_c, and thermal stability Delta less than 2 %.