Magnetic Tunnel Junction Performance Under Mechanical Strain

In this work we investigate the effect of the mechanical stress on the performance of magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy. We developed a 4-point bending setup, that allows us to apply a constant stress over a large substrate area with access to electrical measurements and external magnetic field. This setup enables us to measure key device performance parameters, such as tunnel magnetoresistance (TMR), switching current ($I_c^{50\%}$) and thermal stability ($\Delta$), as a function of applied stress. We find that variations in these parameters are negligible: less than \SI{2}{\percent} over the entire measured range between the zero stress condition and the maximum stress at the point of wafer breakage.Comment: 4 pages, 4 figure

Spin-Orbit Torques in ferrimagnetic GdFeCo Alloys

The spin-orbit torque switching of ferrimagnetic Gd$_x$(Fe$_{90}$Co$_{10}$)$_{100-x}$ films was studied for both transition metal (TM)-rich and rare earth (RE)-rich configurations. The spin-orbit torque driven magnetization switching follows the same handedness in TM-rich and RE-rich samples with respect to the total magnetization, but the handedness of the switching is reversed with respect to the TM magnetization. This indicates that the sign of the spin-orbit-torque-driven magnetic switching follows the total magnetization, although transport based techniques such as anomalous Hall effect are only sensitive to the transition metal magnetization. These results provide important insight into the physics of spin angular momentum transfer in materials with antiferromagnetically coupled sublattices

An all-electrical torque differential magnetometer operating under ambient conditions

An all-electrical torque differential magnetometry (also known as cantilever magnetometry) setup employing piezoelectric quartz tuning forks is demonstrated. The magnetometer can be operated under ambient conditions as well as low temperatures and pressures. It extends the allowed specimen mass range up to several 10 $\mu$g without any significant reduction in the sensitivity. Operation under ambient conditions and a simple all-electrical design of the magnetometer should allow for an easy integration with other experimental setups. The uniaxial magnetic anisotropy of a 25 $\mu$m diameter iron wire, measured under ambient conditions with a high signal to noise ratio, was found to be in good agreement with its literature value. Further applications of the technique are discussed.Comment: 9 two-column pages, 9 figure

Spin-Orbit Torques in Ferrimagnets and Topological Insulators

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 %

Spin-Orbit Torques in Ferrimagnets and Topological Insulators

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 %