Experimental Evidence of Ferroelectric Negative Capacitance in Nanoscale Heterostructures

We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric, Pb(Zr0.2Ti0.8)O3 and dielectric, SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau's mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in MOSFETs, the sub threshold slope can be lowered below the classical limit (60 mV/decade)

Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque.

Spin-polarized electrons can move a ferromagnetic domain wall through the transfer of spin angular momentum when current flows in a magnetic nanowire. Such current induced control of a domain wall is of significant interest due to its potential application for low power ultra high-density data storage. In previous reports, it has been observed that the motion of the domain wall always happens parallel to the current flow - either in the same or opposite direction depending on the specific nature of the interaction. In contrast, here we demonstrate deterministic control of a ferromagnetic domain wall orthogonal to current flow by exploiting the spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO heterostructure in presence of an in-plane magnetic field. Reversing the polarity of either the current flow or the in-plane field is found to reverse the direction of the domain wall motion. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an external magnetic field. Therefore the domain wall motion happens purely due to spin orbit torque. These results represent a completely new degree of freedom in current induced control of a ferromagnetic domain wall

Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque without an External Magnetic Field by Engineering a Tilted Anisotropy

Spin orbit torque (SOT) provides an efficient way of generating spin current that promises to significantly reduce the current required for switching nanomagnets. However, an in-plane current generated SOT cannot deterministically switch a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-density data storage applications due to their significantly larger thermal stability in ultra-scaled dimensions. Here we show that it is possible switch a perpendicularly polarized magnet by SOT without needing an external magnetic field. This is accomplished by engineering an anisotropy in the magnets such that the magnetic easy axis slightly tilts away from the film-normal. Such a tilted anisotropy breaks the symmetry of the problem and makes it possible to switch the magnet deterministically. Using a simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of the magnetization by reversing the polarity of the applied current. This demonstration presents a new approach for controlling nanomagnets with spin orbit torque

Spin orbit torque driven magnetic switching for low power computing and memory

Spintronics has rapidly emerged as a highly pursued research area in solid-state physics and devices owing to its potential application in low power memory and logic as well as the rich physics associated with it. Traditionally in spintronics, spin transfer torque in magnetic tunnel junctions and spin valves has been used to manipulate ferromagnets. Spin orbit torque has recently emerged as an alternative mechanism for manipulating such ferromagnets, which offers advantages of lower energy consumption, simpler device structure, etc. For a ferromagnet- heavy metal bilayer, electrons flowing through the heavy metal separate based on the direction of their spin. This results in the accumulation of spin polarized electrons at the interface, which in turn applies a torque, known as spin orbit torque, on the ferromagnet. A typical such heavy metal is tantalum (Ta) and typical such ferromagnet is CoFeB. The research presented in this dissertation shows how in a perpendicularly polarized Ta/CoFeB/MgO heterostructure, spin orbit torque at the interface of the Ta and CoFeB layers can be used to manipulate the magnetic moments of the CoFeB layer for low power memory and logic applications.The main results presented in this dissertation are fourfold. First, we report experiments showing spin orbit torque driven magnetic switching in a perpendicularly polarized Ta/CoFeB/MgO heterostructure and explain the microscopic mechanism of the switching. Using that microscopic mechanism, we show a new kind of ferromagnetic domain wall motion. Traditionally a ferromagnetic domain wall is known to flow parallel or antiparallel to the direction of the current, but here we show that spin orbit torque, owing to its unique symmetry, can be used to move the domain wall orthogonal to the current direction. Second, we experimentally demonstrate the application of this spin orbit torque driven switching in nanomagnetic logic, which is a low power alternative to CMOS based computing. Previous demonstrations of nanomagnetic logic needed an external magnetic field, the generation of which needed a large amount of current rendering such logic scheme uncompetitive compared to its CMOS counterpart. Here we show that spin orbit torque eliminates the need of an external magnetic field for nanomagnetic logic and hence spin orbit torque driven nanomagnetic logic consumes 100 times lower current than magnetic field driven nanomagnetic logic at room temperature. Though we can demonstrate magnetic logic with spin orbit torque in the absence of the magnetic field, spin orbit torque driven deterministic switching of a perpendicular magnet from up to down and down to up still needs the application of an external magnetic field unless the symmetry of the system is broken. This renders such switching scheme not very useful for real memory devices. In the third part of the thesis, we show through micromagnetic simulations that if the magnet has a wedge shape, the symmetry of the system is broken and the magnet can be deterministically switched from up to down and down to up even in the absence of an external magnetic field. Our simulations are supported by recent experiments, performed in our group. In the last part, we show how a bilayer of two heavy metals (Ta and Pt) can be used to increase the spin orbit torque efficiency. Interfaces of ferromagnet with Ta and that of ferromagnet with Pt exhibit spin orbit torques in opposite directions, so it is expected that their effects will cancel. Instead, in our experiments we find that the spin orbit torque efficiency at the Ta/CoFeB interface increases if a Pt layer exists under the Ta layer. Modeling of the system based on conventional spin transport physics cannot explain this result