Thermal gradient engineering for spin injection and transport in metallic nanomagnetic switches and sensors

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The electronic and magnetic devices that currently enable rapid information processing and massive data storage are formed in part using metallic structures with dimensions approaching 100 nanometers. Often these devices function due to an applied flow of electrons, and this charge flow generates heat. In many cases this heat limits the performance of these tiny devices and prevents gains in technology. This project aims to explore ways to simultaneously avoid or exploit this heating by study of thermal effects in a particular nanoscale metallic magnetic device known as a non-local spin valve. These devices are already known to allow generation of flows of angular momentum, or spin, in a metallic nanowire with no associated charge flow. However, their operation usually requires large applied electron flows in other elements of the device. The main motivation of this work is to provide the transformative knowledge of thermal effects in these devices that could eventually allow operation with only applied heat, eliminating the charge flow altogether. This will enable simplification and further size reduction in future devices that could significantly advance data storage and other technologies.The metallic non-local spin valve is an invaluable, though still not thoroughly understood, device for producing and studying pure spin currents. The use of charge currents to produce spin currents in the NLSV was achieved ~15 years ago, and these sensors are poised to play an important role in near-term magnetic recording. However, the electrical injection of spin causes significant heating and thermoelectric effects that strongly affect the performance of the sensors are not yet well characterized and understood. The demonstration of spin injection in the non-local spin valve via purely thermal effects is of even greater interest. These very recent and novel measurements point the way toward implementation of such sensors without the large charge current, which offer many advantages for the ever smaller and more sensitive sensors or more efficient sources demanded by the information technology community. This project takes advantage of unique expertise in creating and measuring thermal gradients and in measuring Seebeck and Peltier effects in nanoscale systems to understand and control heat and spin flow in metallic nanomagnetic devices. The unique ability to directly measure the thermal properties (thermal conductivity, Seebeck, and Peltier coefficients) of the thin film constituents of nanoscale devices using micromachined thermal isolation platforms is a central focus of the project. By removing the bulk substrate from beneath a nanoscale device or thin film, the uncertainty in the direction of heat flow is dramatically reduced, modeling the structure via finite element methods becomes much simpler, and engineering the thermal gradients applied to the nanoscale structures is possible. Specific tasks include: 1) Understanding interface and materials dependence of the spin-dependent Seebeck effect and the absolute Seebeck effect, 2) the Search for magnon-drag contributions, 3) Studies of thermoelectric effects and the spin injection in "zero substrate" devices, and 4) Thermally engineering response of sensors via external thermal gradients.