Coupled charge, spin and heat transport in metal-insulator hybrid systems

Electrons have intrinsically a charge and a spin. In conventional electronics, electric currents, which are essentially charge flows, are used to transmit information under the control of electric fields. In the burgeoning field of spintronics, however, spin currents, i.e., flows of spin angular momentum of electrons, are utilized as the information carrier, controlled by magnetic fields.

The work presented in this thesis falls into a subfield of spintronics called spin caloritronics, which studies the interaction between charge, spin and heat currents in different material systems. First, a novel effect named magneto-Peltier effect was studied in magnetic tunnel junctions (MTJs). MTJs are the key components in magnetoresistive random-access memory technology, yet their thermal properties were not intensively explored until recently. We found in our experiments that the Peltier coefficient of an MTJ could be altered between its two states, regulated by an applied magnetic field.

Second, we looked into two electrically insulating yet magnetic materials, yttrium iron garnet (YIG) and nickel ferrite (NFO). In these materials, spin is not transported by mobile electrons, but by collective excitations called magnons. This provides an ideal platform to study the direct coupling between spin and heat currents in the absence of charge currents. We studied the transport properties of the magnonic spin currents in YIG and NFO, excited both electrically via spin Hall effect and thermally by the spin Seebeck effect. We believe the work in this thesis helps increase the current knowledge of this field and paves the way for novel spintronic devices.