Magnon Spin Transport in Magnetic Heterostructures

Abstract

In the study of spintronics, one of the most important issues is to identify and manipulate spin currents. The two most profound effects in spintronics, giant magnetoresistance and spin-transfer torque, are the results of the interplay between spin-polarized electron transport and magnetization dynamics. On the other hand, the magnons are also capable of carrying angular momentum currents or spin currents. Magnons are quantized low-energy collective excitations of magnetic materials. Besides the finite energy and crystal momentum, each quasi-particle magnon is also known to have an angular momentum \hbar. This dissertation summarizes my doctoral study about the magnon spin transport in magnetic heterostructures. The first part of my dissertation is devoted to the interplay of magnon and electron currents in heterostructure of ferromagnetic metals. We theoretically formulate the spin-magnon coupled transport by explicitly taking into account the exchange coupling between the conduction electrons and magnons. The intrinsic strong s-d exchange coupling in itinerant ferromagnets inevitably leads to substantial magnon current, in addition to the polarized electron current. Even for a uniformly magnetized conducting ferromagnet, we find an electric charge current is always accompanied with a magnon current at finite temperature. The second part is the study of incoherent magnon transport in heterostructure of magnetic insulators. We propose an insulating spin valve, which is made of an antiferromagnetic insulator sandwiched between two ferromagnetic insulator layers. Instead of conduction electrons, the incoherent magnons in this structure serve as angular momentum carriers. We predict two transport phenomena in such structure in the presence of a temperature gradient: the giant magneto-spin-Seebeck effect and the magnon transfer torque. In the third part, we explore the spin-transfer torques in magnetic tunnel junctions with an antiferromagnetic insulator as the tunnel barrier. The voltage bias induced asymmetric heating would create a magnon current flowing across the antiferromagnetic layer, resulting a magnon transfer torque in addition to the electron spin-transfer torque. This study presents a potential method to realize more energy efficient switching in spin-transfer torque induced magnetization switching in magnetic tunnel junctions. At last, we study the magnon spin transport property in non-collinear antiferromagnets. In such system, the magnon spectra displays non-trivial multi-band structure with unconventional spin-momentum locking. We study the roles of these magnons on the charge and spin transport properties. The magnon spin conductivity tensor has a more complicated symmetry properties compared to the spin Hall conductivity tensor in non-magnetic metals.