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Garnet Materials Development and Spin Transport Phenomena in Magnetic Insulator/ Normal Metal Heterostructures


The Nobel prize was awarded in 2007 due to the big discovery of the giant magnetoresistance effect (GMR) in 1988 giving birth to the new, spin-based electronics or spintronics. In addition to the charge degree of freedom, spin degree of freedom of electrons has attracted a great deal of research interest in the last two decades. Based on electron spin, technology of spintronics has emerged with new device functionalities and decreased electric power consumption. For example, applications such as computer read-heads and magnetoresistive random access memories are based on spin-dependent effects that originate from interaction between spin of the carriers and the magnetic properties of the materials. Therefore, making new magnetic materials and understanding their magnetic properties is very important. This work focuses mainly on rare-earth iron garnet magnetic insulator (REIG-MI) materials and heterostructures of REIG-MI and heavy metals (strong spin-orbit coupling). These REIG-MI materials have unique properties of large electronic band gap, room temperature ferrimagnetism and long spin wave propagation length and thus are ideal materials for low-power spintronics. Heterostructures of REIG-MI and heavy metals give rise to a variety of interesting effects such as the spin Seebeck effect and the spin Hall magnetoresistance effect.

This dissertation summaries all my work in past five years and is divided into seven chapters. The first chapter starts off with a short introduction to magnetism by explaining fundamental concepts as an entrance to spintronics field and then some important transport phenomena in spintronics. The last part is devoted to the importance of thin films and critical role of interface properties and interface engineering.

The second chapter presents the state-of-the-art growth of yttrium iron garnet (YIG). A special emphasis is placed on optimizing growth conditions using pulsed laser deposition (PLD). Epitaxial growth with the layer-by layer mode, growth of strained single crystal YIG and polycrystalline YIG development are discussed in detail with full characterization results.

The third chapter shows a breakthrough of the growth of YIG on the top of Pt, an inverted bilayer structure, where we overcome the Pt stability problems related to high temperature growth or annealing. Mastering growth control leads to single crystal phase for YIG and Pt in the inverted structure with flat and sharp interface.

The fourth chapter mainly discusses the experimental observation of theoretically predicted magnon-mediated current drag effect in HM/FMI/HM heterostructures, here HM denoting heavy metal either Pt or Ta, and FMI denoting ferrimagnetic insulator YIG. This system is ideal to realize SHE in one HM layer and detect ISHE in the second layer of HM. More importantly, distinctive behaviors of this effect are studied via nonlocal voltage response as a function of magnetic field strength, angles and temperature with comparison to local signal of SMR and SSE.

The fifth chapter shows the observation of a giant magnetoresistance (GMR) effect in spin valve-like structure consisting of two identical magnetic insulator layers of YIG where they are separated by a non-magnetic metal layer. Magnetization of anti-parallel state between two layers gives rise to high resistance. SSE measurements confirm individual contribution of thermal spin current from each YIG layer.

Chapter six describes the interface modulated spin dynamics of magnetic insulator (YIG)/Pt bilayers with engineering different resistivities of Pt layer. Gilbert damping constant is tuned based on Pt resistivities and interfacial spin mixing is investigated. Both cavity FMR and broadband FMR with coplanar waveguide set-up are used.

Chapter seven discusses strain effect to tune magnetic anisotropy from in-plane to out-of-plane via magnetostriction effect utilizing strain modulation over a wide range of thicknesses ranging from 5 to 100 nm in TbIG. Study of anomalous Hall and compensation effects on these thicknesses are investigated.

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