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Advancing Magnetoelectric Thin Film Growth for Ultra-Low Power Spintronic Applications
Magnetoelectric antiferromagnetic materials have garnered considerable attention as a fundamental building block for potential energy efficient non-volatile memory technologies. The manipulation of the antiferromagnetic ordering by voltage application is the fundamental prerequisite for magnetoelectric device implementation. This dissertation is motivated by some of the challenges that exist with application of magnetoelectric antiferromagnets in spintronic devices. The primary magnetoelectric antiferromagnet studied here is chromia (Cr2O3) due to its two switchable degenerate 180° antiferromagnetic domains. Chromia can be used to induce exchange bias in a ferromagnetic/antiferromagnetic heterostructure. Through the induction of the exchange bias in these systems, the remnant moment of the adjacent ferromagnetic layer can be switched by the magnetoelectric effect, which allows for a bipolar device concept. Currently, there are still many challenges with using chromia and other magnetoelectrics for voltage-controlled exchange bias. Three main challenges and solutions with realizing a voltage-controlled exchange bias device are discussed in this dissertation with emphasis on the fabrication and characterization of chromia-based exchange bias systems. ^ First, we show the implications and effects of thin film fabrication techniques of chromia on various materials supported on α-Al2O 3(0001) substrates. Al2O3(0001) single crystals are used because they are isostructural with the desired chromia phase. Structural analysis was performed to investigate defects associated with chromia growth on metallic seed layers. By substituting the metallic seed layer materials, we are able to eliminate structural defects in chromia thin films that provides improved dielectric properties. ^ Secondly, we discuss the challenge of raising the antiferromagnetic ordering temperature of chromia to allow for robust temperature device operation. We show the increase of the Néel temperature (TN) of chromia via doping. The antiferromagnetic ordering temperature increase is illustrated by magnetometry and spin polarized inverse photoemission spectroscopy. Lastly, a unique temperature dependence of exchange bias is discussed where a horizontal domain model is used to explain the change in effective magnetic anisotropy of the system. Furthermore, a solution via chemical doping of thin chromia films is proposed and analyzed.^
Street, Michael J, "Advancing Magnetoelectric Thin Film Growth for Ultra-Low Power Spintronic Applications" (2018). ETD collection for University of Nebraska - Lincoln. AAI10979501.