Date of Award

6-15-2024

Document Type

Dissertation

Degree Name

Ph.D.

Organizational Unit

College of Natural Science and Mathematics, Physics and Astronomy

First Advisor

Barry L. Zink

Second Advisor

Xin Fan

Third Advisor

Pavel Salev

Keywords

Antiferromagnet, Charge, Ferrimagnet, Magnetism, Spin, Spintronics

Abstract

Recent efforts have been exploring the use of thin film synthetic ferrimagnets and disordered magnetic oxides for applications in spintronic devices. Due to the antiferromagnetic exchange interaction, ferrimagnetic materials offer the ultrafast dynamics of the antiferromagnetic exchange, with a net magnetization that can be influenced externally. With two, or more, competing ferromagnet sublattices, interesting properties arise that depend on the final magnetic landscape after growth of the material and it’s inherent magnetic anisotropy energies. Properties such as magnetic compensation temperatures, and perpendicular magnetic anisotropy are attractive for applications in spintronic memory and logic devices, some already being implemented in MRAM devices. Observation of increased spin diffusion and spin dephasing lengths at magnetic compensation temperatures has sparked a surge in ferrimagnet spintronics, with novel methods to influence magnetization being realized. With many parameters involved affecting the magnetic landscape it’s imperative that more fundamental knowledge be obtained for ferrimagnetic and antiferromagnetic materials so that useful devices can be achieved. Understanding the magnetic and electrical properties as a function of temperature, and probing for spin transport can help build the essential phase map for these materials and determine how to tune certain properties.

In this dissertation, I begin with a description of the materials being investigated, and what methods are being used to interpret the preceding magnetic and electrical properties. Starting with nonlocal resistance measurements to probe spin transport in amorphous Yttrium Iron Oxide, measurements of nonlocal resistances in amorphous Yttrium Iron Oxide, a disordered antiferromagnetic insulator, are discussed. Inverse spin hall effect (ISHE) measurements in nonlocal geometry show that no spin transport is observed in amorphous Y-Fe-O using this method. In fact, experiments reveal that the material acts as a disordered antiferromagnetic semiconductor since the resistivity follows what you’d expect from Mott’s variable range hopping, leading to a more difficult realization of spin transport across this material. Magnetometry measurements of the amorphous Y-Fe-O also confirm the antiferromagnetism of the material, leading to the conclusion that different methods to probe spin transport in this material are necessary. Next, I focus on the transition metal (TM) rare earth (RE) metallic ferrimagnets with perpendicular magnetic anisotropy. A temperature controlled anomalous Hall effect (AHE) memory device is demonstrated with such a material, utilizing the unique properties around the synthetic ferrimagnet’s magnetic compensation temperature, with details and explanations on the mechanisms that govern it. A closer investigation into the magnetic properties of the TM-RE ferrimagnet Co/Gd then follows, with observations of frustrated interactions at low temperatures. The magnetic anisotropy energies of a couple Co/Gd bilayer samples are quantified, concluding that there is a shift in the bulk perpendicular magnetic anisotropy at low temperatures, confirmed through electrical and magnetic measurements. Finally, a non-local spin valve (NLSV) device is used to measure a pure spin current in Fe/Al, paving the way for implementing ferrimagnetic materials such as TM-RE Co/Gd in these lateral spin valves. Details of fabrication and measurement methods are discussed, with fabrication hurdles overcome to realize a working device that can be fabricated using a two step deposition method.

Copyright Date

6-2024

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Leopoldo A. Hernandez

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

151 pgs

File Size

3.5 MB

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