Semester

Spring

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Nianqiang Wu

Committee Co-Chair

Mikel B. Holcomb

Committee Member

Terence D. Musho

Committee Member

Edward M. Sabolsky

Committee Member

Konstantinos A. Sierros

Abstract

When a thin film thickness becomes ultrathin, the magnetic properties of the thin film can be altered, degraded or even lost. The loss of magnetism has been called the magnetic dead layer (MDL). Considering the trend for miniaturization of information storage and other devices, the MDL is a significant challenge for materials science and engineering. La1-xSrxMnO3 (LSMO) with x=0.3 is an excellent model material that exhibits ferromagnetism at room temperature to study the MDL. This dissertation focuses on understanding the MDL in LSMO films on their own and the effect of this dead layer when coupling with a ferroelectric material [La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 (LSMO/PZT)].

The LSMO/PZT magnetic heterostructures

Ferromagnetic-ferroelectric layers at the interface can show strong magnetoelectric coupling, allowing electrical control of magnetism or vice versa. Images of magnetic domains and interfacial Ti spins were taken at the same locations of the LSMO/PZT heterostructures by utilizing photoemission electron microscopy (PEEM). The interfacial Ti spins prefer to be perpendicular to ferromagnetic domains in the adjacent layer. Using image analysis techniques confirms the population of magnetic switching and the interfacial spins are related to the magnetization within LSMO. In other words, if the ferromagnetic layer begins to lose its magnetic order, the coupling between ferroelectric and ferromagnetic layers will also decrease or even disappear. Thus, this work suggests a magnetoelectric dead layer is about 2.8 nm for the LSMO layer. This result further emphasizes the need to enhance the magnetization in magnetic thin films.

The LSMO/STO magnetic heterostructures

The location of the magnetic reduction can have strong effects on devices for some magnetic applications. Does this reduction occur only at the surface, the interface, or throughout the material? Polarized neutron reflectometry can provide depth dependent magnetic properties. Using this method, we determined that the MDL at the surface of LSMO has a thickness of about 1.7 nm. We attribute the polar discontinuity induced charge reconstruction to interpret the suppressed magnetization at the surface. Unlike the MDL at the surface, the resultant enhanced magnetization at the interface is likely subject to oxygen vacancies. The magnetic moments originate from unpaired electrons which occupy the d orbitals on the Mn site. Oxygen vacancies result in charge accumulation on the interfacial region explained by raising densities of magnetic moments on the Mn site and the enlargement of the ratio of mixed Mn2+/Mn3+ states at the interfacial region.

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