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Investigating quadruple perovskites: synthesis and characterization of CaMn₇O₁₂ films
Dissertation   Open access

Investigating quadruple perovskites: synthesis and characterization of CaMn₇O₁₂ films

Amanda Huon
Doctor of Philosophy (Ph.D.), Drexel University
Dec 2018
DOI:
https://doi.org/10.17918/s5sh-xy51
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Abstract

Molecular beam epitaxy Pulsed laser deposition Perovskite Materials Science Thin Films
The miniaturization trend in conventional electronics is approaching the limits beyond which the reduction of the electronic element is becoming more and more difficult. One way to continue the current trends in computer power and data storage increase without further size reduction is to use new functionalities such as charge-ordering or multiferroic behavior in devices. Complex oxide heterostructures are a materials platform that exhibit an unmatched array of collectively ordered states, including magnetism, ferroelectricity, and charge/orbital ordering, and are being researched for potential device applications. As a member of the quadruple perovskite family, CaMn₇O₁₂ exemplifies the rich physics that complex oxides can host. Previous work shows that this material in bulk form exhibits four distinct phase transitions: a charge-ordering transition at 430 K, an orbital ordering transition at 250 K, and two magnetic transitions at 90 K (T_[N1]) and 45 K (T_[N2]). Additionally, the magnetic ordering at T_[N1] induces ferroelectricity, which is the largest magnetically induced polarization yet reported. Despite the scientific interest and the technological potential for this material, there have been no reports of CaMn₇O₁₂ films prior to this thesis work. This dissertation is focused on establishing growth conditions for CaMn₇O₁₂ quadruple perovskite thin films using molecular beam epitaxy and pulsed laser deposition methods. By synthesizing thin films using both methods, I identify the key parameters in establishing the growth conditions for phase-pure CaMn₇O₁₂ films by evaluating cation stoichiometry and structural properties. To develop insight into the role of thin film structure-property relations, I explore how the electronic and magnetic structures of thin films compare to bulk. Then I develop insight into quadruple manganites by elucidating how the electronic and magnetic properties are tuned by chemical pressure effects through A-site substitution in Ca_[1-x]Sr_xMn₇O₁₂ films. I identified the changes in charge-ordering, magnetic ordering, and helical spin structure through Sr-doping up to x ~ 0.6 by transport, magnetometry, and neutron diffraction measurements. These results provide fundamental insights into quadruple perovskite film synthesis and serve as a catalyst for future studies focused on AMn₇O₁₂ thin films. The material understanding of CaMn₇O₁₂ films obtained from this dissertation offers new avenues in the search and control of new functionalities in oxide heterostructures.

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