Studies of tuning magnetic properties of ferromagnetic heterostructures
Date
2020
Authors
Lauzier, Joshua, author
de la Venta Granda, Jose, advisor
Buchanan, Kristen, committee member
Gelfand, Martin, committee member
Field, Stuart, committee member
Menoni, Carmen, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The magnetic properties of hybrid systems have increasingly become an area of intense focus in both fundamental research and technological application due to the inherent flexibility in material properties by mixing and matching various constituent components. One particularly interesting choice is hybrid heterostructures that consist of ferromagnetic (FM) materials and materials that undergo phase transitions, coupled via structural, electronic, and/or magnetic coupling. Two canonical examples of phase transition materials are vanadium dioxide (VO2) and iron rhodium (Fe50Rh50, abbreviated FeRh). Both materials undergo structural phase transitions (SPT). With increasing temperature, VO2 transitions from a low temperature monoclinic to high temperature rutile structure at 340 K. The SPT is concurrent with a 4-5 orders of magnitude metal to insulator transition (MIT) from a low temperature insulating phase to a high temperature metallic phase. Similarly, FeRh undergoes an isotropic 1% volume expansion at 370 K with increasing temperature. Coincident with the SPT, FeRh also undergoes a magnetic transition from a low temperature antiferromagnetic (AF) to a high temperature ferromagnetic (FM) phase, which is unusual for magnetic materials. The delicate nature of these transitions makes them sensitive to parameters such as stoichiometry, growth conditions, and external stimuli, which allows for high tunability of their respective phase transitions. In this thesis, we first show in Chapter 3 that the surface morphology and MIT properties of sputtered VO2 thin films can be tuned via deposition conditions such as deposition temperature and O2 flow rate during the sputtering process while maintaining the quality of the VO2 transition. Films grown at higher temperatures (>525 ℃) and low O2 flow rate show sub 2 nm surface roughness. Higher temperatures lead to a 'melted'-like surface morphology along with a 5 orders of magnitude MIT, comparable to single crystals. Choice of substrate allows another avenue to strongly tune both the morphology and the MIT characteristics while maintaining a strong VO2 transition due to lattice mismatch. In Chapter 4, we turn to a discussion of VO2/Ni bilayer structures, where the temperature induced VO2 SPT will impart a strain across the interface into the FM layer, which will then influence the magnetic properties via magnetoelastic coupling. Due to an inverse magnetostrictive effect the coercivity and magnetization of the FM layer can be strongly modified. Tuning the VO2 SPT via growth conditions or substrate choice then allows for tuning the coupled magnetic properties of the FM. For sufficiently smooth films, there is a strong enhancement in the coercivity localized close to their respective SPT Tc due to phase coexistence in the SPT material. This chapter is largely based on work previously published as "Coercivity enhancement in VO2/Ni bilayers due to interfacial stress" in Journal of Applied Physics.1 VO2/FM hybrid films also show a dependence on the growth conditions during the FM deposition, which is explored in Chapter 5. Films with the FM deposited above the VO2 phase transition critical temperature (Tc) show a high coercivity below Tc and a low coercivity above Tc, whereas films deposited below Tc show the opposite behavior. Films deposited below Tc also show an irreversibility in their magnetic properties the first time they are thermally cycled. A similar irreversibility is observed in the resistance vs. temperature (R vs. T) properties of bare VO2 films, and cracking as the VO2 crosses the SPT is proposed as a common mechanism. The plausibility of cracking as a mechanism is investigated via computational modeling of the R vs. T properties in a random resistor network, as well as probed directly via Atomic Force Microscopy (AFM). The work shown in this chapter has been previously published under the title "Magnetic irreversibility in VO2/Ni bilayers" in Journal of Physics: Condensed Matter.2 Sputtered FeRh/FM bilayer films show a similar sensitivity as the VO2/FM system to the growth conditions, with the coercivity below Tc tunable whether the FM is initially deposited above or below Tc. Above Tc, the magnetic FeRh phase adds an additional complication, dominating the magnetic response via exchange coupling. This effect is explored in FeRh/Ni bilayer systems in Chapter 6. Polarized neutron reflectometry (PNR) allows for depth dependent structural and magnetic characterization with nanometer resolution. PNR measurements show that the bilayer's magnetic behavior below Tc is likely driven by magnetoelastic effects due to the structural transition of the FeRh, rather than simple magnetic coupling or a pinned interfacial FM layer. The overall magnetic properties of the bilayers are therefore a product of both structural and magnetic coupling between the FeRh and the FM Ni layer. The results of this chapter have been previously published as "Using structural phase transitions to enhance the coercivity of ferromagnetic films" in Applied Physics Letters Materials.
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Subject
magnetism
phase transition
thin film
magnetostriction
hysteresis
structural phase transition