Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Dario A. Arena, Ph.D.

Committee Member

Hariharan Srikanth, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Magnetism, Thin Film, Ferromagnetic Resonance, Polarized Neutron Reflectometry, Ferrimagnets

Abstract

Magnetic thin films are important for technological applications such as giant magnetoresistance,spin valves and magnetic tunnel junctions. A key feature for these applications to be possible is the interface between two materials. We will be studying two types of thin film magnetism; interfaces between molecular carbon, C60, and non-magnetic/ferromagnetic material and two layers of ferrimagnetic rare earth/ transition metal alloys (RE/TM) that have similar composition to engineer a magnetic tunable interface.

One of the most unexpected spin-related properties of C60 is the emergence of room-temperature ferromagnetism in multilayer films of C60 and non-ferromagnetic transition metals such as Cu or Mn that is interfacial in origin. We used a combination of x-ray and neutron scattering techniques to investigate the properties of the interface between the organic molecule and the transition metal. This study led to an intriguing question of how would C60 effect a ferromagnetic material like Fe and Co. When Co/C60 samples are cooled in an external magnetic field there is a development of an asymmetric hysteresis loop can be related to the rotational freezing of the C60 molecule at temperatures below 100K. Ferromagnetic resonance measurements as a function of temperature are used to explore what the rotational freezing does at the Co/ C60 interface.

The second type of thin film magnetism being explored is multilayers of Ferrimagnetic Rare-Earth / Transition Metal (RE/TM) alloys. These alloys can exhibit an unusual effect termed magnetic compensation. A key question in such structures is the interface length between two TM/RE layers with different RE content. Polarized Neutron Reflectometry (PNR) is and ideal tool to study this because it is a well-established technique to examine the magnetic depth profile of materials and is particularly useful in determining the magnetic scattering length density (mSLD) profile of magnetic thin films. We can therefore construct bi-layers, and more complicated structures, that present nearly identical nuclear scattering length density (SLD) but where the magnetic SLD (mSLD) profile is modified significantly with temperature to determine the extent of the magnetic interface in the sample.

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