Influence of Thickness and Capping Materials on the Static and Dynamic Properties of Ferrimagnetic Thin Films
Abstract
Spintronics traditionally relied on ferromagnetic materials for their strong spin properties but faced
issues like large stray magnetic fields. To address these challenges, researchers turned to antiferromagnetic
materials with fast spin dynamics, due to their zero net magnetic moment but the lack of a net spin moment
complicated operation. In this context, ferrimagnetic materials have emerged as promising alternatives.
They combine ferromagnetic and antiferromagnetic traits and feature different magnetic ions in their
sublattices. This dissertation explores how film thickness and capping materials affect the properties of
ferrimagnetic thin films.
Rare-earth transition metal ferrimagnetic amorphous alloys have garnered attention in the realm of
spintronics applications. These alloys provide the means to fine-tune magnetic attributes such as
magnetization, anisotropy, and spin polarization by manipulating factors like composition, film thickness,
temperature, and the selection of capping materials. Additionally, they exhibit a phenomenon called
magnetic compensation, in which the temperature-dependent magnetization behaviors of the transition
metals (Fe & Co) and rare earth (Gd) constituents nearly offset each other. This dissertation focuses on
investigating how the magnetic properties of these materials evolve when altering the capping materials
and adjusting the concentration of Gd within the thin films. This study delves into the magnetic
characteristics of FeCoGd ferrimagnetic thin films by employing magnetometry, magneto-optical Kerr
effect (MOKE), and ferromagnetic resonance (FMR) techniques.
Another class of materials, exemplified by Heusler compounds such as Co2MnGa, has attracted
significant attention. These compounds are renowned for their Weyl fermion lines and topological surface
states, and they have garnered interest due to their negative magnetoresistance, high spin polarization, and
low magnetic damping. We explored the influence of thickness and temperature on ferromagnetic
resonance (FMR) film with the magnetic field aligned along the [110] and [100] directions. Our
investigation unveiled a distinctive double peak pattern in the FMR response.
Description
Magnetism, a fundamental natural force, has left an indelible mark on human existence and scientific
progress. It describes the intriguing behavior of materials and particles with magnetic properties. Its
significance spans a wide range of domains, from everyday conveniences to groundbreaking advancements.
Magnetism arises from the intrinsic magnetic characteristics of elementary particles such as
electrons and protons. It manifests as an attractive or repulsive force between magnetic objects and finds
numerous practical applications. In our daily lives, we encounter magnetism through commonplace objects
like refrigerator magnets and compass needles, as well as in advanced technologies such as MRI scanners
and credit card stripes. Throughout history, magnetism has not only intrigued scientists, but also has carried
cultural and spiritual significance, evident in ancient beliefs surrounding lodestones and modern practices
like magnetic therapy. Furthermore, magnets offer the potential for sustainable energy generation, making
them a compelling focus of research with implications for addressing challenges such as climate change.
Spintronics represents a cutting-edge field at the intersection of magnetism and electronics, offering
significant advantages by harnessing both the intrinsic spin and charge of electrons for electronic devices.
By manipulating electron spin with minimal heat generation, spintronics devices improve energy
efficiency. Non-volatile memory technologies like MRAM (Magnetoresistive Random-Access Memory)
provide quick access times and data retention even when powered off, revolutionizing data storage
solutions. Additionally, the emerging field of quantum spintronics holds the promise of advancing quantum
computing by leveraging the quantum properties of electron spin for complex computational tasks.
Within this realm, magnetic thin films, composed of extremely thin layers of magnetic materials,
typically just a few nanometers thick, exhibit unique magnetic properties due to their reduced
dimensionality. These materials play pivotal roles in various technological applications, powering hard
drives and magnetic tapes that enable high-capacity and long-term data storage by organizing information
in the form of magnetic domains. Thin-film magnetic sensors find applications in diverse fields, including
automotive, aerospace, and medical devices, owing to their high sensitivity and precision in detecting
changes in magnetic fields.
Spintronics has typically relied on ferromagnetic materials due to their strong spin polarization and
ease of manipulation. However, ferromagnetic materials come with disadvantages, such as large magnetic
stray fields. This limitation has inspired researchers to explore antiferromagnetic materials in spintronics,
as they lack stray fields and offer ultrafast spin dynamics with antiferromagnetic resonance frequencies in
the THz region. Nonetheless, antiferromagnetic materials present challenges due to their zero net magnetic
moment, resulting in weak read-out signals and more complex operations.
In this context, ferrimagnetic materials emerge as promising candidates, blending ferromagnetic and
antiferromagnetic characteristics. Unlike ferromagnets and antiferromagnets, the magnetic sub-lattices in
ferrimagnets frequently include different magnetic ions, such as RE-TM alloys made of transition metals
(e.g., Fe, Co) and rare earth elements (e.g., Gd), like FeCoGd. This unique property positions this class of
material as a potential solution for overcoming the disadvantages associated with both ferromagnets and
antiferromagnets.
The influence of thickness and the choice of capping materials on the properties of these magnetic
thin films cannot be overstated. Manipulation of film thickness opens new avenues for tailoring magnetic
behavior, affecting parameters such as coercivity and saturation magnetization. Likewise, the selection of
capping materials can significantly impact the stability and performance of these films, making them
central to our exploration of ferrimagnetic materials in the context of spintronics. In this dissertation, we
delve into a detailed investigation of how variations in thickness and the specific capping materials
employed exert influence on the static and dynamic properties of ferrimagnetic thin films, shedding light on
their potential for next-generation magnetic devices.
In Chapter 2, magnetism is explored, a phenomenon that has fascinated humanity for ages. From its
historical significance to its pivotal role in modern technology, the science behind it is delved into.
Magnetism originates from the properties of electrons, namely their orbital motion and spin. Their
magnetic moments, both orbital and spin-related, are discussed, and how they contribute to magnetism is
explained. Spin-orbit coupling, which combines these angular momenta, is also discussed. The response of
materials to magnetic fields is then examined, with terms like magnetic induction, magnetization, and
susceptibility being introduced. Different material types, including paramagnetic, ferromagnetic,
antiferromagnetic, and ferrimagnetic, are discussed. Magnetic hysteresis loops and their significance are
also explored. Thin film magnetic materials are then examined, with an emphasis on their unique
properties and applications. Lastly, the dynamics of magnetization, as described by the
Landau-Lifshitz-Gilbert (LLG) equation, are discussed, shedding light on precessional motion and
damping effects. This journey enhances our understanding of the mechanisms of magnetism and its broad
scientific and technological implications.
In Chapter 3, the study focuses on amorphous thin films consisting of single layers that exhibit
short-range crystalline order. The sample preparation procedure, involving magnetron sputtering, is
initially discussed, followed by the discussion of the characterization techniques employed to investigate
the structural properties of the thin films, with a specific emphasis on X-ray reflectivity. Furthermore, the
magnetic properties of these thin films are examined through specific characterization methods, including
the utilization of the magneto-optical Kerr effect (MOKE), vibrating sample magnetometry (VSM), and
ferromagnetic resonance (FMR). Additionally, the spin and orbital magnetic moments are obtained through
X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopies.
In Chapter 4, the magnetic properties of 100 nm FeCoGd thin films with the same nominal
composition were fabricated by DC magnetron sputtering from elemental targets. They were subsequently
allowed to cool in two different environments (ultrahigh purity Ar or vacuum) before the initiation of a
capping layer (cap = Ta, Al, or Pt) deposition process. The measurements of hysteresis loops and
magnetization versus temperature (M vs. T) curves were conducted using VSM and MOKE. Valuable
insights into the magnetic behavior of the films at different temperatures and magnetic fields were provided
by these measurements. Notably, a peculiar two-step magnetization reversal process was observed at low
magnetic fields in films subjected to a delay time in the vacuum environment. Additionally, more stable
and consistent magnetic properties were exhibited by films capped with Ta compared to films deposited in
an argon (Ar) environment. In conclusion, the importance of selecting suitable cap materials and
controlling the cooling environment to achieve desired magnetic properties in ferromagnetic thin films is
highlighted by this study.
Chapter 5 investigates the influence of Gd concentration on temperature compensation in amorphous
ferrimagnetic thin films of RE-TM FeCoGd. The films were prepared via DC magnetron co-sputtering at
room temperature. The magnetic properties of the films were examined using magnetometry and FMR
measurements at the magnetic compensation temperature. Magnetic hysteresis loops and magnetization
versus temperature curves were obtained by employing the VSM and MOKE techniques. The alignment of
the magnetic moments of the FeCo sublattice with the applied magnetic field was revealed by the M(H)
curves. FMR measurements provided insights into the behavior of magnetization, resonance fields, and
damping coefficients for FCG23, FCG29, and FCG36 films, revealing the complex interplay between
anisotropy, magnetization, and dynamic interactions.
Chapter 6 explores a distinct class of magnetic materials, namely ferrimagnetic materials, with a
specific focus on Heusler alloys. In this chapter, the dynamic properties of Co2MnGa (CMG) thin films are
investigated. These thin films were grown on single-crystal MgO(001) substrates with varying thicknesses
(20, 40, and 80 nm) and were subjected to in-plane magnetic fields aligned parallel to [100] or [110]
directions of the CMG film. Both orientations were characterized using VSM and FMR techniques,
providing insights into the magnetic characteristics of the films across various thicknesses and
temperatures. Subtle distinctions in saturation behavior and coercivity between the two orientations, as
well as the effect of temperature on these magnetic traits, were unveiled. Effective magnetization declined
with increasing film thickness, and the gyromagnetic ratio remained relatively constant. The dynamic
magnetic properties specific to the CMG film under investigation were aimed to be extracted and examined
by the FMR analysis. Additionally, intriguing findings were revealed in the in-plane FMR data, with
overlapping peaks observed in both orientations.
Keywords
Magnetism, Thin Film, Ferromagnet, Ferrimagnet, Ferromagnetic Resonance
Citation
Alzahrani, Noha S. "Influence of Thickness and Capping Materials on the Static and Dynamic Properties of Ferrimagnetic Thin Films" (2023). USF Tampa Graduate Theses and Dissertations.