Author: Mayank Sharma

Posted on 2024-09-052024-09-06 by Mayank Sharma

From Spin to Spintronics

In the fascinating world of physics and electronics, the term “spin” of an electron holds a pivotal place. While it might sound like something out of a science fiction movie, the electron’s spin is a fundamental property that has revolutionized our understanding of the microscopic world and paved the way for groundbreaking technologies, particularly in the field of spintronics. This blog post aims to demystify the electron spin and introduce some spintronic devices like the Nobel Prize-winning discovery of Giant Magnetoresistance (GMR), which is responsible for the high storage capacities of modern hard drives. We will also explore some exciting applications and future prospects of spintronics.

While the term “spintronics” may have different connotations for different people, this blog post primarily focuses on providing a brief introduction to this field mainly from the perspective of data storage devices. However, the field expands into other domains, involving the science and technology of using the spin degree of freedom of charge carriers to store, encode, access, process, and transmit information. The field is extremely vast, and discussing everything is beyond the scope of this post.

Interest in spintronics was motivated by a longstanding belief that replacing charge with spin could yield significant advantages in terms of processing speed, energy efficiency, and device density on a chip. These advancements have become an absolute necessity in the 21^st century considering the growing demand for energy in the information and communication sector. This demand is expected to consume 20% of global electricity by 2030.

Global energy forecast. Image adapted from ref. 1

The internet has grown exponentially over the last two decades. To put this into perspective, if the data stored on the internet today were printed on paper, it would form a stack reaching beyond the moon! This massive growth highlights the urgent need for more efficient data storage solutions, where spintronics could play a crucial role.

You can also checkout our project video which provides insights on the working principles of spintronics and how it is helping to tranform digital technologies.

What is electron spin?

To grasp the concept of electron spin, it’s essential to understand the basics of atomic structure. Electrons are subatomic particles that, along with protons and neutrons, make up an atom. These elementary particles all possess a quantum mechanical property called spin, which can be measured and has quantized values, including zero.

For the sake of understanding, students often visualize spin as the angular momentum associated with an elementary particle spinning or rotating about its own axis, like a spinning top or a planet. This mental picture is convenient but somewhat crude and incomplete.

There are a few problems with this simplistic picture of an electron’s rotation about its own axis. This model cannot explain the quantization of spin angular momentum, as it suggests that spin should have continuous values rather than discrete ones.

Another problem arises when we try to calculate the spin angular momentum using this semi classical picture.

Where m₀ is the mass of electron, v_s is the speed of the surface of electron and r_e is the Lorentz radius of the electron given by,

Solving the above equation, we find that the speed of rotation on the surface of the electron exceeds 130 times the speed of light. Clearly, this would not be permitted by the Einstein’s theory of relativity.

What does this apparent fallacy imply? It indicates that the concept of spin is inherently quantum mechanical and cannot be described within the framework of classical mechanics. Furthermore, the electron cannot be visualized as a nearly point charge with the Lorentz radius.

Landau and Lifshitz, in their classic textbook on quantum mechanics wrote “[the spin] property of elementary particles is peculiar to quantum theory. [It] has no classical interpretation… It would be wholly meaningless to imagine the ‘intrinsic’ angular momentum of an elementary particle as being the result of its rotation about its own axis.”

Experimental and Theoretical Developments

Throughout the 20^th century, numerous experiments and theoretical developments significantly enhanced our understanding of the electron’s spin. One of the most pivotal experiments was the Stern-Gerlach experiment in 1922, which demonstrated the quantized nature of angular momentum and provided direct evidence of spin. In this experiment, silver atoms were passed through a non-uniform magnetic field, resulting in the atoms being deflected in discrete directions rather than a continuous spread, indicating the presence of quantized spin states.

Stern-Gerlach experiment schematic and the image of the postcard sent by Gerlach to Bohr. Image adapted from ref. 3 and 4

Further theoretical advancements came with the development of quantum mechanics. Paul Dirac’s relativistic quantum theory in 1928 successfully incorporated spin into the framework of quantum mechanics, predicting the existence of antimatter and providing a more comprehensive understanding of the electron’s behaviour.

Experimental techniques continued to evolve, allowing more precise measurements of spin-related phenomena. For instance, the discovery of electron spin resonance (ESR) in 1944 enabled scientists to study the magnetic properties of electrons in various materials. This technique exploits the fact that electron spins can resonate in an external magnetic field, providing detailed information about the electronic structure of substances.

Another significant milestone was the development of the scanning tunneling microscope (STM) in the 1980s, which allowed for the visualization and manipulation of individual atoms and their spins on surfaces. This breakthrough opened new avenues for research in surface physics and nanotechnology.

These theoretical and experimental advances have collectively deepened our understanding of spin and its behaviour under different conditions. They have paved the way for modern applications in fields such as spintronics, where the manipulation of the electron’s spin is used to develop new technologies for data storage and processing. The interplay between theory and experiment continues to drive progress in understanding the fundamental properties of matter.

Spintronics: Electronics with electron spin

In the mid-20^th century, it became clear that electron spin plays a fundamental role in magnetism. Every theoretical model developed to explain the physical origins of magnetism, such as the Bloch model, the Heisenberg model, and the Stoner model, invoked spin in some way.

While magnetism has always been closely linked with spin, in the late 20^th century came a breakthrough realization: spin, either alone or in conjunction with charge, can be harnessed to process information. An electron can have one of two spin states: “spin-up” or “spin-down,” typically represented by arrows pointing up or down. This binary nature of spin makes it an excellent candidate for encoding information, similar to how binary code (0s and 1s) is used in traditional computing.

This role had been traditionally delegated to the “charge” of an electron, not its “spin.” Over the last two decades or so, there has been burgeoning interest in augmenting the role of charge with spin, or even replacing charge with spin in information processing devices.

Comparison between conventional electronics and spintronics

Giant Magnetoresistance

The discovery of GMR was a groundbreaking moment in the field of spintronics and in physics, achieved independently by Albert Fert in France and Peter Grünberg in Germany in 1988. Their work demonstrated how the resistance of multilayered magnetic structures could change dramatically in response to an external magnetic field. This discovery was so impactful that it earned them the Nobel Prize in Physics in 2007.

GMR is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and non-magnetic layers. The resistance of these structures changes significantly in response to an external magnetic field.

Working principle of GMR

Layer Structure: The GMR effect is typically observed in multilayered structures where layers of ferromagnetic materials (like iron or cobalt) are separated by a non-magnetic spacer layer (such as chromium or copper).

Spin-Dependent Scattering: Electrons in ferromagnetic materials have spins that can be either parallel or antiparallel to the magnetization of the layers. The resistance of the material depends on the relative orientation of these spins.

Parallel Alignment: When the magnetizations of the two ferromagnetic layers are parallel, electrons encounter less scattering (i.e. current flowing through the layers will encounter lower resistance).

Antiparallel Alignment: When the magnetizations are antiparallel, the scattering increases (i.e. current flowing through the layers will encounter lower resistance).

Magnetic Field Influence: The magnetization of one of the ferromagnetic layers can be switched at will by applying a magnetic field. This change in resistance of the structure depending on the magnetization state of two ferromagnetic layers is the GMR effect, and it allows external control of the resistance state (high/low) of the structure.

Left schematic (adapted from ref. 5): GMR´s working principle, a spin pointing towards right (represented by black arrow at the bottom) will experience a higher resistance when travelling in a layer with magnetization pointing towards left (solid-pink arrow, pointing towards left) but the same spin will experience low resistance when travelling in a layer with magnetization pointing to the right (solid-pink arrow, pointing towards right). The vertical arrows pointing upwards represent the path of spin through different the layers.
Right schematic (adapted ref. 6): GMR sensor reading individual bits

Applications of GMR

-Hard Disk Drives (HDDs): GMR read heads are used in modern HDDs. The ability to detect small changes in magnetic fields allows for the reading of densely packed data on the disk, significantly increasing storage capacity leading to HDDs with terabyte-level capacities

– Magnetic Field Sensors: GMR sensors are used in various applications to detect magnetic fields with high sensitivity. These sensors are used in automotive applications, industrial positioning, and consumer electronics.

– Biological and Chemical Sensors: GMR-based sensors are being developed for detecting biomolecules and chemical substances, taking advantage of their high sensitivity and specificity.

Giant Magnetoresistance and spin valves have not only advanced data storage technology but also opened new frontiers in sensor technology and spintronics. Their ability to manipulate and detect electron spin with high precision underscores their significance in both scientific research and practical applications. The continued development and application of GMR technology promise to drive further innovations in electronics and information technology.

Outlook

Looking forward, the field of spintronics holds immense promise for revolutionizing the next generation of electronic devices. Spin Transfer Torque Magnetic RAM (STT-MRAM) and Spin-Orbit Torque RAM (SOT-RAM) are emerging as potential replacements for traditional memory technologies, offering faster speeds, higher endurance, and lower power consumption. Beyond memory, spintronics is paving the way for innovative devices like the MESO (Magnetoelectric Spin-Orbit) device, which could lead to even more energy-efficient computing. These technologies are not just theoretical; they are actively being developed and hold the potential to transform everything from data storage to processing, enabling smarter, faster, and more sustainable electronic systems. The future of spintronics is bright, with the potential to push the boundaries of what’s possible in the digital age.

a) Schematic of STT b) Schematic of SOT based device c) Schematic of MESO device Adapted from ref. 7

References:

Jones, N. (2018). How to stop data centres from gobbling up the world’s electricity. Nature, 561(7722), 163–166. https://doi.org/10.1038/D41586-018-06610-Y
Bandyopadhyay, S. ., & Cahay, M. . (2020). Introduction to spintronics. CRC Press. https://www.routledge.com/Introduction-to-Spintronics/Bandyopadhyay-Cahay/p/book/9780367656447
Castelvecchi, D. (2022). The Stern–Gerlach experiment at 100. Nature Reviews Physics 2022 4:3, 4(3), 140–142. https://doi.org/10.1038/s42254-022-00436-4
How the Stern–Gerlach experiment made physicists believe in quantum mechanics – Physics World. (n.d.). https://physicsworld.com/a/how-the-stern-gerlach-experiment-made-physicists-believe-in-quantum-mechanics/
Application of GMR | Evgeny Tsymbal | Nebraska. (n.d.). Retrieved August 21, 2024, from https://unlcms.unl.edu/cas/physics/tsymbal/reference/giant_magnetoresistance/application_%20of_gmr.shtml
CALAVALLE, F., & CALAVALLE, F. (2022). Probing and tuning the electronic properties of low dimensional van der Waals materials. Ph.D thesis UNIVERSIDAD DEL PAÍS VASCO/EUSKAL HERRIKO UNIBERTSITATEA
Manipatruni, S., Nikonov, D. E., Lin, C. C., Gosavi, T. A., Liu, H., Prasad, B., Huang, Y. L., Bonturim, E., Ramesh, R., & Young, I. A. (2018). Scalable energy-efficient magnetoelectric spin–orbit logic. Nature 2018 565:7737, 565(7737), 35–42. https://doi.org/10.1038/s41586-018-0770-2

Posted on 2024-06-252024-06-27 by Mayank Sharma

Behind the Scenes: The making of our SPEAR outreach video

Working on our SPEAR outreach video has been a thrilling and intricate adventure. I volunteered to coordinate this project, but there’s no way it would have been possible without the awesome support and guidance of our project manager, Eli. Big shout out to Eli and all our lovely ESRs!

I jumped in because I love making videos. This outreach video was the perfect way to satisfy my passion while making a meaningful contribution to our project – what better deal than this?

From scripting to finding the right video agency, and even acting in the video, I had the chance to oversee everything. Though we’re still in the post-production phase, I wanted to share a sneak peek behind the scenes, highlighting the creativity, collaboration, and coordination that went into making this video.

Scripting and Conceptualization

The first task was to develop the concept and script for the video. Our project focuses on spintronics, with ESRs working on various subtopics, some closely related and others a bit more distant. For the video, we wanted to move away from the usual “this is our project, and here’s our cool lab” format. Instead, we aimed to create something more meaningful and engaging for young viewers, sparking their interest in the field of spintronics.

The idea was to give the video a documentary-style approach, starting with the basics of electronics and its ubiquitous presence in both personal and commercial sectors. We wanted to illustrate how technology is woven into every aspect of our lives, which naturally leads to a discussion on the resulting excessive energy demands. This would smoothly transition into the need for better, faster, and more energy-efficient technology solutions—enter spintronics.

In the video, we briefly explain that spintronics leverages the spin of electrons to achieve these advancements. From there, we feature short segments where ESRs working on different subtopics within spintronics briefly describe their specific projects while connecting it to real-world applications and challenges.

Our goal was to create a narrative that not only informs but also captivates, making the viewer understand and appreciate the importance and potential of spintronics in addressing modern technological and energy challenges.

Finding the Right Video Agency

Finding the right video agency was a journey in itself. We reached out to contacts all around, from Zurich to the UK, with the initial idea of possibly shooting the video at various locations, showcasing the different labs involved in our project. However, we quickly ran into a series of challenges, from budget constraints to scheduling conflicts with the video agencies.

Additionally, coordinating the schedules of all the ESRs, each heading off to their respective secondments, made availability a major hurdle. After extensive searching and coordination, we finally found an agency in San Sebastian. Fortunately, they had some dates available just before I was set to leave for my secondment in Zurich. It felt like everything finally fell into place, allowing us to move forward with our video production plans.

Working with the Video Agency

Once we found the right video agency, the real fun began. I really liked the director from the start, and once the script was ready and he was happy with it, we dove into the details of how the video should be shot. Since we decided to film at the nanoGUNE, where I’ve been working for the past two years, I had some interesting ideas about the different scenes and locations. We shared these ideas with the director, and despite a small communication gap (my Spanish isn’t the best), he immediately understood our vision. It felt like he could read my mind! Not only did he enhance the ideas, but he also introduced some fantastic new ones.

Filming Day

Filming day was both exhilarating and exhausting, starting bright and early at 8 am and wrapping up around 7 pm. I was filled with excitement and a bit of nervousness. At times, I even forgot the smallest lines! The first scene was nerve-wracking, but as the day went on, I became more comfortable with the crew and the filming process.

Eli was always around, ensuring that every line was delivered correctly and without grammatical errors. Her presence and support were invaluable throughout the day. The whole crew was fun to work with, and the director was especially supportive.

There were some memorable moments that really stood out. In one scene, I was supposed to walk down a busy street, with the chaos of pedestrians in the background. For each take, we had to wait for the traffic light to turn green for pedestrians so that the timing and background would be just right. The shot also involved some tricky handheld camera movements and changes in focus. We faced a series of amusing challenges – sometimes I walked too fast, other times the camera lost focus, or there simply weren’t enough pedestrians crossing the street. Each of these little hiccups made the day more memorable and added a lot of laughter to the mix.

Despite the long hours, I enjoyed every moment of the shoot. The director’s guidance and the team’s camaraderie turned what could have been a stressful day into an incredibly fun and productive experience. By the end of the day, it felt like we had created something beautiful, capturing not just the essence of our project but also the enthusiasm and dedication behind it.

We’re now in the post-production phase, and the excitement continues to build. The final touches are coming together, and I can’t wait to see the finished video and share it with everyone. Stay tuned for its release!

Posted on 2023-11-052023-11-09 by Mayank Sharma

At science museum with Spintronics

Explaining the size of modern day storage devices

Last week I had an exciting opportunity to interact with the high school students at the Inspira Bizitzak (Inspiring Lives) a science outreach event organized by the science museum in San Sebastian. The event was for young students hence it was a good challenge to explain spintronics. I aimed more towards giving them an essence, probably not even the taste, just the smell of the subject.

The day started with talks by the researchers in the morning followed by a poster session. I sat with students in the morning listening to different researchers not realizing that I too have become one.

The event unfolded and soon the poster session started as you can see in the pictures, I was trying to explain them spintronics. Here, I used human hair and human figure nails to give them some idea how tiny the magnetic storage now has become. Although there was some language barrier but still the students were very enthusiastic as if they wanted to learn everything in one day.

The discussions with students went beyond physics or spintronics, a lot of discussions were about being creative or thinking about new ideas in any field. During the session an excellent example of zipper came up and how it has changed a lot of things around us from clothing to tents to covers. The day came to an end with wonderful discussions and curious faces.

I left the place with a feeling that, it is simple ideas with great implications that change the world.

Posted on 2023-02-222023-03-01 by Mayank Sharma

Secondment

The new year started with my academic secondment at ETH. Had a chance to catch up with Marco (ESR3) before he left for his secondment in Grenoble. While Niklas (ESR4) has been constantly there to help me with the institutional and lab stuff. I also had an opportunity to meet my industrial secondment supervisor Dr. Jan Rhensius from QZabre. Zhewen (ESR13) is also around to show me nice trails just behind the ETH Hönggerberg campus.

How can I forget watches, probably the strongest attraction that always pulled me towards Switzerland. Zurich is one good place to see watches thanks to the famous bahnhofstrasse, one whole street full of watch boutiques is no less than a watch paradise. Every week I go to the street to update myself with the new watches on display but still feels like I haven’t seen it enough. Apparently there is a watch museum here which I haven’t managed to see but planning to see soon, never the less I did visit the Lindt chocolate museum. I would recommend to take the guided tour, it’s a fun experience to eat free chocolate like kids.

World’s largest free standing chocolate fountain at the museum

Posted on 2022-06-272022-07-13 by Mayank Sharma

An Afternoon

One afternoon I had some time to play with the microscope during my school days. The moment I was out of sight of my teacher, I started looking at the things that I use every day. I still remember very vividly looking at a dead mosquito. Of course, I ate mosquitos every day ;).

Life have changed and now every afternoon I get to play with the microscope. Below you see a picture of my smartphone screen. On the top row is the picture as we see and in the bottom row is what I saw under the microscope. The matrix of red, blue, and green OLED (Organic Light Emitting Diode) lights up in different combinations and intensities to produce the 16 million colors that we see. We use it every day and technology has been around for a while now. But still to see it in action is very exciting.

These are some of the different types of devices I am working on now. They don´t look as beautiful as a mosquito. On the left we see a device with platinum contacts made with e-beam lithography with a 2D ferromagnet flake encapsulated with boron nitrite. On the right is another device with gold contacts in transverse and longitudinal geometries.

Posted on 2021-10-272021-10-28 by Mayank Sharma

An unexpected meeting…

Perhaps my ear to ear smile itself explains the joy I am experiencing in this moment. It was just two years ago when I was finishing my bachelor’s that I came across the word Spintronics. At that time, I did not realize that soon I would become a part of big project like SPEAR and that I would be working at CIC nanoGUNE (San Sebastián, Spain), where I would get to meet the creator of the Spintronics field itself, Noble laurate Prof. Albert Fert. Prof. Fert is a frequent collaborator of Prof. Felix Casanova, my supervisor and co-leader of the Nanodevices group, and he happened to be visiting our research center last month.

Although it has just been one month since I started my journey as a doctoral researcher at CIC nanoGUNE the experience has been very rewarding. I am learning new preparation and characterization techniques for the 2D magnetic materials which is exciting and challenging at the same time.