Soft Magnetic High Entropy Alloy for Artificial Spin Ice
This project- funded by the DOE EPSCoR program- is all about artificial spin lattices (ASLs)- these cool arrays of tiny magnetic islands that we can tweak by changing their shape and material. I’ve been diving into some cutting-edge research with high-entropy alloys (HEAs), and I can’t wait to tell you about what we’re doing, why it’s exciting, and where we’re headed.
What Are Artificial Spin Lattices?
Picture a grid of tiny magnetic islands, each one so small it’s measured in nanometers. These are ASLs, and they’re like a playground for studying how magnets behave in unique ways. When we arrange these islands in a specific pattern, we call it an artificial spin ice (ASI). ASIs are awesome because they let us explore things like magnetic frustration- where the magnets can’t quite settle into a perfect arrangement- and collective behaviors that you don’t see in natural materials. They’re like a lab for testing ideas from statistical mechanics, which is all about how particles interact in complex systems.
But it’s not just about science for science’s sake. ASIs are showing up as potential game-changers for unconventional computing, like neuromorphic computing, which mimics how our brains process information. Plus, researchers from our team recently discovered that ASIs with specific defects can control something called X-ray orbital angular momentum (OAM), which is a fancy way of saying they can twist X-ray beams in unique ways. We even published this in Physical Review Letters and got a shout-out in Physics magazine! We’ve also shown that ASIs can help us study new X-ray magnetic effects, like X-ray magnetic dichroism, which is a way to probe magnetic properties using X-rays. There’s still so much we don’t know, though, especially about how these lattices interact with X-rays across different energies and how their magnetic properties evolve over super short time scales (think femtoseconds to nanoseconds).
Why High Entropy Alloys?
Most ASL research so far has used permalloy, a nickel-iron alloy that’s great for magnetic studies because it has predictable magnetic properties and works well with X-ray experiments. But permalloy isn’t the only game in town. Enter high-entropy alloys (HEAs)- materials made by mixing several elements in roughly equal amounts to create something with unique properties. HEAs are exciting because they can have magnetic properties similar to permalloy but with some serious upgrades, like lower Gilbert damping (which means less energy loss in magnetic switching) and the potential for intrinsic magnetic frustration. This frustration is like a built-in chaos that makes the magnets flip and fluctuate in ways that could be super useful for computing applications.
Our big idea is to use HEAs to make ASLs that are more tunable and versatile. We’re thinking about creating lattices where fast magnetic changes are driven by the material’s natural frustration, while slower changes come from the shape and arrangement of the islands. This could lead to what we call “hierarchical frustration,” where different scales of magnetic behavior work together to create something totally new. It’s like designing a material that’s frustrated on multiple levels- pretty wild, right?
What We’re Doing
Right now, we’re working on combinatorial synthesis, where we deposit thin films with varying compositions by co-sputtering different metals onto a surface. This lets us test a bunch of HEA recipes at once to find the ones with the best soft magnetic properties. I’ve got experience characterizing both HEAs and magnetic materials. We’re focusing on a few promising HEAs, like AlxCrFeCoNi, which has been predicted to have much lower Gilbert damping than permalloy while still having decent magnetization. Our process starts with making these HEA thin films using magnetron sputtering, where we shoot metal atoms onto a surface to build up a thin layer. Then we put these films through a battery of tests to understand their structure and magnetic properties. We use X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to check the crystal structure and chemical makeup, and focused ion beam scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM) to get a close-up look at the material’s structure. To measure magnetic properties, we’re using broadband ferromagnetic resonance (FMR) spectroscopy to assess Gilbert damping by looking at resonance linewidths- basically, how sharp the magnetic signals are. We’re also using a SQUID magnetometer to measure magnetization, coercive fields (how much magnetic field it takes to flip the magnets), and phase transition temperatures by looking at hysteresis loops and magnetization versus temperature curves under zero-field cooling and field cooling conditions.
Early Findings and What’s Exciting
So far, our work is showing that HEAs could be a big deal for ASLs. The AlxCrFeCoNi films we’ve tested have magnetic properties that rival permalloy, but with the potential for less energy loss, which is huge for applications like neuromorphic computing where efficiency matters. The films with intrinsic frustration, like Fe0.4Mn0.4Co0.1Cr0.1, are showing signs of the fast magnetic fluctuations we were hoping for, which could make them ideal for creating dynamic ASLs. We’re also finding that by tweaking the composition- say, adding a bit more aluminum or adjusting the ratios of iron and manganese- we can fine-tune things like the Curie temperature and magnetization strength.
One of the coolest parts of this project is how it’s opening up new ways to think about ASLs. By using HEAs, we’re not just stuck with the same old permalloy properties. We can design materials that have the exact magnetic behavior we want, whether that’s for studying fundamental physics or building next-gen computing devices. Plus, the combinatorial approach means we can test a ton of compositions quickly, which speeds up the discovery process.
The Bigger Picture
This project is about more than just making cool materials (though that’s definitely part of the fun). We’re aiming to fill some big gaps in our understanding of ASLs. For example, we’re exploring how these lattices interact with X-ray photons that carry both spin angular momentum (SAM) and orbital angular momentum (OAM). We think ASLs could be perfect for studying new phenomena like X-ray magnetic helicoidal dichroism (XMHD) and X-ray spin-orbit conversion (XSOC)- effects that could reveal hidden details about magnetic behavior that traditional X-ray techniques can’t see. We’re also planning to use ultrafast X-ray scattering to watch how these materials’ magnetic properties change on super short time scales, like during femtosecond demagnetization. This has never been done with ASLs before, and it could unlock new insights into how magnets behave under extreme conditions.
For advanced computing, HEAs could make ASLs more practical by reducing energy losses and enabling faster, more complex magnetic behaviors. Imagine a computer chip that mimics the brain’s efficiency but is made from these tiny magnetic islands- it’s a long way off, but we’re laying the groundwork.
What’s Next?
We’re just getting started, and there’s so much more to explore. Our next steps include fabricating full square ASLs using our best HEA candidates and testing their blocking temperatures (the point where the islands’ magnetism gets “locked in”) using cryo-MOKE measurements, which is a way to measure magnetism with light. We’ll also keep using FMR to fine-tune the Gilbert damping and make sure our alloys are as efficient as possible. Down the road, we’re planning to take these ASLs to a synchrotron facility to do X-ray scattering experiments, which will let us see how they interact with X-ray OAM and probe those new dichroic effects we’re hypothesizing about.
Thanks for reading about this project! I’m stoked to keep pushing forward and will definitely share more updates as we make progress. If you’re into materials science, magnetism, or just cool tech, let me know what you think. I’d love to hear from you!