How Ferroelectricity Works, Why It Could Change the World
Ferromagnetism is an all too common phenomenon that you've probably bore witness too, even if the name sounds unfamiliar. Ferroelectricity, on the other hand, you may not have heard of, but it functions along nearly the same principles.
Ferromagnetism is the property of a material that allows it to become magnetic when it is exposed to a magnetic field. This is why we can use a magnet to pick up screws or other metals that aren't magnets in their own right. Ferroelectricity functions in a similar way, but with electrical fields. When a ferroelectric material is exposed to a strong enough electrical field, it becomes an electrically polarized.
Backing up for a second, the name ferroelectricity might cause you to harken back to your school days, where you probably learned about ferrous materials, or materials that consist in some part of iron. Ferromagnetism gets its name from this property, with the main ferromagnetic materials being iron, cobalt, and nickel. Ferroelecticity, on the other hand, doesn't get its name from the ferrous prefix, rather it's named that because it functions in principle similar to ferromagnetism.
How ferroelectric materials work
Ferromagnetic materials are made up of many domains that have north and south poles all pointing in different directions; essentially tiny magnetsscattered throughout the material. When a magnetic field comes the region of a ferromagnetic material, all of the poles of these domains start to align with the field, and as they do, the material itself becomes a magnet.
This is the same for ferroelectric materials. They too have many tiny domains, or crystals, in the material that function as electric dipoles. A dipole simply means that it has a separate positive and negative charge within the crystal. When an electrical field comes into contact with these crystals, the dipoles all start lining up, or pointing in the same direction, giving the material as a whole a positive and negative side.
The coolest and most practical function of ferroelectric materials is that when the electric field is removed, the dipoles remain oriented in the direction they were. This gives ferroelectric materials "memory".
This memory is so strong, in fact, that when a new electrical field is applied to the ferroelectric material, while the orientation of the dipoles does switch polarization, they lag behind one another and change more slowly. The phenomenon - or rather what caused this phenomenon - was a mystery for some time, and it became known as hysteresis.
The principle of hysteresis
It wasn't until 1935 that the true cause of hysteresis in ferroelectric materials was found by German researcher and scientist Ferenc Preisach, where he described the crystals in ferroelectric materials as hysterons. These hysterons could change their dipole direction as we mentioned, but they could also do it at varying speeds, and the strength of the field needed to change each one could be different throughout a material. This means that in a ferroelectric material, there might be a weak hysteron next to a strong one, meaning it would take a strong electrical field to get both to line up. It's this difference in the critical field or the maximum field strength at which polarization switches, which creates hysteresis in a ferroelectric material.
This all fits into something known as Preisach's model, which described the behavior of ferroelectric materials, which had been a mystery for some time. But here's the thing.
Preisach's model described the "how" of the behavior of hysterons and ferroelectric materials. What it didn't answer is the question of "why". For example, What are the hysterons? Why do their critical fields differ as they do? No one really knew what was going on that made hysterons function as they do on the molecular level. That is until it was finally discovered nearly a century later, in 2018.
Researchers observed ferroelectric materials in great detail and found cylindrical stacks of disk-shaped molecules of around a nanometre wide and several nanometres long. These stacks were shown to be hysterons and to strongly interact with each other. However, each stack has a different size. Because of this, the rate of polarization depends on the nanostructure of each stack and the way the stacks interact with each other.
This is illustrated in the image below, from the research released in 2018 that detailed this discovery.
But how exactly is all of this underlying science applicable to modern life? In other words, why should we care about ferroelectric materials The answer to that question comes back to their ability to "remember" polarity. Molecules that store memory are perfect for applications like computer memory ... well almost.
The application of ferroelectric materials
Ferroelectric materials haven't really been ideal for computer memory, or even useful in that industry, until recently. The materials have had a scaling issue, meaning that creating ferroelectric materials that are small enough to fit into a computer efficiently has been difficult for most of their history. When traditional ferroelectric hysterons are scaled down to the level needed, they lose their ability to store memory, or enter hysteresis, and thus don't work for their intended purpose.
In 2018, research was published detailing the creation of ferroelectric materials at a scale that was small enough for computer memory. Even cooler, the substrate that the researchers were able to grow the ferroelectric crystals on was also ferromagnetic, meaning that theoretical, you could pack magnetic and electric storage onto the same drive, exponentially increasing memory capacity.
While this research was a major step forward for ferroelectric materials and for a possible large scale application, there's still a long way to go before the concept is fleshed out enough to be commercially viable and scalable. However, ferroelectric drives may become a computer storage technique of the future.
But all is not lost! There's other applications for ferroelectric materials too, particularly for electrical engineers.
Other applications for ferroelectricity
Ferroelectric materials can exhibit high permittivity, meaning they can be good at storing electric energy. This makes ferroelectrics a suitable material for the creation of capacitors. The underlying reaction to electrical fields that ferroelectric materials demonstrate also means that they could be used as indicators or switches in other devices.
For example, they have a direct piezoelectric effect, making them useful for accelerometers, microphones, and headphones. Breaking that down a little more: ferroelectric materials have the ability to generate a small charge in response to mechanical stress, which is useful in converting physical forces like acceleration or sound into electrical signals, like that needed in the devices mentioned.
The ability for ferroelectric materials to exhibit a piezoelectric effect also works conversely, or rather, they also have a converse piezoelectric effect, which is when mechanical strain is generated through the application of an electrical field. This is a response to the dipoles shifting polarity, and it also makes ferroelectric materials perfect for things like actuators, resonators, and filters.
Ferroelectricity and the materials that possess this property are growing ever more useful as the research surrounding the underlying physics grows more in-depth. It likely won't be long until ferroelectricity is as foundational a principle in the world around us as ferromagnetism. While ferroelectricity has been known for a while now, most of the foundational research on its usage has been made in the last decade.
For now, ferroelectricity is proving to be another vital component of materials science.