THE MAGIC OF MAGNETS
By: Ingrid Stolt
A cristallographic structure of cuprate YBa_2Cu_3O_7 (blue:Cu, brown:Y,green:Ba, red: O), which is a high temperature superconductor. Image courtesy Wikimedia Commons
As a young girl I could spend hours playing with my parents’ refrigerator magnets. I used to pretend that one magnet was a magic wand that was causing the other to move back and forth and rotate through supernatural powers. Magnetism seemed magical because it was so mysterious, yet I wanted to understand how it worked.
In high school I learned that magnetism is inherently intertwined with electricity and that even atoms, the building blocks of all matter, can be magnetic. I continued to study magnetism in college when I started taking physics courses. As I progressed, the material I was learning grew more fundamental; the information in my textbooks appeared more frequently as equations filled with Greek symbols rather than in written English. Each lesson made it increasingly clear that my parents’ refrigerator magnets were not supernatural objects. However, the feeling of a magnet pulling or pushing on another magnet or a piece of metal was more enchanting than ever.
One of the most fundamental and illuminating concepts in magnetism is that of the magnetic field. Scientists use this concept to describe how a magnet is able to influence the motion of distant objects. The basic idea is that all magnets emit a field in the space around them and this field affects the motion of other objects. Scientists like to visualize the magnetic field using a conceptual tool called magnetic field lines. If you look up “magnetic field” on Google images or in an introductory physics textbook, these lines are usually depicted on a cartoon bar magnet with its north and south poles labeled “N” and “S.” The field lines are solid lines that emanate from the north pole and travel on an arc alongside the magnet to the south pole. Arrows along the lines mark the direction of their paths, which always runs outward from the north pole and curve inward toward the south pole.
Consider holding a small bar magnet at its midpoint near a much larger bar magnet, which emanates magnetic field lines. The smaller magnet will rotate so that its north pole is as close to the south pole of the larger magnet as possible. The direction of the south pole of the small magnet is the direction of the larger magnet’s magnetic field at this location.
In addition to having a direction, the field produced by the larger magnet will also have a strength. The field is strongest near the magnet and becomes weaker as you move farther from the source. You can imagine this field strength in terms of how hard you would need to pull on the small magnet to keep it from flying into the large magnet and how it gets easier to keep the two apart as you hold them farther from each other. The density of field lines is proportional to the strength of the field; if there are more field lines in a region this region has a stronger field than one in which there are fewer lines.
These days I work in a physics laboratory with two very strong magnets. We need to keep all our steel tools, such as wrenches and screwdrivers, at least 10 feet from the larger magnet to prevent them from flying into the magnet. Signs on our doors warn people with certain medical implants, such as pacemakers, that the electromagnetic fields in the lab can wreak havoc on these devices. I like to hold a steel wrench near the magnet with as few fingers as possible and pretend that the wrench is floating. If I wear nickel-coated earrings I can feel the attractive force from the magnet twisting them when I get close. I love imagining how the field lines look where I’m standing based on these pulling sensations.
The materials I study in this laboratory belong to a class of man-made superconconductors known as cuprates. If you place a superconductor in a magnetic field the superconductor will behave in one of two ways. All superconductors have the remarkable property that when they are immersed in a magnetic field, the field lines completely avoid them. Just as rain slides off an umbrella, these magnetic field lines steer around a superconductor. However, certain superconductors, including the cuprates, will allow a specific amount of magnetic field lines to penetrate their surface.
Imagine you have identical holes on your umbrella (not a great umbrella). The holes will let raindrops through at these points but you will still have other raindrops following the curve of the umbrella. In the cuprates, the magnetic field lines will run through certain points of the superconductor as if these points had holes in them, while the remainder of the field lines flow around the entire material as if it were field-proof. The points that the magnetic field lines penetrate the superconductors are called vortices and they can form ordered patterns known as vortex lattices. One way to think about a vortex lattice is to imagine the vortices as holes arranged in a certain repeating pattern such that if you drew lines connecting adjacent vortices you would see something resembling a tiled floor. I study the patterns that form when we place a certain cuprate in one of our strong magnets.
The shapes of these vortex lattices—the shape of the tiles from the above analogy—are dependent on the strength of the magnetic field. If we return to the rain metaphor, the strength of the field is similar to the amount of rainfall. A weaker field, a light drizzle, will produce a different vortex lattice (umbrella hole) pattern than a stronger field, or heavier rainfall. The material that I study, for example, has a triangular vortex lattice in weaker fields, but as we increase the field strength the vortices rearrange to form a repeating rectangular pattern. There are other methods of studying the vortex lattice experimentally, but these techniques can only be used to study superconductors in weaker magnetic fields. The advantage of our experimental methods is that they allow us to study systems in some of the strongest fields possible so we can then determine the strong and weak field behavior of our cuprate samples.
People often ask me why my research is important and I am often unsure of how to answer. I think what they mean is “why should I care about the research you are doing?” This is also a tough one. I usually give a vague answer that references supercomputing or developing practical uses for superconductivity. If I am to give a more complete answer, I say that the purpose of my research is to study specific properties of certain superconductors so scientists can use my data to develop an understanding of these materials. By measuring vortex lattices, I am creating a library of patterns using a method that allows us to map these patterns for the strongest magnetic fields out there.
However, if someone were to ask, “why is your research important to you?” my answer would be that the research I do makes the world around me more fascinating and beautiful—and makes refrigerator magnets even more magical to play with now than they were when I was 7.