Paramagnetic species are everywhere. In the right setting, and voiced in a properly somber tone, that phrase could summon images of strange alien invaders running amok all over the globe. Instead, it is a basic statement about a certain quality shared by a well-defined set of particles on and about Earth, and one defined using objective and easily determined criteria.
You have no doubt made use of magnets in your life, and in most cases that you have operated within a non-trivial magnetic field, you haven't been aware of it. You may even know that certain materials function as permanent magnets, and that these can attract metals even though those metals are not themselves apparently magnets. Or are they?
As it happens, the physics world, specifically the sub-discipline of electromagnetism, includes a variety of types of magnetism. One of these is paramagnetism, and it is a property that is often easily verified on sight, because paramagnetic materials are attracted to an externally applied magnetic field. But how does this happen, and where do magnetic "fields" come from, anyway? The chance to learn all of that and more should be strongly pulling you to keep reading!
What Is Magnetism?
In the late 1700s, it was observed that a compass needle, which points toward the north as a result of Earth's magnetic field, can be deflected by the presence of a nearby electric current.
This is the first known evidence that electricity and magnetism were somehow connected. In fact, moving charges (which is the definition of electrical current) generate magnetic fields with "lines" dependent on the geometry of the electrical circuit.
When a current-carrying wire is coiled, or wrapped multiple times, around certain kinds of metal, this can induce the property of magnetism in these metals, at least while the current is being applied. Some of these are used in places like scrap-metal yards and are powerful enough to lift whole automobiles.
The interplay of electric current and magnetic fields is a subject that can and does fill whole textbooks, but for now, you should know that the reason some materials respond differently to magnetic fields than others has to do with the properties of the electrons in the highest ("outermost") energy shell of the atoms in those materials.
The Magnetization of Solids
If a solid substance is placed in an applied magnetic field, you might expect the behavior of the molecules in the substance to depend to some extent on the state of the material. That is, a gas, which has molecules that move about quite freely, and a liquid, in which molecules remain together but are free to slide past each other, might behave differently than a solid, whose molecules are locked in place, usually in a lattice-type structure.
If you picture a solid's basic crystal structure (and the nature of this repeating pattern can vary from substance to substance), you can imagine the nuclei of the atoms being at the centers of cubes, with the electrons occupying spaces in between, free to vibrate and, in the case of metal solids, free to roam about unchained to their parent nuclei.
When the electrons of a solid render the substance a permanent magnet or one that can be made into such a magnet, the substance is called ferromagnetic (from the Latin ferrum, meaning iron). In addition to iron, the elements cobalt, nickel and gadolinium are ferromagnetic.
Most substances, however, exhibit other responses to magnetic fields, making most atoms paramagnetic or diamagnetic. These properties can be found to different degrees in the same materials, and factors such as temperature can affect a material's response to applied magnetic fields.
Diamagnetism, Paramagnetism and Ferromagnetism Compared
Consider three different friends you have chosen as candidates to test your new science gaming app.
One of them only responds to your urges to give it a try by becoming more resistant than she was to game-playing at the outset. The second agrees to install the app and play, but quickly stops playing and uninstalls the app every time you leave him alone, only to reinstall it and keep playing whenever you reappear; and the third friend immediately becomes hooked on the app and never stops using it.
This is loosely how the three kinds of magnetism you are most likely to hear about at the office party work in relation to each other. While ferromagnetism, already described, is a state of permanent magnetism, how does this happen, and what are the alternatives?
As it happens, there are four well-understood alternatives to ferromagnetism. Paramagnetism, again, is the property of being attracted to a magnetic field, and applies to a wide range of metals, including most modern refrigerators. Diamagnetism is the opposite, a tendency to be repelled by a magnetic field. All materials exhibit some degree of diamagnetism. In both cases, critically, the material returns to its previous state when the field is removed.
- Spoken out loud, "ferromagnetism" and "paramagnetism" sound a lot alike, so be careful when discussing these topics in your physics study group.
Ferrimagnetism and antiferromagnetism are less commonly encountered types of magnetism. Ferrimagnetic materials behave much like ferromagnetic materials, and include jacobsite and magnetite. Hematite and troilite are two compounds that demonstrate antiferromagnetism, where no magnetic moment is generated.
Characteristics of Paramagnetic Compounds and Atoms
Paramagnetic elements and paramagnetic molecules share one main trait and that is having unpaired electrons. The more of these there are, the more likely the atom or molecule is to show paramagnetism. This is because these electrons align themselves in a fixed way with the orientation of an applied magnetic field, creating something called magnetic dipole moments around each atom or molecule.
If you are familiar with electron "filling" rules, you know that orbitals within subshells can hold two electrons each, and that there is one of these for an s subshell, three for a p subshell and five for a d subshell. This allows for a capacity of two, six and 10 electrons in each subshell, but these will fill up so that each orbital holds just one electron for as long as possible until the one electron there has to accommodate a neighbor.
This means that you can use the information in a periodic table of the elements to determine if a material will be paramagnetic, and happily, whether it will be weakly paramagnetic (as in Cl, which has one unpaired electron) or strongly paramagnetic (like platinum, which has two unpaired electrons).
List of Diamagnetic and Paramagnetic Atoms and Molecules
One way to quantify magnetism is through the parameter called magnetic susceptibility χm , which is a dimensionless quantity relating a material's response to an applied magnetic field. Iron oxide, FeO, has a very high value of 720.
Other materials considered strongly paramagnetic include iron ammonium alum (66), uranium (40), platinum (26), tungsten (6.8), cesium (5.1), aluminum (2.2), lithium (1.4) and magnesium (1.2), sodium (0.72) and oxygen gas (0.19).
These values range widely and that of oxygen gas may seem modest, but some paramagnetic materials show far smaller values than those listed above. Most solids at room temperature have χm values less than 0.00001, or 1 x 10-5.
The susceptibility, as you might expect, is given as a negative value when the material is diamagnetic. Examples include ammonia ( −.26) bismuth (−16.6) mercury (−2.9) and the carbon in diamond (−2.1).
- OpenStax Physics: Ferromagnets and Electromagnets
- University of Minnesota: Classes of Magnetic Materials
- LibreTexts Chemistry: Magnetic Properties
- Georgia State University Hyperphysics: Magnetic Properties of Solids
- Georgia State University Hyperphysics: Magnetic Susceptibilities of Paramagnetic and Diamagnetic Materials at 20°C
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.