Depending on how their electrons fill the available energy levels, and the degree to which they fill subshells, atoms or molecules of atoms may have net magnetic moments. (Remember from Chapter 12 that electrons have magnetic moments that can be thought of as resembling tiny bar magnets, and the moments result from either (or both) the electron's intrinsic spin or the magnetic component of their spatial-state angular momentum.) These moments tend to be oriented toward or opposing a magnetic field, depending on the plus or minus spin and/or angular-momentum state that the electron occupies.
DIAMAGNETISM
Those substances whose atoms have no net magnetic moment are called diamagnetic substances. (Most people would call these substances nonmagnetic). In fact, all substances will have a small diamagnetic component to their magnetism. It arises from the induced response of the electrons in the quantum spatial states of their atoms as any magnetic field change is applied to them. Classically, one can think of small electrical currents induced within the atom that produce small magnetic fields that oppose the field change. Because these small induced atomic magnetic fields oppose the field that is applied, the atoms are repelled by field change, and so diamagnetic substances are very weakly repelled if a magnetic field is changed on them.1
Most substances are diamagnetic. Living things tend to be diamagnetic. And so it is that scientists have even been able to repel and levitate a frog and other small biological items using the magnetic field produced in one of the most powerful magnets in the world.
MAGNETS AND MAGNETIC FIELDS
In the paragraphs below, I start with a description of “frog levitation” as a mechanism for providing a sense of the magnitudes of magnetic fields. I also mention various devices that produce or use magnetic fields, and I describe materials that produce or duct magnetic fields around.
The source of the levitation field for the frog mentioned above is a 1.25-inch bore 16-tesla Bitter magnet located at the Nijmegen High Field Magnet Laboratory in the Netherlands. (Bore is the available cylindrical open center space in which a patient or specimen may be placed.) Sixteen tesla2 is an extremely high magnetic field. It is approximately 320,000 times the earth's magnetic field; approximately 100 times the magnetic field of household permanent magnets such as those that you stick on your refrigerator doors; approximately 10 times the field of the strongest of permanent magnets; approximately 10 times the field conducted through ferromagnetic materials in the iron magnetic circuits of large transformers, motors, and generators; and approximately 10 times the field inside of the large bore tube of the superconducting magnet that you slide inside of for MRI medical diagnostic imaging (as discussed in Chapter 19).
In the operation of a Bitter magnet, enormous electrical currents are forced through a flat, mechanically strong yet still good-electrically-conducting alloy (that is wound on edge like a Slinky in a tight spiral around the bore) to create the magnetic field. There is some resistance to current flow in a copper or alloy winding, and this resistance produces heat, so coolant is also forced through the winding to remove the heat. This type of magnet is named after Francis Bitter, who developed it to allow a cooling of electromagnets. In 1938, Bitter established a (later called “national”) magnet laboratory at MIT and achieved a record steady magnetic field of 10 tesla. Prior to the Bitter magnet, there was no good way to cool electromagnets, and the magnetic fields that they could produce were limited. (During World War II, Bitter worked on finding ways to demagnetize British ships to protect them against German field-sensing mines. This work Bitter called “Degaussing the Fleet.”3)
The world's highest sustained (rather than pulsed) magnetic field is available for experimental work at the (present) US National High Magnetic Field Laboratory in Tallahassee, Florida. This field is produced by a hybrid magnet system consisting of an outer superconducting solenoid4 that provides an 11.5-tesla field boost for an inner 33.5-tesla Bitter magnet, to produce a total of 45 tesla in a 1.25-inch bore. Huge currents through the resistive Bitter magnet continuously dissipate 33 million watts (megawatts) of power as heat. (This power would light 33,000 average homes!) A water flow of 4,000 gallons per minute is required to cool the magnet. By contrast, the superconducting solenoid requires only a few hundred watts from standard outlets: (1) to run the refrigerators that help to keep the superconductor cold (in a manner similar to that described for MRI magnet systems in Chapter 19) and (2) to supply the power needed (after conversion from AC to DC) to drive sufficient current through the solenoid to supply the energy that is stored as the magnetic field.
PARAMAGNETISM
In many instances, diamagnetism is overcome by the direct pull of a magnetic field on the spin and spatial-state magnetic moments of the atoms. Atoms or molecules may have net magnetic moments that reluctantly align with and are attracted into an applied magnetic field (reluctantly because the magnetic moments are thermally jostled at normal room temperatures toward random orientation). These substances are said to be paramagnetic substances. (They are only weakly magnetic, and most people would also call these substances nonmagnetic). For example, oxygen is paramagnetic. As a demonstration of this, liquid oxygen can be held in suspension between the jaws of a strong horseshoe magnet. (Note that it is the absorption of particular wavelengths of light in paramagnetic oxygen trapped in glacial ice that gives the ice its blue color.)
FERROMAGNETISM
In a few materials (iron, cobalt, and nickel, for example), the magnetic moments of large numbers of atoms or molecules will spontaneously align to create very strong collective magnetic moments within regions of the material called domains. In the absence of a magnetic field, these domains arrange themselves so as to duct the magnetic field of these moments around in closed magnetic circuits. However, even in a weak applied magnetic field in what are called “soft” magnetic substances, these magnetic moments can become fully aligned to create a single domain and a large associated magnetic moment that is strongly attracted into the magnetic field. These ferromagnetic substances are what people normally think of as “magnetic,” and we say that these materials are magnetized when the single domain is created.
Ferromagnetism is a special situation that depends not only on the magnetic properties of the atoms in a solid substance but also on the spacing and magnetic connection between the atoms. The details are explained as follows.
Theoretically, for ferromagnetism to occur, the spatial states of the outermost electrons of adjacent atoms in a solid substance must have a substantial overlap, while at the same time these states must show a limited probability for the electron to reside near the nucleus. Their nuclei must also be spaced appropriately apart. The d and f states are apparently particularly suitable to these requirements. (The probability clouds for these states may somewhat resemble those shown for the d and f states of hydrogen in Figure 3.8.) And so we can understand that iron, cobalt, and nickel in the fourth row of Table IV in Chapter 13 or Table B.2 in Appendix B (with six, seven, and eight outer 3d electrons, respectively) and gadolinium and dysprosium in the sixth row (each with outer 4f electrons) all are ferromagnetic. Interestingly, while manganese (with five outer 3d electrons) by itself is paramagnetic, not ferromagnetic, a couple of compounds of manganese that force the manganese atoms to be slightly farther apart than they are in the pure metal are ferromagnetic, apparently reaching the right separation for the manganese d states.
APPLICATIONS FOR FERROMAGNETIC MATERIALS
Just as copper or superconducting wires are used to duct electrical currents around electrical circuits, ferromagnetic materials are used to duct magnetic fields around magnetic circuits. Magnetic circuits are essential major components of electric motors, generators, and transformers. Since the transformer is an AC device that requires that the magnetic field alternate direction sixty times per second (in the United States and Canada, for example), its magnetic circuit must be made of the “soft” magnetic materials in which the domains are easily and quickly oriented to the continually changing field. Alloys of pure iron with small percentages of silicon have proven to be particularly good for this application.
Magnetic data storage uses magnetizable material to form a nonvolatile memory. Data is “written” into the material (which is usually a surface coating) via heads that magnetize the material. The same heads may be capable of “reading” what is written by sensing the pattern of magnetic fields that are written there. This form of magnetic storage is in the “hard disks,” in hard drives of computers, and in the stripes on our credit cards.
PERMANENT MAGNETS AND THEIR APPLICATIONS
Permanent magnets are made from ferromagnetic materials that tend to retain a single domain (or set of parallel domains) once their domain orientation is established. One primary way of accomplishing this is to create particles so small that they can contain only one domain, and then have these particles oriented in the presence of a magnetic field as the particles are pressed together to make a solid. Another way is to cause phase changes in solids that precipitate out tiny single-domain regions of one material composition (all oriented parallel to an applied magnetic field) within an overall surrounding matrix material of another composition. Permanent magnets are used, for example, in small motors, in devices for magnetically separating materials, in quadrupole magnets for scientific instrumentation, and for refrigerator magnets now popular for advertising or simply mounting notes or business cards. Rare-earth magnets are particularly important for high-field, low-weight applications of these materials. In this regard, neodymium iron boride is still the champ.