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8.3: Natural Magnetism

  • Page ID
    472609
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    Learning Objectives
    • Define diamagnetism, paramagnetism, and ferromagnetism.
    • Describe the role of magnetic domains in magnetization.
    • Explain the significance of the Curie temperature.

    All matter exhibits some sort of magnetism, as it turns out. There are three types of magnetism possible for matter: diamagnetism, paramagnetism, and ferromagnetism. Some materials can also be made into permanent magnets. The reason for these magnetic properties is due to the arrangement of the electrons within the substances that they are composed of.

    Diamagnetism

    As we will learn later in this text, all substances are made of atoms and all atoms contain electrons. Those electrons may exist in pairs within a certain portion of the atom, or they might be the only electron found within that portion of the atom. Most electrons in any atom will exist in pairs. Electrons which are paired have the property of diamagnetism. Diamagnetism is a weak repulsive force to an applied magnetic field. If all of the electrons in all of the atoms of a substance are paired, that substance is said to be diamagnetic. If some of them are not paired, then another type of magnetism will come into play which is much stronger.

    Paramagnetism

    If some of the electrons of an atom are unpaired, the magnetic properties of that substance will be completely different. Instead of being repelled by an external magnetic field, the substance will be attracted to that magnetic field. This phenomena is called paramagnetism. Most substances in their elemental states are paramagnetic, and hence attracted to external magnetic fields. There are some unexpected substances such as molecular oxygen which are paramagnetic. In fact, the electronic structure of oxygen was deduced partially on the experimental evidence that it was paramagnetic.

    photograph of liquid oxygen stuck between two powerful magnets.

    Figure \(\PageIndex{1}\): Liquid oxygen suspended between the poles of a strong magnet. By Bob Burk, work supported by the National Science Foundation under grant numbers: 1246120, 1525057, and 1413739 - [1], frame at 4:26, CC BY 3.0, (Source)

    Ferromagnetism

    Only certain materials exhibit strong magnetic effects. Such materials are called ferromagnetic, after the Latin word for iron, ferrum. Ferromagnetism is more complicated and more rare than the other two types of magnetism. It is this rarity that makes it such a novelty when we first discover it! Ferromagnetic materials have the properties they do because of both the behavior of the electrons within an atom and the ways the the atoms align with each other. Because this property partly depends on the alignment of the atoms with each other, it is possible to use heating, cooling, or applying an external magnetic field to change the properties of a ferromagnetic substance to become either more magnetic or less magnetic.

    Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic effects. A group of materials made from the alloys of the rare earth elements are also used as strong and permanent magnets; a popular one is neodymium. Other materials exhibit the weak magnetic effects we have already discussed, which are detectable only with sensitive instruments. Not only do ferromagnetic materials respond strongly to magnets (the way iron is attracted to magnets), they can also be magnetized themselves—that is, they can be induced to be magnetic or made into permanent magnets.

    drawing, as described in the caption.
    Figure \(\PageIndex{2}\): An unmagnetized piece of iron is placed between two magnets, heated, and then cooled, or simply tapped when cold. The iron becomes a permanent magnet with the poles aligned as shown: its south pole is adjacent to the north pole of the original magnet, and its north pole is adjacent to the south pole of the original magnet. Note that there are attractive forces between the magnets.

    When a magnet is brought near a previously unmagnetized ferromagnetic material, it causes local magnetization of the material with unlike poles closest, as in Figure \(\PageIndex{2}\). (This results in the attraction of the previously unmagnetized material to the magnet.) What happens on a microscopic scale is illustrated in Figure \(\PageIndex{3}\). The regions within the material called domains act like small bar magnets. Within domains, the poles of individual atoms are aligned. Each atom acts like a tiny bar magnet. Domains are small and randomly oriented in an unmagnetized ferromagnetic object. In response to an external magnetic field, the domains may grow to millimeter size, aligning themselves as shown in Figure \(\PageIndex{2}\)(b). This induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped in the presence of other magnets.

    drawing, as described in the caption.
    Figure \(\PageIndex{3}\): (a) An unmagnetized piece of iron (or other ferromagnetic material) has randomly oriented domains. (b) When magnetized by an external field, the domains show greater alignment, and some grow at the expense of others. Individual atoms are aligned within domains; each atom acts like a tiny bar magnet.

    Conversely, a permanent magnet can be demagnetized by hard blows or by heating it in the absence of another magnet. Increased thermal motion at higher temperature can disrupt and randomize the orientation and the size of the domains. There is a well-defined temperature for ferromagnetic materials, which is called the Curie temperature, above which they cannot be magnetized. The Curie temperature for iron is 1043 K \(\left(770^{\circ} \mathrm{C}\right)\), which is well above room temperature. There are several elements and alloys that have Curie temperatures much lower than room temperature and are ferromagnetic only below those temperatures.

    Section Summary

    • Diamagnetic materials are repelled by magnetic fields and Paramagnetic materials are weakly attracted to them.
    • Ferromagnetic materials, such as iron, are those that exhibit strong magnetic effects.
    • The atoms in ferromagnetic materials act like small magnets (due to currents within the atoms) and can be aligned, usually in millimeter-sized regions called domains.
    • Domains can grow and align on a larger scale, producing permanent magnets. Such a material is magnetized, or induced to be magnetic.
    • Above a material’s Curie temperature, thermal agitation destroys the alignment of atoms, and ferromagnetism disappears.

    Glossary

    diamagnetic
    materials with only unpaired electrons that exhibit a weak magnetic repulsion to a magnetic field
    paramagnetic
    materials with unpaired electrons that exhibit weak magnetic attraction to a magnetic field
    ferromagnetic
    materials, such as iron, cobalt, nickel, and gadolinium, that exhibit strong magnetic effects
    magnetized
    to be turned into a magnet; to be induced to be magnetic
    domains
    regions within a material that behave like small bar magnets
    Curie temperature
    the temperature above which a ferromagnetic material cannot be magnetized
    magnetic monopoles
    an isolated magnetic pole; a south pole without a north pole, or vice versa (no magnetic monopole has ever been observed)

    This page titled 8.3: Natural Magnetism is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Jamie MacArthur.

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