By
`
`
`
`
`
`
`
`Webphysics.iupui.edu/webscience/physics_archive/magneticmaterials.himl
`
`»
`
`4 "What is Physics Good For?"
`
`Extra credit is available at the end of this page. Please respond before 9 AM, Monday, October 234, 2000.
`
`Magnetic Materials
`
`For the next several weeks. we will be studying magnetic fields, and the forces that charged particles and currents feel as a result of
`these fields. However, we don't have enough time to cover magnetic materials in any detail. This is too bad for at least two reasons:
`
`* Magnetic materials are an important technology that impacts our daily lives.
`® Your instructor's research is in this area, and he is particularly fond ofit.
`
`This page will provide a VERY BRIEF introduction to the main types of magnetic materials, and to some of their uses.
`
`First, a technical point. All materials are magentic in the sense that they are affected by magnetic fields
`in some way. However, the extent to which they are affected varies widely. There are three categories
`that describe howa material is affected by a magnetic field. There are others, but these are the most
`basic.
`
`1. Diamagnetism: Materials such as copper, lead, quartz, water, acetone, and carbon dioxide are
`diamagnetic. These materials are very weakly affected by magnetic fields. To the extent that they
`are affected, they become magnetically polarized in the opposite direction from the magnetic
`
`field. If the magneticfield is not uniform, they feel a force away from the higher field region.
`
`Diamagnetism results from the the effects of magneticfields on all ofthe electrons in the material.
`Thus, all materials are diamagnetic. However, the other forms ofmagnetism are stronger than diamagnetism, so the
`diamagnetism can usually be ignored unlessit is the only magnetism present.
`
`2. Paramagnetism: Materials such as sodium, oxygen. iron oxide (FeO or Fe)03). and platinum are paramagnetic. They are
`affected somewhat more strongly than diamagnetic materials, they become polarized parallel to a magnetic field. Thus, in a
`nonuniform magnetic field, they feel a force towards the higher field region.
`
`Paramagnetism results from the magnetic forces on unpaired electrons. You mayrecall from a chemistry course that electrons
`move around atoms in "orbitals" and that a maximum of two electrons can go into each orbital. Electrons that are alone in an
`orbital are said to be unpaired.
`
`3. Ferromagnetic: Materials such as iron, nickel, gadolinium,iron oxide (Fe;04), Manganese Bismuth (MnBi), and Cobalt
`Ferrite (CoFe)O04) are ferromagnetic. These materials are very strongly affected by magnetic fields. They become strongly
`polarized in the direction of the magnetic field, thus. they are strongly attracted to the high field region when the field isn't
`uniform. Furthermore, they retain their polarization after the magnetic field is removed. Once polarized, ferromagnetic
`materials produce magnetic fields of thew own. Since these fields are usually not uniform (particularly near the ends of the
`piece) ferromagnetic materials are capable of attracting each other. All of the materials that you are used to calling "magnets"
`are ferromagnetic materials.
`
`Ferromagnetism results from the interactions among the electrons in the material. This is why a ferromagnet can remain
`magnetically polarized even if there is no magnetic field applied to it from the outside.
`
`It should be no surprise that most applications of magnetic materials call for ferromagnetic materials. These are
`the ones that interact most strongly with magnetic fields. Within this category there are several important
`subcategories. These have to do with howeasily the magnetic polarization (magnetization) of the material can be
`changed. In general, the more difficult it is to magnetize a material, the more that material will retain its
`magnetization. This property is known as "magnetic hardness."
`
`
`
`For instance, in a (magnetically) hard material the magnetization is very difficult to change. However, the
`magnetized material will retain almost all of its magnetization once the magnetizing field is removed. Then. the
`material can used in this condition. Hard materials are ideal for use as "permanent magnets." Permanent magnets
`Liquid oxygen
`are used in many ways, the most familiar is probably the "fridge" magnet, which is often used to hold shopping
`©
`I
`:
`7
`i
`between the poles
`lists and children's art to the refrigerator door. Permanent mangets are also used in magnetic catches (e.g., to
`of a horseshoe
`keep the fridge door closed), in electric motors (there's another one in the fridge), in electric generators (which
`magnet.
`U_Ilinois-Urbana Power the fridge), in anti-lock brakes, in electric guitar pick-ups, and in thousands of other common and unusual
`devices.
`
`Ferromagnets that are very easily magnetized, but which give up most of their magnetization when
`removed from the magnetic field are known as soft magnetic materials. These materials are used in
`magnetic field sensors (to read the data on magnetic tape and disks, or in antilock brakes), and in magnetic
`shielding. They are also used in electric transformers, which we will learn about when westudy
`
`
`
`1
`
`APPLE 1032
`
`APPLE 1032
`
`1
`
`

`

`aalternating current ciruits.
`
`Between very hard and very soft materials, are those that can have their magnetization change with
`moderate difficulty, and which retain some or most of their magnetization. These materials are most
`frequently used as recording media. Both analog (cassette tapes, VHS movies) and digital recording
`
`(hard and floppy disks, DAT drives, digital video, etc.) use these materials.FromJean-ellEnterprises
`
`
`
`
`
`‘You can get a lot more information about this subject on the internet. Here are a few search engines
`
`1. Alta Vista
`
`2.Google
`
`3. Northern Light
`
`4. Ask Jeeves
`
`5. Infoseek
`
`And here are a few good links to get you started.
`
`
`
`What is the origin ofthe term "Lodestone"?
`Nameal! ofthe elements that are ferromagnetic
`What is "coereivity"?
`How are magnets used in anti-lock brakes?
`
`
`
`‘You may change your mind as often as you wish. When youare satisfied with your responses click the SUBMIT button.
`
`
`This site is made possible by funding from the National Science Foundation (DUE-9981111).
`©2001 A. Gavrin and G. Novak, all rights reserved.
`
`2
`
`