edited by GEORGE L. GILBERT Denisan University Granville. Ohio 43023
Ferrimagnetism SUBMI-
BY
Kerro Knox princeton Unlversltg Prlncaton, NJ 08544 CmCKED BY Ronald Strothkamp uofstra unlveraity Hempstead, NJ 11550 That unpaired electrons in an ion, such as Fe3+ or the or NO, lead to hypothetical Hg+, or a molecule, such as 0% paramagnetism is a common topic in elementary chemistry. Paramagnetism is then said to be experimentally observed by the fact that substances exhibiting i t are attracted by a magnet. Allvery well, hut the order of magnitude of the force of attraction is not usually made clear. If a student tries to pick up a solid, known to be paramagnetic, such as ferric ammonium sulfate, with an ordinary alnico magnet, he or she finds that no solid particles cling to the faces (except possibly some very fine pieces a t the corners, where the field eradieut is highest). That magnet will erab an iron nail ienaciously, b& not iron ions in-a compou~d. Substances can be divided into three broad classes based on their magnetic behavior': diamagnetic, paramagnetic, and ferromagnetic, with subclasses of antiferromagnetic and ferrimagnet;. Diamagnetic substances have ali electron spins paired, and so have no net magnetic moment: they are repelled by a magnetic field, but the force is extremely weak and can only he measured with strong magnets and quite sensitive apparatus. Paramagnetic substances have some unpaired electron spins isolated on an ion or a molecule, which leads to a net magnetic moment on the species. The direction of alignment of the moment localized on one species is independent of the moments on the other species in the compound, however, so that under the influence of thermal agitation they all point in random directions, and a macroscopic sample of the compound does not havea spontaneous magnetic moment. Under t he intluence of a magnetic field, however, a magnetization is induced in a paramagnetic substance by introducing some net alignment into the collection of magnetic mome&. The order-of alignment is by no means perfect, and i t disappears if the magnetic field is removed. I t is made more random by increasing temperature (Curie's law), but i t does result in attraction by the magnetic field, as measured by the magnetic susceptibility of the substance. The force involved is three orders of magnitude or so greater than that in diamagnetism, so the substance is attracted by a magnet, but only weakly in terms of our everyday experience with a nail. For large pieces, the total volume cannot all get into the space with a high magnetic field gradient near the edges of the pole faces of a magnet. Gravitational pull is then enough to break them away, and even a strong alnico magnet cannot pick them up. In order to get strong attraction by a magnet, the localized magnetic moments on the individual species of a substance
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lntraduction ell wilw Kiiel.. C. ~~~.~ ...- to .-SnlirlState Phvdcc ..,-.--, 6th --., ",. N-w Yo*, 1986: Chapters 14 and 15. Wells, A. F. StructurallnorganicChemist~y,5th ed.: Oxford, 1984; pp 551ff and 593ff. ~
have to he ordered in one direction with respect to each other, even in the absence of a field. If the exchange interaction makes this alignment parallel, so that all moments are in the same direction. a verv. large mametic - s~ontaneous . moment results in the whole substance, which is the iector sum of all the individual moments (the macerial isnot itselfa permanent magnet because of a domain structure in the solid-but that is a subject for more detailed study). This ordering occurs only in the solid state, and it is the kind present in the ferromagnetic elements, iron, cobalt, nickel, dysprosium, and gadolinium, and some alloys and intermetallic compounds. The force is about three orders of magnitude greater than paramagnetism, and so an iron nail is easily picked up by a magnet against the force of gravity. Another kind of coupling occurs in solids con&ning species with individual magnetic moments; the interaction between neighboring species, usually through other adjacent ions, aligns their mutnents antiparallel. In the simplest cases, such as MnF,. half the snlns are in one direction and half in the other, so-that a decrease in magnetic susceptihility is observed: i t is called "antiferromagnetic". Above a certain temperature, called the NEel temperature, the coupling- of the individual moments can he overcome hv thermal energy, and the material behaves like an ~ r d i n a r ~ d h x d e r e d paramagnetic phase. (For a ferromametic solid. that temperature is called the Curie temperakre.) Even more interesting cases occur when the coupling is antiparallel but the number of moments, or the size of the moments, or both, is not equal. Then a net magnetization results because the moments in one direction do not cancel those in the other, and the substance is strongly attracted by a magnet, like a ferromametic solid. This t ". w e of material is called "ferrimagnet>, because it is exemplified by many mixed oxides containing iron and other cations. for which the nonsvstematic nome&lature "ferrites" is used. They are the basis of the ceramic magnets which are so wrevalent nowadavs. One of the fe'rimagnetic materi& is the long.kno& mineral, FesO,, commonly called "magnetite" for that verv reason. I t is a compound in its own right having its own structure, with iron in two different oxidation states. and not a mixture as suggested by its Berzelius formula, 'Fe0.Fez0a (no more than potassium sulfate is K20.SOs). I t has the inverse spinel structure2. Spinel is the mineral, MgA1204, whose structure consists of nearly cuhic-close-packed oxygen atoms with magnesium ions in tetrahedral interstices and aluminums in octahedral. Many mixed oxides of general formula, ABzOa, adopt this structure, while many others
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[llltff111111/11s'2
OCTAHEDRAL INTERSTLCES TETFfAHEDRAL INTERSTICES
Figure 1. The ordering of lhe magnetic moments on the ions in one unit cell of the inverse spinel structure of FeaOl. The octahedral and tenahedral Fe3+ moments cancel each ather, leaving a spoManecus magnetic moment arising Imm the Fez+ions latter Kittei'). : Volume 66
Number 4
April 1989
337
the visible light. This easy electron motion also makes magnetite a reasonably good electrical conductor, which is an undesirable property for practical applications in, for example, transformer cores. There other ferrites are used.
The Absorption of UV Light by Ozone SUBMlTTm BY Edward Koubek U S . Naval Academy Annapolis, YO 21401
James 0. Glanvllle Virginia Tech Blacksburg, VA 24061
Figure 2. The effectofa magnet on slurries of Fe30,(leR)andof Fe20. as seen on the ovehead projector.
have the inverse spinel structure with half of the B ions in the tetrahedral holes, B(AB)O4. Magnetite is then properly written as Fen1(Fe11Fe11')04.At room temperature the compound is ferrimagnetic', with the moments in the tetrahedral interstices ordered antioarallel to those in the octahedral; see Figure 1. There results a permanent magnetic moment due to the iron(I1) . . ions. since the iron(II1) moments cancel. It is possible in a quite simple lecture demonstration to synthesize FesOa right before the students' eyes and to show its interaction with a magnetic field, comparing it to a paramagnetic material. For a sizable synthesis (which can be scaled up for very large classes), the following solutions are prepared: For mixture A: 100 mL of 0.1M Fe3+ (ferric ammonium sulfate) 50 mL of 0.1 M Fez+(ferrous ammonium sulfate) 250 mL of distilled water For mixture B: 150 mL of 0.1M Fe3+ 250 mL of distilled water Mixtures A and B are made on the lecture bench, and then to each is added 75 mL of 1.0 M ammonium hydroxide. Mixture A gives a black precipitate of hydrous FeaOa, while B gives the hydrous red-brown hematite, Fez03. A slurry of each is poured into a Petri dish, and the pair is placed side by side on an overhead projector; they appear as opaque, black circles. A small, strong permanent magnet is placed against the outside of each Petri dish. Soon a light spot develops in the dish with magnetite, shown in Figure 2, as the solid is attracted to the magnet and moves through the liquid phase. Clumps of solid are seen to accumulate near the pole faces, and streams of the oxide are seen to go toward them. After a while no more solid moves, hut the light spot shows the depleted region near the pole faces quite clearly. The movement seen on the screen as the solid moves and the clear space develops is more dramatic than the static end result shown in Figure 2. No action is observed in the dish containing the merely paramagnetic F e 2 0 ~ Two further properties of the ferrimagnetic material can be disrussed. The intense hlack color. comnared to the redbrown hematite, is characteristic of'a mixed valent compound; very strong and broad charge-transfer hands due to easy electron hopping between Fez+ and Fezf ions absorb all
938
Journal of Chemical Education
With all the recent attention eiven to the ozone denletion problem in the popular press, tlhe following demonsbation mav he of interest to vour students. It remesents an extension of a demonstraiion published eariier by King and Templer.' Using a low-pressure mercury vapor UV lamp2 and a recentlv laundered white cotton sheet as a hackmound, one can produce a shadowgraph of ozone emerging frim an &onator.3 The shadowgraph clearly demonstrates the UV-ahsorhing properties of ozone as may be seen in the figure. This demonstration is interesting in that it can also lead to a discussion of fluorescence and the reason for addine a whitening agent to a detergent. Another interestine demonstration with this annaratus involved mercury vapor. A slight squeeze of a plastic bottle containine liauid mercurv oroduces a " ~ u f f "of mercurv vapor as ashadowgraph. a his is an excellent demonstration to use to introduce the topic of atomic absorption. Caution: The toxicity of 0 3 and Hg is well documented. Therefore these demonstrations should be carried out in well-ventilated areas. Care also should he taken with UV light to prevent eye damage.
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'King, L. C.; Templer. A. D. Tested Demonstrations. 6th ed.; Division of Chemical Education: Easton, PA, 1965; p 219. A Model UVS-54. 254 mm Minerallamp available from UkraViolet hoducts. Inc.. San Gabriel. CA, is ideally suited for this. This lamp uses a low-pressure mercury source. We use a homemade device similar to that described by Worstell. R. A. J. Chem. Educ. 1932, 9, 291.
Shadowgraph of ozone demonstrating its UV-absorbing properties
