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I never thought that modern American nickels actually contained nickel anymore. However, according to this wiki article, the coins actually do contain 25% nickel, the rest being copper. And yet, no US coin produced today is officially magnetic. Why is this alloy of nickel not attracted to a magnet?

(and yes, I tried time and again to find and answer elsewhere online.)

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  • $\begingroup$ RE: "... And yet, No US coin is officially magnetic. ..." The 1943 Lincoln penny was steel coated zinc due to a copper shortage during WWII. It is ferromagnetic. There were a few copper blanks that got pressed in 1943 and these specimens are very valuable. A common scam was to plate a steel/zinc penny with copper. Such an altered coin is easily detected with a magnet. $\endgroup$ – MaxW Apr 2 at 0:46
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    $\begingroup$ Be aware that even chrome-nickel stainless steel ( typically 16-20% Cr, 8_12% Ni, rest mostly iron ) is not ferromagnetic ( magnetic in common terms ), even if it contains mostly ferromagnetic metals as Fe and Ni. Because Ni keeps iron in the austenitic non ferromagnetic form. $\endgroup$ – Poutnik Apr 2 at 2:44
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    $\begingroup$ @Poutnik So why are other Fe-Ni alloys, such as permalloy (which have ~45 - 85% Ni), ferromagnetic? Is it becauses the Fe is not austenitic in permalloy? Or because there's enough Ni to overcome the austentic Fe? $\endgroup$ – theorist Apr 2 at 5:26
  • $\begingroup$ @theorist Probably the latter. But I am not expert in metallurgy, relying on googled info and I do as well remember typical CrNi steel is not ferromagnetic. $\endgroup$ – Poutnik Apr 2 at 5:30
  • $\begingroup$ @Poutnik I think it is indeed the latter -- through my own googling, I just found that permalloy has an FCC (i.e., austenitic) crystal structure. Interestingly, I also found that austenitic alloys can be partly converted to ferromagnetic forms (martensitic and ferritic) using cold working (the austentic crystal structure -- which Fe favors in the presence of Ni -- can be restored by melting and recooling). $\endgroup$ – theorist Apr 2 at 5:41
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There are many types of magnetic properties, including ferromagnetism, paramagnetism, diamagnetism, antiferromagnetism, ferrimagnetism, superparamagnetism, metamagnetism, spin glasses, and helimagnetism. Many of these are too weak to cause any noticeable interaction with a magnet. The type of everyday magnetism you're thinking of, which nickel has, is ferromagnetism.

While nickel is ferromagnetic, copper is not. As you said, the American nickel is currently 25% nickel and 75% copper. According to this paper (from 1931!), in order for a nickel-copper alloy to be ferrogmagnetic, it must contain at least 56% nickel:

... 56 percent nickel is required before the alloy shows ferromagnetic properties at ordinary temperatures.

https://journals.aps.org/pr/abstract/10.1103/PhysRev.38.828

E. H. Williams, Magnetic Properties of Copper-Nickel Alloys. Phys. Rev. 38, 828 (1931).

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  • $\begingroup$ So, according to your source, American nickels are only paramagnetic? $\endgroup$ – mpprogram6771 Apr 2 at 0:24
  • $\begingroup$ @mpprogram6771 Yes, that's what's it's saying (at around room temp). $\endgroup$ – theorist Apr 2 at 0:29
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    $\begingroup$ So is it possible to make a ferromagnetic alloy with two or more non-ferromagnetic metals in the right combination? $\endgroup$ – mpprogram6771 Apr 2 at 13:31
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    $\begingroup$ Yes. For instance, Heusler compounds, like Cu-Mn-Al, are composed of non-ferromagnetic metals that, together, can be ferromagnetic. See: en.wikipedia.org/wiki/Heusler_compound $\endgroup$ – theorist Apr 2 at 15:09
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    $\begingroup$ @rcollyer I have added it to the list :). $\endgroup$ – theorist Apr 4 at 3:51
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None of the US coins are magnetic (ferromagnetic), except for the 1943 Lincoln penny (Steel Cents, made in steel and zinc to save copper for ammunition during wartime), which are considered magnetic. Almost all of those coins other than Steel Cents are made with higher percentage of copper ($\ce{Cu}$) and lower percentages of other metals such as nickel ($\ce{Ni}$), zinc ($\ce{Zn}$), etc. (mostly $\ce{Ni}$). For example, current 5-Cent US coin (US Nickel) is made of 75% $\ce{Cu}$ and 25% $\ce{Ni}$, as OP noted in the question. You can find some percentages in US coins in this AZO Materials Article.

Pure $\ce{Ni}$ is magnetic at room temperature while $\ce{Cu}$ is not. Yet, why US Nickel is not magnetic?

The magnetic properties of $\ce{Cu/Ni}$ alloys have been studied by several researchers (e.g., Ref.1 and 2), perhaps because their use in alloys goes back at least two thousand years, at the time, the knowledge of the composition of alloy was unknown (Note: the elemental nickel was only discovered relatively late on). According to this research, pure $\ce{Ni}$ is magnetic with its Curie temperature being around $\pu{527 K}$ (Curie temperature or Curie point is where a metal loses its ferromagnetism with elevating temperature). When $\ce{Ni}$ mixes with $\ce{Cu}$ to make an alloy, this Curie temperature decreases with increasing amount of $\ce{Cu}$ (Ref.3):

The Cu-Ni equilibrium diagram

Apparently, the Curie point has become $\pu{0 ^\circ C}$ or lower for $\ce{Cu/Ni}$ alloys with compositions 67:33 or lower $\ce{Ni}$ amount. Thus, it is safe to say alloys with less than 65% $\ce{Ni}$ in $\ce{Cu/Ni}$ alloys do not show magnetic properties at room temperature. Therefore, the US Nickel with 25% $\ce{Ni}$ would not act on permanent magnets.

However, Kaufmann and Starr (Ref.2) have shown that these alloys with less amount of $\ce{Ni}$ would gain their their magnetism at way lower temperatures (about $\pu{14-77 K}$).

References:

  1. E. H. Williams, "Magnetic Properties of Copper-Nickel Alloys," Phys. Rev. 1931, 38(4), 828 (DOI: https://doi.org/10.1103/PhysRev.38.828).
  2. A. R. Kaufmann, C. Starr, "Magnetic Properties of Solid Solutions. III. The Paramagnetic Alloys of Copper and Nickel," Phys. Rev. 1943, 63(11-12), 445 (DOI: https://doi.org/10.1103/PhysRev.63.445).
  3. E. A. Brandes, G. B. Brook, In Smithells Metals Reference Book, Seventh Edition; Butterworth-Heinemann: Woburn, MA, United States, 1992, Chapter 11: Equilibrium Diagrams, pp. 11-236.
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    $\begingroup$ Nice graph ; Shows why Monel ( 67 Ni , 33 Cu, + little Mn, Si ,etc) is sometimes magnetic and sometime not at room temp. $\endgroup$ – blacksmith37 Apr 3 at 1:13
  • $\begingroup$ Why is there no eutectic point? $\endgroup$ – Vladimir F Apr 3 at 9:00
  • $\begingroup$ so you can freeze a nickel and it will become ferromagnetic? $\endgroup$ – Jim Apr 3 at 13:13
  • $\begingroup$ @Jim: That't the idea (and theory). Even any gas can be magnetic near absolute zero, I read. See ref.2. $\endgroup$ – Mathew Mahindaratne Apr 3 at 13:54
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    $\begingroup$ No eutectic or anything else, just mutual solubility ( as shown in the diagram). $\endgroup$ – blacksmith37 Apr 16 at 21:00
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To the best of my knowledge they are not magnetic to make it very hard to profitably counterfeit our coins. If they are magnetic the vending machines will remove them with a magnet. This is the explanation I was given in the mid 1960's. Yes machines in our town had the magnets, I tried one.

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  • $\begingroup$ Why would be important for the government, how the vending machines work? $\endgroup$ – peterh - Reinstate Monica Apr 16 at 6:23
  • $\begingroup$ Same reason the bills are very hard to duplicate and pass inspection. The easier it is to spot counterfeit money the less chance it will be counterfeited. Same for vendors, if they get a counterfeit coin or bill it is there loss, the government does not buy them back. $\endgroup$ – Gil Apr 17 at 19:09

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