What Materials Are Magnetic? | Hacker News Reader

What Materials Are Magnetic?

sciencenotes.org

75 pointsrwmj42 comments

I reckon holmium deserved a mention, why is it missing?

To quote Wiki:

"Holmium, like all of the lanthanides, is paramagnetic at standard temperature and pressure. However, holmium is ferromagnetic at temperatures below 19 K (−254.2 °C; −425.5 °F). It has the highest magnetic moment (10.6 μB) of any naturally occurring element and possesses other unusual magnetic properties. When combined with yttrium, it forms highly magnetic compounds."

Very cool, the site's entire 'materials' archives is really interesting as a layman in Materials Science. [0]

This kind of reminded of the sense of wonder of reading through Encarta entries in the late 90's and how I assumed the internet would look like in the future.

[0] https://sciencenotes.org/category/chemistry/materials/

That link lists the Piezoelectric Effect near the top. I wondered if there's a corollary for magnetism. I googled the word Piezomagnetic on a whim and was satisfied to find this: https://en.wikipedia.org/wiki/Piezomagnetism

It is quite good. They touch on complex topics but remain understandable and provide sources for further reading. I did not see anything obviously suspect, which is rare in vulgarisation.

Their example of nickel is a U. S. five-cent coin. Nickel is ferromagnetic, but the coin is 75% copper and a magnet will not pick it up.

Interestingly, the Canadian coins are the same dimensions, but are magnetic. US vending machines used to have a magnet that would deflect Canadian coins into the coin return slot.

When I was growing up near Detroit, merchants took US and Canadian coins interchangeably, and gave both back out as change. But the Post Office was strict about it. I managed to collect a set of King George coins -- at least the smaller ones.

And some Canadian coins used be actual sterling, until they went to 80%ag. I think US coins were always 90.

A few nickels from the early 1940s were, I think, 43% Ag, due to wartime metal shortages.

But man, if you ever find a 1943 copper wheat penny in good to fair condition, you can think about retiring. The 1943 steel penny goes for about < $2.00 on average.

But they can be picked up with right kind of electromagnetic arrangement[0].

[0]. https://youtu.be/7ZeBWJLRXqM&t=1231

I have a shrunken nickel from these folks:

https://www.capturedlightning.com/frames/interesting1.html

Don't know if the copper figures into the shrinking...

Tungsten carbide is magnetic. I bought a "tungsten" cube from some shoddy Amazon vendor. It's a lot denser than steel, but not as dense as pure tungsten. It is magnetic. The density of the cube is in the range of tungsten carbide.

> Tungsten carbide is magnetic

Pure tungsten carbide is not. You just got some magnetic contaminants in it.

This has is a good test for fake "gold" that can consist of tungsten core with a thin film of gold on top. It's pretty much impossible to get pure tungsten, so such "gold" ends up being magnetic.

I got one of those too, and it came with a manifest showing that there is about 5% of nickel in the composition so not too surprising.

I would certainly be surprised if they could manufacture a pure cube (> 99.9%) for the price they sell it at :-)

What are you using the cubes for?

The most common use for tungsten cubes is as a kind of spooky ornament. Encountering a 4 inch cube that weighs over forty pounds is pretty weird.

The glue to hold the sintered carbide powder is cobalt or iron or whatever pot-metal crap they have on hand for cheap blocks. That would be where the magnetism is coming from. If it's highly magnetic you probably have little tungsten and a lot of binder.

It's probably not tungsten carbide but rather a common tungsten nickel alloy

At the microscopic level, what determines whether a given set of valence elections has an excess spin, and thus an excess magnetic moment, is still determined by an empirical set of rules, called Hund's rules, instead of from first principles, iirc:

https://en.wikipedia.org/wiki/Hund%27s_rules

https://hackaday.com/2022/09/01/iron-nitrides-powerful-magne...

Iron Nitrides

I think I remember reading, somewhere, that liquid hydrogen is magnetic.

Nah. Hydrogen under enough pressure, becomes metallic. I don’t think it’s magnetic, though.

Good question. The hydrogen atom does have unpaired electrons, but the H2 molecule does not and is diamagnetic. “Standard” solid hydrogen is a molecular solid so it is diamagnetic as well. Liquid H2 is also molecular and should be diamagnetic as well.

I am not really familiar with metallic hydrogen but it would have metallic bonds and delocalised electrons, which can only be the 1s electrons. So the hydrogen atoms would not have any localised electron left, so no (orbital) magnetic moment.

So yeah, I don’t think hydrogen is magnetic in any of its pure forms.

It's the oxygen when in liquid form.

So aluminum and copper are listed as not magnetic yet powerful magnets are used to separate them from non metallic materials in recycling? Seems like a semantic argument.

https://en.wikipedia.org/wiki/Eddy_current_separator

> Seems like a semantic argument.

That kind of phrasing usually indicates someone doesn't understand or is rejecting a meaningful and useful distinction. Is that what you intended to do here?

The article seems to be pretty clear about how there are several different types of magnetism that can be exhibited by materials, and appropriately classifies various materials. Nothing about the existence or mechanism of eddy current separators is a counterexample to anything in the article.

"Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other."

Aluminum is repelled by a strong magnetic field, hence the use of ECS, plastic is not. Aluminum is not Ferro-magentic it does however seem to be magnetic depending on the definition.

https://en.wikipedia.org/wiki/Magnetism

The article itself on contradictory showing aluminium as nonmagnetic then classifying as paramagnetic, which is a type of magentisim, which is it?

From the article: "Usually, when people talk about “magnetic metals,” they’re talking about ferromagnetic and ferrimagnetic metals."

Ferromagnetic and paramagnetic materials behave differently. One sticks to a magnet, the other is slowed by a magnet.

So yes they are both magnetic, thanks.

I'm not a kid, but resemble one when I get a hold of a copper tube and strong neodymium magnet. Probably my favorite toy. I'm kind of glad I have only a kid's understanding of it.

Any electrically conductive material is affected by eddy currents.

Liquid oxygen is magnetic too (I think para in this case)

There are very specific about including 'metals' in every statement. Yep, paramagnetic non metals are: Oxygen (commonly shown on youtube), Nitric Oxide, Boron, Sulfur and some graphites (very slight) due to delocalised electrons.

That's mentioned at the bottom: "However, liquid oxygen and boron fullerenes (B80) are paramagnetic."

Wonder if you could use that to separate gasses.

I also wonder about "sorting" atoms/molecules like Maxwell's demon:

https://en.wikipedia.org/wiki/Maxwell%27s_demon

It's probably possible, but probably is less practical than other methods.

Most interesting sentence:

> 304 stainless steel isn’t normally magnetic, but it becomes partially ferromagnetic if it’s bent at room temperature.

Anyone have a lay-person explanation for this?

Iron exists in many different crystal structures in different kinds of steels. Some of these forms are magnetic, others are not. When you work the metal, such as by bending it, you can cause it to change from one structure to another.

304 is an austenitic stainless steel, and austenite is non-magnetic. But when it's cold worked, it turns into martensite, which is harder and magnetic. If you anneal it by heating it and letting it cool, it turns back into austenite.

Interesting. Do you know if it affects it's anti-rust properties after you work it / make it martensite?

Martensite is less corrosion-resistant. But note that slight working like bending it a bit will only turn a small portion of it into martensite, most of it will remain austenitic.

Not a physicist, but I’d guess that bending the metal causes it to heat up, which in turn could make it easier for electrons to align and respond to magnetic waves.

Nope, heating would make it non-magnetic again.

It's the act of physically working it that makes it change structure.

you're only right about not being a physicist. sorry, had to have a laugh. What made you draw that conclusion?

My smile of course :)

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