

Bacteria make nanomagnets for navigating the oceans  - prat
http://www.newscientist.com/article/dn17585-bacteria-make-nanomagnets-for-navigating-the-oceans.html?DCMP=OTC-rss&nsref=genetics

======
ars
I would assume it's not actually magnets, but rather particles that can detect
magnetic fields, because how would they magnetize it?

~~~
moeffju
The text explains how they create magnetite (by oxidizing iron).

Also: How would you detect magnetic fields without magnetic material - i.e.
magnets?

~~~
ars
No, I mean something that is actually magnetic, i.e. has a magnetic field.

Soft iron is affected by magnetic fields, but does not create a magnetic field
of its own.

~~~
mechanical_fish
_Soft iron is affected by magnetic fields, but does not create a magnetic
field of its own._

Well... yes and no.

Iron is _ferromagnetic_. If you look carefully at this word, and know a little
Latin and/or a little chemistry, you will see that it was basically invented
to describe iron. (Although nickel, cobalt, and other elements and compounds
are also ferromagnetic.)

Individual iron atoms have a magnetic moment, which is to say: they are tiny
magnets. They both create a magnetic field and respond to other magnetic
fields. And when iron crystallizes the atoms which are next to each other tend
to align their magnetic fields.

However, the typical iron-containing crystal has a large number of individual
magnetic _domains_. Within a domain all the tiny magnetic iron atoms are
aligned with each other, but each domain is randomly oriented with respect to
the others. Thus, the tiny magnets tend to cancel out, and outside the iron we
don't see it creating a net magnetic field. However, if you _apply_ a
(nonuniform) magnetic field to the iron it gets attracted, because the
individual atoms do respond to magnetic fields even if they aren't all in
line. [1]

If you apply a magnetic field to the iron -- preferably a strong one --
possibly together with a bit of heat or some banging with a hammer, you can
encourage its magnetic domains to realign themselves and produce a piece of
iron that has a net magnetic moment. Now you have a magnet.

This is what the bacteria are doing. Like most biological things, they are
capable of taking iron apart to its component atoms and then putting those
atoms together again, so this isn't especially astonishing behavior.

The point of the oxidation, to create magnetite, is that a magnetite crystal
bonds the iron particularly tightly, improving the odds that the individual
atoms will retain their orientation. As you know, a rod of pure iron isn't
very stable: It tends to turn into iron oxide all by itself, then flake apart.
We call that _rusting_.

\--

[1] Of course, in the real world pieces of iron do tend to have a net magnetic
moment... because they crystallize on the Earth, and the whole Earth has a
magnetic field. But the Earth's field isn't very strong, so the orientation
effect isn't very strong either.

~~~
ars
> If you apply a magnetic field to the iron -- preferably a strong one .....
> produce .... net magnetic moment.

> This is what the bacteria are doing. Like most biological things, they are
> capable of taking iron apart to its component atoms and then putting those
> atoms together again, so this isn't especially astonishing behavior.

That's what I was saying - how are the bacteria doing this? They don't have
access to a strong magnetic field (except the earth).

Are you suggesting they are aligning the magnetic moments of the iron atom by
atom? Is that really possible?

~~~
mechanical_fish
_They don't have access to a strong magnetic field (except the earth)._

When you assemble ferromagnetic atoms together one at a time they tend to line
up with each other, even if the applied field is weak to nonexistent. This is
why the orientation of the atoms in an iron bar aren't random from one _atom_
to the next: instead they form "domains" where all the atoms point in one
direction.

If you are building a tiny enough iron bar it's probably pretty easy to have
the whole thing come out as one domain. And bacteria aren't very big, so the
bars of magnetite that they're building inside themselves are pretty small.

 _Are you suggesting they are aligning the magnetic moments of the iron atom
by atom? Is that really possible?_

Absolutely. This is how all crystal growth works. One atom at a time. You may
not be used to thinking about chemistry in this way, but that is what
chemistry is.

Of course, you shouldn't think of the bacteria the way you think of human
model-makers, consciously assembling tinkertoys. That kind of tedious fiddling
is for people like us, who are long on understanding but short on design time.
The well-designed bacterium uses chemistry instead: Metaphorically, you throw
properly shaped tinkertoys into a bucket and shake it around and they assemble
themselves. ;)

In fact, if you take a liquid and chill it below the freezing point, what
usually happens is that crystals start to form in more than one place, and
then those crystals expand atom by atom until they bump into each other and
fuse. For example, this is the reason why there is more than one magnetic
domain in the typical iron bar. If you grew your iron bar under perfect
temperature-controlled conditions from a single seed crystal, you might be
able to grow an entire bar with one domain. That is what the bacteria are
doing, but they get to cheat because the bars they are growing are so small,
so they get to quit while they are ahead. :) If you try to grow a meter-long,
centimeter-thick bar like this, you'll find that you've got trouble with
temperature control, trouble with impurities, trouble with rust (as I said),
and that because you're operating at a temperature above absolute zero there's
always some randomness going on, trying to scramble your work.

Having said that, it might be possible for humans to do this. We do similar
things with silicon every day. The chips inside your computer were cut from a
single crystal of silicon 25 cm in diameter and perhaps 1 meter long:

<http://en.wikipedia.org/wiki/Boule_(crystal)>

<http://en.wikipedia.org/wiki/Single_crystal>

But understanding the detailed atomic behavior of ferromagnetic materials is
actually quite a hard problem. Lots of difficult thermodynamics. Amusing
chaotic systems problems. I basically hit the wall in my graduate course on
the subject. ;)

