63
votes
Why does the Earth even have a magnetic field?
The crucial part is that earth's outer core is fluid, and that it's conductive. That the material happens to be iron which we know as ferromagnetic is actually rather unimportant, because the ...
- 15k
26
votes
Accepted
Why does the Earth even have a magnetic field?
The core of the Earth isn't a giant bar magnet in the sense that the underlying principles are different. A bar magnet gets its magnetic field from ferromagnetism while Earth's magnetic field is due ...
23
votes
Would a compass with unmagnetized needle work?
For an unmagnetized iron needle to align with an external magnetic field, the field would need to be able to induce a magnetization in the needle. This is definitely possible with a large enough field....
- 479
23
votes
Why do little chips break off so easily from strong neodymium magnets?
The Neodymium magnets that you use are not actually made from solid chunks of metal, but are rather made by compressing a large amount of powder into blocks through a process called sintering. This is ...
- 11.5k
14
votes
Accepted
What is so special about spontaneous symmetry breaking? (time reversal example)
Indeed, one of the definitions of spontaneous symmetry breaking is in terms of its susceptibility:
Suppose we add a symmetry breaking perturbation $h \; \delta H$ to our
Hamiltonian (as you do), ...
- 9,311
13
votes
Accepted
Area in hysteresis loop = energy lost?
Consider the simple ferromagnetic system shown below. The instantaneous power consumed by the system at any time $t$ is simply:
$$p(t) = v(t)i(t)$$
Where $v(t)$ and $i(t)$ are simply the voltage and ...
- 3,877
9
votes
Accepted
Why Curie temperature is bigger for smaller lattice in 2D Ising model
Why numerical simulation gives seemingly wrong result?
The exact expression for $T_c$ is given by
$$T_c=\frac{2J}{\ln\left(\sqrt{2}+1\right)}\approx 2.269\,J,$$
where we assume that $k_B=1$. This ...
- 2,704
8
votes
Accepted
Microcanonical ensemble, ergodicity and symmetry breaking
First, I strongly advise you to read sections 24 and 25 of Tolman's excellent book on statistical mechanics. My answer will mainly go along the lines of what is in the book.
The ergodic hypothesis ...
- 1,510
8
votes
Would a compass with unmagnetized needle work?
A magnetic dipole would be induced in the iron bar and the iron bar would try and align itself along the magnetic field lines because of the torque applied on it by the interaction of the induced ...
- 99.9k
8
votes
Accepted
I've read that spontaneous symmetry breaking is similar to what happens in ferromagnetism, but are they similar mathematically?
Spontaneous symmetry breaking isn't defined to refer only to what the Higgs field does; it's a much more general phenomenon, of which ferromagnetism is an example. In fact, both examples result from ...
- 25.4k
8
votes
Accepted
Metals and Magnets
Newton's Third Law tells us that if object A exerts a force on object B then object B will exert an equal and opposite force on object A (this is required by the principle of conservation of momentum)....
- 60.5k
7
votes
Accepted
Free energy functions are analytic or non-analytic in phase transitions?
Landau free energy is just an approximation to the real free energy in the thermodynamic limit. For that reason, Landau free energy can be analytic, while the real one is not. Let me show how the ...
- 1,208
7
votes
Why do spinning electrons generate magnetic dipole moments?
Currently our best model of the electron is that it is a point particle. Measurements give an upper bound of $10^{-22}$ meters on its radius. A point particle has no radius, and thus cannot rotate. ...
- 2,785
7
votes
Why do little chips break off so easily from strong neodymium magnets?
These rare earth magnets are very brittle as you said.
From Wikipedia:
There are two types: neodymium magnets and samarium–cobalt magnets. Rare-earth magnets are extremely brittle and also vulnerable ...
- 13.8k
7
votes
Metals and Magnets
To enlarge slightly upon gandalf61's answer, when a piece of ferromagnetic metal is placed near a magnet, it (temporarily) becomes a magnet too, and so the two magnets then attract each other.
- 97.8k
6
votes
Accepted
Is the boundary between ferromagnetism and paramagnetism well-defined?
It turns out that if you work out the physics behind the magnetism of a material, the magnetisation $M$ obeys a transcendental equation of the form
$$M=\tanh\left(\frac{\alpha M+\gamma H}{T}\right)$$
...
- 9,463
6
votes
Accepted
Does a crystalline ferromagnetic solid break the rotational symmetry twice?
It seems like you are confusing spatial rotation symmetry and spin rotation symmetry. They are different symmetries (and both are present in nature).
In terms of group theory we will say that spin ...
- 1,510
6
votes
Why classical mechanics is not able to explain the net magnetization in ferromagnets?
Often, a statement like in the question is linked to the Bohr-van Leeuwen theorem. However, such a theorem does not say that classical electromagnetism can't explain any form of magnetism in materials....
6
votes
Accepted
Low temperature behavior for ferromagnets: theoretical and experimental discrepancies
Start from the definition of $\tanh x$ in terms of exponentials
\begin{align}
\tanh x &= \frac{e^x -e^{-x}}{e^x +e^{-x}}\\
&= \frac{1 - e^{-2x}}{1 + e^{-2x}}\\
&=(1 - e^{-2x})\left(1 - e^{-...
- 9,460
5
votes
Free energy functions are analytic or non-analytic in phase transitions?
The Wikipedia page is being sloppy. They mean that the free energy density is an analytic function of the mean-field order parameter, whereas at a thermal phase transition the free energy density is ...
- 49.4k
5
votes
Accepted
How to understand the difference of spin wave excitation for ferromagnetism (quadratic dispersion) and anti-ferromagnetism (linear dispersion)?
There is a classical picture of spin waves in ferromagnets that answers your first question. Think first of a single spin in an external magnetic field, say along the $z$-direction. We know what ...
- 2,032
5
votes
Why are there gaps between two lines of iron filings in a magnetic field?
The magnetic field originating from a bar magnet is continuously decreasing when moving away from the bar magnet. It does not have "lines" of stronger field strength. The lines that are ...
- 1,149
5
votes
What does the $B$-field refer to?
$B$ is the magnetic field. In the Lorentz force law $\vec{F}=q(\vec{E} + \vec{v}\times \vec{B})$, which tells you how charges respond to electric and magnetic fields, the field that appears is $B$, ...
- 55.4k
5
votes
Low temperature behavior for ferromagnets: theoretical and experimental discrepancies
For the first question you can rewrite $\tanh$ in terms of exponentials, see e.g. this Math.SE post for details. For the second question, yes, $M(0)$ is the magnetization in the zero temperature limit,...
- 2,749
5
votes
What is the difference between paramagnetic and ferromagnetic?
The definitions of the two kinds of material is the following:
A paramagnetic material (in short paramagnet) is not magnetized when there is no external field. However, when put in an external field ...
- 12.2k
5
votes
Accepted
Are there any "induced" forces which are repulsive in nature?
Yes, diamagnetic materials are repelled from the source of magnetic field (pushed in direction of field decrease). For example, bismuth, copper and even water at normal temperatures are diamagnets (...
- 41.4k
4
votes
Free energy functions are analytic or non-analytic in phase transitions?
I'll provide one possible answer to your question. I use the language of the Ising ferromagnet/paramagnet phase transition, but this is just for convenience as the same holds much more generally. ...
- 10.7k
4
votes
Accepted
Why does dipole-dipole interaction not lead to ferromagnetic ordering?
The magnetic dipole dipole interaction is just by far too weak. In Stephen Blundell: "Magnetism in Condensed Matter" this is estimated to be on an energy scale of about $10^{-23}$J which may play a ...
- 1,178
4
votes
What is the reason of hysteresis loss?
Hysteresis is a kind of non-time-reversible transition, so it is
associated with entropy (which must, in theory, generate heat).
Because magnetic materials have crystal structure, the internal
...
- 10.2k
Only top scored, non community-wiki answers of a minimum length are eligible