I have a question regarding the magnetization of an object. Let's say this object has magnetic moments pointing in random directions, such that the total magnetization is null. If we apply an external magnetic field, the magnetic dipoles will align themselves with the field to cause a magnetization field. This magnetization field is in the same direction as the applied field, and thus causes an increase in the total magnetic field strength inside. My question is: if the dipoles always want to point in opposite directions, e.g. north to south, why would the dipoles align themselves with the field, and not against it? In electric polarization the negative side of the dipole is attracted to the positive side of the field, this decreasing the total $\mathbf{E}$ field strength inside the material. Why is this not the case with magnetic fields?
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$\begingroup$ Shouldn't this have something to do with the fact that there is no magnetic monopole, i.e, magnetic field is solenoidal, i.e., $0=\nabla\cdot \vec B\neq \nabla\cdot \vec D=\rho_{f}$? $\endgroup$– NewbieJan 28, 2022 at 16:23
1 Answer
If we apply an external magnetic field, the magnetic dipoles will align themselves ... in the same direction as the applied field, describes the behavior of ferromagnetic materials.
the dipoles always want to point in opposite directions describes the behavior of antiferromagnetic materials.
Antiferromagnetism has to do with the process during solidification of magnetic materials from the liquid phase. At the beginning of solidification, neighboring atoms permanently influence each other and form magnetic domains, i.e. permanent magnets. Since this occurs simultaneously and independently, these spatial alignments of these domains are random.
It is clear that a permanent magnetization of such Weissian domains by an external magnetic field is difficult.
Ferromagnetism has to do with the existence of unpaired electrons in atoms.
Electrons are the smallest detectable permanent magnets (Electron magnetic moment) and tend to be aligned in pairs in the atom (Pauli Exclusion Principle). Atoms with an odd number of electrons therefore also have a magnetic moment.
The alignment of these magnetic moments under the influence of an external field succeeds much easier than in the crystalline domains of antiferromagnets.
In electric polarization the negative side of the dipole is attracted to the positive side of the field,... Why is this not the case with magnetic fields?
Magnets are always diople, breaking the dipole into two monople is impossible. Therefore, macrosocopic magnetic fields do not result from the separation of charges but exclusively from the common alignment.
