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Why Does The Magnet Lose Its Magnetism When It Is Left For A Long Time

- Jul 26, 2018 -

The magnetism of the magnet is not so easy to eliminate, so it is more likely that the magnet will suddenly change from the state of magnetization to the external structure after a sudden stimulation by the external field or other mechanical and thermal stimuli. But in addition to this sudden change, there is still an entropy effect, I think, perhaps the meaning behind you is actually to know if the entropy effect dominates. For ferromagnetic materials, it must be emphasized that when discussing the problem of ferromagnetism, we do not take into account the entropy part, because ferromagnetics also contain longer-range correlations, stronger spins between each other. Function, so it is more likely that the magnet is only in a frozen state, and not like the liquid crystal, which ultimately dominates the entire phase transition by orientation. There are many different types of magnetic materials, but since they exhibit magnetic properties, there is a tendency to have some orientation within them. Today we know that this is mainly caused by the orbital angular momentum of electrons (and sometimes the nucleus), the spin angular momentum, and so on. After the material is magnetized, some wonderful changes have taken place. Intuitively, it is magnetic. Looking at it carefully, the material is a disordered state (all directions are the same, this is isotropic, this case Can be imagined as a ball), now there is some directionality (there is some special direction, this is anisotropy, this situation is similar to a stick), obviously the symmetry of the ball is more abundant than the symmetry of the stick Then, the process of magnetization is the reduction of symmetry, which is usually called symmetry breaking. Generally speaking, when this state of magnetization is used, the energy of the system will be relatively low (or even very low), but this is only one aspect of the problem. Think about the expression of free energy: F=U-TS, we It is hoped that the free energy will decrease, although it is important to consider the energy reduction, but if the temperature is gradually increased, the increase of the system entropy can also reduce the free energy. The increase in entropy corresponds to an increase in the number of states in the microscopic state, that is, an increase in the number of states in the orientation can result in a decrease in free energy. In short, when the temperature is relatively high, the magnet can allow more kinds of orientations, which is one of the reasons that may cause the magnetic weakening. But this effect is very different for different magnetic materials. When the interaction strength between magnetic moments is strong enough (U dominates), the effect of entropy effect (TS) may actually be very weak. In fact, this is also the case. In general, this part of the effect of entropy may not be the main factor in the problem of the disappearance of magnet magnetism in what we usually call normal temperature.

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