Magnetic Moment
The generation of the magnetic field can be divided into two aspects: one is based on the moving current (electromagnetic induction), and the other is based on the spin of the basic particles of matter. The first is the magnetic effect of current that we are more familiar with, which is the magnetic field generated by the directional movement of free electrons after the wire is energized. The second is the magnetic field generated by the matter itself.
Matter is made of atoms, and atoms are made of nuclei and electrons. In an atom, electrons have an orbital magnetic moment due to their movement around the nucleus; electrons have a spin magnetic moment due to their spin, and the magnetic moment of an atom is mainly derived from the magnetic moment of the electron, which is the source of all material magnetism.
Magnetic moment of a single isolated atom
Magnetic moment is a vector with direction. The spin modes of electrons in atoms are divided into two types: up and down. In most substances, there are as many electrons with up spins and down spins. The magnetic moments generated by them will cancel each other out, and the whole atom is not magnetic to the outside. The number of electrons in different spin directions in only a few material atoms is different. In this way, after the magnetic moments of the electrons with opposite spins cancel each other out, the spin magnetic moments of the remaining part of the electrons are not canceled, and the whole atom has a total magnetic moment. The magnetic moment of a single atom depends on the atomic structure, and atoms of all elements in the periodic table have their own magnetic moments.
magnetic moments of atoms in a crystal
Above we discussed the magnetic moment of a single atom, but in solid crystal or amorphous, the atoms are on the crystal nodes, these atoms will be affected by the nuclear electric field and electron electrostatic field of the adjacent atoms, so the magnetic moment of the atoms in the crystal Moments are not the same as single isolated atoms. For example, iron, cobalt, and nickel, they are called 3d transition metals. In the crystal, the electrons of some atoms will become publicized electrons of adjacent atoms, the electronic structure of the atoms will change, and some orbital magnetic moments will be frozen. At this time, only the spin magnetic moment is left to contribute to the magnetic moment of the atoms in the crystal, so the magnetic moment of the atoms in the crystal is different from the theoretical value.
Magnetic moment of macroscopic matter
Because the magnetic moments of different atoms are different, resulting in the interaction between the atomic magnetic moments of the macroscopic substances, the arrangement of the atomic magnetic moments at room temperature is different. According to the magnetic properties of the macroscopic substances, we divide them into paramagnetic substances, diamagnetic substances, Ferromagnetic substances, ferrimagnetic substances and antiferromagnetic substances, these characteristics include the following three.
1. Magnetization M
The macroscopic magnetism of a substance is contributed by the magnetic moment of the atoms or molecules that make it up. We call the total magnetic moment of a material per unit volume V the magnetization of the material, represented by M, and the unit is A/m.
Suppose the volume of a certain substance is V, it has n atoms, and the magnetic moment of each atom is μJ, then M=μJ1+μJ2+…+μJn , that is, M=∑μJ /V
2. Magnetization curve of magnetization intensity (M~H curve)
When the external magnetic field is zero, the atomic magnetic moments may be arranged chaotically, but when we apply a non-zero external magnetic field, under the action of the external magnetic field, each atomic magnetic moment can turn to the direction of the external magnetic field. When the magnetization M of the substance changes. The relationship curve of the magnetization M with the change of the external magnetic field H is called the magnetization curve, which is abbreviated as the M~H magnetization curve. The magnetization curves of different substances are also different.
3. Magnetic susceptibility χ
On the M~H magnetization curve, the ratio of M to H at any point is called the magnetic susceptibility, expressed by χ. , where the unit of M is A/m, and the unit of H is also A/m, so it is the relative magnetic susceptibility, which has no unit.
We use the size and arrangement of the atomic magnetic moments, the shape of the M~H magnetization curve, and the magnetic susceptibility parameters to describe the magnetism of the substance and classify the substance.
Paramagnetic substance: It is a kind of substance that can be magnetized according to the direction of the magnetic field when it is moved close to the magnetic field, but it is very weak and can only be measured with precision instruments; if the external magnetic field is removed, the internal magnetic field will also return to zero, resulting in It is not magnetic. Such as aluminum, oxygen and so on. Each atom of a paramagnetic substance has a magnetic moment, which makes the paramagnetic substance have an inherent atomic magnetic moment; in addition, there is no interaction between adjacent atoms of the paramagnetic substance, so at room temperature, the atomic magnetic moments are arranged chaotically, and the atoms The projected value of the magnetic moment μJ in any direction is zero. When there is an external magnetic field H, the atomic magnetic moment of this kind of substance can only turn a very small angle along the direction of the external magnetic field, and its magnetization increases slowly with the increase of the external magnetic field. Its magnetic susceptibility χ>0, the value of χ is generally 10-5~10-3. In order to make the atomic magnetic moments of paramagnetic substances completely align with the direction of the external magnetic field, according to a rough estimate, this requires an external magnetic field strength of 109~1010A/m, which is difficult to achieve with artificial magnetic fields.
Diamagnetic substance: It is a substance with a negative magnetic susceptibility, that is to say, the direction of the magnetic field after magnetization is opposite to the direction of the external magnetic field. All organic compounds are diamagnetic, and graphite, lead, water, etc. are diamagnetic substances. The projection of the atomic orbital magnetic moment and spin magnetic moment of the diamagnetic material in the magnetic field is zero, that is to say, the diamagnetic material has no net atomic magnetic moment, but under the action of the external magnetic field, the electron orbit will produce an induced additional Magnetic moment, and this induced magnetic moment is opposite to the direction of the external magnetic field, so negative magnetism appears. The magnetization direction of the diamagnetic material is negative, contrary to the external magnetic field, and its absolute value increases linearly with the increase of the external magnetic field.
Ferromagnetic substance: After being magnetized under the action of an external magnetic field, even if the external magnetic field disappears, it can still maintain its magnetized state with magnetic properties. Iron, cobalt, and nickel are all ferromagnetic substances.
Ferrimagnetic substance: The macroscopic magnetism is the same as ferromagnetism, but the magnetic susceptibility is lower. The typical ferrimagnetic substance is ferrite. The most significant difference between them and ferromagnetic substances is the difference in the internal magnetic structure.
Antiferromagnetic substances: do not generate a magnetic field. This kind of substance is relatively uncommon. Most antiferromagnetic substances only exist at low temperatures. Assuming the temperature exceeds a certain value, it usually becomes paramagnetic. For example, chromium, manganese, etc. are all antiferromagnetic.
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