Magnetism and magnetic moment
All things in the world have magnetism, from the tables and chairs around us to the planets and the sun in the universe. No matter what state they are in (crystalline, amorphous, liquid or gas), high or low temperature, high or low pressure, they all have magnetism. The difference is that some substances have strong magnetism, while others have weak magnetism. However, it can be said that substances without magnetism do not exist. Substances can be classified into five categories based on their characteristics in an external magnetic field: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances. What causes all substances to have magnetism? What are the reasons why different substances have the above different characteristics? This starts with the basis of matter - atoms. Matter is composed of atoms, and atoms are composed of atomic nuclei and electrons. In atoms, electrons have orbital magnetic moments due to their motion around the atomic nucleus; Electrons have spin magnetic moments due to their spin, and the magnetic moments of atoms mainly come from the orbital and spin magnetic moments of electrons, which are the source of all material magnetism. The magnetic moment of an atomic nucleus is only 1/1836.5 of that of an electron, so the magnetic moment of an atomic nucleus is generally ignored
Magnetic moment of a single isolated atom
Magnetic moment is a directional vector. The spin mode of electrons in atoms can be divided into two types: up and down. In most substances, there are as many electrons with up and down spin, and the magnetic moments they produce cancel each other out. The entire atom has no magnetism to the outside world. Only a few substance atoms have different numbers of electrons in different spin directions. Therefore, after the magnetic moments of electrons with opposite spins cancel each other out, there are still some electrons whose spin magnetic moments are cancelled out, and the entire atom has a total magnetic moment. The magnetic moment of a single atom depends on its atomic structure, namely the arrangement and number of electrons. All atoms of elements in the periodic table have their own magnetic moments. The magnetic moment of atoms in a crystal we discussed above is the magnetic moment of a single atom, but in solid crystals or non crystals, atoms are located at crystal nodes and are affected by the nuclear electric field and electron electrostatic field of neighboring atoms. Therefore, the magnetic moment of atoms in a crystal is different from that of a single isolated atom. For example, iron, cobalt, and nickel are known as 3D transition metals. In a crystal, some atoms' electrons become public electrons of adjacent atoms, causing a change in the electronic structure of the atoms. Some orbital magnetic moments are frozen, leaving only the spin magnetic moment contributing to the atomic magnetic moment in the crystal. As a result, the magnetic moment of the atoms in the crystal deviates from the theoretical value. We already know from the previous content that all things in the universe have magnetism, and magnetism mainly originates from atomic magnetism. Due to the different magnetic moments of different atoms, the interaction between atomic magnetic moments in macroscopic substances is caused. The arrangement of atomic magnetic moments at room temperature is different. We divide macroscopic substances into paramagnetic substances, diamagnetic substances, ferromagnetic substances, sub ferromagnetic substances, and antiferromagnetic substances according to their magnetic properties, including the following three characteristics. 1. The macroscopic magnetism of a substance is contributed by the magnetic moments of its constituent atoms or molecules. We refer to the total magnetic moment of a material per unit volume as its magnetization, denoted by M and measured in A/m. If the volume of a substance is V, it has n atoms, and the magnetic moment of each atom is μ J, then M=μ J1+μ J2+...+μ Jn, that is, M=∑ μ J/v. 2. The magnetization curve (M~H curve) of magnetization intensity: When the external magnetic field is zero, the atomic magnetic moments may be randomly arranged. However, when we apply a non-zero external magnetic field, each atomic magnetic moment can turn in the direction of the external magnetic field, and the magnetization intensity M of the substance changes. The relationship curve between the magnetization intensity M and the external magnetic field H is called the magnetization curve, abbreviated as the M~H magnetization curve. The magnetization curves of different substances are also different.
3. Magnetic susceptibility x2
On the M~H magnetization curve, the ratio of M to H at any point is called the magnetic susceptibility, represented by chi. χ=M/H , The unit of M is A/m, The unit of H is also A/m, so it is relative magnetic susceptibility and has no unit. We use the size and arrangement of atomic magnetic moments, the shape of the M~H magnetization curve, and parameters such as magnetic susceptibility to describe the magnetism of substances and classify them.
Paramagnetic substances are substances that can magnetize according to the direction of the magnetic field when they are moved close to it, but they are 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 its lack of magnetism. Such as aluminum, oxygen, etc. Each atom in paramagnetic materials has a magnetic moment, which gives paramagnetic materials an inherent atomic magnetic moment; There is no interaction between adjacent atoms in paramagnetic materials, so at room temperature, the atomic magnetic moments are randomly arranged, and the projection value of the atomic magnetic moment μ J in any direction is zero. When subjected to an external magnetic field H, the atomic magnetic moment of such substances can only rotate a very small angle along the direction of the external magnetic field, and their magnetization strength slowly increases with the increase of the external magnetic field. Its magnetic susceptibility is greater than 0, with a value generally ranging from 10-5 to 10-3. In order to align the atomic magnetic moments of paramagnetic substances completely in the direction of the external magnetic field, it is estimated that an external magnetic field strength of 109-1010 A/m is required, which is currently difficult to achieve with artificial magnetic fields. Antimagnetic substances are substances with negative magnetic susceptibility, which means that the direction of the magnetic field after magnetization is opposite to the direction of the external magnetic field. All organic compounds have diamagnetism, such as graphite, lead, water, etc. The projection of the atomic orbital magnetic moment and spin magnetic moment of diamagnetic substances in a magnetic field is zero, which means that diamagnetic substances do not have a net atomic magnetic moment. However, under the action of an external magnetic field, the electronic orbital will generate an induced additional magnetic moment, and this induced magnetic moment is opposite in direction to the external magnetic field, resulting in negative magnetism. The magnetization direction of diamagnetic substances is negative, opposite to the external magnetic field, and its absolute value increases linearly with the increase of the external magnetic field. A ferromagnetic substance is a substance that can maintain its magnetic state even when the external magnetic field disappears after being magnetized by an external magnetic field. So far, there are 83 metal elements discovered, of which 4 are ferromagnetic elements above room temperature, namely iron, cobalt, nickel, and gadolinium; At extremely low temperatures, there are five elements that can transform into ferromagnetic elements, namely terbium, dysprosium, holmium, erbium, and thulium. In ferromagnetic materials, atoms have inherent atomic magnetic moments, and some electrons are shared. The spin magnetic moments of adjacent atoms are arranged parallel to each other in the same direction (also known as spontaneous magnetization). The M~H magnetization curve of ferromagnetic materials is nonlinear, and the magnetic susceptibility x varies with the magnetic field. The magnetic susceptibility x of ferromagnetic materials is very large, reaching up to 105~107. Antiferromagnetic substance
It does not generate a magnetic field, and this substance is relatively uncommon. New antiferromagnetic substances are still being discovered. Most antiferromagnetic materials only exist at low temperatures, and assuming the temperature exceeds a certain value, they usually become paramagnetic. For example, chromium, manganese, etc. all have antiferromagnetic properties. Atoms in antiferromagnetic materials also have inherent atomic magnetic moments, with some electrons being shared, but adjacent atoms having opposite magnetic moments (also known as antiferromagnetic ordering). The M~H magnetization curve of ferromagnetic materials is linear, with a magnetization rate of χ>0 and a value of about 10-4~10-5, which is very small and a constant. This means that when antiferromagnetic materials are magnetized in an external magnetic field, their atomic magnetic moment changes very little with the external magnetic field, similar to paramagnetic materials, and belongs to weak magnetism. The magnetic susceptibility of antiferromagnetic materials varies with temperature, as shown in the figure below, where Tn is referred to as the Niel temperature. The macroscopic magnetism of ferromagnetic materials is the same as that of ferromagnetism, except that their magnetic susceptibility is lower (with a susceptibility of 102~105). Typical ferromagnetic materials, such as ferrites, differ most significantly from ferromagnetic materials in their internal magnetic structure (arrangement of magnetic moments). The atomic magnetic moments of ferromagnetic materials are not zero, and there is indirect exchange or RKKY exchange between adjacent atomic magnetic moments, which causes the atomic magnetic moments of adjacent sublattices to be arranged in reverse parallel, but the atomic magnetic moments of adjacent sublattices are of different sizes (as shown in the figure above). This phenomenon is also known as ferromagnetic ordering or ferromagnetic spontaneous magnetization. The M~H magnetization curve of ferromagnetic materials is nonlinear, similar to ferromagnetic materials, except for a slightly lower magnetic susceptibility, but still belongs to strong magnetism.