Ferromagnetism

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Ferromagnetism refers to the magnetic moment Due to their interaction, they are arranged roughly in the same direction in some areas magnetic field intensity When increasing, the degree of directional arrangement of the magnetic moment in these regions will increase to a certain limit value.

Chinese name: Ferromagnetism
Foreign name: ferromagnetism

Discipline: power
Discovery date: 1907
Principle: adjacent atom Or ion Interaction

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Ferromagnetism

Ferromagnetism refers to the magnetic state of a material with spontaneous magnetization Phenomenon. The magnetism of transition metals (such as iron) and their alloys and compounds is called ferromagnetism.^[1]

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Inside ferromagnetic materials, like paramagnetic materials, there are many unpaired electrons. because Exchange action (exchange interaction), these electronic spin Tends to be in the same direction as the spin of adjacent unpaired electrons. because Ferromagnetic material There are many internal divisions Magnetic domain Although the spins of all electrons inside the magnetic domain will be arranged in one direction, resulting in "saturation magnetic moment", the direction and size of magnetic moment are different between magnetic domains. Therefore, it has not been magnetization The net magnetic moment and magnetization vector of ferromagnetic materials are equal to zero.

Assuming an external magnetic field is applied, the magnetic moments of these magnetic domains also tend to be in the same direction as the external magnetic field, thus forming a possibly quite strong magnetization vector and its induced magnetic field. As the external magnetic field increases, the magnetization will also increase until“ Saturation point ”, net magnetic moment Is equal to saturation magnetic moment. At this time, the magnetization will not be changed by increasing the external magnetic field. It is assumed that the magnetization will also decrease when the external magnetic field is weakened. But it will not be the same as the previous magnetization for the same external magnetic field. Magnetization The relationship with external magnetic field is not One-to-one correspondence Relationship. The curve of magnetization compared with external magnetic field forms Hysteresis loop 。

Assuming that after reaching the saturation point again, the external magnetic field is removed, the ferromagnetic material can still retain some magnetization state, and the net magnetic moment and magnetization vector are not equal to zero. Therefore, after magnetization treatment Ferromagnetic material It has "spontaneous magnetic moment".^[1]

Discoverer

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The founder of ferromagnetic theory, French physicist P E. Weisi In 1907, he proposed the phenomenological theory of ferromagnetic phenomena. He assumed that Ferromagnet There is a strong "molecular field" inside, which can make the inside spontaneously magnetization ； Spontaneous magnetization The small area of is called magnetic domain, and the magnetization of each magnetic domain reaches magnetic saturation. Experiments show that magnetic domains magnetic moment Electrogenic Spin magnetic moment 。 1928 W K. Heisenberg first used quantum mechanics The spontaneous motion of ferromagnet is calculated Magnetization Weiss's "molecular field" is explained by quantum mechanics. In 1930, F. Bloch proposed Spin wave Theory. Heisenberg and Bloch's ferromagnetism theory believes that ferromagnetism comes from mismatched electron spin The direct exchange effect of.

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I.e Magnetic domain Inside each atom The unpaired electron spins of. Therefore, the magnetism in the magnetic domain is very strong, but the whole material may not reflect strong magnetism, because the magnetic orientation of different magnetic domains may be randomly arranged. If we add a tiny magnetic field , such as solenoid The magnetic field will make the originally randomly arranged magnetic domains have the same orientation. At this time, we say that the material is magnetization 。 Material is magnetization After that, you will get a strong magnetic field, which is electromagnet The physical principle of. When adding magnetic field After removal, the material will still have some magnetic field left, or the material "remembers" the history of their magnetization. This phenomenon is called remanence , so-called Permanent magnet After being magnetized, the remanence is very large.

When the temperature is very high, the magnetism will disappear due to the enhancement of irregular thermal movement. This critical temperature is called Curie temperature (Curietemperature)。

If we investigate Ferromagnetic material Under external magnetic field Mechanical response It will be found that in the direction of the external magnetic field, the length of the material will change slightly. This property is called Magnetostriction (magnetostriction)。

Hysteresis loop

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Fig. 1 Hysteresis Loop

and Ferroelectrics Similarly, the magnetization of ferromagnetic materials is similar to that of external magnetic field Nonlinear relation 。 This relationship is a closed curve, which is called hysteresis loop (as shown in Figure 1). Generally speaking, the magnetization M or magnetic induction B of ferromagnets and other strong magnetic substances is not the same as that of magnetic field H Single valued function It depends on the history of its magnetic state. Starting from H=M=B=0, when the magnetization curve is from OABC to C, the magnetization tends to be saturated, which is recorded as Ms. If the magnetic field is reduced, M deviates from H starting from B Initial magnetization curve The change of M lags behind that of H. When H decreases to zero, M is not zero, but equal to Remanent magnetization Mr。 In order to make M zero, a reverse magnetic field is required, that is, the magnetic coercive field Hc. When the reverse magnetic field continues to increase to - Hs, the magnetization M will be magnetized to - Ms in the opposite direction. The curve BDEGB is the hysteresis loop.^[2-3]

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Spontaneous magnetization of ferromagnetic materials:

Although ferromagnetic phenomena were discovered very early, the essential causes and laws of these phenomena were only recognized at the beginning of the last century. In 1907, French scientist Weiss systematically put forward the ferromagnetic hypothesis, which mainly includes: there is a strong "molecular field" inside the ferromagnetic material, Atomic magnetic moment Tend to be arranged in the same direction and parallel, i.e Spontaneous magnetization To saturation, called spontaneous magnetization; The spontaneous magnetization of ferromagnet is divided into several small areas (such small areas where the spontaneous magnetization reaches saturation are called magnetic domains). Since the magnetization directions of each area (magnetic domains) are different, their magnetism counteracts each other, so the large ferromagnet does not show magnetism externally.

Weiss's hypothesis has achieved great success. Experiments have proved its correctness, and on this basis, modern ferromagnetic theory has been developed. Based on the molecular field hypothesis Spontaneous magnetization (spontaneous magnetization) theory, which explains the essence of ferromagnetism; Based on the magnetic domain hypothesis Technical magnetization The theory explains the behavior of ferromagnet in magnetic field. iron magnetic material The magnetism of is spontaneous. The so-called magnetization process (also known as magnetic induction or magnetization) is just to show the magnetism of the material itself, rather than the process of providing magnetism to the material from the outside. Experiments prove that, Ferromagnetic The root of spontaneous magnetization is atoms（ Positive ion ）The magnetic moment, and what plays a major role in the atomic magnetic moment is Electron spin magnetic moment 。 And atoms Paramagnetism Similarly, in atomic Electron shell The existence of a state that is not filled with electrons is a necessary condition for ferromagnetism. For example, there are four vacancies in the 3d state of iron, three vacancies in the 3d state of cobalt, and two vacancies in the 3d state of nickel. If the filled electron spin magnetic moment is aligned in the same direction, a larger magnetic moment will be obtained. Theoretically, iron has 4 μ B, cobalt has 3 μ B, and nickel has 2 μ B.

However, for other transition group elements, such as manganese, there are five vacancies in the 3d state. If they are aligned in the same direction, their spin magnetic moment should be 5 μ B, but it is not a ferromagnetic element. Therefore, the existence of a state (d or f state) in an atom that is not filled with electrons produces ferromagnetism necessary condition , but not a sufficient condition. Therefore, ferromagnetism is not only generated by whether the atomic magnetic moment of the element is high, but also by considering whether the bonding between atoms is beneficial to the formation of ferromagnetism when forming crystals. This is the second condition for the formation of ferromagnetism.

According to the bonding theory, when atoms are close to each other to form molecules, Electronic cloud They should overlap each other, and the electrons should exchange with each other. For transition metals, the energy difference between the 3d state of the atom and the s state is not large, so their electron clouds will also overlap, causing the redistribution of electrons in the s and d states. This exchange produces a Exchange energy Eex (related to exchange integral), this exchange energy may make the spin magnetic moment of the d layer in the adjacent atoms to be aligned in the same direction. Quantum mechanical calculations show that when Magnetic material The electron exchange integral of the internal adjacent atoms is positive (A>0), and the magnetic moments of the adjacent atoms will be aligned in the same direction and parallel, thus realizing Spontaneous magnetization 。 This is the reason of ferromagnetism. The essence of this electron exchange effect of adjacent atoms is still that electrostatic force forces the electron spin magnetic moments to be arranged in parallel, and the effect is like a strong magnetic field. Weiss molecular field is so named. Theoretical calculation proves that the exchange integral A is not only related to the electron motion state And strongly depends on the wave function of Nucleus Distance between Rab（ lattice constant )。 Only when the ratio of the distance between atomic nuclei Rab and the distance between the electron participating in the exchange action and the nucleus (electron shell radius) r is greater than 3 can the exchange integral be positive. Iron, cobalt, nickel and some rare earth elements meet Spontaneous magnetization Conditions. Chromium and manganese A are negative and not ferromagnetic metals, but iron can be obtained by changing their lattice constants through alloying so that the Rab/r ratio is greater than 3 Magnetic alloy 。

To sum up, the conditions for ferromagnetism are: ① there should be an unfilled electron shell inside the atom; ② And Rab/r ratio is greater than 3, so that the exchange integral A is positive. The former means that the atomic intrinsic magnetic moment is not zero; The latter refers to a certain crystal structure.

According to the process and theory of spontaneous magnetization, many ferromagnetic properties can be explained. For example, the influence of temperature on ferromagnetism. When the temperature rises, the atomic spacing increases, reducing the exchange effect, while the thermal motion constantly destroys the regular orientation of the atomic magnetic moment, so Spontaneous magnetization Ms drops. Until the temperature is higher than Curie point So that the regular orientation of the atomic magnetic moment is completely destroyed, the spontaneous magnetic moment does not exist, and the material changes from ferromagnetism to paramagnetism. Similarly, it can be explained Magnetocrystalline anisotropy , magnetostriction, etc.^[4]

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As the first choice for making 2D magnets, chromium triiodide has three important characteristics: first, chromium triiodide crystals contain many layers, which are separated from each other like "transparent tape", and 2D layered structure is easy to obtain; Secondly, the compound is a ferromagnetic material, in which the spin direction of electrons is uniform and can produce permanent magnetism like refrigerator magnets; Finally, chromium triiodide also has anisotropy, which makes its internal electrons spin in a direction perpendicular to the crystal surface.

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Only four metallic elements are ferromagnetic above room temperature, namely iron, cobalt, nickel and gadolinium. At extremely low temperatures, there are five elements that are ferromagnetic, namely terbium, dysprosium, holmium, erbium and thulium. And praseodymium and neodymium of fcc. Curie temperature: iron 768 ℃, cobalt 1070 ℃, nickel 376 ℃, gadolinium 20 ℃

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