Magnetism has long fascinated people.
Over 3,000 years ago, the Chinese discovered natural magnets in nature that could attract one another or pieces of iron. People used their rich imaginations to compare this phenomenon to a mother’s loving care for her child.
This was recorded in “Lushi Chunqiu – Jiqiuji”: “Kind stones call for iron and they are attracted.”
The compass, one of China’s four great ancient inventions, is an example of how ancient Chinese people made use of magnetism.
As we know, a magnetic stone is actually iron ore (usually magnetite Fe3O4). We also know that iron can be attracted and magnetized by a magnet.
But why do they have magnetism or become magnetized?
How is magnetism produced?
To explain the macroscopic properties of magnetism in materials, we need to start with atoms and investigate the origin of magnetism.
1. The origin of magnetism
“Structure determines properties”. Of course, magnetism is also determined by the internal structure of material atoms.
The relationship between atomic structure and magnetism can be summarized as follows:
(1) The magnetic property of an atom comes from the spin and orbital motion of electrons.
(2) The presence of unfilled electrons inside the atom is a necessary condition for the material to have magnetism.
(3) The “exchange interaction” between electrons is the fundamental reason why atoms have magnetism.
1. Generation of Electron Magnetic Moment
Atomic magnetism is the basis of magnetic materials, and atomic magnetism comes from electron magnetic moment.
The movement of electrons is the source of electron magnetic moment. Electrons have both rotational motion around the atomic nucleus and intrinsic spin motion.
Therefore, electron magnetic moment consists of two parts: orbital magnetic moment and spin magnetic moment.
According to Bohr’s atomic orbit theory, electrons inside atoms move around the atomic nucleus on a certain orbit.
The motion of electrons along the orbit corresponds to a circular current, which will produce an orbital magnetic moment accordingly.
The plane of electron orbital magnetic moment in an atom can take different directions, but in a directional magnetic field, the direction of electron orbit can only be in several fixed directions, that is, the direction of the orbit is quantized.
The origin of magnetism stems from the spin of the electron charge, which is known as the electron spin magnetic moment.
Under the action of an external magnetic field, the spin magnetic moment can only be parallel or antiparallel to the orbital magnetic moment.
In many magnetic materials, the electron spin magnetic moment is larger than the electron orbital magnetic moment.
This is because in a crystal, the direction of the electron’s orbital magnetic moment is modified by the crystal lattice field, and thus it cannot form a composite magnetic moment that projects outside the material, leading to what is commonly referred to as “quenching” or “freezing” of the orbital angular momentum and orbital magnetic moment.
Therefore, the magnetism of many solid-state materials does not primarily arise from the electron orbital magnetic moment but rather from the electron spin magnetic moment.
Of course, there is also a nuclear spin magnetic moment, but it is generally much smaller than the electron spin magnetic moment (by three orders of magnitude), so it can be ignored.
2. Atomic Magnetic Moment
In an atom, due to the Pauli exclusion principle, it is not possible for two electrons to be in the same state.
Only two electrons can be accommodated at most in an orbit, so when an orbit is filled with electrons, their spin magnetic moments will cancel out because they must have opposite spins.
To make the atom form a magnetic moment externally, there must be an unfilled electron orbit.
Of course, as we can see from examples, this is only a necessary condition. Metals such as Cu, Cr, V and many lanthanides have unfilled electron orbits, but they do not show magnetism (specifically ferromagnetism).
3. Classification of Magnetism
Before discussing the exchange interaction of electrons, let us first look at the macroscopic manifestation of material magnetism.
According to the different magnetic properties shown on a macroscopic level by superimposing the action of atomic magnetic moments, magnetic materials can be classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic.
Diamagnetism refers to the fact that when there is no magnetic field, the magnetic moment of atoms with fully filled electron shells is equal to zero, or the total magnetic moment of some molecules is zero, and it does not exhibit macroscopic magnetism.
But under the action of a magnetic field, the orbital motion of electrons will produce an additional motion, resulting in an induced magnetic moment opposite to the direction of the external magnetic field but with a very small value.
This phenomenon is called diamagnetism.
Common diamagnetic materials include Na+, K+, Ca2+, F-, Cl, etc.
Paramagnetism refers to the fact that atoms have magnetic moments that are not completely canceled out, and therefore have a total magnetic moment.
However, because the direction of atomic magnetic moments is chaotic, the external effects cancel each other out, and it does not exhibit macroscopic magnetism.
But under the action of an external magnetic field, each atomic magnetic moment is aligned more often with the direction of the magnetic field, and less often against it, which can manifest as weak magnetism at the macroscopic level. In fact, the material is magnetized in this way.
Experiments show that the higher the temperature, the lower the magnetization of paramagnetic materials. This is because thermal motion destroys the regular orientation of atomic magnetic moments.
The higher the temperature, the greater the thermal energy of the atoms, making it harder for atomic magnetic moments to align with the external magnetic field, and therefore the magnetization is lower.
Ferromagnetism refers to the phenomenon in which adjacent atoms can be aligned orderly towards the direction of an external magnetic field due to mutual interactions.
Generally, ferromagnetic materials can achieve high magnetization even in weak magnetic fields; after the external magnetic field is removed, they can still retain strong magnetism.
Why can ferromagnetic materials be magnetized to saturation even in weak magnetic fields?
This is because the internal atomic magnetic moments of these materials have already been aligned in a certain direction to a certain extent, without the action of an external magnetic field, which is commonly referred to as spontaneous magnetization.
This spontaneous magnetization is divided into small regions, and within each region, the atomic magnetic moments are parallel to each other. These small regions are called magnetic domains.
The spontaneous magnetization orientations of the various magnetic domains inside the material are different from each other, and they cancel out each other’s effects externally, so the entire material does not exhibit macroscopic magnetism.
In other words, ferromagnetic materials are composed of small “magnets” that are irregularly arranged and do not exhibit magnetism externally under statistical regularities.
However, when an external force (external magnetic field) arranges the polarity of each “small magnet” in the same direction, it exhibits strong magnetism externally.
The spontaneous magnetization of the magnetic domains inside ferromagnetic materials is an important reason for their ferromagnetism.
This explains why “atoms with unfilled electron shells” are only a necessary condition for material magnetism.
In strict sense, what we usually call magnetism should actually be ferromagnetism.
Therefore, elements such as Mn and Cr, although they also have atomic magnetic moments, do not have magnetism (ferromagnetism) internally.
Antiferromagnetism refers to the phenomenon in which, under the action of a magnetic field, adjacent atoms or ions with the same spin arrange themselves in opposite directions, causing their magnetic moments to cancel each other out, making them similar to paramagnetic materials and not exhibiting magnetism.
Ferrimagnetism is essentially antiferromagnetism where the reverse magnetic moments on two sublattices do not completely cancel out.
It is similar to ferromagnetism in that it exhibits strong magnetism, but different from ferromagnetism in that its magnetism comes from the difference between two oppositely directed and unequal magnetic moments.
Currently, many ferrites (composite oxides composed of iron and one or more metals) that have been studied belong to ferrimagnetic materials.
Ferrimagnetism and antiferromagnetism are closely related. Starting from a known antiferromagnetic structure, it can be reconfigured through element substitutions into a ferrimagnetic material that maintains the original magnetic structure but has two sublattices with unequal magnetic moments.
Ferromagnetic and ferrimagnetic materials are collectively referred to as strong magnetic materials, and they represent the main direction of development of magnetic materials.
Interaction Next, let’s take a look at how the exchange interaction of electrons affects the spin magnetic moment of electrons and thus affects the macroscopic magnetism of materials.
The exchange interaction between atoms generally refers to the electrostatic interaction caused by the mutual exchange of positions of electrons in adjacent atoms.
Specifically, when two atoms are close together, in addition to considering electron 1 moving around nucleus 1 and electron 2 moving around nucleus 2, since electrons are indistinguishable, we must also consider the possibility of exchanging the positions of the two electrons, so that electron 1 appears to be moving around nucleus 2, and electron 2 appears to be moving around nucleus 1.
For example, in a hydrogen atom, this kind of electron exchange occurs at a frequency of about 1018 times per second. The energy change caused by this exchange interaction is called the exchange energy, denoted as Eex.
In general, the energy of atomic binding can be expressed as:
Where E0 is the total energy of each atom at its ground state;
C is the energy increment resulting from the static electric Coulomb interaction between nuclei and electrons;
A is the energy increment resulting from the exchange of electrons, generally referred to as the exchange energy constant.
A depends on the degree of proximity of partially filled electron shells of neighboring atoms, and it is an energy that measures the magnitude of the exchange interaction.
Experimental evidence shows that the energy change (i.e., exchange energy Eex) caused by the exchange interaction of two electrons in a hydrogen molecule can be approximately expressed as follows:
Where Sa and Sb represent the spin quantum numbers of the two electrons. φ is the angle between the directions of the spin magnetic moments of the two electrons, and its possible range of variation is 0° to 180°.
Although the above equation is obtained from the exchange interaction between hydrogen atoms with only one electron, it has a general significance for the qualitative analysis of the exchange interaction of multi-electron atoms. Further analysis reveals that:
(1) When A>0, if φ=180°, cosφ=-1, indicating that the directions of the spin magnetic moments of the two electrons are opposite, i.e., the spin magnetic moments of electrons are arranged antiparallel, and Eex(180)=+2ASaSb; if φ=0°, indicating that the directions of the spin magnetic moments of the two electrons are the same, and the spin magnetic moments of the electrons are arranged parallel, Eex(0)=-2ASaSb.
Moreover, if 0°<φ<180°, then the spin directions of the two electrons are neither the same nor the opposite but rather separated by an angle φ, and their exchange energy Eex lies between the two, i.e., Eex(0°)<Eex<Eex(180°). According to the basic law of energy minimization being the most stable state, it can be seen that the system’s energy is minimized only when φ=0°, at which point the system is in the most stable state.
When the directions of the adjacent spin magnetic moments of the two electrons are the same, the electron spin magnetic moments are necessarily arranged parallel, giving rise to spontaneous magnetization and leading to the existence of ferromagnetism in matter.
(2) When A < 0, only when φ = 180°, the energy of the entire system is minimized, which means that the direction of electron spin is arranged in an anti-parallel way, which is antiferromagnetism.
(3) When |A| is very small, the exchange interaction between these two adjacent atoms is weak, and the exchange energy Eex is very small. When φ is around 90o, the energy is low, so the direction of the magnetic moment is chaotic, and the material is paramagnetic.
In summary, the specific properties of material magnetism depend on A, that is, the degree to which the unfilled electron shells of neighboring atoms are close to each other.
Therefore, the magnetism of materials is determined by the distribution of electrons in atoms and the crystal structure of the material.
The characteristics of magnetism make magnetic materials crucial to the development of high-tech industries, and they are an important pillar for the advancement of science and technology. They are also a highly active research area in modern technology.
Given the prominent role of magnetic materials in today’s information society, a country’s level of technological development can be reflected by its magnetic materials, and the demand for this type of material can be used to gauge a country’s economic and average living standards.
Next, we will briefly describe some common magnetic materials in everyday life.
2. Applications of Common Magnetic Materials
The term “magnetic materials” mainly refers to ferromagnetic and ferrimagnetic materials.
Based on their magnetic distribution, they can be divided into hard (permanent) magnetic materials, semi-hard magnetic materials, and soft magnetic materials.
(1) Soft Magnetic
Materials Soft magnetic materials refer to materials that are easily magnetized and demagnetized by alternating current, usually with ferrimagnetic properties.
They have some special properties:
(1) Through external magnetic field magnetization, they can have a high maximum magnetic induction intensity;
(2) Under the magnetization of an external magnetic field of a certain strength, soft magnetic materials themselves can have a higher magnetic induction intensity;
(3) The resistance to magnetic domain movement in soft magnetic materials is small.
Because of these properties, soft magnetic materials are widely used in communication, broadcasting, television, instrumentation, and modern electronic technology. They are commonly used as cores for generators and distribution transformers.
In these fields, magnetic materials are required to have a high sensitivity to changes in external magnetic fields.
If the material is difficult to magnetize or if the magnetic properties are not easily released after magnetization, it cannot meet the requirements of these applications. Soft ferrimagnetic materials are ideal for these purposes.
Therefore, soft ferrimagnetic materials are among the earliest developed, most diverse, highest yielding, and most widely used magnetic materials.
(2) Hard Magnetic Materials
Hard magnetic materials, also known as permanent magnets, can maintain strong magnetization after being magnetized and can provide a constant magnetic field to a given space for a long time without consuming electrical energy.
They are usually ferromagnetic materials. Hard magnetic materials are widely used in electric motors, generators, speakers, bearings, fasteners, and transmission devices.
The permanent magnetism of hard magnetic materials is precisely what these fields require.
For example, electric motors and generators require a magnetic body with a constant magnetic field to operate, and permanent magnets are ideal because they do not consume electrical energy to maintain their magnetic properties.
However, due to the low variability of hard magnetic materials, while they offer high stability, their range of use is limited.
(3) Semi-Hard Magnetic Materials
Semi-hard magnetic materials have properties that fall between soft magnetic materials and hard magnetic materials.
They are characterized by a stable residual magnetic induction intensity under external magnetic fields smaller than a certain value (similar to hard magnetic materials), but they also have a tendency to change their magnetization direction under reverse magnetic fields greater than a certain threshold, similar to soft magnetic materials.
Therefore, semi-hard magnetic materials are used as dynamic materials, and with the increasingly intelligent society, there is a growing demand for dynamic materials, making semi-hard magnetic materials a promising field of development.
Applications include relays, semi-fixed storage devices, and alarm devices.
Magnetic recording media is an important type of semi-hard magnetic material, which is widely used in information storage devices such as hard disks, magnetic tapes, and credit cards.
Semi-hard magnetic materials play a vital role in these applications because of their dynamic properties.
Taking hard disk drives as an example, the semi-hard magnetic material is mainly used in the disk portion.
When the disk rotates, if the head remains in one position, each head will create a circular track on the surface of the disk.
These circular tracks are called tracks, which are basically magnetic circuits with gaps.
During the writing process, the computer converts information into electrical current and sends it to the coil around the head.
The current in the coil magnetizes the head, and the magnetic field generated by the magnetized head magnetizes the medium on the track.
Because the current size is different, the magnetic field of the head changes, which in turn changes the magnetization of the magnetic medium and records different data.
As the head and disk move, large amounts of information are recorded on the disk.
The readout process runs in the opposite direction of the write process, using the magnetic field of the magnetic medium to produce a change in magnetic flux on the head, generating varying current in the coil, which serves as an electrical signal that can be used by the computer.
Magnetic materials play a significant role in our daily lives, and their importance is self-evident. We believe that with a deeper understanding of magnetism and advances in magnetic material technology, it will have even wider applications in our lives.
The above analysis is relatively general and simple.
Understanding the deeper principles and how to control the magnetic properties of magnetic materials for our use will be the direction we need to continue advancing in the future.
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