Magnetism has long fascinated people.
More than 3,000 years ago, the Chinese discovered natural magnets in nature that could attract each other or pieces of iron. People used their rich imagination to compare this phenomenon to a mother's loving care for her child.
This was recorded in “Lushi Chunqiu – Jiqiuji”: “Gentle stones ask for iron and are attracted.”
The compass, one of China's four great ancient inventions, is an example of how the ancient Chinese made use of magnetism.
As we know, a magnet 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 empty electrons within 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 Electronic Magnetic Moment
Atomic magnetism is the basis of magnetic materials, and atomic magnetism comes from the magnetic moment of the electron.
The movement of electrons is the source of the electron's magnetic moment. Electrons have rotational movement around the atomic nucleus and intrinsic spin movement.
Therefore, the electronic magnetic moment consists of two parts: orbital magnetic moment and spin magnetic moment.
According to Bohr's atomic orbit theory, electrons within atoms move around the atomic nucleus in a certain orbit.
The movement of electrons along the orbit corresponds to a circular current, which will produce a corresponding orbital magnetic moment.
The plane of the orbital magnetic moment of the electron in an atom can take different directions, but in a directional magnetic field, the direction of the electron's orbit can only be in several fixed directions, that is, the direction of the orbit is quantized.
The origin of magnetism arises 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 spin magnetic moment of the electron is greater than the orbital magnetic moment of the electron.
This is because in a crystal, the direction of the electron's orbital magnetic moment is modified by the field of the crystal lattice, and therefore it cannot form a composite magnetic moment that projects outward from the material, leading to what is commonly referred to as “ extinction” or “freezing” of orbital angular momentum and orbital magnetic moment.
Therefore, the magnetism of many materials in the solid state does not arise primarily from the electron's orbital magnetic moment, but rather from the electron's 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 empty electron orbit.
Of course, as we can see from the examples, this is just a necessary condition. Metals such as Cu, Cr, V, and many lanthanides have empty electron orbits but do not exhibit magnetism (specifically ferromagnetism).
3. Classification of Magnetism
Before discussing the electron exchange interaction, let us first look at the macroscopic manifestation of material magnetism.
According to the different magnetic properties presented at a macroscopic level by the superimposition of the action of atomic magnetic moments, magnetic materials can be classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic.
(1) Diamagnetism
Diamagnetism refers to the fact that when there is no magnetic field, the magnetic moment of atoms with fully filled electronic shells is equal to zero, or the total magnetic moment of some molecules is zero, and does not exhibit macroscopic magnetism.
But under the action of a magnetic field, the orbital movement of the electrons will produce additional movement, 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.
(2) Paramagnetism
Paramagnetism refers to the fact that atoms have magnetic moments that are not completely canceled out and therefore have a total magnetic moment.
However, as the direction of atomic magnetic moments is chaotic, the external effects cancel each other out and do 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 itself 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 occurs 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 difficult to align the atomic magnetic moments with the external magnetic field and, therefore, the magnetization is lower.
(3) Ferromagnetism
Ferromagnetism refers to the phenomenon in which adjacent atoms can be aligned orderly in 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 called 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 within the material are different from each other and cancel out each other's effects externally, so that the entire material does not exhibit macroscopic magnetism.
In other words, ferromagnetic materials are composed of small “magnets” arranged irregularly and do not exhibit external magnetism under statistical regularities.
However, when an external force (external magnetic field) arranges the polarity of each “little magnet” in the same direction, it exhibits strong magnetism externally.
The spontaneous magnetization of magnetic domains within ferromagnetic materials is an important reason for their ferromagnetism.
This explains why “atoms with empty electron shells” are just a necessary condition for material magnetism.
In a strict sense, what we normally 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.
(4) Antiferromagnetism
Antiferromagnetism refers to the phenomenon in which, under the action of a magnetic field, adjacent atoms or ions with the same spin organize themselves in opposite directions, causing their magnetic moments to cancel each other out, making them similar to paramagnetic materials and not exhibiting magnetism. .
(5) Ferrimagnetism
Ferrimagnetism is essentially antiferromagnetism where the reverse magnetic moments in two sublattices do not completely cancel each other out.
It is similar to ferromagnetism because it exhibits strong magnetism, but different from ferromagnetism because its magnetism comes from the difference between two magnetic moments of opposite and unequal directions.
Currently, many ferrites (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 in a ferrimagnetic material that maintains the original magnetic structure, but has two sublattices with unequal magnetic moments.
Ferromagnetic and ferrimagnetic materials are collectively called strong magnetic materials and represent the main development direction of magnetic materials.
4. Exchange
Interaction Next, let's take a look at how the electron exchange interaction affects the spin magnetic moment of electrons and therefore affects the macroscopic magnetism of materials.
Exchange interaction between atoms generally refers to the electrostatic interaction caused by the mutual exchange of electron positions 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 the electrons are indistinguishable, we must also consider the possibility of exchange of 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 type of electron exchange occurs at a frequency of about 1018 times per second. The change in energy caused by this exchange interaction is called exchange energy, denoted as Eex.
In general, atomic binding energy can be expressed as:
E=E0+E '=E 0 +(C+A)
Where E 0 is the total energy of each atom in its ground state;
C is the increase in energy resulting from the static electrical Coulomb interaction between nuclei and electrons;
A is the increase in energy resulting from the exchange of electrons, generally referred to as the exchange energy constant.
A depends on the degree of proximity of the partially filled electronic shells to neighboring atoms and 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 expressed approximately as follows:
E ex =ΔE=-2AS a S b cosφ
Where a and S b 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 from 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 multielectron atoms. A more in-depth 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, that is, the spin magnetic moments of the electrons are arranged antiparallel, and E ex (180)=+2AS a S b ; 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, E ex (0)=-2AS a S b .
Furthermore, if 0°<φ<180°, then the spin directions of the two electrons are neither equal nor opposite, but rather separated by an angle φ, and their exchange energy E ex lies between the two, i.e. E ex (0°)
When the directions of the adjacent spin magnetic moments of the two electrons are equal, the electron's spin magnetic moments are necessarily arranged in 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 electron spin direction is arranged in an antiparallel manner, which is antiferromagnetism.
(3) When A is very small, the exchange interaction between these two adjacent atoms is weak and the exchange energy E ex 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, on the degree to which the unfilled electronic 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 crystalline structure of the material.
The characteristics of magnetism make magnetic materials crucial to the development of high-tech industries and are an important pillar for the advancement of science and technology. They are also a highly active area of research 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 demand for this type of material can be used to assess a country's economic and average living standards. country.
In the following, 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, generally with ferrimagnetic properties.
They have some special properties:
(1) Through the magnetization of the external magnetic field, they can have a high maximum intensity of magnetic induction;
(2) Under the magnetization of an external magnetic field of a certain intensity, soft magnetic materials themselves can have a higher magnetic induction intensity;
(3) The resistance to movement of the magnetic domain in soft magnetic materials is small.
Due to 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, it is necessary for magnetic materials to have a high sensitivity to changes in external magnetic fields.
If the material is difficult to magnetize or 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 oldest 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 certain space for a long time without consuming electrical energy.
They are generally 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 function, 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, although they offer high stability, their range of use is limited.
(3) Semi-Hard Magnetic Materials
Medium-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 less 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 society becoming more and more intelligent, there is an increasing 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 that is widely used in information storage devices such as hard drives, magnetic tapes, and credit cards.
Medium-hard magnetic materials play a vital role in these applications due to their dynamic properties.
Taking hard disk drives as an example, semi-hard magnetic material is mainly used in the disk part.
When the disc rotates, if the head remains in the same position, each head will create a circular track on the surface of the disc.
These circular tracks are called tracks, which are basically magnetic circuits with gaps.
During the writing process, the computer converts the 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 in the track.
As the size of the current 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 written to the disk.
The reading process occurs in the opposite direction to the writing process, using the magnetic field of the magnetic medium to produce a change in the magnetic flux in the head, generating variable 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 evident. We believe that with a deeper understanding of magnetism and advances in magnetic materials technology, it will have even broader 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 moving forward in the future.
