What Is Magnetic Susceptibility?

Magnetic susceptibility, a physical quantity that characterizes the properties of a magnetic medium. The common symbol cm indicates that it is equal to the ratio of the magnetization M to the magnetic field strength H, that is, M = cmH for paramagnetic properties, cm> 0, and for antimagnetic properties, cm <0, their values are very small. For ferromagnetism, cm is large and it is also related to H (that is, there is a complex nonlinear relationship between M and H). For isotropic magnetic media, cm is a scalar; for anisotropic magnetic media, the susceptibility is a second-order tensor.

Any material will be magnetized under the action of a magnetic field and show a certain characteristic of magnetism. This magnetism is not only characterized by the magnitude of magnetization or magnetic induction, but also should be reflected by the characteristics of the change of magnetization with external magnetic field. For this reason, the ratio of the magnetization M to the magnetic field strength H under the action of a magnetic field is defined as the magnetic susceptibility: = M / H

Generally, the magnetization refers to the vector sum of the magnetic moments of atoms or ions in a unit volume of a material, so the susceptibility defined by the above formula is also called volume susceptibility. If the density of the material is known as , the magnetic susceptibility per unit mass of the material is _m = /

In addition, it is also possible to define the magnetic susceptibility of a mole of 1 mole _M = _m M

In the formula, M is a molecular weight.

The type of magnetic properties of a material can often be judged based on the susceptibility, its positive and negative values, and its behavior with temperature.

In the International System of Units (SI), the susceptibility cm is a dimensionless pure number.

The magnetic susceptibility of a substance can be used

Matter in an external magnetic field,

Magnetization :

The process of magnetic materials in a magnetically neutral state gradually changing from macroscopically non-magnetic to displaying magnetism under the action of a magnetic field is called magnetization.

Magnetization process :

Under the action of a magnetic field, the process of the magnetization of a magnetic material from a magnetically neutral state to zero to a very strong magnetic field strength approaching saturation magnetization is called a magnetization process.

Magnetization curve :

Under the action of a magnetic field, a magnetic material in a magnetically neutral state will have its magnetization M increase with the increase of the magnetic field strength H, and finally reach a saturation magnetization value Ms at a certain saturation magnetic field strength Hs. At this time, the inside of the material The atomic magnetic moments of are basically oriented along the magnetic field. If the magnetic field strength is increased, the magnetization value will not increase significantly. The corresponding curve of magnetization as a function of the magnetic field strength is plotted on the MH diagram, which is called the magnetization curve, also called the initial magnetization curve. Accordingly, a curve in which the magnetic induction intensity B of the magnetic material changes with the magnetic field intensity H is referred to as a BH magnetization curve.

Hysteresis loop :

After a magnetic material is saturated and magnetized under the action of a sufficiently strong magnetic field (called the saturation magnetization field Hs), the strength of this forward magnetic field is reduced to zero, and the magnetization of the material will decrease from Ms to Mr. Obviously, the Changes lag behind changes in magnetic field strength, a phenomenon called hysteresis. Mr is called residual magnetization, referred to as residual magnetization. For Mr to be zero, a reverse magnetic field Hci or MHc must be applied to the material. This magnetic quantity is called intrinsic coercivity. If the reverse magnetic field is gradually increased to -Hs, the material will reach saturation magnetization again. Reduce the reverse magnetic field to zero and continue to increase the magnetic field strength to Hs in the forward direction. The magnetization will reach Ms through -Mr and Hci. Therefore, a closed curve will be formed on the MH diagram because the change in magnetization always lags behind. Due to the change in magnetic field strength, such a closed curve is called the MH hysteresis loop. Correspondingly, if the magnetic field intensity undergoes a periodic change, that is, Hs 0 HC Hs HC Hs, the change in the magnetic induction intensity B will also form a closed loop on the BH diagram, which is called the BH hysteresis loop. On such a hysteresis loop, the magnetic induction intensity retained by the material after the magnetic field is removed after saturation magnetization is called the residual magnetic induction intensity, also referred to as the residual magnetization Br. The reverse magnetic field required to reduce Br to zero is called coercive force.

Show. In addition, when the magnetic field intensity is Hs, the magnetization intensity is the saturation value Ms, and the corresponding magnetic induction intensity is called the saturation magnetic induction intensity, which is expressed by Bs. At this time, Bs = 0 (Hs + Ms). 0 is the vacuum permeability.

Demagnetization curve :

The second quadrant of the saturation hysteresis loop is called the demagnetization curve, which is a characteristic curve reflecting the magnetic properties of hard magnetic materials.

Permeability :

The material is magnetized under the action of the magnetic field H and has a certain magnetic induction intensity B. The ratio of the two is called absolute permeability ', that is, ' = B / H

The ratio of absolute permeability to vacuum permeability 0 is called relative permeability : = '/ 0

Numerically, 0 = 4 × 10-7H / m. Relative permeability is also often referred to simply as permeability. In the International System of Units, the relationship between relative permeability and susceptibility is = 1 +

The magnetism of a substance is related to the microstructure of the atoms, ions, or molecules that make up it. In diamagnetic substances, because the electron spins are paired, there is no permanent magnetic moment. However, the orbital motion of the internal electrons, the pull-in precession generated under the action of the external magnetic field, will induce an induced magnetic moment that is opposite to the direction of the external magnetic field, so it shows diamagnetism. Its M is equal to the inverse susceptibility inverse, and M <0. In a paramagnetic substance, there are spin unpaired electrons, so they have a permanent magnetic moment. In the external magnetic field, the permanent magnetic moments are aligned along the direction of the external magnetic field, resulting in paramagnetism. The molar susceptibility M of a paramagnetic substance is the sum of the molar paramagnetic susceptibility and the molar antimagnetic susceptibility, that is,

M = along + inverse (5)

Usually cis ratio is about 1 to 3 orders of magnitude inverse, so these materials always show paramagnetism, with M > 0. The relationship between paramagnetic susceptibility and molecular permanent magnetic moment obeys Curie's law

(6)

In the formula, NA is Avogadro constant; K is Boltzmann constant (1.38 × 10erg · K); T is thermodynamic temperature; m is molecular permanent magnetic moment (erg · G). Therefore

(7)

Since the inverse does not change (or has a very small change) with temperature, as long as M is plotted against 1 / T at different temperatures, the intercept is inverse, and m can be obtained from the slope. Because it is much smaller than , it can be ignored in the inaccurate measurement.

(8)

The relationship between m of a paramagnetic substance and the number of unpaired electrons n is

(9)

In the formula, it is a Bohr magneton, and its physical meaning is: the magnetic moment generated by a single free electron spin.

B = 9.273 × 10erg · G = 9.273 × 10J · G = 9.273 × J · T