1. The main magnetic properties of permanent magnet materials are: remanence (Jr,Br), coercivity (bHc), intrinsic coercivity (jHc), magnetic energy product (BH)m. What we usually call the magnetic properties of permanent magnet materials refers to these four items. Other magnetic properties of permanent magnet materials include Curie temperature (Tc), working temperature (Tw), temperature coefficient of remanence and intrinsic coercivity (Br θ,jHc θ), recovery permeability (μrec.), squareness of demagnetization curve (Hk/jHc), high temperature demagnetization performance and uniformity of magnetic properties.

Magnetic Property Test Report of Permanent Magnet Material

In addition to magnetic properties, the physical properties of permanent magnet materials include density, electrical conductivity, thermal conductivity, thermal expansion coefficient, etc.; mechanical properties include Vickers hardness, compressive (tensile) strength, impact toughness, etc. In addition, there is an important one in the performance index of permanent magnet materials, that is, the surface state and its corrosion resistance.
2. What is the magnetic field strength (H)?
In 1820, Danish scientist Oster (H.C.Oersted) found that the wire with current can deflect the magnetic needle nearby, thus revealing the basic relationship between electricity and magnetism, and electromagnetism was born. Practice shows that the strength of the magnetic field generated by the infinite wire with current is proportional to the size of the current and inversely proportional to the distance from the wire. It is defined that the magnetic field strength of an infinite wire carrying 1 ampere of current is 1A/m (A/m, International System of Units SI) at a distance of 1/2π meter from the wire; In the CGS unit system (cm-g-sec), in order to commemorate Oster's contribution to electromagnetism, the magnetic field strength of an infinite wire carrying 1 ampere of current is defined as 1Oe (Oster) at a distance of 0.2cm, 1Oe = 1/(4π x 10?)A/m. The magnetic field strength is usually denoted by H.
3. What is magnetic induction intensity (B), what is magnetic flux density (B), and what is the relationship between B and H,J,M?
Both theory and practice show that when a magnetic field h is applied to any medium (the magnetic field can be provided by external current or external permanent magnet, or by permanent magnet to permanent magnet medium itself, and the magnetic field provided by permanent magnet to permanent magnet medium itself is also called demagnetization field-the concept of demagnetization field), the magnetic field strength inside the medium is not equal to h, but is expressed as the sum of h and the magnetic polarization strength j of the medium. Since the magnetic field strength inside the medium is expressed by the magnetic field H through the induction of the medium, in order to distinguish it from H, it is called the magnetic induction strength of the medium, which is recorded as B:

B = μ0H J(SI system of units)(1-1)
B = H 4 π M(CGS system of units)

The unit of magnetic induction intensity B is T, and the unit of CGS is Gs(1T = 104Gs).
For non-ferromagnetic media such as air, water, copper, aluminum, etc., the magnetic polarization strength J and magnetization M are almost equal to 0, so the magnetic field strength H is equal to the magnetic induction strength B in these media. Since the magnetic phenomenon can be vividly represented by the magnetic lines of force, the magnetic induction intensity B can be defined as the density of the force flux, and the magnetic induction intensity B and the magnetic flux density B can be used conceptually.
4. What is magnetic polarization (J), what is magnetization (M), and what is the difference between the two?
Modern magnetic research shows that all magnetic phenomena originate from electric current. Magnetic materials are no exception, the ferromagnetic phenomenon is originated from the material internal atoms of the extranuclear electronic movement of the formation of micro-current, also known as molecular current. The collective effect of these micro-currents makes the material exhibit a variety of macroscopic magnetic properties. Because each micro-current produces a magnetic effect, a unit micro-current is called a magnetic dipole. It is defined that the maximum torque generated by each unit of external magnetic field on a magnetic dipole in vacuum is the magnetic dipole moment pm, and the vector sum of the magnetic dipole moment in each unit of material volume is the magnetic polarization intensity j, and its unit is t (Tesla, in CGS unit system, j is Gs,1T = 10000Gs). The magnetic moment of a magnetic dipole is defined as pm/μ0,μ0 is the vacuum permeability, the vector sum of the magnetic moment per unit volume of material is the magnetization M, the SI unit is A/m, and the CGS unit is Gs (Gaussian). The relationship between M and J is: J = μ0M, in CGS unit system, μ0=1, so the magnetic polarization intensity is equal to the value of magnetization; In SI unit system, μ0 = 4π × 10-7 H/m (Heng/m).
5. What is remanence (Jr,Br), and why is the magnetic polarization intensity J value and magnetic induction intensity B value at any measurement point on the demagnetization curve of permanent magnet materials necessarily less than the remanence Jr and Br values?

After the permanent magnet material is magnetized to saturation by the external magnetic field in the closed circuit state, when the external magnetic field is removed, the magnetic polarization strength J and internal magnetic induction strength B of the permanent magnet material will not disappear due to the disappearance of the external magnetic field H, but will maintain a certain value, which is called the residual magnetic polarization strength Jr and residual magnetic induction strength Br of the material, collectively referred to as remanence. The units of remanence Jr and Br are the same as the units of magnetic polarization and magnetic induction. According to the relation (1-1), on the demagnetization curve of the permanent magnet material, when the magnetic field h is 0, Jr = Br, and when the magnetic field h is negative, j and B are not equal, and they are divided into J-H and B- H curves. It can also be seen from relation (1-1) that with the increase of reverse magnetic field H, B changes from the maximum value Br = Jr to 0, and finally becomes negative. For modern permanent magnet materials, the change rule of B demagnetization curve is often straight line. The change rule of J demagnetization curve is different: with the increase of reverse magnetic field H, the value of B decreases linearly, since the decrease of the B value is always greater than or equal to the increase of the reverse magnetic field H, a relatively straight line can be maintained in a certain region on the J demagnetization curve, but the J value is always less than Jr.
6. What is coercivity (bHc) and intrinsic coercivity (jHc)?
On the demagnetization curve of the permanent magnet material, when the reverse magnetic field H increases to a certain value bHc, the magnetic induction intensity B of the magnet is 0, and the value of the reverse magnetic field H is called the coercivity bHc of the material; When the reverse magnetic field H = bHc, the magnet does not show magnetic flux to the outside, so the coercivity bHc represents the ability of the permanent magnet material to resist external reverse magnetic field or other demagnetization effects. The coercive force bHc is one of the important parameters in the design of magnetic circuit. It is to be noted that the coercive force bHc is always smaller in value than the remanence Jr. Since it can be seen from equation (1-1) that at H = bHc, B = 0, then μ0bHc = J, as described above, the magnetic polarization intensity value at any point on the J demagnetization curve is always less than the remanence Jr, so the coercivity bHc is always less than the remanence Jr in value. For example, a magnet with Jr = 12.3kGs cannot have a bHc greater than 12.3kOe. In other words, the remanence Jr is numerically the theoretical limit of the coercive force bHc. When the reverse magnetic field H = bHc, although the magnetic induction intensity B of the magnet is 0 and the magnet does not show magnetic flux to the outside, the vector sum of the microscopic magnetic dipole moment inside the magnet is often not 0, that is to say, the magnetic polarization intensity J of the magnet in the original direction often still maintains a large value. Therefore, bHc is not enough to characterize the intrinsic magnetic properties of the magnet; when the reverse magnetic field H increases to a certain value jHc, the vector sum of the microscopic magnetic dipole moment inside the magnet is 0, and the value of the reverse magnetic field H is called the intrinsic coercive force jHc of the material. Intrinsic coercivity jHc is a very important physical parameter of permanent magnetic materials. For magnets with jHc much greater than bHc, when the reverse magnetic field H is greater than bHc but less than jHc, although the magnet has been demagnetized to the degree of reversal of magnetic induction intensity B, after the reverse magnetic field H is removed, the magnetic induction intensity B of the magnet can still return to the original direction because the vector sum of the internal microscopic magnetic dipole moment is in the original direction. That is, as long as the reverse magnetic field H has not reached jHc, the permanent magnet material has not been completely demagnetized. Therefore, the intrinsic coercivity jHc is one of the main indicators of the ability of the permanent magnet material to resist the external reverse magnetic field or other demagnetization effects to maintain its original magnetization state. The units of the coercive force bHc and the intrinsic coercive force jHc are the same as the units of the magnetic field intensity.
7. What is magnetic energy product (BH)m?
On the B demagnetization curve (two quadrants) of the permanent magnet material, different points correspond to different working states of the magnet. Bm and Hm (abscissa and ordinate) corresponding to a point on the B demagnetization curve respectively represent the magnetic induction intensity and magnetic field inside the magnet in this state, the product of the absolute values of Bm and Hm (BmHm) represents the ability of the magnet to do external work in this state, which is equivalent to the magnetic energy stored by the magnet, and is called the magnetic energy product. At Br point and bHc point on demagnetization curve B, the (BmHm)= 0 of the magnet indicates that the ability of the magnet to do external work is 0, I .e. the magnetic energy product is 0; The maximum value of the magnet in a certain state (BmHm) indicates that the magnet has the maximum ability to do external work at this time, which is called the maximum magnetic energy product of the magnet, or magnetic energy product for short, and is recorded as (BH)max or (BH)m). Therefore, it is generally desirable for the magnets in the magnetic circuit to operate at their maximum energy product. The unit of the magnetic energy product is J/m3 (joule/cubic meter) in the SI system, MGOe (megagoersted) in the CGS system, and 100/4πJ/m3 = 1MGOe.