Permanent NdFeB Magnet
The permanent ndfeb magnet is made into a certain shape. For example, if the magnetization curve in the open circuit state is measured along the x, y, and z directions of the magnet, it can be found that along the x direction, only a lower magnetic field (H1) is needed to magnetize it to technical saturation, while along the y and z directions, higher magnetic fields are needed to magnetize it to technical saturation.
This phenomenon is called the shape anisotropy of the permanent ndfeb magnet . The direction that requires a smaller magnetic field to magnetize it to technical saturation is called the easy magnetization direction; the direction that requires the largest magnetic field to magnetize it to technical saturation is called the difficult magnetization direction.
Why do permanent magnets of a certain shape (non-spherical) have shape anisotropy? This is related to the demagnetization field of permanent ndfeb magnet.
We know that permanent magnets in block or cylindrical shapes always have N poles and S poles. The presence of magnetic poles will generate magnetic fields around and inside the magnets. The magnetic field generated by the magnetic poles of permanent magnets always goes from N pole to S pole, both around and inside the magnets.
The demagnetization field inside the permanent ndfeb magnet is represented by Hd, and its direction is from N pole to S pole. The direction of the permanent ndfeb magnet’s magnetic moment Mm or the direction of the magnetization intensity is from S pole to N pole. The Hd inside the permanent magnet is opposite to the direction of the magnetization intensity M, and plays a demagnetization role, so it is called the demagnetization field.
How is the demagnetization field of a permanent ndfeb magnet generated? We can regard a permanent magnet as composed of two magnets, A and B. Magnet A is just in the magnetic field generated by magnet B, and magnet B is also in the magnetic field generated by magnet A. These magnetic fields are just from N pole to S pole, so it is opposite to the direction of the magnetization intensity of the magnet, so it is called the demagnetization field.
The strength of the demagnetization field of a permanent ndfeb magnet is related to the shape and size of the permanent magnet. For example, the strength of the demagnetization field of a ring-shaped permanent magnet varies with the size of the gap of the ring-shaped permanent magnet (i.e., the length of the permanent magnet).
When magnetizing along the axis of the ring-shaped permanent ndfeb magnet, no demagnetization field is generated; when the ring-shaped permanent magnet is cut in half, the demagnetization field of the half ring is Hd; the shorter the ring-shaped permanent magnet is cut, the greater the absolute value of the demagnetization field Hd. The order in which the absolute value of the demagnetization field gradually increases is Hd(e)>Hd(d) >Hd(c) >Hd(b) >Hd(a)→0.
It can be inferred that when the coercive force Hcj of the permanent magnet is not large enough, if the permanent magnet is made thin, it may not be able to be magnetized along the thin direction because the demagnetization field is too large.
The above example shows that the size of the demagnetization field of a permanent magnet is related to the shape and size ratio of the magnet. Experimental and theoretical calculations can prove that the demagnetization field of a permanent magnet is Hd=-NM, where M is the magnetization intensity of the permanent magnet and N is the demagnetization factor. The negative sign in the formula indicates that Hd is in the opposite direction to M.
For an infinitely long round rod-shaped permanent ndfeb magnet, the demagnetization factor N1 along the length direction and the demagnetization factor Nd in the diameter direction can be expressed as follows, where k is the size factor, k=l/d, d is the diameter of the rod-shaped magnet, and l is the length.
The demagnetization factor of any permanent ndfeb magnet in three mutually perpendicular directions (N1+Nd+Nd=1) is equal to 1.0. It can be seen that k=l/d=0, which is equivalent to a very thin plate, and its demagnetization factor N1→1.0;
for a round rod-shaped permanent magnet magnetized along the axial direction, the change of its demagnetization factor along the length l is shown in the following figure permanent ndfeb magnet. When the length l is equivalent to 200 times the diameter, its demagnetization factor can be ignored and its demagnetization field can be considered.
For permanent magnetic materials such as NdFeB, we mainly use its ability to generate and preserve magnetism, which is the most basic functional requirement for magnetic materials. So how to measure the ability of magnets to generate and preserve magnetism? We usually use four indicators: remanence Br, coercivity Hcb, intrinsic coercivity Hcj and maximum magnetic energy product BH (max).
To truly understand the above indicators, we must first understand the demagnetization curve. Because permanent magnetic materials need to be magnetized first, and permanent ndfeb magnet after magnetization need to withstand various factors that are unfavorable to magnetism during use, the demagnetization curve can effectively reflect the overall performance of the magnet.
The above picture is a typical permanent ndfeb magnet hysteresis loop. The first quadrant is the magnetization curve, the second quadrant is the demagnetization curve, the third quadrant is the reverse magnetization curve, and the fourth quadrant is the reverse demagnetization curve. The horizontal axis represents the external magnetic field intensity H, and the vertical axis represents the magnetic induction intensity B and the magnetic polarization intensity J.
[Simply put, the magnetic induction intensity B is the accumulation of the external magnetic field and the internal magnetic field of the magnet, and the magnetic polarization intensity J is the internal magnetic field of the magnet]
The four magnetic performance parameters of remanence, coercive force, intrinsic coercive force and maximum magnetic energy product are all derived from the demagnetization curve. The red line in the above picture is called the J-H demagnetization curve (the curve of the change of magnetic polarization intensity J and the external magnetic field H), also called the intrinsic demagnetization curve permanent ndfeb magnet, and the blue line is called the B-H demagnetization curve (the curve of the change of magnetic induction intensity B and the external magnetic field H). We use small boxes to represent magnetic domains.
Magnetic domains can be understood as tiny magnets. A magnet is composed of a lot of magnetic domains. The arrow is the spontaneous magnetization direction C axis of the magnetic domain. For a permanent magnet in a magnetically neutral state (in simple terms, a permanent ndfeb magnet that has not been charged), most of the magnetic domains are in the same coordinates but their directions cancel each other out, so that no magnetism is displayed to the outside.
When the applied external magnetic field increases in the opposite direction, the magnetic domains of the magnet gradually shift and rotate, as shown in Figure 5. When the magnetic field strength reaches a certain value, the magnetic induction intensity B of the magnet drops to 0. Simply put, the magnetic field strength retained inside the magnet and the external reverse magnetic field strength cancel each other out.
The corresponding magnetic field strength value at this time is called the magnetic coercivity Hcb (also written as bHc), with units of Oe or kA/m. The magnetic coercivity Hcb is closely related to the slope of the J-H demagnetization curve. If the magnetic domains of the permanent ndfeb magnets are unlikely to shift or rotate in a short period of time, the J-H line will be very straight, so that the value of Hcb will be infinitely close to Br, and the upper limit of Hcb is Br, Hcb=Br.
This is an ideal situation, which means that there is no tendency for the magnetic domains to reverse before the reverse magnetic field strength reaches Br in value. In this way, the magnet is very stable, that is, the magnet has a very strong ability to resist demagnetization in the initial state of the reverse magnetic field value below Br.
When the reverse magnetic field continues to increase and reaches a critical value, reverse magnetic domains appear quickly, causing the magnetic polarization intensity of the magnet to drop quickly to 0. In simple terms, the magnetic field intensity retained inside the magnet drops to 0, as shown in Figure 6. The corresponding magnetic field intensity value at this time is called the intrinsic coercive force Hcj (also written as jHc), with units of Oe or kA/m.
The intrinsic coercive force is a physical quantity that reflects the magnitude of the permanent ndfeb magnet’s ability to resist demagnetization. The greater the intrinsic coercive force, the stronger the ability to resist demagnetization, or more precisely, the stronger the ability to resist complete demagnetization. Special attention should be paid to the difference between Hcj and Hcb. When Hcj is greater than Br, the limit value of Hcb is Br. When Hcj is less than Br, the limit value of Hcb is Hcj.
The product of B and H corresponding to any point on the B-H demagnetization curve is called the magnetic energy product. The maximum value is the maximum magnetic energy product (BH)max. Theoretically, the maximum magnetic energy product (BH)max=½ Br². The maximum magnetic energy product takes into account both remanence and magnetic coercivity. Its value represents the magnitude of the magnetic energy contained in the magnet and can also reflect the initial inclination of the J-H line. The unit is GOe or j/m³.
If the above four parameters are still difficult to understand, you can simply think that the magnet is a cup of water, magnetization is heating, remanence is the heat of the water after heating stops, demagnetization is like cooling the water permanent ndfeb magnet, Hcb is when the ambient low temperature reaches a certain value, it offsets the heat of the water, and the overall heat is not reflected to the outside, and Hcj is the ambient low temperature value required to completely reduce the heat of the water to 0.
Para os parâmetros magnéticos acima referidos, devemos prestar atenção aos seguintes pontos em aplicações práticas:
1. For magnets that require strong magnetic fields, we usually need to increase their remanence as much as possible to release more magnetism, but it should be noted that this is under the premise of no demagnetization. If demagnetization exists, simply increasing the remanence may be ineffective permanent ndfeb magnet, and Hcj needs to be increased to reduce demagnetization. The simplest example is a D10*1 disc magnet.
The surface magnetism of 54H is higher than that of N54. The reason is that the Hcj of 54H is higher, which allows its Br to be fully utilized, while the Hcj of N54 is very low and cannot maintain it in a non-demagnetized state. No matter how high the Br is, its magnetism cannot be fully utilized.
2. For magnets that require strong anti-demagnetization ability and stability, we usually need to increase Hcj, and the Hcb value needs to be as close to Br as possible permanent ndfeb magnet.
3. For magnets that need good temperature resistance, we usually choose to increase Hcj, because the temperature coefficient of coercivity of NdFeB permanent magnets with the same Hcj is not much different. The most direct effect of increasing Hcj is to make the inflection point of the B-H line appear as late as possible or not appear at all. If strict requirements are placed on temperature stability, the requirements of Hcb must also be considered to minimize the slope of the B-H line to reduce the irreversible attenuation amplitude of the magnet.
