On Magnetism

Magnetic Property
1. In magnetism, main magnetic properties of permanent magnetic materials are remanence (Jr, Br), coercivity (Hcb), intrinsic coercivity (Hcj) and magnetic energy product (BH). Generally, they are the most important four properties of permanent magnetic materials.
2. Other properties in magnetics are Curie temperature (Tc), operation temperature (Tw), temperature coefficient of remanence and coercivity (α, β), recoil permeability (μrec), demagnetization at high temperature, etc.
3. Apart from of properties about magnetism, we should also have knowledge of main physical properties like density, electric conductivity, heat conductivity, and coefficient of thermal expansion.
4. Main mechanical properties of permanent magnet materials are Vickers Hardness, compressive strength, impact toughness, etc. Besides, surface states and decay resistance should be taken into consideration when judging permanent magnetic materials.

Magnetic Field Strength (H)
1. Electromagnetism is resulted from research of H. C. Oersted, a Danish scientist who discovered the correlation between electricity and magnetic field in 1820. He noticed that a compass needle deflected from magnetic north pole when the electric current from battery was switch on and off. This deflection convinced him that magnetic fields radiated from all sides of a wire carrying an electric current, just as light and heat do. This experiment confirmed a direct relationship between electricity and magnetic force and brought up a new scientific branch called electro-magnetics.
2. Through intensive research on magnetism, H. C. Oersted founds that a magnetic field is caused by the current flowing in a long wire, and the magnetic field around the wire is directly proportional to the current intensity. Hence, he defined the magnetic field strength whose perpendicular distance to the wire is 1/(2π) meter is 1 A/m. To memorize his devotion to science, magnetic field strength whose perpendicular distance to the 1 A current flow is 1 Oe (Oe means Oersted), here, there is an equation, 1 Oe=1/(4π) × 103 A/m. Magnetic field strength is denoted by symbol H in magnetism.

Magnetization Polarization (J) and Magnetization (M)
1. Research shows that magnetic fields arise from current flow. To be exact, origin of magnetic fields is microscopic electric currents. These currents are resulted from either the motion of electrons or the spin of electrons or the nuclei in atoms. Because every closed loop of electric current can generate magnetic field line, here, we call the unit closed loop of electric current as a magnetic dipole in magnetics.
2. In magnetism, magnetic dipole moment (p m) is the largest magnetic moment caused by magnetic dipoles in unit magnetic field. Here, in unit quantity permanent magnet material, there is a vector sum of these magnetic dipole moments. We defined the vector sum as magnetization polarization with symbol J. The unit of magnetization polarization is T, which is also denoted as Gs in CGS unit system (Centimeter, Gam, Second). There is a transformation relation between T and Gs as follow, 1 T=104 Gs.
3. Magnet dipole moment of one magnet dipole is p m /μ0, here, μ0 is vacuum permeability. In magnetism, vector sum of the magnetic moments in unit quantity magnet material is defined as magnetization. The symbol of magnetization is M, and the unit is A/m (in standard international unit system) or Gs (in CGS unit system).
4. There is an equation explaining the correlation between magnetization and magnetization polarization as follow, J=μ0M. In CGS unit system, μ0=1, here, J=M. While in standard international unit system, μ0=4π × 10-7H/m, that is to say, J=(4π × 10-7H/m)M.

Magnetic Induction Strength (B), Magnetic Flux Density (B) and Relevant Physical Relation
1. In magnetism, theories and experiments have shown that, if a magnetic medium is inflicted with an external magnetic field whose magnetic field strength is H, the magnetic field magnet field in the medium is not equal to H. Actually it is the sum of the external magnetic field strength and the medium’s magnetization polarization (J). In magnetics, it is defined as magnetic induction strength. Here, the sum is denoted with symbol B, it is the magnetic field strength in the magnetic medium. Here, B = μ0H+J (in standard international unit system), or B = H+4πM (in CGS unit system).
2. In non-ferrite magnetic mediums such as air, water, cooper and aluminum, J≈0, M≈0, hence, H≈B.
3. Sometimes, symbol B also refers to magnetic flux density because the invisible magnetic induction strength is able to be directly observed through the visible magnetic flux density. Generally in magnetism, these two physical conceptions mean the same physical parameter.

Coercivity (Hcb) and Intrinsic Coercivity (Hcj)
1. In the demagnetization curve of one certain permanent magnet material, if the reversing magnetic field strength H is large enough, the magnetic induction Strength B will be reduced to zero, here, the physical parameter which is equal to this H is called coercivity (Hcb) of this magnet material. When H is equal to Hcb, magnet shows no magnetic flux. Besides, coercivity (Hcb) is always less than remanence (Jr).
2. In magnetism, there is another property which should be distinguished with coercivity (Hcb), namely intrinsic coercivity (Hcj). As we know, when the magnetic induction strength B is zero, magnet shows no magnetic flux. But, the vector sum of magnetization polarization (J) is always not reduced to zero, because the magnetic field strength H is not enough. Therefore, a new larger H is brought in magnetics to make the vector sum of magnetization polarization (J) reduce to zero. Here, intrinsic coercivity (Hcj) is equal to the new H. Intrinsic coercivity is an important property to magnets materials, because if Hcj is not reduced to zero, magnet does not be thoroughly demagnetized.
3. Unit of these two magnetism properties (Hcb and Hcj) is respectively A/m or Oe.

Neodymium Magnet
1. Neodymium magnet is the strongest type of artificial permanent magnet. Its main components are neodymium (Nd), Iron (Fe) and Boron (B). Its tetragonal Nd₂Fe₁₄B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy. Nowadays, neodymium magnets are commonly applied in various fields, including computers, medical equipments, communication devices, electron devices and magnetic machineries. Hence, this product in magnetism lab is largely put into industrial applications.
2. Neodymium magnets can be divided into sintered magnets and bonded magnets. Generally, sintered magnets are generated through powder metallurgy. They are made from isotropic magnetic powders. While microcrystalline NdFeB powders for bonded magnets are generated by chilling procedure. These powders contain a large amount of Nd₂Fe₁₄B crystals. They are bonded into bulky magnets which are known as bonded magnets. Hence, sintered magnets have much better magnetism performance than bonded magnets. But bonded permanent magnet materials have various irreplaceable advantages. They can be used to produce magnets which are small in size, or in complex shapes, or require high geometric accuracy. As products in magnetics, these bonded magnets can also be massively produced under automatic controlling. Compared with the sintered magnets, bonded magnets are more erosion-resistant.

Producing Procedures of Neodymium Magnets
1. Common Producing Procedure of Neodymium Magnets
(1) Melting Alloy---(2) Milling Powder---(3) Aligning Powder into Dense Blocks---(4) Heat Treatment---(5) Tempering Magnet---(6) Magnetism Inspection---(7) Finishing---(8) Curving---(9) Accurate Grinding---(10) Inspection of Semi-finished Product---(11) Electroplating---(12) Product Inspection---(13) Packaging Products and Putting Them in Storage
2. Basic Mechanical Properties

Bending Strength (N/mm2)	Compressive Strength (N/mm2)	Young Modulus (kN/mm2)	Ductility %	Hardness (HV)
250-345	1100	150-160	~0	600-620

3. Neodymium magnet is very brittle. Hence, drastic impact and tensile stress should be avoided. Besides, magnetism of the neodymium magnets is rather strong, so operators should pay more attention to operational safety during magnet production.

Electroplating Procedure for Neodymium Magnets
1. Surface Treatment before Electroplating Procedure
Firstly, remove oil on magnets. Then clean and etch magnets, and cleanout them again. At last, magnet surface should be clean and oil-less.
2. Electroplating the Magnets
Whether electroplating quality is good depends on if plating material and operation condition meet demands. So during electroplating procedure, various aspects, such as electroplating material, operation temperature, current density, etc. should be strictly controlled according to regulated standards.
3. Treatment after Electroplating Procedure
Apart from properties in magnetics, such as magnetism, sometimes we should also make permanent magnet materials meet some special functions. Hence, treatments, such as neutralizing, bright dipping, passivating, organic material coating, etc. should be conducted.