Magnet Glossary

Magnetic Parameters

B magnetic flux density

Magnetic induction is the magnetic field induced in a medium by a magnetic field strength at a given point, denoted as B, measured in gauss (Gs, CGS unit) or teslas (T, SI unit), 1 T= 104 Gs. It is the vector sum, at each point within the medium, of the magnetic field strength and resultant polarization strength. Magnetic induction is also defined as the number of flux line per unit area normal to the direction of the magnetic path.

J magnetic polarization

Magnetic polarization strength is also called intrinsic induction, denoted as J, measured in gauss or teslas. It is the vector sum of the magnetic dipole moments of a unit material. It indicates the vector difference between the magnetic induction in the material and the magnetic field strength. This relation is expressed by the following equations:
B= m0H +J                (SI)                    (1-1)
B= H +4pM               (CGS)               (1-1a)

Where μ0 is the magnetic permeability in vacuum. In CGS unit, μ0=1 Gs/Oe, while in SI unit, m0=4p´10-7 Wb×A-1×m-1 (webers per ampere and meter). M is known as magnetization, similar as polarization J, representing the vector sum of individual atomic magnetic moments per unit volume, and there has J=μ0M.

H magnetic field strength

Magnetic field strength is one of two ways that the intensity of a magnetic field can be expressed. Technically, a distinction is made between magnetic field strength H, measured in amperes per meter (A/m), and magnetic flux density B, measured in Newton-meters per ampere (Nm/A), also called teslas (T).

The magnetic field can be visualized as magnetic field lines. The field strength corresponds to the density of the field lines.
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. It is also the measure of that magnetization.[1] Colloquially, when a magnet is "magnetized" it has remanence.[2] The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism.

The equivalent term residual magnetization is generally used in engineering applications. In transformers, electric motors and generators a large residual magnetization is not desirable (see also electrical steel) as it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing.

Hcb coercive force

In electrical engineering and materials science, the coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. An analogous property, electric coercivity, is the ability of a ferroelectric material to withstand an external electric field without becoming depolarized.

For ferromagnetic material the coercivity is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC. It can be measured using a B-H analyzer or magnetometer.

Ferromagnetic materials with high coercivity are called magnetically hard materials, and are used to make permanent magnets. Materials with low coercivity are said to be magnetically soft. The latter are used in transformer and inductor cores, recording heads, microwave devices, and magnetic shielding.

Hcj intrinsic coercive force

The Intrinsic Coercive Force (Hcj) indicates the magnet alloy’s ability to withstand heat and demagnetization from external magnetic fields.  A quick review of the table of Neo alloy grades shows an increase in operating temperature with an increase in the intrinsic coercive force.  This also holds true for the magnet’s ability to withstand external demagnetizing field.  The higher the intrinsic coercive force, the better the magnet will withstand external demagnetizing fields.

The Intrinsic Coercive Force of a magnet material adds cost and that is why the Hcj level should be matched to the application.  A Neo magnet’s Hcj is enhanced by adding various materials to the crystal lattice.  The most prominent material is dysprosium which is very expensive.  Also, by adding materials to the lattice, the effective energy of the Neo alloy is reduced.  This means that high grades of Neo with high operating temperatures are very difficult to manufacture, are expensive or not available.  This is why Neo grades of 50 have an Intrinsic Coercive Force of ~ 14 kilo-Oersted and a lower tolerance to heat.

BH max energy product

The maximum energy product is a measurement for the maximum amount of magnetic energy stored in a magnet. It concerns the product maximally attainable with a material made out of flux density B and field strength H.

The standard unit of measurement is kJ/m³ (Kilojoule per cubic meter) or MGOe (Mega-Gauss-Oersted).BHmax is a volume independent magnetic characteristic, meaning a small and large magnet made from the same ND-48 Neodymium Iron Boron magnet alloy will have the same BHmax, although though they create vastly different magnetic fields or flux.  The Maximum Energy Product, or BHmax, is attained when the magnet is operating at the highest Induction level (Gauss) and at the smallest volume for a particular pole cross-section.  Larger volumes of the magnet will produce larger magnetic fields and more flux, though the magnet may not be operating at BHmax.

Additional Magnetic Properties

(α(Br))/(%/K) Temp.Coeff. Of Br

The fractional change in remanence Br of a permanent magnet material for every 1° increase in temperature near ambient temperature. Because this change is reversible, it is also called the reversible temperature coefficient.

(α(Hcj))/(%/K) Temp.Coeff. Of Hcj

The fractional change in Intrinsic Coercive Force Hcj of a permanent magnet material for every 1° increase in temperature near ambient temperature. Because this change is reversible, it is also called the reversible temperature coefficient.

(Tc)/K

In physics and materials science, the Curie temperature (TC), or Curie point, is the temperature at which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism. The Curie temperature is named after Pierre Curie, who showed that magnetism was lost at a critical temperature.

The force of magnetism is determined by the magnetic moment, a dipole moment within an atom which originates from the angular momentum and spin of electrons. Materials have different structures of intrinsic magnetic moments that depend on temperature; the Curie temperature is the critical point at which a material's intrinsic magnetic moments change direction.

Permanent magnetism is caused by the alignment of magnetic moments and induced magnetism is created when disordered magnetic moments are forced to align in an applied magnetic field. For example, the ordered magnetic moments change and become disordered at the Curie temperature. Higher temperatures make magnets weaker, as spontaneous magnetism only occurs below the Curie temperature.

Recoil Permeability: µr

The ratio of change in flux density as a function of incremental change in applied field (H) in the vicinity of H=0. It has no dimensions in either the MKSA or CGS system.

Physical & Mechanical Properties

Vickers Hardness

The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests.

Electrical Resistivity

Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity) is a fundamental property that quantifies how strongly a given material opposes the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current.

Compressive Strength

Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size, as opposed to tensile strength, which withstands loads tending to elongate. In other words, compressive strength resists compression (being pushed together), whereas tensile strength resists tension (being pulled apart).

Tensile Strength

Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or Ftu within equations,[1][2][3] is the capacity of a material or structure to withstand loads tending to elongate, as opposed to compressive strength, which withstands loads tending to reduce size. In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together). Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking.

Bending Strength

The flexural strength is stress at failure in bending. It is equal or slightly larger than the failure stress in tension.
Flexural strength, also known as modulus of rupture, or bend strength, or transverse rupture strength is a material property, defined as the stress in a material just before it yields in a flexure test. The transverse bending test is most frequently employed, in which a specimen having either a circular or rectangular cross-section is bent until fracture or yielding using a three point flexural test technique. The flexural strength represents the highest stress experienced within the material at its moment of yield. It is measured in terms of stress, here given the symbol {\displaystyle \sigma } \sigma.

Thermal Conductivity

Thermal conductivity (often denoted k, λ, or κ) is the property of a material to conduct heat.

Young's Modulus

Young's modulus, also known as the elastic modulus, is a measure of the stiffness of a solid material. It is a mechanical property of linear elastic solid materials, and will be more or less dependent on temperature, depending on the material being considered. It defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material.

Coefficient of Thermal Expansion

The coefficient of thermal expansion(Y) describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure.