Here are some magnetic terms that you may hear and want to understand the meaning of.
A low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.
A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.
A bar magnet is exactly as it sounds, it is a permanent magnet which has a magnetic length which is greater than its diameter or effective diameter for rectangular bar magnets. A bar magnet has a north and a south pole typically at opposite ends of the bar
B-H curve (magnetic flux density-magnetic field curve)
Sometimes referred to as the ‘magnetisation curve’ or ‘demagnetisation curve’, the B-H curve is a graphical representation showing the relationship between ‘magnetic flux density’ (B) and the ‘magnetic field strength’ (H) required to demagnetise a specific magnet. This graph is the second quadrant of a four quadrant hysteresis curve.
The magnetic flux density increases in proportion to the field strength until it reaches a point of saturation and becomes constant even as the field strength continues to increase.
Magnetic flux density is measured in Gauss (G) or Tesla T, where 10,000 Gauss equals 1 Tesla. Magnetic field strength is measured in Oersteds (Oe). When analysing anisotropic materials if the magnetic field is not applied in parallel or perpendicular to the objects anisotropy axis then the measurements of the B-H curve are not valid.
This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.
Coercive Force, Hc
The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, B, to zero after the magnet has previously been brought to saturation.
Curie Temperature, Tc
The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetization.
The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.
Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs, which should be accounted for or minimized.
A magnet, consisting of a solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.
Indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve. Calculated as Bd x Hd, and measured in Mega Gauss Oersteds, MGOe.
A material whose permeability is very much larger than 1 (from 60 to several thousand times 1), and which exhibits hysteresis phenomena.
The condition existing in a medium subjected to a magnetizing force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The cgs unit of flux is the Maxwell.
An instrument that measures the change of flux linkage with a search coil.
Leakage flux particularly associated with edge effects in a magnetic circuit.
Lines of magnetic flux per square centimeter, cgs unit of flux density, equivalent to lines per square inch in the English system, and Webers per square meter or Tesla in the SI system.
An instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.
A closed curve obtained for a material by plotting corresponding values of magnetic induction, B, (on the abscissa) against magnetizing force, H, (on the ordinate).
The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.
Intrinsic Coercive Force, Hci
Measured in Oersteds in the cgs system, this is a measure of the material’s inherent ability to resist demagnetization. It is the demagnetization force corresponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.
Intrinsic Induction, Bi
The contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relationship is expressed as: BI = B-H.
Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by re-magnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.
A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetized in any direction without loss of magnetic characteristics.
A piece of soft iron that is placed on or between the poles of a magnet, decreasing the reluctance of the air gap and thereby reducing the flux leakage from the magnet.
Knee of the Demagnetization Curve
The point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. Alnico 5 exhibits a knee at room temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without re-magnetization.
That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gaps, and is therefore unable to be used.
Length of air-gap, Lg
The length of the path of the central flux line in the air-gap.
A line drawn from the origin of the Demagnetization Curve with a slope of -B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see Permeance Coefficient.
An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.
The total magnetic induction over a given area. When the magnetic induction, B, is uniformly distributed over an area A, Magnetic Flux = BA.
Magnetizing Force, H
The magnetomotive force per unit length at any point in a magnetic circuit. Measured in Oersteds in the cgs system.
Magnetomotive Force, F
Analogous to voltage in electrical circuits, this is the magnetic potential difference between any two points.
Maximum Energy Product, BHmax
The point on the Demagnetization Curve where the product of B and H is a maximum and the required volume of magnet material required to project a given energy into its surroundings is a minimum. Measured in Mega Gauss Oersteds, MGOe.
Magnetic materials such as permanent magnets are split into individual microscopic domains. The magnetic domain structure of a material is responsible for its magnetic characteristics such as those displayed by metallic elements and alloys like permanent magnets.
Each domain is a region which has a uniform direction of magnetisation, however, different domains may have different directions of magnetisation. During the process of manufacturing magnetic material, electromagnets align each domain, providing the greatest magnetic energy and giving the finished material anisotropy.
Magnetic field strength (H-field)
Magnetic field strength is the measure of a magnetising field originating from an electrical current or a permanent magnet. Magnetic field strength is measured in Oersteds (Oe).
Magnetic length refers to the dimension of a magnet which follows the direction of a magnet’s magnetic axis. A magnet’s magnetic length is always listed last when referring to a magnet’s physical dimensions. For example, a bar magnet magnetised along its length might be described as 50mm x 50mm x 100mm
Magnetisation refers to an object producing a magnetic field.
A material, or magnet is defined as magnetised when it exerts a magnetic field, either because of its interaction with an electromagnet or another permanent magnet.
Magnetomotive force (mmf)
Magnetomotive force is the magnetic field produced by a coil of wire when current is passed through it. The more current that is passed through a solenoid coil and the more coils the solenoid has, the larger the magnetic field produced. A magnetomotive force is expressed in ampere-turns; a value of the amount of applied current multiplied by the number of turns in a solenoid. Alternatively magnetomotive force is sometimes measured in Gilberts.
The term material refers to the physical composition of a magnet. For example, neodymium magnets are made out of a neodymium alloy (NdFeB) material containing neodymium (Nd), iron (Fe) and boron (B).
There are five main types of magnetic material and they are:
Maximum energy product (BHmax)
The maximum energy product of a magnet is measured in ‘Mega-Gauss Oersteds’ (MGOe). Known as the maximum energy product value, this is the primary indicator of a magnet’s ‘strength’. In general, the higher the maximum energy product value, the greater the magnetic field the magnet will generate in a particular application. In neodymium grading, the two numbers in a grade name (e.g. N42) represent the maximum energy product for that grade. The higher the value, the greater the magnetic field strength the magnet will exert in a particular application and the smaller the volume of magnet required.
(BH)max is a product of remanence (Br) and coercivity (Hc) and represents the area under the graph of the second quadrant hysteresis loop.
Each grade of neodymium magnet has an associated maximum energy product, displayed on the ‘neodymium magnet grades’ page of the Tech Centre.
Maximum Operating Temperature (Tmax)
The maximum operating temperature is exactly as it sounds, it represents the maximum temperature that a particular grade of magnet will be able to function at, before it becomes permanently demagnetised.
All permanent magnets weaken in relation to their temperature coefficient, but as long as the maximum operating temperature is not exceeded, this is fully recoverable on cooling. If the maximum operating temperature is exceeded, then the losses will not be fully recovered on cooling. Repeatedly heating a magnet above its maximum operating temperature and cooling will significantly demagnetise the magnet.
Neodymium magnets operate best in cold temperatures down to approximately -130oC. Regular neodymium magnets will maintain their magnetism in operating temperatures up to 80oC whereas different variants of neodymium magnets can operate up to temperatures of 230 oC.
The maximum operating temperature for each grade of magnet material is displayed on the ‘How does temperature affect neodymium magnets’ page of the Tech Centre.
Maxwell is a measurement for magnetic flux on the CGS scale where 1 Maxwell is equal to 1 line of flux. The measurement is named after James Clerk Maxwell who was a Scottish theoretical physicist born in 1831. Maxwell’s most high-profile achievement was formulating a set of equations that united electricity, magnets and optics into one consistent theory. Maxwell’s achievements were widely acclaimed as the second great unification in physics after those realised by Isaac Newton.
Mega Gauss Oersteds (MGOe)
Mega Gauss Oersteds is the CGS measure of the maximum energy product of a magnet (BHmax).
The five main types of magnet material have the following typical maximum energy products:
Neodymium upto 52 MGOe
Alnico upto 5.5 MGOe
Ferrite up to 3.5 MGOe
Samarium Cobalt up to 32 MGOe
Flexible magnets up to 2 MGOe
M-H loop (Hysteresis loop)
Also known as the hysteresis loop, the M-H loop is a four quadrant graph, showing magnetising force relative to resultant magnetisation of a permanent magnet material as it is successively magnetised to its saturation point, then demagnetised, magnetised in the reverse polar direction and then finally re-magnetised.
When the cycles are complete, this four quadrant graph will be a closed loop which illustrates the magnetic characteristics of the magnetic material under test. Magnetically ‘hard’ materials have a large area inside the loop which denotes the level of magnetic energy. Magnetically ‘soft’ materials lose magnetism when the magnetising field is removed and therefore these have very small areas inside the loop. The second quadrant within the four quadrants (+X and -Y) is the most important of the four curves and is known as the demagnetisation curve.
Currently, the existence of magnetic monopoles remains theoretical as their existence has not yet been proven. In theory, every magnet must have a north and south pole and magnetism flows from one to the other. Without both poles there is no flow of magnetism.
That pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often best to clarify by using “north seeking pole” instead of “north pole” in specifications.
The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties. Also known as the “axis”, “easy axis”, or “angle of inclination”.
A cgs unit of measure used to describe magnetizing force. The English system equivalent is Ampere Turns per Inch, and the SI system’s is Ampere Turns per Meter. The Oersted (Oe) is a measure for magnetic field strength and is named after the Danish physicist and chemist Hans Christian Oersted. In 1820, Oersted discovered the magnetic effect of electric current, contributing significantly to the study of magnetism. The Oersted is closely related to the Gauss measurement for flux density and is used to measure external electromagnetic forces usually produced in magnetisers and demagnetisers.
A magnet is said to be in open circuit when it is not attached to any other ferrous material, meaning that its lines of magnetic flux make their way from the north pole to the south pole through the air alone, rather than through a ferromagnetic material. Because it is more difficult for lines of magnetic flux to travel through air rather than other parts of a circuit, a magnet produces less Gauss when in open circuit.
A magnet’s orientation refers to the physical location and direction of its magnetic poles, e.g. through length, thickness, diameter, axially ,radially or diametrically.
A material having a permeability slightly greater than 1.
The inverse of reluctance, analogous to conductance in electrical circuits.
Plating is another term for coating. Platings or coatings are applied to raw neodymium magnets to prevent corrosion and demagnetisation. The most common coating is a layer of nickel, followed by a later of copper and then another layer of nickel.
At HERE, we can provide many different coatings and platings for bespoke applications, including:
Titanium Nitride (TiN)
Polytetrafluoroethylene (PTFE, also known as Teflon Ni-Cu-Ni plus Epoxy)
Nickel-Copper-Nickel, plus Rubber
Zinc, plus Rubber
Nickel-Copper-Nickel, plus Parylene
Nickel-Copper-Nickel, plus PTFE
Tin, plus Parylene
All magnets have both a north and a south pole, usually 180o apart. Polarity refers to a magnet’s magnetic orientation with regards to its poles. Opposite poles attract each other but similar poles repel.
Ratio of the magnetic induction, BD, to its self demagnetizing force, HD PC = BD / HD This is also known as the “load line”, “slope of the operating line”, or operating point of the magnet, and is useful in estimating the flux output of the magnet in various conditions. As a first order approximation, BD / HD = Lm/Lg, where Lm is the length of the magnet, and Lg is the length of an air gap that the magnet is subjected to. PC is therefore a function of the geometry of the magnetic circuit.
Ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.
A pull-gap curve plots the ‘pulling power’ of a magnet in direct contact with a thick and flat piece of steel and then though a steadily increasing range of air gaps. Pull follows an inverse square law relationship with distance.
High field gradient magnets have the highest clamping forces in direct contact with ferrous material (zero air gap), but the weakest pull through steadily increasing air gaps.
Low field gradient magnets have the lowest clamping forces in direct contact with ferrous material (zero air gap), but the highest pull through steadily increasing air gaps.
A high field gradient magnet’s pull-gap curve and a low field gradient magnet’s pull-gap curve will cross over if plotted on the same graph.
The pull strength is the highest possible holding power of a magnet, measured in kilograms. It is the force required to prise a magnet away from a flat surface of steel when the magnet and metals have full and direct surface-to-surface contact. The grade of the metal, surface condition and angle of pull all have an impact on the pull strength.
The ratio of permeability of a medium to that of a vacuum. In the cgs system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the cgs system.
Analogous to resistance in an electrical circuit, reluctance is related to the magnetomotive force, F, and the magnetic flux by the equation R = F/(Magnetic Flux), paralleling Ohm’s Law where F is the magnetomotive force (in cgs units).
The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remanence will be less than the residual induction, Br.
Residual Induction, Br:
This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials
Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux.
Reversible Temperature Coefficient
A measure of the reversible changes in flux caused by temperature variations.
Rare-earth metals are categorised in the periodic table in the group known as Lanthanides. The most common elements in this category are neodymium, samarium and dysprosium. Despite the name, rare-earth elements are relatively abundant in the earth’s crust, however, they are not typically found in economically exploitable deposits and are often dispersed, deriving the term ‘rare-earth.’
Remanence is described as the magnetism that is left in a magnet, after the removal of the external magnetic force applied to magnetise it. When a material has been magnetised, it has remanence, as the magnetism has at some point been induced by an external magnetic field.
When two magnets are placed close together with the same poles facing each other, e.g. north facing north or south facing south, they will always repel one another. The reason for this is because the magnetic fields being generated by each magnet are trying to flow in the same direction and when placed closed together they collide, having a repellent effect.
The condition under which all elementary magnetic moments have become oriented in one direction. A ferromagnetic material is saturated when an increase in the applied magnetizing force produces no increase in induction. Saturation flux densities for steels are in the range of 16,000 to 20,000 Gauss.
A coil conductor, usually of known area and number of turns that is used with a fluxmeter to measure the change of flux linkage with the coil.
Exposure of a magnet to demagnetizing influences expected to be encountered in use in order to prevent irreversible losses during actual operation. Demagnetizing influences can be caused by high or low temperatures, or by external magnetic fields.
Surface Field / Surface Gauss
The surface field strength is measured in Gauss and is the magnet’s maximum field strength taken from the magnet’s pole surface. Measurements are usually taken using a gauss meter.
Stacking refers to the process of placing magnets together to increase the net pull strength . When five magnets are stacked together to make one magnet which is five times thicker, then this magnet will be substantially more powerful because of the increase in its L/d ratio (length to diameter). Once the length of the magnet exceeds the diameter of the magnet, the magnet is working at an optimum level and further additions to magnetic length will provide only small increases in performance.
In magnetic terms this is the specific pole of the magnet which ‘seeks’ the earths geographic South Pole. The earth’s geographic South Pole actually has a magnetic north polarity, thus greatly confusing the issue.
Temperature coefficient is a factor that is used to calculate the decrease in magnetic flux corresponding to an increase in operating temperature. The loss in magnetic flux is recovered when the operating temperature is decreased, providing the maximum operating temperature is not exceeded. The temperature coefficient for magnetic materials are typically;
Neodymium 0.11 % per degree C rise in temperature
Alnico 0.02% per degree C rise in temperature
Ferrite 0.2% per degree C rise in temperature
Samarium cobalt 0.03 % per degree C rise in temperature
Flexible magnets 0.2 % per degree C rise in temperature
The Tesla is a unit of measurement for magnetic flux density. It is named after Nikola Tesla who was a Serbian-American inventor, engineer and physicist. One Tesla is equal to 10,000 Gauss.
Some magnets are manufactured to include a thread for fixing in their applications. Neodymium magnets themselves are generally not threaded as they are too brittle, instead a neodymium magnet will be fixed to or encased within another material which will be threaded.
The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.
Weight refers to the weight of one single magnet made of magnetic material.