What is magnetism?
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- What is magnetism?
- Ferromagnetic materials
- Permanent magnetism
- Permanent magnets
- Geographic North and South pole
- Influence of magnetism
- Magnetizing magnets
- Hysteresis curve
- Remanence, Br
- Coercivity, intrinsic: HcJ
- Coercivity, normal: HcB
- Demagnetization curve
- Ferrous metals
- Undesired magnetism
- Irreversible loss
- Curie temperature
- Flux density B
- Br value
- Eddy current
- Measuring magnetism
- Dangers of magnetism
- Ferrous metal
- Force index
- Gauss G
- Magnetic field
- Magnetic induction, B
- Magnetic polarization, J (I)
- Magnetic field strength, H
- Maximum energy density
- Maximum application temperature
- Reversible loss
- Specific force index
- Temperature coefficient
- Field lines / Lines of force
- Free poles
- Weiss domain
- Working point/working line
- Foucault's current
- Pauli paramagnetism
Alternative names: lead stone, magnetic iron ore.
Back in ancient times, people discovered that magnetite crystals attract or repel each other, depending on their orientation. We call this physical phenomenon magnetism. The words magnetite and magnesium are both derived from Magnesia, the name of an area in the Thessalyë region of ancient Greece where magnetic stone can be found in abundance.
It is the iron in the rock that is responsible for the magnetic properties of magnetite. Many iron alloys possess magnetic properties. Besides in iron, we find magnetic properties in nickel, cobalt and gadolinium as well.
Magnetically 'hard' or 'soft'?
Of all magnetic materials, ferromagnetic materials are the only materials that are strong enough to be attracted by a magnet or to be used as magnet material.
We divide ferromagnetic materials into magnetically soft and hard materials. Soft magnetic materials, such as annealed iron, are easily magnetized, but do not stay magnetic after magnetization. The magnetism disappears quickly and almost completely. Hard magnetic materials do stay magnetic.
A permanent magnet is a ferromagnetic hard material.
Ferromagnetically hard materials retain their magnetic properties permanently. They have sufficient resistance to demagnetization.
All magnets have 2 poles; the north pole (N) and the south pole (S). North and south poles attract. The attraction decreases by the square of the distance between them.
The north pole of a magnet repels the north pole of another magnet. Two south poles also repel each other.
View the range of permanent magnets from Goudsmit Magnetics.
Large range of permanent magnet materials and qualities.
You can choose between four types of magnetic alloys. Each alloy serves a specific purpose. The most important differences are in the strength and resistance to demagnetization. The resistance to demagnetization depends on the material and quality, and the ratio of the dimensions.
Goudsmit magnets are of such a high quality that over time they barely lose any magnetic force at all. This is provided that you apply them within the specifications, such as temperature range and external magnetic fields.
For all magnets, the magnetic force decreases with increasing temperature. Some materials are more affected by this than others. The resistance to demagnetization generally decreases with increasing temperature. An exception is ferrite, of which the resistance to demagnetization actually increases when the temperature rises.
Please click on a magnet material for more information about the alloy, its various qualities and specific applications:
- Aluminium-Nickel-Cobalt (AlNiCo) magnets
- Ferrite magnets
- Samarium-Cobalt (SmCo) magnets
- Neodymium-Iron-Borium (NdFeB) magnets (also marketed by Goudsmit under the brand name Neoflux®)
- Plastic-bonded neodymium magnets
Demagnetizing a magnetic metal can be done in several ways:
At a certain temperature - the Curie temperature - the magnet permanently loses its magnetism because the atoms vibrate so intensely that there is no longer any global orientation. The same thing can happen as a result of mechanical shocks or oxidation. This loss of magnetism is irreversible.
Materials can also become undesirably magnetic, for example as a result of mechanical processing. We can demagnetize this material by deliberately applying a sufficiently strong magnetic counter-field (-H) to it. We use this principle in demagnetization equipment.
Geographic North and South pole
The earth also has a magnetic field.
What is confusing is that we call the south pole of the earth magnet the magnetic north pole and the north pole of the earth magnet the magnetic south pole. The pole names of a magnet are derived from this.
The magnetic south pole is found near the geographic north pole and the magnetic north pole is found near the geographic south pole. Therefore, a free-spinning magnet always takes on a north-south orientation.
With a pole indicator, you can see where the north or south pole of a magnet is located.
Influence of magnetism
Reaction of different materials on magnetism.
Ferromagnetic materials are the only materials strong enough to be drawn to a magnet. That is why we call them magnetic.
However, all other substances also respond weakly to a magnetic field, via one or more other types of magnetism. When we expose a material to a magnetic field, it can respond in various ways. We distinguish between the following kinds of magnetism:
- Pauli paramagnetism
- Super paramagnetism
- Spin glass magnetism
When we speak of magnetic material, we mean that the material shows ferro- or ferrimagnetic behaviour.
The forces that occur with dia- and paramagnetic behaviour are much weaker. Moreover, these materials do not spontaneously produce their own magnetic field. We therefore consider them to be non-magnetic.
Diamagnetic materials tend to repel field lines from their core, while ferromagnetic, ferrimagnetic and paramagnetic materials tend to concentrate them.
A practical example of diamagnetism: water is weakly diamagnetic, about forty times less than, for example, pyrolytic carbon. This is enough for light objects - containing a lot of water - to float if they are in a strong magnetic field.
This frog started floating for example, using a 16 tesla electromagnet at the High Magnetic Field Laboratory at Radboud University Nijmegen in the Netherlands.
Magnetization with preferred direction.
The most common permanent magnets are anisotropic, i.e. the magnet has a preferred direction of magnetic orientation and can only be magnetized along one axis. It is possible, however, to reverse the polarity of the magnet, which exchanges its north and south pole. Anti-isotropic magnets have a higher holding power than isotropic magnets.
View our range of permanent magnets and find out what magnetic forces they possess.
Magnetic material that is not pressed in a magnetic field is called isotropic.
A form of magnetism.
Antiferromagnetism is a form of magnetism that occurs in materials containing unpaired spins. The interactions that try to put these unpaired spins in opposite directions are stronger than the interactions that try to put the spins in parallel.
For more information go to the Wikipedia page on antiferromagnetism.
The opposite of magnetism.
A form of magnetism in which the relative permeability is less than or equal to 1.
Diamagnetic materials have a magnetic susceptibility that is less than or equal to 0, because this susceptibility is defined as χv = μv − 1.
Magnetic fields repel diamagnetic materials. They form induced magnetic fields in the direction opposite to that of the applied magnetic field.
For more information go to the Wikipedia page on diamagnetic materials.
With very powerful magnetization equipment, we can magnetize permanent magnets to their saturation level.
We magnetize magnets by placing them in a coil. With a pulse generator we then send a high current through the coil for a very short time. As a result, the coil generates a very strong magnetic field, causing the magnet to take over the direction of that magnetic field.
Isotropic versions of ferrite magnets have not been pressed into a magnetic field and can later be magnetized in all directions.
With a magnetization unit, we completely saturate non-magnetized magnets. This is, however, limited to the maximum dimensions of the coils present and the desired direction(s) of the magnetic field.
Please contact our service department if you have any questions concerning the magnetization of permanent magnets.
A hysteresis curve shows the relationship between the induced flux density -B and the magnetic field strength -H.
The hysteresis or BH curve provides insight into the following magnetic properties:
(De-) magnetization curve - BH curve = hysteresis curve
When a periodically alternating external magnetic field H is applied, the magnetization of a ferromagnetic material follows a magnetization curve. Starting from 'virgin' material without net magnetization, we follow the blue curve the first time we do this (see image below).
Upon reaching the saturation flux density - with magnetic field strength Hs - the magnetization does not increase further.
Remanent field strength BR
If we then invert the field, the magnetization at field strength H = 0 has not fully decreased to zero. There is a remanent field strength BR left as a result of the ‘Weiss domains’ not returning to their original state.
Coercive field strength Hc
Only when the externally applied field strength has reached an oppositely directed value - the coercive field strength Hc - does the magnetization B = 0 and the product is demagnetized. The area of the loop passed through with alternating magnetization is a measure of the loss. Materials with low values of Hc and therefore small hysteresis loops are called soft magnetic materials. If Hc is very large, they are called hard magnetic material.
'Hysteresis' is present in ferromagnetic material. You can also see this in the figure below. The magnetic field strength H is shown along the x-axis and the degree of magnetization (magnetic flux density) B is shown along the y-axis. If there is no magnetic field, there is no magnetization at the beginning and we therefore find ourselves at the origin of the graph.
If we apply a magnetic field, the ferromagnetic material will become magnetic. This continues until all the 'Weiss domains' in the material have the same orientation. The material is now at its maximum magnetization and increasing the magnetic field has no further influence on the degree of magnetization. If we reduce the magnetic field, the Weiss domains will mostly maintain their position.
When the field becomes more negative, the total magnetization also changes direction. This continues until all the spins are oriented in the other direction and the magnetization is reversed. The product is now demagnetized.
See YouTube video: Flux density vs remance
Hysteresis curve (BH curve)
The maximum energy density BH-max is the largest possible product of Bd and Hd on the demagnetization or hysteresis curve.
In other words, in the second quadrant of the hysteresis loop. In general, the higher the BHmax of the magnetic material, the smaller the magnet required for a particular application.
How do you calculate the maximum product of the residual magnetism Br and the intrinsic coercivity Hcj in MGOe from the MH curve?
First transfer the M-H curve into the B-H curve, using B=mu0(H+M). Then calculate (B.H) and get the maximum value (BH). Make sure that all units are correct; B is in Oe, H is also in Oe.
The B-H curve is the curve that characterises the magnetic properties of a material, element or alloy. It tells you how the material reacts to an external magnetic field. This information is important when designing magnetic circuits.
Do you have a specific question about the application of magnets in your product? Contact our engineers.
2nd quadrant of the hysteresis curve, the saturated part of the curve.
The demagnetization curve of magnetic material is determined by placing a sample in a closed system in which coils are used to generate a magnetic field, first magnetizing the material to saturation (+H) and then demagnetizing it (-H).
Metals with magnetic properties.
Ferrous metals include iron, cobalt and nickel. Due to its magnetic properties, gadolinium is sometimes also considered a ferrous metal. We consider all other metals to be non-ferrous metals.
Ferrous metals play an important economic role. This stems not from their scarcity, rather from their abundance. This has led to the development of innumerable technical applications. The economic value of ferrous metals is determined by their quantity. In contrast, the value of non-ferrous metals, which are much less abundant, is determined by their quality: there is little available and demand is high.
The distinction between ferrous and non-ferrous metals is also economically important in the waste processing industry. That is why it is interesting to separate the two groups at an early stage of the recycling process.
Goudsmit Magnetics supplies various magnetic separators for the recycling and sorting of metals and non-ferrous metals.
Ferromagnetic materials can become undesirably magnetic.
Ferromagnetic, also called magnetically conductive materials, such as iron and steel, can very easily become magnetic. Depending on the type of material or alloy, the product remains magnetic. This is referred to as remanent magnetism. Even non-ferritic stainless steel can become magnetic as a result of deformation or during welding.
In such cases, the induced magnetism often originates from other magnetic sources such as lifting magnets, clamping tables, loudspeakers or magnetic conveying systems. Magnetic fields near transformers, welding cables and welding processes can also induce magnetism. Furthermore, certain processes such as drilling, grinding, sawing and sanding the material sometimes result in remanent magnetism. Even stainless steel can become undesirably magnetized.
The consequences of residual magnetism can be problematic or even very costly. A nut that clings to the end of a screwdriver is handy, but two products that stick together in a mould disrupt production, resulting in financial losses. Other possible consequences of undesired magnetism: a coarse surface after galvanization, welds that only adhere on one side, rapid wear of bearings, or metal chips that stick to the parts.
These consequences can be avoided by demagnetizing the material. We supply demagnetization systems and also offer on-site demagnetization of your products. Read more about demagnetization on location or contact us if you have a problem with undesired magnetism.
Loss of magnetic properties.
If we increase the temperature to the Curie temperature, a magnet will permanently lose its magnetism. The atoms vibrate so intensely that there is no longer any global orientation. The material demagnetizes. Mechanical shocks, oxidation or exposure to very strong external fields can also make the magnetism disappear permanently.
This loss cannot be repaired = irreversible.
On the other hand, we have reversible loss: temporary loss of magnetism, e.g. due to a change of temperature. This loss can be reversed by cooling or remagnetization.
The Curie temperature is named after Pierre Curie (1859-1906).
The Curie temperature is the temperature above which ferromagnetic materials no longer possess a permanent magnetic field. This happens because the atoms vibrate so intensely that there is no longer any global orientation. Above the Curie temperature the material behaves paramagnetically.
As the temperature rises, the molecular excitement gradually disrupts the spin alignment. When the Curie temperature is reached, the alignment collapses because the thermal energy exceeds the energy of the magnetic interaction.
It is difficult to measure the Curie temperature exactly. For one thing, the permanent magnetic field around the material only gradually disappears. Secondly, the Curie temperature varies greatly based on even small quantities of contaminants in the material.
For example, if we heat an AlNiCo magnet above its Curie temperature of 850°C, it will no longer be ferromagnetic. It then becomes paramagnetic. Even after the magnet cools down, the permanent magnetic field does not return. There will, however, be new magnetic fields present in small areas within the material, the so-called Weiss domains (Weiss 1865-1904), but these fields are aligned in random directions so their vector sum does not result in an external magnetic field. Nevertheless, it is possible to remagnetize the magnet.
The ferromagnetic elements and alloys with their Curie temperatures:
Material Curie temp.
Sm Cobalt 750-825°C
Pierre Curie (1859-1906)
Magnetism, generated by an electric current.
Electromagnetism is generated by an electric current. In essence, all magnetism is caused by either rotating or revolving electrical charges in eddy currents.
Physics of electromagnetism
A magnetic field is generated around a conductive wire through which an electric current flows. The generated magnetic flux density B is expressed in tesla (T), gauss (G = Vs/m2) or weber (Wb/m2):
Φ = L * I
B = ΔΦ/ΔS, with ΔS as surface[m2].
Φ is the magnetic flux expressed in weber (Wb)
L is the self-induction in henry (H)
I is the current in ampere (A)
We get a strong magnetic field from high currents or high self-induction. High currents are not always applicable or desirable; they can be dangerous and generate heat. That is why we usually generate a high self-induction by winding a wire around an iron core, which is referred to as a ‘solenoid’. The fields generated with each winding act collectively, resulting in a strong and harmless magnetic field.
Magnetism by electric current.
Electromagnets only become magnetic under the influence of an electrical current.
If you need a very strong and deep magnetic field, choose an electromagnet instead of a permanent magnet. The main advantage is that you can quickly turn off or change the magnetic field by controlling the amount of electric current in the windings.
Electromagnets generally consist of a core of ferromagnetic material, such as soft iron, around which a coil has been wound. The core is only magnetic as long as an electric current flows through the coil.
View the range of Goudsmit Magnetics electromagnets.
Flux density B
Fluxdensity is a value for magnetic strength.
The flux density is the number of magnetic field lines that pass through a certain point on a surface. Another indication is magnetic induction. The unit of magnetic flux is the weber (Wb).
The SI unit is T (tesla), which is equal to weber per square metre (Wb/m2). The unit in the CGS system is G (gauss). 1 tesla is equal to 10,000 gauss.
At any given point in a magnetic field, you can see the magnetic flux density as a vector in the field direction with a magnitude equal to the Lorentz force that an electrical wire experiences when it is oriented perpendicular to the field lines.
The higher the flux density, the stronger the magnet is at that point and therefore the better it can hold iron particles at that point.
In a homogeneous field where the surface is perpendicular to the magnetic field lines, it is the product of the surface and the magnetic field strength. Magnetic flux density is usually represented in formulae with the symbol, a (pseudo) vector field.
FluxΦ = B·AΦ = magnetic flux (Wb)
B = magnetic field strength (T)
A = surface (m2)
Goudsmit calculates the flux density using the Finite Elements Method (FEM calculation). This allows us to develop the right magnet for a new or existing product or application, faster and better. You can read more about magnet calculations and simulations here. Or you can read our whitepaper on this subject.
See YouTube video: Flux density vs remance
Maximum value of magnetic flux density that a magnetic material can provide.
The Br value is a magnetic property of permanently magnetic materials, expressed in the unit [T] (tesla). You can derive the Br value from the BH curve, where the line intersects with the y-axis.
In practical magnet system applications, the flux densities are generally lower than the maximum value that the material can theoretically deliver.
See also Remanence.
Induction current, generated by an alternating magnetic field around an electrically conductive material.
Another name for eddy current is Foucault current.
Eddy currents are the electric currents intentionally or unintentionally induced in a flat conductor. It is a physical phenomenon that occurs when, for example, a changing magnetic field is in a metal plate. This could be an alternating field from an electric coil, but it could also be the result of movement that causes the plate to cut through the field lines. When a conductor cuts through magnetic field lines, a current is introduced in the conductor. Read more about eddy currents on Wikipedia.
Goudsmit Magnetics eddy current separators use this principle. These remove non-ferrous metal particles, such as copper and aluminium, in a continuous process, with the aim of recovering, recycling or disposing of metals.
Gauss or tesla meter.
The easiest way to determine whether magnetism is present is with a paperclip. By attaching one to a string and dangling it above the surface, you can locate the magnetic areas. If the product actually draws the paperclip towards it, and the paperclip sticks to it, the magnetic flux density is at least 20 gauss. Below 20 gauss, the paperclip will fall off, and above 40 gauss it will be firmly held in place.
Iron filings will be held in place at levels above just 10 gauss. This is very little, as the Earth’s magnetism (depending on the location on Earth) is around 0.5 gauss.
By using a gauss or tesla meter, also called a magnetic field meter, we can measure the exact field strength and direction of the field. Order them easily online.
Dangers of magnetism
Strong magnets can cause injuries.
Neodymium-iron-borion or Nd-Fe-B magnets, are marketed by Goudsmit under the brand name Neoflux®. These magnets are very strong. Neodymium magnets smaller than one cent are powerful enough to lift over 10 kilograms!
These magnets are dangerous because they can pinch the skin or trap fingers when suddenly attracted to iron or steel.
Neodymium magnets are made with special powders and coatings and are therefore brittle and fragile. They can easily break at temperatures above 150 ºC or when they slam together. When they break, this occurs so suddenly and violently that the flying pieces can cause injuries. Watch our safety film on magnets.
Neodymium magnets should always be kept far away from electrical appliances, magnetic (bank)cards, old (deep) monitors, pacemakers, watches, etc., because otherwise they can cause permanent damage to these devices.
Download our safety guidelines here.
A special form of antiferromagnetism.
Ferrimagnetic material has populations of atoms with opposing magnetic moments (spins), as in antiferromagnetism. However, in ferrimagnetic materials the opposing spins are not of equal strength, which results in a residual magnetic moment.
See also the explanation under antiferromagnetism.
For more information go to the Wikipedia page on ferrimagnetism.
The basic mechanism by which certain materials (such as iron) are attracted by magnets or form permanent magnets.
Ferromagnetism occurs in materials with unpaired spins. Due to the interaction between these spins, the atomic magnetic moments align parallel to each other.
This creates permanent magnetic fields around an object made of a ferromagnetic material.
We use ferromagnetic materials for permanent magnets and the cores of electromagnets, for example soft iron.
Ferrous metals include iron, cobalt and nickel.
Due to its magnetic properties, gadolinium is sometimes also considered a ferrous metal. All other metals are non-ferrous metals.
The distinction between ferrous and non-ferrous metals is also important in the waste processing industry. The economics of further processing makes it attractive to separate the two groups at an early stage of the recycling process. Magnets achieve this separation relatively easily.
View our range of magnet systems for the recycling industry here.
Value for the force of attraction exerted by a magnet on a ferromagnetic object at a particular distance.
The strength of the attraction is determined by the extent to which a magnetic field is ‘inhomogeneous’.
The force index is calculated by multiplying the local flux density in a particular direction by the amount of change of flux density per unit of length in that direction.
That is: Force index = flux density * (change of flux density per unit of distance).
As a formula: FI = B (ΔB/Δx)
The following video (in English) explains things further: What is important when it comes to a magnet attracting a part?
Unit of magnetic flux density, from the CGS unit system.
Gauss is an outdated, but still commonly used unit for magnetic flux density, especially in the magnet industry. The official unit is tesla (T).
1 gauss is equal to one maxwell per square centimetre.
1 G (gauss) = 10-4 tesla; 1 mT = 10 G
1 gauss is equal to 0.0001 tesla in the SI system.
The gauss is named after the German geodesist, mathematician and physicist Carl Friedrich Gauss.
The situation in and around a magnet.
The magnetic field can be compared to the attraction of the earth. However, it has an orientation and a certain value, a magnetic field strength.
In physics and the study of electricity, a magnetic field is a field that permeates space and which exerts a magnetic force on moving electrical charges and magnetic dipoles. Magnetic fields surround electric currents, magnetic dipoles and changing electric fields.
The magnitude and orientation are expressed as a vector, the magnetic field strength H. A related quantity is the magnetic flux density B, also called magnetic induction.
With magnetic viewing film you can see the field lines in a magnet.
Magnetic polarization, J (I)
Material share in magnetic flux density.
Magnetic induction, in the unit tesla or gauss, can consist of two components:
- one part caused by magnetised material
- one part from an externally applied field.
Magnetic polarisation, also called intensity of magnetization - I or J - is the part caused by the magnetized material.
Magnetic field strength, H
Vector unit that expresses the strength of a magnetic field.
In the SI system, the magnetic field strength is expressed in amperes per metre or A/m.
An older unit, from the Gaussian-CGS system, is the Oersted (≈ 79.5775 A/m).
The magnetic field strength is usually given as the symbol H and is the counterpart of the magnetic flux density B. Also called magnetic induction. Read more about Maxwell’s laws on Wikipedia.
Maximum energy density
Point on the demagnetization or hysteresis curve at which the product of B and H reaches its maximum. In general, the higher the BHmax of the magnetic material, the smaller the magnet required for a particular application.
A form of magnetism where certain materials are attracted to an externally applied magnetic field.
The attracted materials form internally generated magnetic fields in the direction of the applied magnetic field. In contrast to this behaviour, diamagnetic materials are repelled by magnetic fields, thereby inducing magnetic fields in the direction opposite to that of the applied magnetic field.
Paramagnetic substances have a relative permeability that is slightly greater than 1 and are therefore considered weak ferrous magnets. The non-ferromagnetic materials can be divided into diamagnetic and paramagnetic materials.
Paramagnetic materials are most chemical elements and certain compounds. They have a relative magnetic permeability greater than or equal to 1 and therefore a positive magnetic susceptibility. As a result, magnetic fields attract these materials. The magnetic moment induced by the applied field is linear with the field strength and rather weak.
Specific force index
A value of a ferromagnetic object that determines when the object is attracted by a magnet.
The specific force index depends on the shape of the object. If this is lower than the force index of a magnet at a certain distance, the magnet will attract the object.
This enables us to predict the distance at which a magnet attracts objects, so that we can choose the right magnet for the application.
Magnets, such as plate and block magnets, have a deeply penetrating field. They also attract ferromagnetic particles at a distance. The capacity to attract certain objects is dependent on the magnetic conductivity of the object and its shape. Not its weight. An elongated shape is easiest to attract. This lessens as the shape becomes more cubic. A spherical shape is the most difficult to attract.
Goudsmit Magnetics plate and block magnets are magnets that you can easily build into existing installations.
Br and HcJ.
Br en HcJ indicate the reversible change (in percentage) of Br and/or HcJ in connection with temperature change. The values depend on the type of material, the quality and the temperature, among other things.
Field lines / Lines of force
Imaginary lines that indicate the orientation of the magnetic field at a given point.
Magnetic field lines run outside a magnet from the north pole to the south pole and inside a magnet in reverse. They never intersect. The density of the magnetic field lines represents the strength of the magnetic field, also defined as flux density.
In a homogeneous magnetic field, the strength and direction of the field is the same everywhere. This is the case, for example, with horseshoe magnets. In an inhomogeneous magnetic field, the magnetic force of one pole is greater than that of the other, causing particles to be deflected.
Magnetic field lines become visible by placing a sheet of paper on a magnet and sprinkling some iron filings on it. The iron filings will cluster along the field lines therefore showing them. A compass needle also points in the direction of the field lines, so you can also follow them that way.
The field lines leaving the magnet return to the magnet through the air.
For free poles, the field lines therefore do not pass through magnetically conductive material.
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The term Weiss domain refers to microscopically small magnetized domains in the crystals of the magnetic materials.
They were discovered by the French physicist Pierre-Ernest Weiss (1865–1940).
When you create an external magnetic field, the walls of the domains shift. The domains magnetized in the direction of the external field become larger. This is at the expense of domains magnetized in other directions.
As the field strength increases, this process continues until all the domains are magnetized in the direction of the external field. The material is then magnetically saturated.
Working point/working line
The working point -Bm, Hm- of a magnet is the intersection of the working line with the hysteresis curve.
This establishes the strength and resistance to demagnetization of a magnet. For magnets with free poles and no external magnetic field, the angle between the working line and the B-axis is dependent on the magnet's length-to-diameter ratio.
Read more about the term hysteresis curve on this page.