O zjawisku magnetyzmu
- O zjawisku magnetyzmu
- Ferromagnetic materials
- Permanent magnetism
- Goudsmit permanent magnets
- The earth is a magnetic field
- Influence of magnetism
- Magnetizing magnets
- (De-)Magnetizing curve
- Ferrous metals
- Undesired magnetism
- Loss of magnetic properties
- Magnetic flux density B
- Eddy current / Foucault's current
- Measuring magnetism
- Dangers of magnetism
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.
Ferromagnetic hard materials can remain magnetic
A permanent magnet is a ferromagnetic hard material. It keeps its permanent magnetic properties for a long time after magnetization, and has enough resistance against demagnetization.
All magnets have 2 poles that we call the North (N) pole and the South (S) pole. 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 other magnets and attracts the south pole of other magnets. 2 south poles also repel each other.
Magnetic field lines
Field lines are imaginary lines that indicate the orientation of the magnetic field at a given point. They become visible by placing a sheet of paper on a magnet and its magnetic field, and sprinkling some iron filings on it. The iron filings will cluster along the field lines thus 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 define both path and direction. Outside the magnet, they travel from north to south and inside the magnet from south to north. The density of the magnetic field lines represents the strength of the magnetic field, also defined as flux density.
Goudsmit permanent magnets
Various magnetic materials and qualities available
Goudsmit magnets are of such high quality that they hardly lose any magnetic force over time. Provided that you apply them within the specifications, such as temperature range and external magnetic fields.
We deliver, and use 4 sorts of magnet alloys in our systems. 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.
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. The exception is ferrite, of which the resistance to demagnetization decreases when the temperature is lowered.
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®)
The earth is a magnetic field
Geographic North and South pole
The earth also has a magnetic field. A magnetic south pole near the geographic North pole and a magnetic north pole near the geographic South pole. Therefore, a free-spinning magnet will always take on a north-south orientation. The pole names of a magnet are derived from this. What’s confusing is that we call the south pole of the earth magnet magnetic north pole and the north pole of the earth magnet the magnetic south pole.
Influence of magnetism
Reaction of different materials on magnetism
Ferromagnetic materials are the only kind with strong enough magnetic properties to be drawn to a magnet (which is why they are considered magnetic). But 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 it shows ferro- or ferrimagnetic behaviour. The forces that occur with dia- and paramagnetic behaviour are much weaker, and 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. But this is enough for light objects containing much water, to float if it 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.
Direction of magnetization - anisotropic – isotropic
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. Goudsmit has very powerful magnetization equipment with which permanent magnets can be magnetized to their maximum saturation.
We magnetize magnets by placing the magnet 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 briefly generates a very strong magnetic field, causing the magnet to take over the direction of that magnetic field.
Ferrite magnets are also available in isotropic versions. These magnets are not pressed in a magnetic field, and can therefore later be magnetized in any direction.
Segment-shaped ferrite and Neoflux® magnets are also available. These are radially anisotropric and can therefore only be magnetized in the radial direction.
Hysteresis in ferromagnetic materials - BH curve = hysteresis curve
‘Hysteresis’ is present in ferromagnetic material. You can 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 are in the origin of the graph.
If we apply a magnetic field, the ferromagnetic material will become magnetic. This continues until all the ‘Weiss regions’ 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 regions will mostly maintain their position.
Only when the field becomes more negative will the total magnetization also change direction. This continues until all the spins are oriented in the other direction and the magnetization is reversed. The product is now demagnetized.
Hysteresis curve (BH 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, the blue curve is followed the first time an external field H is applied. Upon reaching the saturation flux density, with magnetic field strength Hs, the magnetization does not increase further.
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 ‘areas of Weiss’ not yet returning to their original state.
Only when the externally applied field strength has reached an oppositely directed value - the coercive field strength Hc - the magnetization B becomes zero. 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 calledsoft magnetic materials. If Hc is very large, they are called hard magnetic material.
Remanence is the magnetic induction in magnetic material at field strength zero (H=0), and after complete saturation.
Hysteresis curve (BH curve)
Metals with magnetic properties
The 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, but 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. This can be easily achieved through the use of recycling & sorting magnets.
Undesired magnetization of ferromagnetic materials
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 welding.
In that case, the induced magnetism often originates from other magnetic sources such as lifting magnets, clamping tables, loudspeakers or magnetic transport 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 can result in remanent magnetism. This can even occur in stainless steel.
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.
Loss of magnetic properties
If we increase the temperature to the Curie temperature, the magnet will permanently lose its magnetism. The atoms then vibrate so intensely that there is no global orientation anymore, as a result of which the magnet demagnetizes. The same can happen due to mechanical shock, oxidation or exposure to very strong external fields.
This loss can not be repaired = irreversible.
On the other hand we have reversible loss: temporary loss of magnetism, e.g. due to change of temperature. This loss can be reversed by cooling and/or remagnetization.
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, 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.
They are often preferred to permanent magnets when a deep and very strong magnetic field is necessary. A main advantage of an electromagnet over a permanent magnet is that one 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.
Magnetic flux density B
A value for the magnetic strength
The flux density is the number of magnetic field lines that pass through a certain point on a surface. 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 thus the better it can hold iron particles at that point.
Goudsmit can calculate the flux density using the Finite Elements Method (FEM calculation). This allows us to develop the right magnet faster and better, for a new or existing product or application. You can read more about magnet calculations and simulations here.
Eddy current / Foucault's current
Induction currents, generated by an alternating magnetic field
An eddy current is an induction current, generated by an alternating magnetic field around an electrically conductive material.
Another name for eddy currents is Foucault currents. These 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.
For our Eddy current separators, we make use of this principle to separate non-ferrous metals from material flows for recycling purposes.
Gauss or tesla meter
The easiest manner 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.
Using a gauss or tesla meter, also called a magnetic field meter, we can measure the exact field strength and direction of the field.
Dangers of magnetism
Dangers of magnetic field
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!
As a result, these magnets are also dangerous, as they can pinch the skin or fingers when suddenly attracted to iron or steel.
Neodymium magnets are made with special powders and coatings and are therefore brittle and easily broken. They can easily break at temperatures above 150 ºC or when they slam together. When they break, this occurs so suddenly and violently that flying pieces may cause eye- or other injuries.
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.
Some magnet-related warning and prohibition signs that you can find or affix yourself: