Micromag Magnetic Filter
Compact, high-performance grinding coolant filtration systems (flow rates up to 150 litres per min.)
We deal with a broad range of new projects for diverse application. One of the common misconceptions we encounter is the belief that all metals can be attracted by magnets or that all metals are magnetic. It is common knowledge that magnets don’t attract materials like wood, plastic, glass but, when it comes to metals, things are often less straightforward.
Any material can be classified in terms of its magnetic behaviour when exposed to an external magnetic field. Although many think materials are either magnetic or non-magnetic, there are, in fact, five (5) categories of magnetic behaviour. But, for most of the elements of the periodic table, diamagnetism and paramagnetism are two of the most common types of magnetism (at room temperature).
To properly define how a material responds to an applied magnetic field, we introduce a term called Magnetic Susceptibility, which is a value that indicates the degree of magnetization of a material in response to the applied magnetic field (i.e. it indicates to what extent the material gets magnetised by the applied field). Every material will have a Magnetic Susceptibility value. Relative magnetic permeability is another value that is used.
If you take the value of 1 away from the relative magnetic permeability, you will have the magnetic susceptibility value. Magnetic susceptibility is used in material with very weak magnetic responses to external magnetic fields because such materials tend to have a magnetic permeability close to 1, so using magnetic susceptibility can give values that allow easier comparisons. Read more about our range of magnetic materials.
In simple terms, magnetism is all about the spin of electrons around the nucleus of an atom. Going into very technical detail (including quantum mechanics), there is an enormous amount to be said about how this all works – the below is a simplified summary.
Each electron behaves like its own tiny magnet with a north and south pole, when the electrons around a nucleus are lined up in the same direction all pointing north or all pointing south then the atom becomes magnetic. As electrons can rotate or spin around the nucleus of an atom the atom can also possess a magnetic field, even when the poles aren't all in alignment due to the electrons' spinning – the atom acts similarly to an electromagnet. The number of electrons, their spin and their alignment all affect the magnetic properties that will be detected.
Only a vacuum is truly non-magnetic as it has a magnetic permeability of exactly 1. It has no material within it to interact with a magnetic field is applied.
Every other material interacts with an applied magnetic field because every material has a magnetic permeability with a value other than 1. Some materials are obviously ‘magnetic’, but even then there are some differences to how they respond (ferromagnetic, ferrimagnetic materials).
The other materials, thought of as non-magnetic, actually have extremely weak interactions with magnetic fields (usually barely perceivable) but they may actually be very weakly attracted or repelled by magnetic fields, so are actually paramagnetic, diamagnetic or antiferromagnetic. Every material that exists can be classified into one of 5 magnetic categories.
Whether something is magnetic or not is determined by its atomic structure.
If it is magnetic, it is regarded as being attracted to an externally applied field (from an electromagnet or a permanent magnet), but even this isn’t the full picture.
Some materials can be repelled by externally applied magnetic fields – they are still ‘magnetic’, as such as they are interacting with the applied magnetic field. Therefore, we need to define magnetic responses into one of five categories: - diamagnetism, paramagnetism, ferromagnetism, ferrimagnetism, and antiferromagnetism.
In a diamagnetic material, the atoms have no net magnetic moment when there is no applied magnetic field. When a magnetic field (H) is applied the spinning electrons move to cause a type of electric current which produces a magnetisation (M) in the opposite direction to that of the applied field – you may experience this as a very weak repulsion (look up “push me a grape” or “levitating frog” on the internet for an example) - the opposing flux grows slightly more slowly with the applied field than it does in a vacuum.
All materials will show a diamagnetic effect but often the materials may also have larger paramagnetic or ferromagnetic effects. Water, hydrogen, ammonia, bismuth, copper, gold, silver, brass, lead, zinc, mercury, graphite and other diamagnetic materials will be repelled by a nearby magnet (although the effect is extremely feeble). Think of it as a manifestation of Lenz's law. Diamagnetic materials are those whose atoms have only paired electrons.
There are various theories of paramagnetism, which are valid for specific types of material. In the Langevin model, application of a magnetic field creates a slight alignment of magnetic moments, hence a low magnetisation in the same direction as the applied field, but as the temperature increases thermal agitation makes it harder to align the atomic magnetic moments (Curie law) – thermal vibrations continually knock the permanent moments out of alignment.
In paramagnetic molecules, there are unpaired electrons but the cancellation of magnetic moments from the electrons results in a net or permanent magnetic moment, even in the absence of an externally applied field. An externally applied magnetic field moves the electron orbits to line up the ‘poles’ in parallel with the applied field to increase it, causing a weak attraction to the applied field.
The flux grows slightly faster with the applied field than it does in a vacuum. Oxygen, tin (Sn), aluminium (Al), chromium (Cr), tungsten (W), austenitic stainless steels, and hydrated copper sulphate are examples of paramagnetic materials. The thermal agitation causes the permanent moments to continually be moved out of alignment with the field – this is why such materials can have stronger magnetic responses at much colder temperatures e.g. liquid oxygen at -183 °C can be pulled around by a powerful magnet.
Ferromagnetism is only possible when atoms are arranged in a lattice and the atomic magnetic moments interact to align parallel to each other to give a strong magnetic response to an externally applied magnetic field.
Weiss, in 1907, postulated the presence of magnetic domains within the material, which are regions where the atomic magnetic moments are aligned. The movement of these domains affects how the ferromagnetic material responds to a magnetic field. The effect is so strong that magnetic susceptibility becomes a function of the applied magnetic field, which is why the magnetic performance of many mild steels is shown as first quadrant BH curves – the permeability changes with the magnitude of the applied field.
Magnetic susceptibility is large. Ferromagnetic materials are often compared in terms of saturation magnetisation, Bs, (when all the domains are fully aligned) rather than magnetic susceptibility. Iron (Fe), Cobalt (Co) and Nickel (Ni) are elements that are ferromagnetic at or above room temperature. Some alloys of manganese (Mn) are also ferromagnetic.
Many mild steels, with strong magnetic responses, are ferromagnetic. As ferromagnetic materials are heated then the thermal agitation of the atoms means that the degree of alignment of the atomic magnetic moments decreases and hence the saturation magnetisation also decreases. Thermal agitation of the atoms does affect magnetic performance and at a temperature above the Curie temperature for that material, the materials will show paramagnetic performance.
Most grades of mild steel are ferromagnetic. Ferritic stainless steels are also ferromagnetic (e.g. 400 series).
Ferrimagnetism is only observed in compounds because they have more complex crystal structures than elements – they have a combination of parallel and antiparallel aligned magnetic moments in some of the crystal sites when an external magnetic field is applied to them. They have magnetic domains, but they have lower saturation magnetisations than ferromagnetic materials. Magnetic susceptibility is large. Examples include Barium ferrite (BaO.6Fe2O3), Strontium Ferrite and haematite.
The only element exhibiting antiferromagnetism at room temperature is chromium (Cr). When an external magnetic field is applied the exchange interaction between neighbouring atoms leads to an anti-parallel alignment of the atomic magnetic moments causing the magnetic fields to cancel each other out, making the material behave like a paramagnetic material. Antiferromagnetic materials become paramagnetic above the Néel temperature.
Different materials - because they interact with applied magnetic fields differently - can make better or worse performance choices in your magnetic application. You may want magnetic shielding, therefore ferromagnetic or ferrimagnetic materials would be a better choice, but that links to magnetic permeability values that are very high.
You may be wanting to try to filter out materials using magnetic fields – in which case you need to look at the magnetic permeability (and even magnetic susceptibility) to determine how much success you may get in your work.
And if you need a material that is truly non-magnetic, low permeability and low magnetic susceptibility materials are required so the material choice (and also magnet type) is made to suit the application. For working with materials with low magnetic permeability you will need more powerful magnet systems, if you want magnetism to interact best with them. You have to consider the entire system, including environmental conditions and application requirements.
Most steels are ferromagnetic – they will be attracted to a magnetic field. But, there are exceptions, such as austenitic stainless steels (300 series e.g. SS304).
Steel is an alloy of iron plus other elements. Mild steels tend to contain some carbon stainless steels tend to contain chromium. Iron makes up the majority of each composition, which is why most are magnetic, but the addition of other elements to the alloy and the overall composition determines whether the steel will be magnetic or not. Stainless steels are usually non-magnetic (some magnetic versions exist).
This is all linked to alloy structure, the free electrons, and how they spin and interact with an applied magnetic field as well as domain theory.
Stainless steels contain principally iron and a minimum of 10.5% chromium (to give corrosion resistance). Increasing the chromium content beyond the minimum of 10.5% confers still greater corrosion resistance. Corrosion resistance may be further improved, and a wide range of properties provided, by the addition of 8% or more nickel. The addition of molybdenum further increases corrosion resistance (in particular, resistance to pitting corrosion), while nitrogen increases mechanical strength and enhances resistance to pitting.
Ferritic stainless steels consist of chromium (typically 12.5% or 17%) and iron, are nickel-free and contain very little carbon. They are ferromagnetic magnetic due to high iron content and the structure having the metallic atoms are located on a body-centred (bcc) lattice - each bcc crystal is a cube with one atom at each of the eight corners and a single atom at the geometric centre of the cube.
Ferritic stainless steels should be magnetic. Ferritic stainless steels lose their ferromagnetism and become paramagnetic when heated above their Curie temperature so will still attract to a magnetic field even when very hot. Stainless steel alloyed with chromium, molybdenum and silicon is more to give a bcc crystal structure at room temperature, so these are more likely to be magnetic.
Martensitic stainless steels consist of carbon (typically 0.2-1.0%), chromium (typically 10.5-18%) and iron. They are ferromagnetic.
Austenitic stainless steels consist of chromium (typically 16-26%), nickel (typically 6-12%) and iron, possibly with added alloying elements (e.g. molybdenum). The austenitic group is used in greater quantities than any other category of stainless steel (e.g. SS304, SS316). Austenitic stainless steels are often described as non-magnetic but may become slightly magnetic when machined or worked. Austenitic stainless steels have a face-centred cubic (fcc) lattice – each fcc crystal consists of a cube with an atom at each of the cube's eight corners and an atom at the centre of each of the six faces. Austenitic stainless steels should be non-magnetic.
Stainless steel alloyed with nickel, manganese, carbon or nitrogen is more likely to give an fcc crystal structure at room temperature, so these are more likely to be non-magnetic.
Duplex stainless steels consist of chromium (typically 18-26%) nickel (typically 4-7%), molybdenum (typically 0-4%), copper and iron. These stainless steels have a microstructure consisting of austenite and ferrite so are magnetic but usually not as magnetic as ferritic or martensitic stainless steels.
Some non-magnetic steels (e.g. SS304) have an fcc structure and is non-magnetic normally, but when bent, drilled, welded, etc. or worked, those areas become ferromagnetic (by changing in those regions to a ferritic phase) so, the affected areas may start to attract to a magnet. The only fix for this is re-annealing the steelwork.
Some steels retain magnetism after the applied field has been removed simply due to their structure (magnetic domains) – imperfections prevent the magnetic domains from rotating back to a random state to cause cancellation of the magnetisation with the structure. The material is said to have coercivity. Some materials have this purposely (semi-hard materials).
The materials can be demagnetised with the right equipment (demagnetising fixtures) but it will return when the magnetic field is applied then removed again (re-annealing might solve the problem if the issue was caused by the part being worked, machined, bent, drilled, welded, etc).
In most cases, alloys containing materials such as Iron, Cobalt and Nickel will exhibit magnetic qualities or be attractive to a magnetic field - this is usually determined by the ratio of magnetic to non-magnetic ingredients and the alloying process. Common alloys such as Bronze, Pewter or Gun Metal have minimal magnetic material in their structure so they often exhibit almost zero receptiveness to magnetism.
Some stainless steels are naturally magnetic so will attract a permanent magnet system for collection/recovery.
Likewise, some non-magnetic stainless materials become magnetic when machined or worked, so stainless steel filings/shards may be attracted to magnets (stronger magnets may be required). We can do this with our magnetic filtration systems, for example. if you need help on this, please contact us so we can talk to you about your application to determine how we can best provide you with a magnetic solution.
Possibly. As you will see from the above, it depends on how magnetic the contamination is. If it is magnetic like mild steel, then yes; if it is slightly magnetic we’d need to factor in your application to determine what is possible – if you have this problem we can talk to you about it to determine what we can do for you, including the possibility of trial tests for suitability. Read more about magnetic filtration
Ferromagnetic and ferrimagnetic materials react strongly to applied magnetic fields, offering noticeable levels of pull towards the source of the magnetic field. Paramagnetic, diamagnetic and antiferromagnetic materials offer weak interaction forces to the applied magnetic field, often so weakly they are not really noticed. But this isn’t the full story. The magnitude of the applied magnetic field, the size (thickness) and shape of the material the magnetic field is being applied to, air gap between material and magnetic source, environmental conditions (e.g. temperature) and the total magnetic circuit all have an impact. So a very thin mild sheet steel is very likely to offer less pull force than a large slab of mild steel. A small NdFeB disc magnet will probably give less pull than a large NdFeB pot magnet on a large ferritic stainless steel component.
A pot magnet will give much less pull force as the air gap to the ferromagnetic part increases. Steels with higher magnetic permeability will probably pull stronger to the applied magnetic field. Materials with higher magnetic susceptibility (which is linked to magnetic permeability) will offer increased levels of pull force. So the material type does affect magnetic performance - but so too does the applied magnetic field; the complete system needs to be taken into account. The real world problem is that some materials simply do not have reliable magnetic data (and in some situations no magnetic data exists for some materials). So in a few scenarios, the intended application is such that empirical tests are required to determine likely safe performance – often the magnetic solutions are application specific and we work with the customer to get the best solution for their needs.
You can learn more about magnets by reading our quick guide here.
If you would like to discuss your specific magnet needs with our team of experts, why not get in touch today? We offer free consultations to understand your requirements and will tailor a solution suited to your business. Click here to find out more.