Ferrite materials and magnetization - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 11, 2020, 02:57:28 pm
content: Hi,can all types of ferrite material be magnetised?For example, can ferrite rods used for antennas or those used for EMI suppression on cables or those used in transformers be permanently magnetised?What is the property (name) that describes the ability to maintain in time the magnetisation applied?Thank you

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 11, 2020, 03:57:41 pm
content: The property is called magnetic remanence. Ferrite grades used for transformers and inductors are actually optimized to minimize hysteresis loss, which means that they also have very little remanence if I'm not mistaken. Note that the field strengths needed to make decent permanent magnets are extremely high, in the order of tens to hundreds of kiloamp-turns if done with an electromagnet.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 11, 2020, 04:26:35 pm
content: Thank you Wolfram,comparing for example two magnets, one a Ferrite magnet (low magnetic remanence) and one a Neodinium one (high magnetic remanence) which are then both magnetised with the same current (OR with the same final strength)... If they are then left fully isolated from each other and external interferences, in time, does the ferrite magnet become demagnetised faster and sooner than the Neodenium one? Or not?Thank you

author: duak
author_rank: Super Contributor
timestamp: June 11, 2020, 05:34:52 pm
content: Ricko, ferrite materials have a range of magnetic remanence as well as other magnetic properties. Some have medium to high remanence and are used to magnets for loudspeakers and magnetrons. Others have low remanence and are used to make cores for switching power supply inductors.The way I think about ferro-magnetism is that it's about how the iron atoms in a core or magnet line up. Because of the way the electron orbitals in an iron atom (and nickel, cobalt and a few others) line up, the atoms have a natural north and soutn pole. In a piece of pure iron the atoms are pointing in differing direction and the overall magnetic field is pretty much zero. If you apply an external magnetizing force, like a current in a coil or another magnet you can get the iron atoms to line up and induce a magnetic field in the piece. When the external field is removed, most of the iron atoms snap back to a random orientation and the remanent field almost disappears, but not quite. Some iron atoms still line up and so there is now a field. If the iron is alloyed with other elements like carbon, some iron atoms are more likely to be locked into the magnetized orientation and so have greater remanence. This is why steel tools tend to get magnetized and bits and bobs get stuck to themSome elements are better than others at locking the iron atoms into a magnetized orientation than others. Aluminum-Nickel-Cobalt (ALNICO), Neodymium-Boron and Samarium-Cobalt are particularly effective. ALNICO has a problem in that it has a high remanence only when the magnetic field can be maintained at a high level after the magnetizing force is removed, ie., in a magnetic circuit. That's why ALNICO magnets had keepers between their poles or were magnetized in place like in loudspeakers. The latter two types of magnets are far better at retaining fields when taken out of a magnetic circuit.Ferrites use other elements to give the appropriate properties. Ferrite magnets aren't as strong as they have only medium remanence but they're a lot cheaper as they use more common alloying elements. Neodymium, Samarium, Nickel and Cobalt are not so common and are more expensive.Temperature is the enemy of all magnets. Higher temperatures tend to cause the iron atoms to randomize their orientations even if they are locked in place. Supermagnets are generally better in this respect than ferrite magnets.

author: Benta
author_rank: Super Contributor
timestamp: June 11, 2020, 05:47:37 pm
content: The "technical marketing" names are:"Soft Ferrite" = low remanence and coercivity for inductors"Hard Ferrite" = high ditto for permanent magnetsThe names are misleading, as they suggest mechanical properties. Nevertheless, these are common terms and will help you when searching.

author: uer166
author_rank: Super Contributor
timestamp: June 11, 2020, 06:20:59 pm
content: What about square ferrites that have high remnance and high permeability with a very sharp (square) BH curve?

author: duak
author_rank: Super Contributor
timestamp: June 11, 2020, 08:29:13 pm
content: Uer166, square B-H curve ferrites' composition is optimized for the application. Ideally, you'll want the maximum number of iron atoms to store the greatest amount of magnetic flux but iron atoms don't want to stay lined up ("Something about free will" - from Time Bandits). Adding other elements helps lock the iron atoms in place but dilutes and reduces the maximum flux. Core memory was just before my time so I'm not familiar with the exact composition. I'd expect that the cores should not be electrically conductive otherwise the eddy current would affect things.It also helps that they are formed into toroids that close the magnetic circuit to retain the flux. If you were to break a magnetized magnetic core into two pieces the field strength would drop and let some of the energy stored in the flux escape. If you reassembled the core, it would still have a field but it wouldn't be as strong. It's very much like the ALNICO case above.A few years ago I tried to remagnetize a machinists' base with a switchable magnet that's used to clamp a gauge onto a an iron surface. I had a big coil of wire and a piece of round steel that fit inside of it. I used a bunch of steel pieces about 2 cm thick and 5 cm wide to make a magnetic circuit that coupled the base. I applied a DC current to the coil to create a magnetic field. All the steel pieces stuck together strongly enough even after the current was removed that I could pick the whole thing up (7 or 8 kg) even though it wasn't fastened or welded together. Levering two pieces apart caused the whole thing to fall apart. The steel pieces had almost no residual magnetism afterwards. I don't think I had a strong enough field to recharge the magnet in the base. It probably has an ALNICO magnet and would require a fair bit to do the job.I can't remember all of my college electromagnetics and I'm quite amazed that people that actually understand it are able to exploit materials to get them to do the things that they do.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 11, 2020, 09:19:23 pm
content: No; there are ferrites, i.e. compounds of metal oxides and Fe2O3, which are antiferromagnetic. These have ferromagnetic ordering but the atoms cancel each other out, giving no external magnetic field.Sounds pretty useless, but it's not entirely so, because the key is the number of atoms contributing towards the net field. IIRC, YIG (yttrium iron garnet, a compound of Y2O3, FeO and F2O3 -- a ferrite) has a net sum so is magnetic, but fairly pitifully so (Bsat like 50mT or something, IIRC?). Fortunately, that's almost irrelevant in its typical application -- microwave resonators, circulators and etc., the signal flux is quite small indeed.Most ferrites are of the spinel type, MeO.Fe2O3, where MeO is some metal oxide. (Spinel, the mineral, being MgAl2O4, but FeO freely substitutes for MgO, and Fe2O3 for Al2O3.) Magnetite for example is a hard magnet, not a very good one, but it is of this form (FeO.Fe2O3).Conventional soft ferrites for power, RF and filtering are (Zn,Mn)O.Fe2O3 and (Ni,Zn)O.Fe2O3, the former having higher Bsat (up to 450mT or so) and the latter having higher resistivity and cutoff frequency.Conventional hard ferrites are SrO.Fe2O3 and others, I forget what all exactly; they have reasonable Bsat (~0.4T) and coercivity (takes lots of amp-turns to de/magnetize them). They're nowhere near the strong magnets, but they're cheap as hell so you can just slap on some cheap mild steel pole pieces and make a powerful speaker for example.Among strong materials: old school AlNiCo is Bsat ~ 1.5T but lower coercivity, low enough that it can't be left in free space (no pole pieces) without losing magnetization. SmCo is the second best, with Bsat ~ 1T and high coercivity. NdFeB is the current "supermagnet" with Bsat ~ 1.5T and quite high coercivity.As for what makes a hard or soft magnet, eh, I don't recall that there's much known? Hysteresis losses in soft materials can be due to flux pinning, where a magnetic domain or pole gets stuck on an impurity or defect site; I don't know if the same mechanism can apply on an atomic scale, which might explain high-coercivity materials. Need to do more reading, it's been a while.Tim