My understanding of magnetization in core memories is as follows: a bit is written to an inductor's core by magnetizing it, by applying a large current. Later, the bit is read by observing the effect of this magnetization on the saturation point. Is this correct?

What types of inductor cores exhibit magnetization? I have attempted to magnetize ferrite cores (of various common iron alloy powders) by using a nice big bank of electrolytics to pulse a current that is 500 times larger than the saturation current. Then I measure saturation current in both polarities. However, I do not see any shift in the saturation point. Does this mean that the typical inductor core cannot be magnetized?

  • \$\begingroup\$ You will need to go back to the late 1950's and early to mid 1960's to get what you need to see in published papers. I can provide some of those papers, if you care (I've got them stored on disk here.) I don't believe anyone is making those kinds of cores, anymore. When I was looking around, I had to buy them as 'old stock' taken from large memory boards from old computers. I couldn't find any modern manufacturer making those cores, today. There's no market. So no one makes them. They can, of course. But they don't because there are no buyers for them. Old ones work well, though. \$\endgroup\$ Aug 2 at 6:05
  • \$\begingroup\$ Here's a reference: Theory and Design Techniques for Magnetic-Core Memories, Vol I of II. I never did find volume II. Maybe never happened. But the document covers some details well. Around the same time there is a thesis by Wolde-Ghiorghis, "Some developments in high-speed ferrite-core memories". And if you want to get back to the early phases there is, from 1957 by Gabriel E. Valenty, "A Medium-Speed Magnetic Core Memory" in 1957 Western Computer Proceedings. Best wishes. \$\endgroup\$ Aug 2 at 6:48
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    \$\begingroup\$ You might try looking up the keywords "square loop ferrites". \$\endgroup\$ Aug 2 at 6:51
  • \$\begingroup\$ Note that ferrite cores you need are very "small" (diameter < 1 mm). \$\endgroup\$
    – Antonio51
    Aug 2 at 7:00
  • \$\begingroup\$ @Antonio51 Yeah. And there's a reason for that which goes beyond just being able to have enough bits to be useful. Making them smaller means that far less energy is required to traverse the hysteresis loop. And less energy per bit is also much better. \$\endgroup\$ Aug 2 at 7:07

1 Answer 1


Extreme currents aren't necessary; the core material is relevant, and can be switched at much lower currents depending on type.

The easiest way to experiment with core today is probably to find a magamp core in an oldish ATX power supply (they were used to separately regulate the 3.3V output from the 5V (AC) supply, and look like any old toroid choke, except for details about how they're wired). These are either ferrite, or amorphous/nanocrystalline strip, with a "square" B-H curve. Flux (volt-seconds) is only delivered in response to sufficient current flow to push the operating point around the hysteresis loop.

Magnetic core takes advantage of hysteresis, by using the core itself as a SUM gate, i.e. it only flips when N of the M inputs (read: wires through the center) are active. Which, when N = M, this is simply an AND function. So there's a row, column, and sometimes plane or bit line as well, which, only when all activated, flux is generated and thus a pulse detected (or not), successfully reading the core.

The core then needs to be remagnetized, of course (by the same process), to furnish a lossless read operation, that is (or perform a write while discarding the previous value).

Magamp cores are a poor choice, because they're optimized for minimum losses -- that is, minimum hysteresis loop area. The height of the loop is still about the saturation point (0.25-0.4T for ferrite, 0.8-1.5T for iron), which means the width is made low (i.e., few amp-turns required to switch it). Which is good for the application, that's less bias current required to drive it, less power dissipated. But you can still demonstrate core memory this way, scaling drive current appropriately; it's just more easily corrupted.

Conversely, hard magnets are made with maximal width (lots of amp-turns required to magnetize), which is good for them so they can magnetize things in turn (i.e., be a permanent magnet), and not good here because you need massive currents to flip even small cores.

If you must -- you can of course find some either vintage, or modern equivalents or substitutes (including perhaps magamp cores like this), and make a more authentic test circuit.

Oh, and to be clear -- "square" materials are in contrast to "soft" materials used for inductors and transformers, which have minimal hysteresis loss by reducing loop area along either axis -- this can sometimes lead to the same thing (some high-μ nanocrystalline materials for transformer service are nearly square*) but mostly leads to lower remenance. Since the zero-bias ("initial") slope (μ = dB/dH) is significant in these types, you don't really observe "switching" behavior.

*Which are likewise suitable for mag-amp use, but due to how narrow the loop is (coercion as low as a few mAt?), it may be quite difficult to actually demonstrate. Like I said, they may be a poor choice.

Ah, I should also mention the correct terms. Remenance is the flux density (B) at zero magnetization (H = 0). Coercion is the H required to force B to zero. On the B-H curve, coercion is the width (or rather, half of it), and remenance, height-wise.

Furthermore, airgap (when applicable, as in most inductors) reduces μeff, stretching out the B-H curve in such a way that the hysteresis loop is a smaller fraction of the total VARs being cycled; remenance is reduced, and Q factor increases.


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