When a ferromagnetic conductor, such as carbon steel, experiences a sudden change of external magnetic field, the magnetization of the material does not follow the external magnetic field simultaneously. Namely it takes time for the magnetic field to reach its final steady state. Two effect exist in the material contribute to the relaxation time:
1.eddy current rises in the material and produce a magnetic field which opposes the original magnetic field change
2.magnetic domain reversal and domain wall movement in the ferromagnetic material does not complete instantaneously
How can I separate these two effects in carbon steel, only measure the relaxation time caused by eddy current effect or only measure the time caused by magnetic domain movement?
I would suggest scaling method. See for instance: arXiv.1309.0022, arXiv.1111.0939, arXiv.1304.6380 and there in.
When using pulsed eddy current in Carbon steel the signal drops of as function of time according to time^-1.5 while for domain relaxation it drops off as time^-1. See European Patent EP 2 256 489 A1
A possible solution: 1. try to freeze the magnetic effect by lowering the temperature (not to much, in order for the resistivity to be almost constant) and 2. Change the resistance (the perimeter of Eddy currents). Thus, you will be able to increase the isolation of the two effects. Also, try to work with monochromatic, this means frequency locked. I would adventure to say that the magnetic effect is slower and it is predominant in the tail of the impulse response, but the perimeter length and the magnetic properties caused by structural peculiarities certainly affect the two behaviors.
Thanks for all the answers, really appreciate your help. There are still some questions that I want to consult.
I think lowering temperature can be a good way to isolate the two effects since the influence of temperature on magnetic relaxation is more remarkable. The amount of temperature to be changed is very significant in this method
As for changing the perimeter of eddy currents, I'v thought about this approach before. One way to do this is using laminated steel plate instead of a steel block. Certainly, eddy currents will be suppressed, but I'm not sure whether the domain movements will be suppressed accordingly.
I've read the European Patent EP 2 256 489 A1. In this patent, a tertiary magnetic field is used to neutralizing temporal relaxation processes in the magnetisable layer. I am to know how to determine the magnitude of the tertiary magnetic field.
The simplest and reliable way is possible, f you can change the form of sample.
The dynamics of domen wall almost do not depend of a size and form of your sample. The eddy currets are localised at surface. Make a stare-form sample and the effect of eddy currenst is to be chenged.
Reversal of domains can be forced by applying a reversed field. The trick is to relax as much as needed so that net relaxation is 0.
Dear all
there are very well defined formulae for the frequency dependent skin depth avialable which take the(frequency dependent and complex) mu of the ferromagnetic material into account (from any good textbook) .Also this kind of problem can be nicely solved by numerical methods for all kind of geometric structures.
However in some case one has to look also at remanence and hysteresis in the ferromagnetic material as well as magentic anisotropy..and then its getting more difficult.
We use a patented technique known as F2F for the measurement of NdFeB magnets in an industrial situation with Puled Field Magnetometers.
We are able to deduce eddy currents and remove them from measurement and obtain measurements that agree with other techniques to better than 1%.
We apply large +/-7T pulses at two frequencies. As we know the relationship of the two frequencies, and therefore the relationships of the eddy currents, we can calculate what is going on.
Some info at http://www.hirst-magnetics.com/character/pfm14_p1.shtml
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