Hi, I am measuring the FMR from a thin permalloy film, placed on top of a CPW (the very usual setup with VNA FMR).
Once I get the measurements for several values of B, I see that the noise level has increased at high frequencies (range is from 10 MHz to 10 GHz) and there are more fluctuations above 8 or 9 GHz, for the latest (and therefore highest) values of B. Is the increasing of B the reason I get that higher noise? Or could be the increase of temperature when we evaluate at those values of B? Any idea on the matter will be appreciated.
Thanks
Hi, we have made a little more research and we have seen that this noise increases not with the increase of B but with time (each swept of B takes about half minute). So, why is this noise at high frequencies increasing when time goes on? Is there any kind of reflection or feedback in the network? I am just guessing.
Thanks
I think the increase in the noise power is as a result of increasing the temperature, because noise power is directly proportional to temperature. Also, why increase in bandwidth doesn't affect the noise power? It is only when you have constant change in bandwidth which is ideal in this case.
Noise is supposed to increase with temperature at all levels of frequencies.
Very interesting.
The noise of a magnetic material can be relied on several factors. Thermal-Electronic noise (Nyquist) noise is caused by temperature and a frequency band across a resistance R, according to the formula:
V = (4kTRΔf)1/2
where V = rms noise voltage, k Boltzammn's constant, R resistance, Δf the f band
It should be noticed that in magnetic materials a higher B results in higher thermal losses (in the magnetic core) due to a higher no load voltage, resulting also to a higher noise (because noise is directly connected with this no load voltage). This is well known from transformers.
Moreover, a higher noise level is expected for higher frequencies, as higher thermal losses are produced in this case. Resistance is also augmented bringing on more losses, therefore more noise.
If you let the equipment exposed to high f and B for a longer time, a significant amount of heat is also produced, as heat is equal to magnetic losses*time. This heat leads also to a temperature rise, provoking a noise elevation.
Additionally, you have to consider the Barkhausen effect. Fast changes in B will move Weiss-Bloch walls even faster, causing more noise. In my opinion you have to take into account not only the B, f and t, but also the dB/dt (and df/dt) when applying your measurements.
Perhaps permalloy as an anisotropic material appears a stronger B-H microfluctuations resulting to a significant Barkhausen jumps.
Hi all, many thanks for your answers, they are really very helpful. In any case, last experiment was leaving the VNA alone showing S12 from the sample with the electromagnet switched off and this fluctuations, rather than noise, still appear. Funny thing is that it starts from around 10 GHz till the end of our freq range (40 GHz). My guess is that it has something to do with losses in the material as you said, or something related to the CPW...
Update of the situation, after tighten everything up and one hour and a half later, it seems that fluctuations have definitely stop growing. Why so? Still on it. Any idea?
Update: found this morning that the fluctuations have gone higher, so false alarm.
Cheers
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