I currently have the magnetic moment vs temperature curves for one set of plots and magnetic moment vs. applied field at three temperatures. I am told that you can get the critical temperature for superconducting materials from the SQUID measurements. I am just not sure if I can get it from the data I already have or if there is another run of measurements I need to do.
Hi, superconducting state is purely diamagnetic. If you perform M(chi) vs T measurement, it will show paramagnetic region (positive moment) to diamagnetic region (negative moment) at the superconducting transition. In addition to this, if you have resistivity vs T data, it will show a sharp drop at superconducting transition temperature.
So in my moment vs temperature, the moment is always positive and never has a negative value. That is the zero field cooled and field cooled plots and a constant field strength.
Only in the moment vs applied field does it change from positive to a negative susceptibility (chi) value. But those are all at a given single temperature so all I can tell is that it changes from paramagnetic to diamagnetic between 20-300 Oe. Also is negative chi from say -20- -300 Oe. So a small region.
On the temperature vs moment plot I get a peak but not sure what that actually means. That is the ZFC side of the plot.
This graph shows that you have at some temperature a small drop of the magnetic moment, but it remains positive at all temperatures. There are two possibilities: one is that this drop is not due to superconductivity. Another possibility is that your sample is mainly paramagnetic, but it contains a small fraction of the superconducting phase. The diamagnetism of this small fraction is not sufficient to win against the paramagnetism of the main nonsuperconducting phase, so magnetic moment stays positive. It would be helpful to perform FC experiments in the same field. In case of superconductivity the drop of the magnetic moment will decrease compared to ZFC measurement.
It is clear from your graph that you do not have a system that is only a superconductor. As stated above there is something paramagnetic in the system as well. Also it appears from the plot that you need to go lower in temperature to be sure you are below the superconducting transition temperature. The following reference has some nice examples of both a standard superconductor Al, and UPt3 a novel superconductor http://dx.doi.org/10.1103/PhysRevB.52.4446
I had one more question. This paper I read measured the percent of their material that is superconducting.
"The superconducting volume fraction was estimated from the ZFC magnetic susceptibility at 2 K is 84%"
I am planning to do the same measurement but still not sure how you get the volume fraction. Not sure how you differentiate between what particles are and aren't superconducting in that type of a plot.
Since you know a completely superconducting sample is a perfect diamagnet, the assumption is usually made that if you have a diamagnetic response that is less than perfect then that reduced fraction is what is not superconducting. However this means you have to know what a perfect superconductor looks like in your measurement device. I suspect your squid is already calibrated so a perfect superconductor should have a susceptibility value of -1. In the case of http://dx.doi.org/10.1103/PhysRevB.52.4446, they used a well known piece of pure Al to calibrate a tunnel diode oscillator and their mutual inductance rig. Also you have to know what else in your sample or device could give you a diamagnetic or paramagnetic signal. This is why the FC measurements would help you understand what is going on. The Dissertation that has more details to the above paper is at http://uf.catalog.fcla.edu/permalink.jsp?20UFDC000010972
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