SQUID: What and How Does It Measure?

[Anthus]

What exactly does SQUID measure?

Hi,

I am not sure, if Superconducting Quantum Interference Device measures total magnetization of sample, or only the component of magnetization that is parallel to axis perpendicular to Josephson junction?
http://www.nature.com/nature/journal/v492/n7429/fig_tab/492358a_F1.html

 

[marcusl]

Technically, a SQUID responds to the line integral of the vector potential around the loop. In practice, that is related to the amount of magnetic flux \Phi=\int{\mathbf B \cdot d\mathbf S} through the loop, which is basically normal to the junction as you say. It is magnetic induction, not magnetization, that matters however. Whether that flux arises from the field of an electric current or from the magnetization of a solid is irrelevant.

 

[f95toli]

Marcusl is 100% correct

However, since you referring to "magnetisation of samples" I suspect you might have come across a SQUID used as a magnetometer in some measurement application. In many (I'd say most) such applications the SQUID loop is arranged in such a way that the SQUID measures the gradient of the field (and in some cases even higher order derivatives).

This is -not surprisingly- known as a gradiometer, and is what would typically be used for studies of materials, bio-applications etc.

 

[Anthus]

Thank you for your anwers!
Magnetometer I will use has three detection coils that work as second-derivative gradiometer.
However, what about situation:
https://docs.google.com/file/d/0BzvjZoZeQg3nT1NSeEpENjk2STQ/edit?usp=sharing
when the sample is magnetized only perpendicular to coil axis? It seems that there is no magnetic flux through any coil in this case. Do coils (SQUID) detect signal?
I suppose they don't. Wikipedia also says, that SQUID is a "vector magnometer"
http://en.wikipedia.org/wiki/Magnetometer#Vector_magnetometers, so it measures only component of magnetic field - that parallel to coil axis.

 

[marcusl]

Correct, you need some component of field that is along your gradiometer's axis to get a reading.