How to apply Butterworth High Pass Filter
If you just want to do thresholding, then you don't have to use any filter. Just throw away all those samples whose values are below your threshold (in this case, $100 \ \mathrm{pT}$).

In MATLAB, you could do something like this:

data(data &lt; 100) = [];


If you want to keep the array size, then you might consider replacing those values with NaN or 0 instead of [].