Latetly we recognized a parasitic signal in our voltage measurements. It is a perfect sine wave with a period of a few hours (f = uHz). What could be the origin of such a signals?
Ok, a few more details:
We do Hall-Effect measurements in a cryostat. The temperature is held constant at a few hundred mili Kelvin. We use a standart six terminal measurement to obtain the hall resistance and the magnetoresistance. We use standart lock-in amplifiers at a detection frequency of ca 22 Hz to measure the voltage along and transverse the current path through the sample.