So... we were measuring RLC circuit, and while we inserted core into solenoid, the amplitude on osciloscope decreased. It would imply, that the core is diamagnetic. But it is original core from PHYWE solenoid and I don´t see a reason, to create solenoid with diamagnetic core. I´ve heard, that FeII could be diamagnetic, so is it posible, that iron core can change to be diamagnetic? I´m going to measure density of the core next time, just to be sure, that it really is iron.
Not completely clear from your description, but do you mean that you tuned your frequency to achieve a current resonance and then inserted the iron core?
Did you not consider that the resonant frequency is completely changed by the increased inductance, so you would then be far from resonance at the same frequency? Depending on the resistance in the circuit, this could diminish the current very significantly.