From data in frequency domain to time domain (IFFT)

Hello everyone. I have a big problem. I really need your help.

My main purpose of work is to find out impedance(t) from impedance(f). When the variable frequency source(1 Hz ~ 1MHz) is injected, the impedance(f) is calculated from measured voltage and current. The x-axis of impedance(f) is a frequency(1 Hz ~ 1MHz) and y-axis is an amplitude(ohm).

In this case, I would like to ifft the impedance(f). I have a question. The impedance(f) is consisting of only positive frequency. Can i obtain the original impedance(t) from the positive frequency?

I think it is impossible. so I made the data in the negative frequency(symmetry). but the result was not good.

Only if I have data in positive frequency, how can i get the data in time domain with ifft?