Core loss appearing in buck converter ripple current
Core loss is caused by current induced into the magnetic core, which is effectively a secondary winding with a resistance across it. 

Since the voltage going into this 'transformer' is a square wave, the current and voltage across the core resistance is also a square wave. If you remove the slope caused by current flowing into the buck converter's load then this low amplitude square wave is what's left.

Here is an LTspice simulation showing the effect. L2 and R2 represent the core 'secondary winding' and its resistance (shown as the equivalent 1:1 transformer - in reality it is a single turn with extremely low resistance):-



I(L1) is current going through the inductor. V(n002) is voltage across the core resistance.