Irll024N logic level mosfet + atmega 2560 problem
You must use a p-Channel MOSFET. 

IRLL024N is an n-Channel MOSFET, therefore there will be a Vth-Vov drop, between the gate and the source (Vov = overdrive with respect the threshold voltage, to allow the load current to flow into the MOSFET).

Otherwise, you must use a VGS value much higher than the drain voltage (e.g. the gate must be 10V).