The Bank of Japan unexpectedly adopted a negative interest rate policy Friday, stunning investors with a move aimed at shielding the country's sluggish economy from volatile markets and slowing global growth.

The BOJ said it would use a three-tiered system to charge for excess reserves parked with the institution, an aggressive policy pioneered by the European Central Bank that penalizes banks for holding cash and encourages them to loan it out.

"The BOJ will cut the interest rate further into negative territory if judged as necessary," the central bank said in a statement announcing the decision.