Resistive switching in HfO2 in the sub-forming regime (before an electroforming step had been fully performed) is studied by electrical measurements using a very low current compliance of 1 mu A. Electroforming under low current limitation results in reduced self-heating and partial filament formation. Following the reset process in this sub-forming regime, the device fully recovers its pristine resistive state. Furthermore, a dual bipolar resistive switching (DBRS) effect is observed, which we model as two antiparallel bipolar resistive switches. We attribute this phenomenon to intermittent formation and rupture of filaments originating from opposite electrodes. Following the rupture of a filament, originating from one of the electrodes, another filament originating from the opposite electrode is formed. (C) 2015 Elsevier Ltd. All rights reserved.