5 FIG. 5: Resistive hysteresis loops obtained for an increasing degree of asymmetry. A$_{L}$ = 1000 and A$_{R}$ = 1000, 100, 50 and 25 in panels a , b , c and d respectively. The right panels show experimental data for a manganite ( e ) and a cuprate sample ( f ) that were rendered asymetric by means of intense and fixed polarity pulsing. The former was pulsed in current control mode and the latter in voltage control, similarly as in Refs. 21 and 25, respectively. Notice the qualitative similarity of data of panels e and f with panel d ; and of data in Fig. 2b with panel a . The data of Fig. 2c seem to be in between the results of panel a and b . Support from CONICET (grants PIP 5254/05 and PIP 112-200801-00047) and ANCTyP (grants PICT 483/06 and PICT 837/07) is gratefully acknowledged. ACKNOWLEDGMENTS 1 G.I. Meijer, Science 319 , 1625 (2008). 2 R. Waser and M. Aono, Nature Materials 6 , 833 (2007). 3 A. Sawa, Materials Today 11 , 28 (2008). 4 M. Janousch, G.I. Meijer, U. Staub, B. Delley, S.F. Karg, and B. P. Andreasson, Adv. Mat. 19 , 2232 (2007). 5 K. Szot, W. Speier, G. Bihlmayer, and R. Waser, Nature Materials 5 , 312 (2006). 6 I.H. Inoue, S. Yasuda, H. Akinaga, and H. Takagi, Phys. Rev. B 77 , 035105 (2008). 7 K. Fujiwara, T. Nemoto, M.J. Rozenberg, Y., and H. Tak- agi, Jpn. J. App.Phys 47 , 6266 (2008). 8 I.G. Baek, M.S. Lee, S. Seo, M.J. Lee, D.H. Seo, D.-S. Suh, J.C. Park, S.O. Park, H.S. Kim, I.K. Yoo, U.-In Chung, and J.T. Moon, Tech. Dig. IEDM. 587 (2004). 9 S.Q. Liu, N.J. Wu, and A. Ignatiev, Appl. Phys. Lett. 76 2749 (2000). 10 A. Beck, J.G. Bednorz, Ch. Gerber, C. Rossel, and D. Widmer, App. Phys. Lett. 77 , 139 (2000). 11 Y. Watanabe, J.G. Bednorz, A. Bietsch, Ch. Gerber, D. Widmer, A. Beck, and S.J. Wind, App. Phys. Lett. 78 , 3738 (2001). 12 B.J. Choi, D.S. Jeong, S.K. Kim, C. Rohde, S. Choi, J.H. Oh, H.J. Kim, C.S. Hwanga, K. Szot, R. Waser, B. Re- ichenberg, and S. Tiedke, J. Appl. Phys. 98 , 033715 (2005). 13 A. Baikalov, Y.Q. Wang, B. Shen, B. Lorenz, S. Tsui, Y.Y. Sun, Y.Y. Xue, and C.W. Chu, Appl. Phys. Lett. 83 957 (2003). 14 Y.B. Nian, J. Strozier, N.J. Wu, X. Chen, and A. Ignatiev, Phys. Rev. Lett. 98 , 146403 (2007). 15 N.A. Tulina, S.A. Zver’kov, Y.M. Mukovskii and D.A. Shulyatev, Europhys. Lett. 56 , 836 (2001). N.A. Tulina, Physics-Uspekhi 50 , 1171 (2007).