Nonvolatile Multilevel States in Multiferroic Tunnel Junctions

Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed multiferroic tunnel junctions (MFTJs), can be achieved not only by the magnetic alignment of two ferromagnets, but also by the electric polarization of the ferroelectric interlayer; this provides great opportunities for next-generation multistate memory devices. Here, we show that a ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3(\mathrm{LSMO})/{\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3(\mathrm{PZT})/\mathrm{Co}$ structured MFTJ device can exhibit multilevel resistance states in the presence of gradually reversed ferroelectric domains via tunneling electroresistance and tunneling magnetoresistance, respectively. Nonvolatile ferroelectric control in the MFTJ can be attributed to separate contributions that arise from two independent ferroelectric channels in the PZT interlayer with opposite polarization. Our study shows the dominant role of “mixed” ferroelectric states on achieving accumulative electrical modulation of multilevel resistance states in MFTJs; thus paving the way for multifunctional device applications.

Figure 1

Multiferroic tunnel junctions. (a) Schematic image of the LSMO/PZT/Co MFTJs. (b) Magnetic hysteresis loops of the LSMO and Co films used from FM electrodes with coercive fields of 40 and 500 Oe, respectively. (c) I-V curves of the junction (measured at 10 K and 2000 Oe) with the poled-up and -down PZT. (d) Dynamic conductance (red square dots, poled-up PZT) as a function of voltage calculated from the I-V curve and fitted by the Brinkman model (blue solid line) in small voltage range. (e) Energy diagram of the MFTJ with poled-up and -down PZT.

Figure 2

Multilevel resistance states of the MFTJ tuned by external magnetic and electric fields. (a)–(d) Magnetic and electric field programing of resistance: the voltage is ramped to V_max= 0.5, 1.0, 2.0, 3.0, −0.5, −1.0, −1.5, −2.0, −2.5, –3.0 V (b), to modulate the FE states of the PZT layer (c), and then to V_MR (=−0.25 V) for magnetoresistance measurements (d) by sweeping the magnetic field (a). (e) TMR as a function of V_MR measured after application of different V_max.

Figure 3

Modulation and simulation of the TER of the MFTJ with an intermediate ferroelectric polarization state (x). (a) Schematics of the two-channel model consisting of pole-up and -down ferroelectric domains, which correspond to different resistivities. (b) TER defined as the resistance ratio, $R_x/R_{\uparrow }$, as a function of the poling voltage (V_max). (c) TER as a function of the fraction of FE pole-down domains (x) calculated from the two-channel model and maximum and minimum resistance values (see text). (d) The x value, as a function of V_max, extracted according to (b),(c). The arrows in (b),(d) indicate measurement sequence.

Figure 4

Modulation and simulation of the TMR of the MFTJ with an intermediate ferroelectric polarization state (x). (a)–(c) Schematics of the MFTJ with magnetization of the ferromagnetic electrodes aligned parallel or antiparal and with the ferroelectric layer fully poled up (x = 0%), partly poled down (x), or fully poled down (x = 100%). (d) R(H) of the MFTJ measured at V_MR = 0.2 V and (e) V_MR = −0.25 V for x = 0% [(a) state] and x = 100% [(c) state]. (f) Measured (scatters) TMR compared with TMR calculated (lines) from the two-channel model for V_MR = 0.2 V and V_MR = −0.25 V.