Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling

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1 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling Pedram Khalili Amiri 1,2,JuanG.Alzate 1, Xue Qing Cai 1, Farbod Ebrahimi 1,2,QiHu 2, Kin Wong 1, Cécile Grèzes 1, Hochul Lee 1, Guoqiang Yu 1,XiangLi 1, Mustafa Akyol 1,QimingShao 1, Jordan A. Katine 3, Jürgen Langer 4, Berthold Ocker 4, and Kang L. Wang 1 1 Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA USA 2 Inston Inc., Los Angeles, CA USA 3 HGST Inc., San Jose, CA USA 4 Singulus Technologies AG, Kahl am Main 63796, Germany We review the recent progress in the development of magnetoelectric RAM (MeRAM) based on electric-field-controlled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance effect for readout in a two-terminal memory element, similar to other types of magnetic RAM. However, the writing of information is performed by voltage control of magnetic anisotropy (VCMA) at the interface of an MgO tunnel barrier and the CoFeB-based free layer, as opposed to current-controlled (e.g., spin-transfer torque or spin orbit torque) mechanisms. We present results on voltage-induced switching of MTJs in both resonant (precessional) and thermally activated regimes, which demonstrate fast (<1 ns) and ultralow-power (<40 fj/bit) write operations at voltages V. We also discuss the implications of the VCMA-based write mechanism on memory array design, highlighting the possibility of crossbar implementation for high bit density. Results are presented from a 1 kbit MeRAM test array. Endurance and voltage scaling data are presented. The scaling behavior is analyzed, and material-level requirements are discussed for the translation of MeRAM into mainstream memory applications. Index Terms Magnetic RAM (MRAM), magnetoelectric RAM (MeRAM), nonvolatile memory, spin-transfer torque (STT), voltage control of magnetic anisotropy (VCMA). I. INTRODUCTION MAGNETOELECTRIC effects enable new devices where magnetic properties are controlled by the application of an electric field. A number of such emerging devices are based on electric-field-induced switching of nanomagnets [1] [5], which may have important applications for memory and logic in electronic systems. A key advantage of a magnetoelectric device, compared with those of existing current-controlled devices utilizing spin-transfer torque (STT) [6] [11], Oersted fields [12], [13], or even spin-orbit torques [14] [17], is the potential for dramatic reductions in power dissipation. By eliminating the need for currents to operate the device, Ohmic dissipation which in most cases is the predominant loss mechanism in magnetic memory is significantly reduced, resulting in a very low dynamic (i.e., switching) energy dissipation [1] [4]. While nonvolatile memories such as magnetic RAM (MRAM) have been known to reduce or eliminate standby power, the additional gain in dynamic power dissipation is also of considerable importance, in particular for applications where frequent rewriting of the memory bits takes place during operation, e.g., in embedded cache or logic-in-memory architectures [18], [19]. This has motivated much research on novel mechanisms such as voltage control for switching of magnetization. In this paper, we discuss the development status and challenges of electric-field-controlled magnetoelectric Manuscript received March 20, 2015; revised May 28, 2015; accepted May 28, Date of publication June 10, 2015; date of current version October 22, Corresponding author: P. Khalili Amiri ( pedramk@ ucla.edu). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMAG RAM (MeRAM) based on voltage control of magnetic anisotropy (VCMA) in magnetic tunnel junctions (MTJs). In addition to reduced power dissipation, the use of electric fields for writing in MeRAM offers an advantage in terms of enhanced bit density. In particular, magnetoelectric writing does not impose a current-drive-based size limit on the access devices (e.g., transistors) when integrated in a circuit, hence allowing for a much smaller overall cell area [5], [20]. At the same time, MeRAM in principle retains all key advantages of STT-MRAM, namely, high endurance, high speed, radiation hardness, and a possibility for nonvolatile operation. The paper is organized as follows. Section II presents a brief overview of the VCMA effect, followed by a discussion and comparison of different switching regimes (i.e., thermal and precessional). This section will also briefly discuss possible crossbar array configurations utilizing unipolar VCMA-based writing. Section III will then present experimental data on prototype MeRAM bits and 1 kbit test arrays. Write speed and energy will be discussed based on recent experimental data on MTJ devices with a perpendicular magnetization. This is followed by a discussion of resistance distributions, endurance, and write voltage of MeRAM bits (down to 60 nm in size). Scaling requirements of magnetoelectric memory and their implications for the required VCMA effect in MTJ material systems will be discussed as well. II. DEVICE CONCEPT AND MEMORY OPERATION A. Voltage Control of Magnetic Anisotropy In metallic ferromagnetic films such as those typically used in MTJ devices, electric fields are screened by the conductivity of the material and hence only penetrate a few angstroms into the film surface. Hence, the concentration of IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015 Fig. 1. Measured magnetoresistance curves of an 80 nm 80 nm perpendicular MTJ under an out-of-plane magnetic field. Note that the device has out-of-plane free and fixed layers when no electric field is applied (V = 0), while the application of a negative (positive) voltage increases (decreases) the coercivity of the free layer (bottom ferromagnet in this case) due to VCMA. the electric field near the surface is in principle a limitation for electric field control of magnetic properties. However, by utilizing ultrathin ( <2 nm) ferromagnetic films, the magnetic properties may be sensitive to or even dominated by interface effects, hence providing a mechanism for coupling the applied electric field to the magnetic anisotropy of the material. Thus, manipulating metallic ferromagnets via voltage-controlled interfacial perpendicular magnetic anisotropy (PMA) can be used to realize electric-field controlled nanomagnetic devices [7], [21] [33]. The VCMA effect can be explained in terms of the electricfield-induced change of occupancy of atomic orbitals at the interface, which, in conjunction with spin orbit interaction, results in a change of anisotropy [34] [36]. The VCMA effect can also be qualitatively described based on the interfacial Rashba effect [29]. Fig. 1 shows the measurements of resistance versus the out-of-plane magnetic field in a perpendicular 80 nm 80 nm MTJ device [37]. The application of a voltage results in an electric field through the MgO oxide and, hence, in the accumulation of charges near the interface between the MgO and the CoFeB free layer. Negative voltages are observed to increase the coercivity of the free layer along the perpendicular axis for this particular material stack, i.e., the perpendicular magnetic anisotropy is increased at 1 V, while it is decreased for the opposite voltage (+1 V). This reconfiguration of the magnetic anisotropy of the free layer via the VCMA effect is critical to allow for switching using electric fields, as described next. The effective magnetic anisotropy in a thin ferromagnetic film can be written as K 1,eff (V ) = M s H k,eff (V ) 2 K i (V ) t 2π M 2 s (1) where M s is the saturation magnetization, H k,eff is the effective anisotropy field, K i (V ) is the voltage-dependent interfacial anisotropy energy, and t is the thickness of the ferromagnetic layer. The external electric field applied to the device is E ext = V/d, wherev is the applied voltage and d is the thickness of the MgO layer. In general, K i (V ) may have a nonlinear dependence on the applied voltage. Fig. 2. Schematic of VCMA-induced switching mechanisms in an MTJ with perpendicular magnetization. At equilibrium (V = 0), the energy barrier E b separates the two stable states of the free layer magnetization (pointing up and down, indicated by the arrows). Due to VCMA, a voltage applied across the device can reduce the energy barrier so that switching can be achieved via thermal activation (top panel). If the voltage is increased further beyond the critical voltage V c (4), full 180 switching can be achieved by timing the resulting precessional motion of magnetization. However, considering the linearized form of the dependence observed in most experiments to date, we have K i (V ) = K i (V = 0) ξ V/d (2) where ξ is the linear VCMA magnetoelectric coefficient used to parameterize the PMA dependence on the applied electric field. The VCMA coefficient (fj/v m) is a material stack and interface-dependent parameter quantifying the change of interfacial anisotropy energy (mj/m 2 ) per unit electric field (V/nm). Typical values of interface anisotropy and VCMA in the widely used Ta CoFeB MgO-based MTJ material structure are K i 1 2 mj/m 2 [6], [7], [25], [38], while the magnitude of the VCMA coefficient is typically in the range of fj/v m [2], [24], [25], [38]. The choice of capping/seed metal layer has an important effect on PMA and VCMA. Hence, recent works have explored the use of different metal layers in conjunction with the CoFeB MgO structure in MTJ devices. As an example, the use of Hf and Mo seed layers has resulted in a larger K i compared with that of the Ta layer, which is an important requirement for scaling of MeRAM [39], [40] (see Section III-E), while very large VCMA values have been observed in V Fe-based films [41]. However, further work is required to integrate these structures into high-tunneling magnetoresistance (TMR) MTJs and evaluate their potential at the level of memory arrays. B. Precessional Versus Thermally Activated Switching Recent results have demonstrated VCMA-based switching in in-plane as well as perpendicularly magnetized MTJs in both thermally activated and precessional switching regimes (see [1] [4], [42], [43]). The switching process is shown in Fig. 2. The application of a voltage with the correct polarity can reduce K i, lowering the energy barrier between the free layer states. Depending on the amplitude of the applied voltage, switching can then take place by thermal activation across the barrier or a precessional reorientation if the anisotropy is reduced sufficiently to eliminate the barrier.

3 KHALILI AMIRI et al.: ELECTRIC-FIELD-CONTROLLED MeRAM: PROGRESS, CHALLENGES, AND SCALING Fig. 3. Thermally activated VCMA-induced switching: the measured probability of switching for the VMCA-induced switching of MTJ devices. Note that to achieve deterministic switching, the write process is assisted by STT in this experiment [3], compared with the experiment for nondeterministic precessional switching (see Fig. 4), where no STT is required. Importantly, for both types of VCMA-induced switching (Figs. 3 and 4), the writing is unipolar, i.e., it uses only one polarity of voltage. The device has an R A product of 170 -μm 2,TMR 20%, and 40, with the applied pulses having a length of 100 ms. Thermally activated switching, using a combination of STT and VCMA, provides deterministic switching in both directions, using voltage pulses of different amplitudes (see Fig. 3) [2], [3], [43]. Notably, in this approach, deterministic switching is achieved by: 1) incorporating a nonzero stray field from the fixed layer, which determines the switching direction for the lower voltage pulse amplitude (i.e., where current-induced torques are small), and 2) designing the device such that for a larger voltage pulse, current-induced spin torques overcome the effect of this stray field and realize the opposite switching direction. However, this approach has only been successfully realized at slow speeds (μs ms) and requires the realization of both significantly larger VCMA and STT efficiencies, before being of practical interest for high-speed memory and logic applications. Precessional (i.e., resonant) switching, by contrast, takes place at a much faster time scale, but requires timing of the voltage pulse width to achieve 180 reorientation. Experimental results on precessional (i.e., resonant) switching of perpendicular MTJs based on VCMA demonstrate write times of <1 ns with relatively low write voltages (see Fig. 4). This type of switching, however, is nondeterministic and presents new considerations in terms of circuit design. Notably, the state of the MTJ needs to be read out before writing, to determine whether a reversal of the free layer magnetization is needed to write the required information [44]. Given the fast read times possible in high-tmr MTJ devices ( <2 ns)andthevery short time required for precessional switching ( ns), this can still provide competitive total programming times of <3 ns [44]. C. Array Structure and Select Devices Similar to STT-MRAM, MeRAM can be realized using a 1-transistor/1-MTJ cell structure with CMOS transistors as the access devices. In STT-MRAM, however, the relatively large currents required to switch STT-based MTJs require large transistors to drive them [20], [45]. As a result, the density of STT-MRAM arrays is typically limited not by the dimensions of their MTJs, but rather by the switching current of the magnetic bit itself. In addition, the use of Fig. 4. Precessional VCMA-induced switching: the measured probability of precessional voltage-induced switching in a perpendicular MTJ, for both parallel (P) to antiparallel (AP) and AP to P directions. Note the oscillatory dependence of the write probability on the voltage pulse duration, which is a signature of the precessional write process. No current is required for switching in this case. The device has an R A product of 175 μm 2 and a TMR of 40%, providing a minimum write energy of fj/bit. Fig. 5. (a) Illustration of two adjacent diode-mtj magnetoelectric memory cells in a crossbar MeRAM array. The use of diodes as access devices is possible due to the unipolar writing characteristic of VCMA-based devices (see Figs. 3 and 4). (b) Measured time-domain traces showing the operation of the diode-mtj structure. To write or read information, the bit line of the selected bit is set to the appropriate write or read voltages, respectively. Bit selection is accomplished by grounding the desired source line (SL) and pulling the undesired SL to a high enough voltage to turn OFF its select device [20], [46]. three-terminal CMOS transistors also imposes a layout-based limit of approximately 6 8F 2 on the maximum cell density. The unipolar writing process of MeRAM (see Figs. 3 and 4), however, allows it to be used also in a 1-diode/1-MTJ crossbar cell structure [20], [46]. In principle, crossbars are the densest memory arrays possible (with a 4F 2 /N cell size, where N is the number of stacked MTJ layers in the backend of the line process), and hence, the realization of a diode-controlled memory device for crossbar arrays can greatly increase the density and scalability of the overall memory. Fig. 5 shows a schematic of such a crossbar cell structure, along with experimental data from a prototype array,

4 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015 Fig. 6. Distributions of low- and high-resistance states on the measured test arrays, indicating a separation of 6σ for 60 nm devices (left) and 12σ for 120 nm devices (right), respectively. using devices with VCMA-induced switching in the thermally activated regime [20], [47]. The combined action of STT and VCMA in this example allows for a unipolar set/reset write scheme, where voltage pulses of the same polarity but different amplitudes can be used to switch the device between the parallel (P) and antiparallel (AP) states. Voltage pulses of the opposite polarity will not switch the device, but rather reinforce the initial state [20], [47]. It should be noted that the same cell structure can be applied to precessionalswitched MeRAM bits as well, which similarly use only one polarity (and amplitude) of write voltage pulse. Fig. 5(b) shows the experimentally measured waveforms demonstrating the functionality of the crossbar structure. MTJ 1 and MTJ 2 are first initialized into the P state. MTJ 1 is then switched from P to AP, and back to P, without disturbing the value of MTJ 2. This is followed by a similar process for MTJ 2, with intermediate reading steps demonstrating that the correct information has been written into the selected bit in each case. III. DEVICE AND TEST ARRAY MEASUREMENT RESULTS This section presents measurement results on MeRAM devices and 1 kbit test chips, discussing the key performance metrics. A discussion of the material-level requirements, future prospects, and scaling behavior of MeRAM is also presented. A.1kbitTestArrays Voltage-controlled perpendicular MTJ arrays were fabricated on silicon wafers for statistical measurements. The samples consisted of 1 kbit test arrays without select devices. Arrays were designed to provide electrical access to individual MTJ bits for measurements using a probe-card system. We performed measurements on devices with bit diameters ranging from 60 to 400 nm. For the smallest bit size measured (60 nm), we obtained the MTJ resistance distributions shown in Fig. 6 (left), which show TMR 40% with low- and high- state resistance standard deviations (σ) of 6% and 5%, respectively, corresponding to a 6σ separation. The read margin further increases with bit size, with resistance distributions at 100 and 120 nm bit size showing separations of 8σ (not shown) and 12σ [see Fig. 6 (right)], respectively. The dependence on bit size may point to sidewall damage during etching of the MTJ pillars, as well as lithographic variations in the device size, resulting in wider distributions at Fig. 7. Endurance measurements on an MTJ bit using 1 ns voltage pulses with an amplitude of 1.5 V. The device shows no significant degradation for the range of applied pulses, indicating an endurance >10 11 cycles. smaller nodes. The read margin is expected to further increase with improved MTJ devices having higher TMR >100%. B. Subnanosecond Write Performance Fig. 4 shows the measured VCMA-driven precessional write probability for an 80 nm 80 nm MeRAM bit with perpendicular magnetization. The material stack consists of a bottom electrode FeCoB (1.1) MgO (1.1) FeCoB (1.4) Ta (0.3) [Co (0.3) Pt (1.0)] 10 top electrode (thickness in nanometers) multilayer structure. The device has a parallelstate resistance R p 37 k, 20, and TMR 40%, corresponding to a resistance area (R A) product of 175 -μm 2. The observed oscillatory behavior of the switching probability is a signature of precessional switching. The results show 100% switching probability in a pulse width window of 700 ± 250 ps on the first peak of the switching probability, using a voltage pulse of 1.9 V, corresponding to a minimum write energy of fj/bit. C. Endurance Endurance is a key requirement for scenarios where frequent memory access and rewrites are required, including working memory (e.g., dynamic RAM) and embedded (e.g., static RAM) types of applications. Magnetic memories including MeRAM are expected to have very high endurance due to the operation mechanism of their storage device, which does not require physical displacement of atoms or ions, hence preventing physical fatigue. The results of a preliminary endurance measurement with 1 ns voltage pulses on a sample memory bit are shown in Fig. 7, indicating a lower bound of write cycles for the endurance. It is expected that MeRAM can achieve very high endurance levels (>10 15 ) similar to other types of MRAM, provided a material system with sufficiently large VCMA is available to ensure low enough write voltages. It should also be noted that a typically thicker MgO used in magnetoelectric devices (as opposed to low R A values required for STT writing) may have an additional beneficial effect to increase endurance. D. Voltage Scaling and Thermal Stability To analyze the scaling behavior of magnetoelectric memory, we consider as a figure of merit the ratio of switching voltage over the thermal stability (V c / ) basedonthevcma effect [37], [48].

5 KHALILI AMIRI et al.: ELECTRIC-FIELD-CONTROLLED MeRAM: PROGRESS, CHALLENGES, AND SCALING Fig. 8. Measured dependence of V c / on device dimensions (60 and 80 nm) of perpendicular MTJs switched by VCMA, where = E b /kt is the thermal stability parameter. From (1), the voltage-dependent thermal stability factor for a perpendicular MTJ can be calculated as = E ( b(v ) Ki ξ V/d 2π Ms 2 t) A kt kt = (V = 0) ξ A dkt V (3) where the demagnetization factor in the perpendicular (out-of-plane) direction is approximated to be N z 1, A is the bit area, k is the Boltzmann constant, T is temperature, and E b is the voltage-dependent energy barrier between the stable free-layer states. The standby thermal stability ratio of the memory bit follows from V = 0. From (3), it is seen that as the device is scaled down (i.e., A is decreased), increasing the VCMA coefficient ξ is required to keep the same rate of control of the energy barrier by the applied voltage. In the thermally activated switching regime, E b is reduced by voltage such that a switching event is realized due to thermal fluctuations of the free-layer magnetization. The critical switching voltage V c in this scenario can be defined as the voltage required to reduce the energy barrier between the perpendicular states to zero, i.e., (V c ) = 0. Hence V c = dkt (V = 0). (4) ξ A For any write voltages V w > V c, the magnetic easy-axis is reoriented to the in-plane condition. In this scenario, full 180 switching can be obtained by timing the pulse voltage to take advantage of the magnetization precession, as shown in Fig. 2. The VCMA-induced switching performance was characterized using pulsed switching probability testing for an ensemble of devices in the size range of nm, allowing for write voltage characterization. The thermal stability factor at V = 0 was estimated from magnetic bias field-dependent dwell-time measurements. The results are given in Fig. 8, which shows the measured dependence of V c / on the dimensions of perpendicular MTJs switched by VCMA. E. Device and Material Requirements Equation (4) suggests that for scaling with a constant electric field (V c /d), scaling of the magnetic bit size for Fig. 9. (a) Scaling requirements for the interfacial magnetic anisotropy K i to meet different nonvolatile retention limits ( = 40, 60, and 80) [37]. Note that this projection is valid for any magnetic memory utilizing interfacial perpendicular anisotropy to realize out-of-plane memory bits, regardless of the writing mechanism. As device dimensions are scaled, larger K i values are required. The arrow indicates the typical values reported for the CoFeB MgO material system, which are suitable for the 32 nm node. (b) Scaling of the VCMA coefficient ξ across technology nodes. The current CoFeB MgO-based systems provide typical values <100 fj/v m (right arrow), while newer materials have been reported to provide values potentially suitable for 14 nm and below. Note that the requirements on both K i and ξ may be significantly relaxed for applications where nonvolatility is not required, i.e., where <40. perpendicular VCMA-switched MTJ devices is marked by a tradeoff between the device area A and the VCMA coefficient ξ. At the same time, similar to other types of MRAM, an increase in the perpendicular anisotropy (i.e., K i ) is required with device scaling, if a constant thermal stability factor (i.e., retention time) is required. These two requirements i.e., the increase in K i and ξ and maintenance of a high TMR to ensure a sufficient read margin, are the primary material considerations for the development of MeRAM devices. Fig. 9(a) provides a scaling analysis in the single-domain approximation, showing the required interfacial perpendicular anisotropy energy K i and the VCMA coefficient ξ for bit dimensions ranging from 90 to 5 nm [37]. Three target values of the thermal stability factor are considered ( = 40, 60, and 80). In a practical setting, the required value for depends on the target application, which determines the retention time needed for memory operation. As an example, for a single bit of information at room temperature, the values of = 30 and 40 correspond to retention times of 3 hand 10 years, respectively. Hence, while low values of <30 may be sufficient for applications in embedded memory with relaxed retention requirements, higher values of may be needed for long-term nonvolatility in larger ( Mbit Gbit) arrays. The scaling analysis of K i in Fig. 9(a) is based on (3).

6 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015 For better accuracy, the demagnetization factors were included (calculated using the elliptical/circular cylinder approximation described in [49]). A saturation magnetization of M s = 1000 emu/cm 3 was assumed. It can be seen that for technology nodes down to 45 nm with = 40, the typical values of K i 1erg/cm 2 (=1 mj/m 2 ) in the Ta CoFeB MgO system may be sufficient to scale the device size, provided the CoFeB layer thickness is reduced accordingly to increase overall PMA [see the inset in Fig. 9(a)]. To prevent reducing the free-layer thickness below 1 nm (which may affect TMR), an increase in K i is needed for technology nodes below 32 nm. Note that this K i requirement is common to all types of MRAM using interfacial anisotropy, regardless of the writing mechanisms used. The corresponding scaling analysis for the VCMA coefficient ξ is shown in Fig. 9(b). The analysis is based on (4), assuming scaling at a constant electric field of 1 V/nm (i.e., corresponding to write voltages 1 V at each technology node). It can be seen that for nonvolatile operation at scaled technology nodes below 45 nm, higher values of ξ (compared with the experimental and theoretical values for Ta CoFeB MgO, which are typically <100 fj/v m) are required. Several experiments have already demonstrated possible routes to realizing such large values of ξ (see [41], [50]), with numbers up to 7500 fj/v m demonstrated when a strain-mediated mechanism is used to control the interface anisotropy K i [51]. Further work is required to integrate these materials into devices with high TMR and assess their performance at the level of arrays. In addition, uniformity of the precessional switching across memory arrays needs to be studied for implementation into products. IV. CONCLUSION The switching of magnetoelectric MTJs using pulsed voltages provides a pathway to fast, ultralow-power, and highdensity memory arrays. The unipolar write scheme used in MeRAM devices facilitates the realization of crossbar memory arrays, potentially resulting in a very small cell area. We presented recent results from MTJ devices as well as 1 kbit test arrays, showing subnanosecond switching times with energies <40 fj/bit. Existing switching measurement data from devices with different dimensions show good agreement with the expected scaling trend. The scaling roadmap of MeRAM was discussed along with the projected material-level requirements on VCMA and anisotropy at different technology nodes. ACKNOWLEDGMENT This work was supported in part by the NSF Nanosystems Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems (TANMS), in part by the DARPA Program on Nonvolatile Logic, and in part by Inston through an NSF Phase II Small Business Innovation Research Award. P.K.A. and J.G.A. contributed equally to this work. REFERENCES [1] Y. Shiota, T. Nozaki, F. Bonell, S. Murakami, T. Shinjo, and Y. Suzuki, Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses, Nature Mater., vol. 11, pp , Nov [2] W.-G. Wang, M. Li, S. Hageman, and C. L. Chien, Electric-fieldassisted switching in magnetic tunnel junctions, Nature Mater., vol. 11, no. 1, pp , [3] J. G. Alzate et al., Voltage-induced switching of nanoscale magnetic tunnel junctions, in Proc. IEEE Int. Electron Devices Meeting (IEDM), Dec. 2012, pp [4] S. Kanai, M. Yamanouchi, S. Ikeda, Y. Nakatani, F. Matsukura, and H. Ohno, Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction, Appl. Phys. Lett., vol. 101, no. 12, p , [5] K. L. Wang, J. G. Alzate, and P. K. Amiri, Low-power non-volatile spintronic memory: STT-RAM and beyond, J. Phys. D, Appl. Phys., vol. 46, no. 7, p , [6] D. C. Worledge et al., Spin torque switching of perpendicular Ta CoFeB MgO-based magnetic tunnel junctions, Appl. Phys. Lett., vol. 98, Jan [7] S. Ikeda et al., A perpendicular-anisotropy CoFeB MgO magnetic tunnel junction, Nature Mater., vol. 9, pp , Sep [8] Y. M. Huai, F. Albert, P. Nguyen, M. Pakala, and T. Valet, Observation of spin-transfer switching in deep submicron-sized and lowresistance magnetic tunnel junctions, Appl. Phys. Lett., vol. 84, no. 16, pp , Apr [9] Y. Huai, Spin-transfer torque MRAM (STT-MRAM): Challenges and prospects, AAPPS Bull., vol. 18, no. 6, pp , [10] J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. Ralph, Current-driven magnetization reversal and spin-wave excitations in Co/Cu/Co pillars, Phys. Rev. Lett., vol. 84, no. 14, pp , Apr [11] J. A. Katine and E. E. Fullerton, Device implications of spin-transfer torques, J. Magn. Magn. Mater., vol. 320, no. 7, pp , Apr [12] S. Tehrani et al., Magnetoresistive random access memory using magnetic tunnel junctions, Proc. IEEE, vol. 91, no. 5, pp , May [13] M. Durlam et al., A 1-Mbit MRAM based on 1T1MTJ bit cell integrated with copper interconnects, IEEE J. Solid-State Circuits, vol. 38, no. 5, pp , May [14] L. Liu, O. J. Lee, T. J. Gudmundsen, D. C. Ralph, and R. A. Buhrman, Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect, Phys. Rev. Lett., vol. 109, p , Aug [15] L. Liu, C.-F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman, Spin-torque switching with the giant spin Hall effect of tantalum, Science, vol. 336, pp , May [16] I. M. Miron et al., Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection, Nature, vol. 476, pp , Aug [17] I. M. Miron et al., Current-driven spin torque induced by the Rashba effect in a ferromagnetic metal layer, Nature Mater., vol. 9, pp , Jan [18] F. Ren and D. Markovic, True energy-performance analysis of the MTJ-based logic-in-memory architecture (1-bit full adder), IEEE Trans. Electron Devices, vol. 57, no. 5, pp , May [19] S. Matsunaga et al., Fabrication of a nonvolatile full adder based on logic-in-memory architecture using magnetic tunnel junctions, Appl. Phys. Exp., vol. 1, no. 9, p , Sep [20] R. Dorrance et al., Diode-MTJ crossbar memory cell using voltageinduced unipolar switching for high-density MRAM, IEEE Electron Device Lett., vol. 34, no. 6, pp , Jun [21] P. K. Amiri et al., Switching current reduction using perpendicular anisotropy in CoFeB MgO magnetic tunnel junctions, Appl. Phys. Lett., vol. 98, no. 11, p , Mar [22] T. Maruyama et al., Large voltage-induced magnetic anisotropy change in a few atomic layers of iron, Nature Nanotechnol., vol. 4, pp , Jan [23] M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon, and D. Givord, Electric field-induced modification of magnetism in thinfilm ferromagnets, Science, vol. 315, no. 5810, pp , [24] M. Endo, S. Kanai, S. Ikeda, F. Matsukura, and H. Ohno, Electricfield effects on thickness dependent magnetic anisotropy of sputtered MgO/Co 40 Fe 40 B 20 /Ta structures, Appl. Phys. Lett., vol. 96, May 2010, Art. ID [25] J. Zhu et al., Voltage-induced ferromagnetic resonance in magnetic tunnel junctions, Phys. Rev. Lett., vol. 108, p , May [26] T. Nozaki, Y. Shiota, M. Shiraishi, T. Shinjo, and Y. Suzuki, Voltageinduced perpendicular magnetic anisotropy change in magnetic tunnel junctions, Appl. Phys. Lett., vol. 96, no. 2, p , Jan

7 KHALILI AMIRI et al.: ELECTRIC-FIELD-CONTROLLED MeRAM: PROGRESS, CHALLENGES, AND SCALING [27] T. Nozaki et al., Electric-field-induced ferromagnetic resonance excitation in an ultrathin ferromagnetic metal layer, Nature Phys., vol. 8, pp , Apr [28] C.-G. Duan, S. S. Jaswal, and E. Y. Tsymbal, Predicted magnetoelectric effect in Fe/BaTiO 3 multilayers: Ferroelectric control of magnetism, Phys. Rev. Lett., vol. 97, p , Jul [29] S. E. Barnes, J. Ieda, and S. Maekawa, Rashba spin-orbit anisotropy and the electric field control of magnetism, Sci. Rep., vol. 4, Feb. 2014, Art. ID [30] Y. Shiota, T. Maruyama, T. Nozaki, T. Shinjo, M. Shiraishi, and Y. Suzuki, Voltage-assisted magnetization switching in ultrathin Fe 80 Co 20 alloy layers, Appl. Phys. Exp., vol. 2, no. 6, p , [31] M. Endo, S. Kanai, S. Ikeda, F. Matsukura, and H. Ohno, Electricfield effects on thickness dependent magnetic anisotropy of sputtered MgO/Co 40 Fe 40 B 20 /Ta structures, Appl. Phys. Lett., vol. 96, no. 21, p , [32] T. Nozaki, Y. Shiota, M. Shiraishi, T. Shinjo, and Y. Suzuki, Voltageinduced perpendicular magnetic anisotropy change in magnetic tunnel junctions, Appl. Phys. Lett., vol. 96, no. 2, p , [33] P. Upadhyaya et al., Electric field induced domain-wall dynamics: Depinning and chirality switching, Phys.Rev.B, vol. 88, p , Dec [34] J. P. Velev, S. S. Jaswal, and E. Y. Tsymbal, Multi-ferroic and magnetoelectric materials and interfaces, Philos. Trans. Roy. Soc. A, Math.,Phys.Eng.Sci., vol. 369, pp , Aug [35] C.-G. Duan et al., Surface magnetoelectric effect in ferromagnetic metal films, Phys.Rev.Lett., vol. 101, p , Sep [36] M. K. Niranjan, C.-G. Duan, S. S. Jaswal, and E. Y. Tsymbal, Electric field effect on magnetization at the Fe/MgO(001) interface, Appl. Phys. Lett., vol. 96, no. 22, p , [37] J. G. Alzate, Voltage-controlled magnetic dynamics in nanoscale magnetic tunnel junctions, Ph.D. dissertation, Dept. Elect. Eng., Univ. California, Los Angeles, CA, USA, [38] J. G. Alzate et al., Temperature dependence of the voltage-controlled perpendicular anisotropy in nanoscale MgO CoFeB Ta magnetic tunnel junctions, Appl. Phys. Lett., vol. 104, no. 11, p , [39] T. Liu, J. W. Cai, and L. Sun, Large enhanced perpendicular magnetic anisotropy in CoFeB/MgO system with the typical Ta buffer replaced by an Hf layer, AIP Adv., vol. 2, no. 3, p , [40] T. Liu, Y. Zhang, J. W. Cai, and H. Y. Pan, Thermally robust Mo/CoFeB/MgO trilayers with strong perpendicular magnetic anisotropy, Sci. Rep., vol. 4, Jul. 2014, Art. ID [41] A. Rajanikanth, T. Hauet, F. Montaigne, S. Mangin, and S. Andrieu, Magnetic anisotropy modified by electric field in V/Fe/MgO(001)/Fe epitaxial magnetic tunnel junction, Appl. Phys. Lett., vol. 103, no. 6, p , [42] Y. Shiota et al., Pulse voltage-induced dynamic magnetization switching in magnetic tunneling junctions with high resistancearea product, Appl. Phys. Lett., vol. 101, no. 10, p , [43] P. K. Amiri, K. L. Wang, and K. Galatsis, Voltage-controlled magnetic anisotropy (VCMA) switch and magneto-electric memory (MeRAM), U.S. Patent 14/ , [44] H. Lee et al., Design of a fast and low-power sense amplifier and writing circuit for high-speed MRAM, IEEE Trans. Magn., vol. 51, no. 5, May 2015, Art. ID [45] K. Lee and S. H. Kang, Development of embedded STT-MRAM for mobile system-on-chips, IEEE Trans. Magn., vol. 47, no. 1, pp , Jan [46] P. K. Amiri and K. L. Wang, Systems and methods for implementing magnetoelectric junctions, U.S. Patent , [47] J. G. Alzate et al., Voltage-induced switching of nanoscale magnetic tunnel junctions, in Proc. IEEE Int. Electron Devices Meeting (IEDM), San Francisco, CA, USA, Dec. 2012, pp [48] P. K. Amiri, P. Upadhyaya, J. G. Alzate, and K. L. Wang, Electric-fieldinduced thermally assisted switching of monodomain magnetic bits, J. Appl. Phys., vol. 113, no. 1, p , [49] M. Beleggia, M. De Graef, Y. T. Millev, D. A. Goode, and G. Rowlands, Demagnetization factors for elliptic cylinders, J. Phys. D, Appl. Phys., vol. 38, no. 18, pp , [50] F. Bonell, S. Murakami, Y. Shiota, T. Nozaki, T. Shinjo, and Y. Suzuki, Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films, Appl. Phys. Lett., vol. 98, no. 23, p , [51] G. Yu et al., Strain-induced modulation of perpendicular magnetic anisotropy in Ta/CoFeB/MgO structures investigated by ferromagnetic resonance, Appl. Phys. Lett., vol. 106, no. 7, p , 2015.