Three-state memory using tunnel junctions

IOP PUBLISHING

JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 40 (2007) 5819–5823

doi:10.1088/0022-3727/40/19/003

Three-state memory combining resistive and magnetic switching using tunnel junctions J Ventura1,2 , A M Pereira1,2 , J P Araujo1,2 , J B Sousa1,2 , Z Zhang1,3 , Y Liu1,3 and P P Freitas1,3 1 2 3

IN, Rua Alves Redol 9-1, 1000-029 Lisbon, Portugal IFIMUP, Rua do Campo Alegre 678, 4169-007, Porto, Portugal INESC-MN, Rua Alves Redol 9-1, 1000-029 Lisbon, Portugal

E-mail: [email protected]

Received 16 May 2007, in final form 29 July 2007 Published 21 September 2007 Online at stacks.iop.org/JPhysD/40/5819 Abstract Magnetic fields generated by current lines are the standard way of switching between resistance (R) states in magnetic random access memories. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current crossing the tunnel barrier. Such current induced magnetization switching (CIMS) or current induced switching (CIS) effects were recently observed in thin magnetic tunnel junctions, and attributed to spin transfer (CIMS) or electromigration of atoms into the insulator (CIS). In this work, electromigration-driven resistance changes (resistive switching) are superimposed with thermally induced pinned layer reversal (magnetic switching), producing a reproducible, three-state memory device. The tunnel junctions under study show a tunnel magnetoresistance of 14% with a RA product of 50
µm2 . The reversible electromigration-driven resistance changes amount to 7–8% of the full resistance change and more than 104 R-switching events can be current induced without significant damage to the tunnel junction. Typical critical current densities are of the order of 2 × 106 A cm−2 .

1. Introduction Semiconductor-based random access memories (RAMs) are expected to have severe difficulties in satisfying the ever increasing demand for smaller solid-state memories. Therefore, a wide variety of next generation non-volatile memories has been proposed and is being intensively investigated. The list currently includes phase changing (PCRAM) [1], ferroelectric (FeRAM) [2–6], magnetic (MRAM) [7] or resistive (ReRAM) [8] RAMs. From the above, resistive change-based RAMs should be scalable beyond capacitance based RAMs, thus offering the possibility of very high density integration. However, large currents are needed for resistance switching in PCRAMs, as opposed to ReRAMs, where a low power consumption is expected. Many different materials and structures have been shown to display resistance switching between two or more states 0022-3727/07/195819+05$30.00

© 2007 IOP Publishing Ltd

without phase change. These devices usually consist of simple metal–insulator–metal sandwiches and large reversible resistance (R) changes can be induced by an electrical current leading to high (ON) and low (OFF) conductance states. Insulating materials can be as diversified as CuO [9], ZnO [10], NiO [11], TiO2 [12], MgO [13], Al2 O3 [14, 15], SiO2 [16], perovskites [17] or organic films [18]. However, the microscopic origin of the R-switching mechanism is still unclear and several models have been proposed, based on bulk or interfacial effects, such as charge trapping at interfacial sites [19], oxygen diffusion [20], Mott metal–insulator transition [21], variable Schottky barrier [22] or metallic filamentary path formation/rupture [23]. Magnetic tunnel junctions (MTJs) consist of two ferromagnetic (FM; pinned and free) layers separated by an insulator [24]. Two resistance (R) states can then be obtained by the application of a small magnetic field, corresponding

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to parallel or antiparallel magnetizations. Thus, naturally, the first proposed process to switch the magnetization of the free layer in a MRAM cell was to apply two orthogonal magnetic fields, along the easy and hard axis directions. However, new writing schemes are needed to decrease the MRAM cellsize into the 100 nm range. Such schemes are under intense investigation and include toggle- [25], spin transfer- [26] or thermally assisted-switching [27]. In 1996, Slonczewski [28] and Berger [29] independently predicted that a spin polarized current exerts a torque on the magnetization of a FM layer, which could ultimately lead to magnetization reversal. The spin transfer driven switching was in fact later observed in magnetic structures, including exchange-biased spin valves [30] and MTJs [31]. For the thermally assisted switching scheme, the MTJ structure includes two AFM layers, with high and low blocking temperatures, respectively, exchange-biasing the pinned (reference) and the ‘free’ (storage) layers. To write a bit one then needs to heat the junction above the blocking temperature of the AFM biasing the storage layer (significantly lower than that of the reference layer), and to cool it under a small applied magnetic field. Such thermomagnetic switching shows reduced power consumption and better thermal stability than other switching processes, although writing times are still relatively high. The possibility of fabricating a four-resistance state magnetic device was recently demonstrated resulting from a combination of the FM and ferroelectric properties of multiferroic La0.1 Bi0.9 MnO3 (LBMO) [32]. In fact, by growing a La2/3 Sr1/3 MnO3 (LSMO)/LBMO/Au structure, two resistance states can be obtained by applying a magnetic field, when changing the relative magnetization orientation between the LBMO barrier and the LSMO electrode. On the other hand, a bias voltage controls the polarization of the LBMO barrier, giving rise to two more resistance states [33]. However, note that such LBMO-based devices would require liquid nitrogen temperatures, so that there is still a search for a room temperature multiferroic device [34]. Electromigration (EM) of metallic ions from the electrodes into the barrier driven by an applied electrical current were also shown to lead to reversible R-changes in thin Al2 O3 MTJs [35, 36], an effect that was called current induced switching (CIS). In fact, for a sufficiently intense current (critical switching current, Ic ) high electrical fields occur across the thin insulating barrier and may lead to EM of ions from the FM electrodes (Co, Fe) into the barrier, decreasing the effective barrier thickness and consequently the junction resistance (‘thin’ barrier; Rb ). If the electrical field is reversed one observes a R-increase due to the return of the previously displaced ions back into the FM electrodes (‘thick’ barrier; RB ). Note that neither the spin transfer effect nor the Oersted field created by the electrical current flowing through the MTJ can account for the observed reversible resistance changes. In fact, such R-changes were recently observed in tunnel junctions, deposited under similar conditions, but having non-magnetic electrodes [15]. Here we present an alternative route to obtain a device with, at least, three-resistance states at room temperature, using MTJs and a combination of the EM-driven and the thermomagnetic-driven [37, 38] switching processes. In fact, the exchange bias direction of the TJ can be reversed while 5820

measuring a CIS cycle under a negative magnetic field. This occurs because the electrical current can both induce the CIS effect and also locally heat the AFM layer above its blocking temperature (TB ). One is then able to switch the resistance from the [RAP , RB ] to the [RP , Rb ] state. When large enough current pulses are applied in the opposite direction, EM occurs in the reverse sense, increasing R to a new, distinct state (RP , RB ).

2. Experimental details The complete structure of the ion beam deposited series of tunnel junction used in this work is glass/bottom lead/Ta (90 Å)/NiFe (50 Å)/MnIr (90 Å)/CoFe (80 Å)/Al (3 Å+ 4 Å) + oxidation/CoFe (30 Å)/NiFe (40 Å)/Ta (30 Å)/TiW(N) (150 Å)/top lead. The AlOx barrier was formed by two-step deposition and natural oxidation processes (50 mTorr, 3 min, 100 mTorr, 20 min). NiFe, CoFe, MnIr and TiW(N) stand for Ni80 Fe20 , Co80 Fe20 and Mn78 Ir22 , Ti10 W90 (N). The bottom and top leads are made of Al 98.5%, Si 1%, Cu 0.5%, 600 Å and 3000 Å thick, respectively, and are 26 µm and 10 µm wide. The junctions were patterned to a rectangular shape with areas (A) ranging from 1 × 1 µm2 to 4 × 2 µm2 by a selfaligned microfabrication process. The samples were annealed at 550 K under an external magnetic field (3 kOe) to impress an exchange bias direction between the AFM and FM pinned layers, taken here as the positive direction. The electrical resistance, magnetoresistance (MR) and current induced switching were measured with a four-point dc method and an automatic control and data acquisition system [39]. The CIS cycles were performed using the pulsed current method: current pulses (Ip ) of 1 s duration and 5 s repetition period are applied to the TJ, starting with increasingly negative pulses from Ip = 0 (junction resistance ≡ Rinitial ), in Ip = 2 or 3 mA steps until a negative maximum −Imax . One then positively increases the current pulses (with the same Ip ), following the reverse trend through zero current pulse (Rhalf ) up to positive +Imax , and then again to zero (Rfinal ), to close the CIS hysteretic cycle. The junction remnant resistance is always measured in the 5 s waiting periods between the above mentioned consecutive current pulses, using a low current of 1 mA, thus providing an R(Ip ) curve for each CIS cycle. This low-current method allows us to discard non-linear I (V ) contributions to the resistance. Positive current is here considered as flowing from the bottom to the top lead. The CIS coefficient is defined as CIS =

Rinitial − Rhalf , (Rinitial + Rhalf )/2

(1)

and the resistance shift (δ) in each cycle δ=

Rfinal − Rinitial . (Rinitial + Rfinal )/2

(2)

The tunnel magnetoresistance is defined as TMR =

RAP − RP , RP

(3)

where RP (RAP ) is the tunnel junction resistance when the free and pinned layer magnetizations are parallel (antiparallel).

Three-state memory using tunnel junctions

Figure 2. Maximum temperature inside the tunnel junction as a function of time during heating (under an applied electrical current) and cooling (under zero current).

(when the Ip magnitude is reduced in the CIS (Ip ) cycle) leaves the TJ in the inverted exchange bias direction. We also performed dynamical simulations of TJ-heating and cooling under an applied electrical current using [41] Figure 1. Consecutive sets of MR and CIS cycles. The thicker (thinner) arrows refer to the pinned (free) layer magnetization.

cp d

3. Experimental results and discussion Figure 1(a) shows a standard magnetoresistance MR(H) cycle, giving TMR = 14% under a small and constant bias current (I = 1 mA  Ic ). The strongly pinned layer is fixed into the direction of positive fields. The initial state (A) under H = −200 Oe then corresponds to antiparallel free/pinned layer magnetizations (the free layer is easily oriented into the H-direction), giving a high-R state [RAP , RB ]. When the magnetic field is increased towards positive values, this state essentially remains until H again reverses the magnetization of the free layer (resulting in parallel free/pinned layer magnetizations), to produce a lower R state (B). The reverse sequence occurs when H is decreased from +200 Oe to −200 Oe, closing the MR(H) cycle in the initial antiparallel state (A). Keeping H = −200 Oe (state A), a full CIS cycle was performed under a negative applied magnetic field R = R(Ip ; H = −200 Oe), for Ip varying up to Imax = 39 mA (figure 1(b)). Starting with increasing negative current pulses, a sharp resistance decrease is observed for Ip
−25 mA (CIS = 15%; notice the different denominators in the TMR and CIS definitions), leading to a junction resistance (at −Imax ) considerably smaller (state C) than in the parallel state (B) of the previous MR(H) cycle. Part of this R-decrease is caused by EM of ions from the electrodes into the barrier [35]. The remaining decrease is due to the reversal of the magnetization of the pinned layer (under a H = −200 Oe field), since the following MR(H) cycle appears inverted (figure 1(c)). This means that in the CIS cycle under H = −200 Oe (figure 1(b)) the tunnel junction has reverted to its parallel state (C) under sufficiently large negative current pulses. Such a pinned layer magnetic switch was made possible through heating under high current pulses, which raises the local temperature inside the TJ above the blocking temperature of the 90 Å thick MnIr AFM layer (TB = 520 K [40]). The corresponding magnetization then easily aligns with the applied negative magnetic field, opposite to the initial exchange bias direction. Notice that such negative magnetic field applied during current pulse cycling only needs to be large enough to overcome the coercive field of the pinned layer (≈30 Oe). Subsequent cooling below TB

∂T j V −x/ lin ∂ 2T e , − K 2 = ρj 2 + ∂t ∂ x lin

(4)

where cp is the heat capacity, d is the mass density, K is the heat conductivity, T is the temperature, x is the stack position, ρ is the electrical resistivity, j = V /(RA) is the current density, V is the bias voltage and lin is the inelastic scattering mean free path. Values of the parameters used can be found in [41]. Figure 2 shows the obtained results. One observes that both heating up to TB and cooling back to room temperature occur in a very small time frame of less than 1 ns. In fact, the thermomagnetic write scheme [37,38] was recently demonstrated for current pulses as short as 5 ns [42]. Returning to the CIS(Ip ) cycle of figure 1(b), high positive current pulses induce ionic EM in the reverse direction, switching the tunnel junction resistance [35] into the previous intermediate resistance state (B). This shows virtual reversibility of the EM effects induced by comparable magnitude positive and negative current pulses. Therefore, the large difference between Rinitial (A) and Rfinal (B; δ = −7%) observed in figure 1(b) is only due to magnetic switching (antiparallel to parallel state). If pinned layer magnetic switching had not taken place under negative Ip , one would have had simply a recovered barrier and low δ-shift, i.e. the initial state A. The CIS cycle measured subsequently (figure 1(d)), under H = +200 Oe (opposite to the previously impressed exchange bias direction), shows essentially the same features. Three well separated (∼7%) resistance states were therefore demonstrated (figure 3) in the same TJ device, when one adequately plays with the thermomagnetic and EM effects. Starting in the antiparallel, thick barrier state [RAP , RB ], one can directly obtain a parallel, thin barrier state [RP , Rb ] using simultaneously a large electrical current and an external magnetic field opposite to the exchange bias direction. When a current pulse of opposite direction is applied, the parallel, thick barrier state is obtained [RP , RB ]. Studies with thicker MTJs are underway aiming to enhance both the CIS signal and the TMR ratio. We also performed tests on the stability of the structural switching. For measurements up to 104 CIS cycles, a stable junction resistance (indicative of barrier stability) is achieved after the first ≈6000 pulses. Once this equilibrium resistance is reached for a certain current pulse intensity, one can, in principle, apply much larger pulse numbers. 5821

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CoFe/AlOx /CoFe tunnel junctions was presented. When CIS cycles are measured under suitable external magnetic fields, one is able to current-induce both a change in the sign of the exchange bias of the TJ (thus in the magnetic state: antiparallel/parallel) and a change in the TJ effective barrier, due to EM. A CIS device offering three different electrical resistance states was demonstrated.

Acknowledgments

Figure 3. Three resistance states. Notice the reversal of the magnetization of the pinned layer due to heating effects.

This work is supported in part by POCTI/CTM/36489/2000, POCTI/CTM/45252/02 and IST-2001-37334 NEXT MRAM projects. J Ventura is thankful for the FCT grant (SFRH/BPD/21634/2005).

References

Figure 4. Envisaged four resistance states in thin tunnel junctions using both CIS and MR effects.

A four-resistance state device can be further envisaged as illustrated in figure 4. For each MR state (RP or RAP ) two R-states due to EM are possible: the low resistance state characterized by a thin barrier and the high resistance state characterized by a thick barrier. The alternative process can also be envisaged: for each state in the CIS effect (large or small barrier), two distinct MR states can be produced using an external magnetic field and thermally induced switching. The magnetic antiparallel, and ‘thick’-barrier state is obtained with a negative magnetic field and a large, positive electrical current pulse [RAP , RB ]. The antiparallel and ‘thin’-barrier state [RAP , Rb ] can be obtained with a negative magnetic field (to reverse the free layer magnetization) and a high enough negative electrical current (to return ions to the electrodes). The [RP , RB ] state can be obtained with a positive magnetic field and a sufficient positive electrical current. Finally the [RP , Rb ] state can be obtained with a positive magnetic field and a high enough negative electrical current. A fourstate device is thus envisaged, but the TJ characteristics and experimental procedure have to be carefully designed to obtain well separated resistance states. Work is in progress to achieve such a device with a reliable performance over the successive structure/magnetic resistive states.

4. Conclusions In conclusion, a detailed study of the current induced switching effects on low resistance and thin (7 Å barrier) 5822

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