Time resolved transport studies of magnetization reversal in orthogonal spin transfer magnetic tunnel junction devices

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1 Invited Paper Time resolved transport studies of magnetization reversal in orthogonal spin transfer magnetic tunnel junction devices Georg Wolf a, Gabriel Chaves-O Flynn a, Andrew D. Kent a, Bartek Kardasz b, Steve Watts b, and Mustafa Pinarbasi b a Department of Physics, New York University, 4 Washington Place, New York, NY 2; b Spin Transfer Technologies Inc., 4 North Port Loop, Fremont, CA 9438 ABSTRACT In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel (AP) states and the reverse transitions (AP to P) under the influence of ns voltage pulses. The electrical response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process is obtained. The P to AP transition switching time decreases with increasing current and shows only direct switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession of the magnetization, and lead to reversal of the magnetization at later times. Keywords: MRAM, orthogonal spin transfer, time resolved magnetization reversal. INTRODUCTION The prospect of developing a magnetic random access memory (MRAM), with its advantages such as the nonvolatility and fast operation times, has been a driving force in research on spin transfer torque driven magnetization reversal in magnetic nano-structures., 2 A memory cell typically has a magnetic tunnel junction (MTJ), consisting of at least two ferromagnetic layers separated by a thin insulating tunnel barrier. The relative orientation of the magnetizations of these layers, parallel (P) or antiparallel (AP), determines the digital state of the cell. The write operation is achieved by the reorientation of the magnetization of one layer the free layer while the orientation of the magnetization other layer the reference layer is fixed. The reorientation of the magnetization of the free layer can be achieved by either a magnetic field or more efficiently by applying an electric current through the MTJ via a spin transfer torque, which can be described with Eq. : 3 ( d m ) dt ST T = γ h η(θ) j( m ( m p)), () 2e M S d where γ a is the gyromagnetic ratio, h the reduced plank constant, e the electron charge, M S the saturation magnetization and d the thickness of the free layer, η(θ) is a ratio of the spin current to the charge current density j (i.e. related to the spin-polarization of the current). However, from the form of the spin-transfer torque (Eq. ) it is clear that there is no torque when the magnetization vector m of the free layer and the polarization vector of the incoming electrons p is absolutely parallel or antiparallel. Therefore changing the memory state of a device requires a misalignment between the spin polarization of the incoming electrons and the magnetization of the free layer. In conventional collinear devices this is usually provided by thermal fluctuations, which leads to an incubation time to reverse the magnetization and a stochastic switching process. 4, Orthogonal spin transfer (OST) devices provide a large initial torque through an additional layer that is magnetized perpendicular to the free layer (i.e. a perpendicular polarizer), as illustrated in Fig. 2(a). 6 The combination of a current polarized by the reference layer and the perpendicular polarizer leads to an effective Further author information: (Send correspondence to G. Wolf.) G. Wolf.: gw42@nyu.edu Spintronics V, edited by Henri-Jean Drouhin, Jean-Eric Wegrowe, Manijeh Razeghi, Proc. of SPIE Vol. 967, 967H 24 SPIE CCC code: X/4/$8 doi:.7/ Proc. of SPIE Vol H-

2 spin polarization axis that is tilted at an angle out of the layer planes, denoted ω p. The spin transfer torque from the reference layer favors a direct switch into one of the two states, where the final state (P or AP) depends on the current polarity. The perpendicular spin torque component drives the magnetization out of the film plane and can generate precessional motion about the magnetic hard axis of the free layer (i.e. the axis perpendicular to the plane of the free layer). The perpendicular polarizer can excite this precession motion for both current polarities. The switching mechanism, whether it is direct or occurs by precession about the hard axis, depends sensitively on the magnitude of the spin-polarization angle ω p and the ratio between the magnetic hard axis and easy axis anisotropies. 7 It can also depend on other factors, such as the current magnitude. In order to operate these structures as a memory device the write mechanism needs to be very reliable. Therefore reducing the write error rate (WER) is a crucial focus of STT-MRAM research. The WER can be influenced by multiple factors, such as thermal activation, structural defects of the device, or stray fields from the other magnetic layers in the device. A simple pulse voltage write and later readout test scheme for different pulse amplitudes and durations will reveal write failures, but the interpretation of theses failures is usually difficult because they can only be correlated to variations of the pulse parameters. For example, at room temperature thermal activation can lead to variable switching times for the same write pulses. But the excitation of precessional states 8 or back hopping 9 can also occur and lead to write failures. In a simple write/read test these failures are not distinguishable. Time-resolved investigation of the magnetization reversal time can reveal the actual switching mechanism, and distinguish between different write failures modes MEASUREMENT TECHNIQUE In this work we measure in real-time the electrical response of an OST structure to ns voltage pulses. An arbitrary waveform generator applies a voltage pulse to the MTJ, which is in series with a fast oscilloscope (2 GHz bandwidth) (see Fig. 2). The measured voltage across the Ohm input impedance of the oscilloscope gives us a direct measurement of the current through the junction. This measurement allows one to identify the timing of an individual switching (or write) events. Two bias tees provide a DC measurement path to monitor the resistance state of the device, to measure the device initial and final state before and after each pulse. The experiment is repeated multiple times and for different amplitudes of the voltage pulse. For each event, the voltage trace on the oscilloscope is recorded. For each trace we subtract a reference measurement, obtained in a high magnetic field, where no switching is observed. The resulting trace is denoted V. This reference measurement is also used as a calibration for the current for the switching events, since the resistance of the device and thus the measured signal is not altered through any dynamic motion of the magnetization. From the V traces we can extract the switching time t sw of an individual event (defined as the % mark between and the high voltage level). This setup allows to us to measure the switching probability as well as the switching time distribution for a given pulse amplitude in a single run. e - top electrode PPL FL RL I + PL AFL bottom electrode (a) Arb. Waveform Gen. Ohm Bias Tee OST device R Bias Tee Ohm Scope Figure. (a) Schematic layout of an OST device layer stack. FL: free layer. RL: reference layer. PL: pinned layer coupled to the antiferromagnetic layer (AFL). RL and PL are coupled antiferromagnetically. PPL: perpendicular polarizer. Positive current corresponds to electron flow from the bottom to the top electrode. Time resolved electrical measurement setup: an arbitrary waveform generator supplies ns-voltage pulses to the OST stack, which is in series with a fast realtime oscilloscope (2 GHz bandwidth), which detects the response of the OST device. A DC measurement circuit is included via a set of bias tees to monitor the device resistance before and after the voltage pulse. Proc. of SPIE Vol H-2

3 3. EXPERIMENTAL DATA Figure 3(a) shows the switching probability data for a 6 nm x 24 nm lateral dimension device as a function of the applied current. In both transitions the switching probability increases with increasing current and reaches almost % for large currents. This OST device is designed to have a low polarization tilt angle ω p, which leads to this direct switching behavior, where the final device states depends on the current polarity, a characteristic associated with the spin-torque from the reference layer. Fig. 3(c) and (d) show the histograms in a color scale of the switching time for different amplitudes and for the two transitions AP to P and P to AP, respectively. The red color denotes a high probability of this switching time to occur, while blue denotes a low probability. For low currents the distributions are wide and give a large mean switching time, as seen in Fig. 3. As the absolute value of the current increases a reduction of the mean switching time is observed, as well as a reduction of the width of the distribution (standard deviation is given as error bars in Fig. 3). However, for the P to AP transition the width decreases continuously, which can be interpreted as a transition from a thermally assisted current induced switching regime to more deterministic switching regime at higher currents. On the other hand, the AP to P transition has a wider distribution at higher current that the P to AP case, which also leads to a saturation of the mean switching time at the highest currents studied. The wider switching distribution means there would be a higher probability for write failures in a memory device. In order to identify the cause of that behavior we have investigated individual switching events. Figure 3(a) and show the density of the V levels on a color scale for pulses for P to AP (2.2 ma) and AP to P (2.4 ma), respectively. Again red denotes a high probability to find that voltage value and blue low probability. In the P to AP case a very narrow band for the switching is observed, which can be linked to a direct switching mechanism. While in the AP to P case a much longer tail of voltages is observed. This feature indicates that a fraction of the events switch at a later time. Figure 3(c) shows individual voltage traces representing those events with small switching times. It is obvious that these events, which still are in the majority of the switching events, go through a direct switch. Fig. 3(d) and (e) show individual traces of switching events with a later switching time. In these minority cases it can be observed that there are pre-oscillations before the actual switching occurs. The voltage signal reaches almost half of the full amplitude and decreases again. We found that there can be up to three oscillations before the full switch occurs (Fig. 3(e)). Probability t sw [ns] Current [ma] (a) Current [ma] Current [ma] t sw [ns] Figure 2. (a) Switching probability for P to AP and AP to P transitions for ns pulses. Mean switching time vs. current. The error bars indicate the standard deviation of the distribution. (c) Switching time distribution for AP to P transition. (d) Switching time distributions for P to AP transition. Blue color indicates low probability and red high probability for t sw to occur. (c) (d) high low high low Proc. of SPIE Vol H-3

4 I I I I A Fourier analysis of the rare events time traces show characteristic oscillation frequencies of less than GHz. The in-plane ferromagnetic resonance frequency calculated based on the saturation magnetization (M S = 97 ka/m) and the shape is above GHz. In a macrospin model the spin transfer torque switching is characterized by an initial exponential increase in the oscillation amplitude and then a direct switch. We observe the direct switch in most of cases, but events in the long-time tails of the switching time distribution show oscillations of similar amplitude; they do not show a rapid increase in oscillation amplitude with time mA I I I I LI i ilii 2.4mA IliiuiiI i i I.6m I - - Time [ns] (a) I Time [ns] Figure 3. Density plots of the Voltage traces for switching events for a) P to AP transition for 2.2 ma and b) AP to P transition for 2.4 ma. c)-e) Individual Voltage traces for the AP to P transition for 2.4 ma. c) Direct switching events. d) One oscillation before the switch. e) Multiple oscillations before the switch. (c) (d) (e) 4. MICROMAGNETIC SIMULATIONS We performed micromagnetic simulations using the mumax3 3 code to investigate this behavior, with an element defined as noted in, 4 with a tilt angle ω p of the polarization vector of. Fig. 4(a) shows on the left y-axis the spatially averaged magnetization component along the major axis of the element < m x >, which corresponds to the MR signal measured in the experiment, for two different current densities. The simulations were done at zero temperature and cannot capture the effect of thermal fluctuations. The curve for a current density of 2 GA/m 2 shows a typical direct switch, like that found in a macrospin model. On the right y-axis of Fig. 4(a) a parameter that represents the non-uniformity of the magnetization in the element is plotted. This is ɛ = (< m x > 2 + < m y > 2 + < m z > 2 ), which gives the deviation from the uniform magnetization, where the <> indicate a spatial average. Close to the switching point (the time at which < m x >= ) this parameter increases slightly and decreases again after the switch. The slight increase is due to slightly different precession frequencies at the edge and the center of the element, because of local variations in the effective field. The second curve is < m x > for 4 GA/m 2 and Figures 4-(e) illustrate the spatial magnetization distributions for 4 points in time. The < m x >-component increases first exponentially up to 7. ns and the magnetization precesses almost uniformly (Fig. 4). Then < m x > decreases at 8. ns (Fig. 4(c) and (d)) and around 8.67 ns (Fig. 4(e)) increases again and finally reverses. Together with this decrease of < m x > the non-uniformity increases. The magnetization distributions at 8. ns and 8. ns show that the magnetization at the short edges Proc. of SPIE Vol H-4

5 R R and the center part precess in opposite directions. This non-uniform precession causes the decrease in < m x > and suppresses the coherent reversal and leads finally to the later switching time. The observation of such a non-uniform behavior is highly sensitive to the current amplitude used in the simulations, as well as the shape of the element. In the experimental situation it still remains unclear what exactly is causing the pre-oscillation and why it is only observed in the AP to P case. Local variations in the structure as well as a different stray field distribution from the reference layer may promote these events. Since the majority of the events are still direct switches, there is also a stochastic component to the occurrence of the pre-oscillation, which suggests that thermal fluctuations of the magnetization also play a role. Averaged Mx GA/m 2 4 GA/m Time [ns] Non-uniformity (a) ti l l R R R R R R R R R R R R R l R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R l l l l l l R R R R l R R R R R R R R R R R R R l l l l l l R R R R R R R R R R R R R R R R R R l l l l l l l l R R R R R R R R R R R ti ti ti l l R R R R R R R R R R R\\\\ l l l R \ R R R R R R\ R R. R. R i i ttiii44444 rttiiiiii4 rrrrlli{i rrrrrll rrrrrr L L L L L\ L L L L L S L L L L L L L L L L i i L L L L i i i i L\\\\\ L L L \ L\ L L\ L\\\\ L L L i i i i i L L L L L L L\ L L L L L \ i 4 L L L 7.ns rrrrrr ii{ilrrrr ilrrrrr 4iiiiiottt 4444iiiiol o Figure 4. (a) Left y-axis: Spatially averaged magnetization component < m x > along the major axis of the element vs. time obtained from micromagnetic simulations for an element defined as in. 4 Right y-axis: non-uniformity parameter (defined in the text) vs. time. -(e) Spatial magnetization distribution for a current density of 4 GA/m 2 at four points in time 7. ns, c) 8. ns, d) 8. ns, e) 8.67 ns. 8.ns (c) rrrrrrrrrrii rrrrrrrrrrrr i i i i i r r r r r r i i i i i i i i i i i i i i i i i i i i i i i i i i 8.ns yi+i yiiiil+iii ++++iiiiii, (d) 8.67ns (e). CONCLUSION In summary, we were able to observe fast magnetization reversal in OST devices and could monitor the reversal in real-time. We observe a decrease of the mean switching time and the width of the distribution with increasing current. This is associated with a transition from thermally assisted spin-transfer switching toward deterministic switching. The P to AP transition clearly shows direct switching for the whole current range, while at high currents the AP to P transition show a wider distribution which is caused by pre-oscillatory behavior of the magnetization. We attribute this behavior to a breakdown of the homogeneous magnetization motion, as we have observed in micromagnetic simulations. The findings demonstrate the importance of time resolved measurements in understanding the operation of MRAM devices as well as in improving device performance. 6. ACKNOWLEDGEMENTS This research was supported by Spin Transfer Technologies, Inc. REFERENCES [] Katine, J. A. and Fullerton, E. E., Device implications of spin-transfer torques, J. Magn. Magn. Mater. 32, 27 (28). [2] Brataas, A., Kent, A. D., and Ohno, H., Current-induced torques in magnetic materials, Nat. Mater., 372 (22). [3] Slonczewski, J., Current-driven excitation of magnetic multilayers, J. Magn. Magn. Mater. 9, L L7 (996). [4] Bedau, D., Liu, H., Sun, J. Z., Katine, J. A., Fullerton, E. E., Mangin, S., and Kent, A. D., Spin-transfer pulse switching: From the dynamic to the thermally activated regime, Appl. Phys. Lett. 97, 2622 (2). [] Heindl, R., Rippard, W. H., Russek, S. E., Pufall, M. R., and Kos, A. B., Validity of thermal activation model for spin transfer torque switching in magnetic tunnel juctions, Jour. Appl. Phys. 9, 739 (2). Proc. of SPIE Vol H-

6 [6] Kent, A. D., Ozyilmaz, B., and del Barco, E., Spin-transfer-induced precessional magnetization reversal, Appl. Phys. Lett. 84, 3897 (24). [7] Pinna, D., Kent, A. D., and Stein, D. L., Thermally assisted spin-transfer torque magnetization reversal in uniaxial nanomagnets, Phys. Rev. B 88, 44 (23). [8] Liu, H., Bedau, D., Backes, D., Katine, J. A., and Kent, A. D., Precessional reversal in orthogonal spin transfer magnetic random access memory devices, Appl. Phys. Lett., 3243 (22). [9] Min, T., Sun, J. Z., Beach, R., Tang, D., and Wang, P., Back-hopping after spin torque transfer induced magnetization switching in magnetic tunneling junction cells, Jour. Appl. Phys., 7D26 (29). [] Devolder, T., Hayakawa, J., Ito, K., Takahashi, H., Ikeda, S., Crozat, P., Zerounian, N., Kim, J.-V., Chappert, C., and Ohno, H., Single-shot time-resolved measurements of nanosecond-scale spin-transfer induced switching: Stochastic versus deterministic aspects, Phys. Rev. Lett., 726 (28). [] Cui, Y., Finocchio, G., Wang, C., Katine, J. A., Buhrman, R. A., and Ralph, D. C., Single-shot timedomain studies of spin-torque-driven switching in magnetic tunnel junctions, Phys. Rev.Lett. 4, 972 (2). [2] Liu, H., Bedau, D., Sun, J. Z., Mangin, S., Fullerton, E. E., Katine, J. A., and Kent, A. D., Time-resolved magnetic relaxation of a nanomagnet on subnanosecond time scales, Phys. Rev. B 8, 224 (22). [3] Vansteenkiste, A. and de Wiele, B. V., Mumax: A new high-performance micromagnetic simulation tool, J. Magn. Magn. Mater. 323(2), (2). [4] mumax3 simualtion parameters: Saturation Magnetization M S = 97 ka/m, Perpendicular Anisotropy K p = 38 kj/m 3, Damping α =., Exchange stiffness A = 3 2 J/m, Elliptical shape nm x nm, layer thickness d =.8, spin polarization tilt angle ω p =. Proc. of SPIE Vol H-6