Ultrafast magnetization switching by spin-orbit torques

   

Ultrafast magnetization switching by spin-orbit torques Kevin Garello,1,2 Can Onur Avci,1,2 Ioan Mihai Miron, 3 Olivier Boulle, 3 Stéphane Auffret, 3 Pietro Gambardella,1,2 Gilles Gaudin3 1

Department of Materials, ETH Zurich, Schafmattstrasse 30, CH-8093 Zürich, Switzerland

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Catalan Institute of Nanoscience and Nanotechnology (ICN2), UAB Campus, E-08193, Barcelona, Spain 3

SPINTEC, UMR-8191, CEA/CNRS/UJF/GINP, INAC, F38054 Grenoble, France

Spin-orbit torques induced by spin Hall and interfacial effects in heavy metal/ferromagnetic bilayers allow for a switching geometry based on in-plane current injection. Using this geometry, we demonstrate deterministic magnetization reversal by current pulses down to 380 ps in square Pt/Co/AlOx dots with lateral dimensions of 135 nm. Contrary to precessional switching in spin-transfer torque devices, we observe that the switching is bipolar with current and field and that the switching probability does not oscillate with the pulse duration or intensity. The comparison between experimental results and macrospin simulations indicates that magnetization reversal proceeds via nucleation and propagation of domains.

The development of an electrically addressable non-volatile memory combining high speed and high endurance is essential to improve the performance and reduce the power dissipation of data storage devices [1]. The integration of non-volatility as a new feature of a cache memory, for example, would minimize the static and dynamic energy consumption that is otherwise predicted to limit the performances of microprocessors [2,3]. Spin transfer torque magnetic random access memory (STT-MRAM) [4] has been identified as one of the most promising technology candidates for scalable non-volatile memories [5]. Despite its higher energy efficiency and better scalability compared to field-induced switching, the development of STT-MRAM as a “universal” memory, particularly for cache memory applications, is hampered by the large current densities required for sub-nanosecond switching [4]. In the case of a simple in-plane magnetized spin valve structure, the spin torque from the pinned layer simply counteracts the damping from the free layer. For sufficiently large currents the effective damping becomes negative and the magnetization precession is enhanced until the magnetization switches in the opposite direction. The simplest way of speeding up this process is to increase the injected current. A second way of improving the switching time is to develop 1   

   

materials with low damping. Even though this approach can lead to significant gain in terms of switching time, it has a built-in flaw: when the magnetizations of the two layers are parallel or anti-parallel, the torques are zero. The resulting non-negligible incubation delay induced by thermally activated pre-switching oscillations is a limitation for ultrafast switching and induces a broad switching time distribution [6]. Several solutions have been explored to reduce the incubation delay, such as biasing the device with a hard axis field [6] or adding an out-of-plane polarizer and an in-plane analyzer [7,8]. This has led to switching times as low as 100 ps, providing that the duration of the current pulse can be controlled with sufficient accuracy. The situation is similar for STT switching in magnetic tunnel junctions, for which reliable switching has been observed with pulses of 500 ps using this last solution [9] and even faster, sub-200 ps, using an in-plane magnetized free layer with perpendicular magnetic interface anisotropy [10]. In any case, the very thin oxide tunnel barrier and the large current densities required for fast switching lead to reliability issues and accelerated aging of the oxide barrier, which limit the write endurance [5]. An alternative switching mechanism of the magnetization based on orbital-to-spin momentum transfer and in-plane current injection geometry has been demonstrated recently [11]. An electrical current flowing in the plane of a trilayer with structural inversion asymmetry such as Pt/Co/AlOx (Fig. 1) creates two torques originating from the spin orbit interaction [12]: an outof-plane torque T┴~ m × y equivalent to an effective field B┴ perpendicular to the current direction [13,14] and an in-plane torque Tǁ ~ m × (y × m) equivalent to a rotating magnetic field Bǁ perpendicular to the magnetization m [11]. Tǁ is responsible for the switching of the magnetization. In the absence of an external magnetic field, Tǁ destabilizes both directions of the magnetization. The application of a small bias field Bext (≥ 5 mT [11]) along the current direction stabilizes one of the two configurations for a given current polarity. Consequently, switching is bipolar with respect to both current and bias magnetic field. Although the physical origin of these torques is not yet fully understood and different mechanisms (spin Hall, Rashba effect) have been proposed to contribute to such torques [15,16,17], magnetization switching has been reported for several other systems such as Pt/Co/MgO [18] (perpendicular magnetization), W/CoFeB/MgO [19] (in-plane magnetization) and Ta/CoFeB/MgO [20,21,22] (perpendicular and in-plane magnetization). Such spin-orbit torque (SOT) induced switching presents several advantages for magnetic memory cells compared to STT-MRAM. First, the current used for switching the magnetization is injected in-plane, avoiding electrical stress of the tunnel barrier during writing; second, for an equivalent writing current density, the absolute current can be reduced by one order of magnitude because of the reduced effective area through which it is injected; third, the trilayers used for demonstrating SOT induced switching are natural elementary bricks of tunnel junction constituting MRAM cells, reading being performed via the tunnel magneto-resistance (TMR) signal. For all these reasons, novel architectures of MRAM, called SOT-MRAM, have been proposed both for in-plane and out-of-plane magnetization [23]. Some have been recently demonstrated for in-plane [19,20,24] and out-of-plane [22] magnetized layers. 2   

   

In this letter we show that SOT-induced magnetization switching can be very fast, thus making SOT-MRAM a realistic candidate for introducing non-volatility in random access and cache memories. We demonstrate deterministic magnetization switching of perpendicularly magnetized Pt/Co/AlOx dots with lateral dimensions of 135 and 210 nm by applying current pulses from 20 ns down to 380 ps. We investigate the critical switching current as a function of pulse width and external applied field and compare our results with the prediction of macrospin simulations. Although the timescale of the current pulses used in our measurements is the same as that of precessional switching in STT experiments, we find that the switching probability is close to 1 above the critical current, i.e., it does not oscillate with the pulse duration and intensity. The comparison of experimental results with macrospin simulations suggests that magnetization reversal proceeds via nucleation and propagation of domains. Pt(3nm)/Co(0.6nm)/AlOx layers with perpendicular anisotropy were deposited by magnetron sputtering and patterned into 200nm and 100nm wide dots on top of Pt Hall bars, as described in [11]. Fig. 1 shows a schematic of the measurement setup. The current pulses are applied along the x direction using a Kentech RTV40 pulse generator with 0.15 ns rise and fall time (inset of Figure 1). To ensure the transmission of fast current pulses without significant reflection, a 100 resistance is connected in parallel with the sample. To avoid the spreading of high amplitude pulses into the Hall cross branches, 100k resistances are connected in series as represented in Figure 1. A magnetic field is applied nearly in-plane (B = 890.5°) along the x direction, parallel to the current. The perpendicular component of the magnetization is measured via the anomalous Hall effect resistance (RAHE) using a DC current. A bias tee separates the current pulses and the DC current. All measurements were performed at room temperature. Figure 2a shows the switching behavior of a 135 nm dot using 380 ps long current pulses. While the external field is swept in steps from -750 mT to 750 mT, positive and negative current pulses of amplitude Ip = 1.5 mA (3.1x108 A cm-2) are applied at each field step. RAHE is measured after each pulse: solid black squares show RAHE after positive current pulses and open orange circles after negative pulses. For the entire field range delimited by the coercivity of the Co layer the orange and black curves follow separate lines indicating that for positive field values the positive pulses switch the magnetization downwards and the negative pulses switch it upwards, whereas for negative field values the effect of the current polarity is reversed. Remarkably, although the switching is achieved using much shorter current pulses, this behavior is similar to that reported for pulse lengths ranging from tens of ns to s [11,18]. The variation of the critical current as a function of pulse length p is shown in Fig. 2b. The critical current is defined as the lowest current value for which switching occurs at every pulse for a series of ten consecutive bipolar pulses. Obviously, this value depends also on the magnitude of the in-plane static magnetic field, which we set here at 100mT.The critical current increases monotonically from less than 0.3 mA (0.6x108 A cm-2) at p = 20ns to approximately 1.6mA (3.3x108 A cm-2) at p = 380 ps. Since switching occurs on such short timescales and considering the analogy between STT with out-ofplane polarizer (OSTT) and SOT, effects related to the magnetization precession are expected to 3   

   

be important. Typically, in the case of OSTT, the switching probability has a non-monotonic behavior and oscillates with the pulse duration or/and the current density [8,9,25]. In contrast with this expectation, the monotonic behavior of the critical current for p lower than 5 ns indicates non-precessional switching behavior of the magnetization. In order to characterize the magnetization reversal in more detail, we measured the switching of magnetization of a 210 nm dot as a function of pulse intensity and length for current amplitudes up to Ip ~ 2.5x108 A cm-2 and 380 ≤ p ≤ 560 ps. Figure 3a shows the switching diagrams as a function of Bext (from 0 to 450mT) and Ip (from 1.6 to 2.5x108A.cm-2) for a fixed pulse duration of 550ps. Figure 3b shows the switching diagram as a function of Bext and p (from 380 to 560 ps) at constant current density of 2.5x108A.cm-2. The red (blue) color represents successful (unsuccessful) switching. In both cases, the range of successful switching events decreases monotonically as the current density or pulse length is reduced, whereas it increases with the inplane field amplitude (within the coercive field range), which sets the initial tilting of the magnetization. To shed light on the qualitative features of the magnetization reversal on such short time scales particularly the absence of precessional behavior - we rely on a simple macrospin model including the SOTs reported recently for the same heterostructure. We compute the Landau∥ Lifshitz-Gilbert equation including the SOTs described in Ref. [12]: T∥ with 8 -2 8 ∥ with = 50 mT / 10 A cm and T = 32 mT / 10 A cm-2 and = 23 mT / 108 A cm-2. The external field Bext is aligned along the current direction with an angle B = 89° and the magnetic parameters previously reported for our layers are used: anisotropy field BK = 1T, damping parameter  = 0.5, and saturation magnetization MS = 1T [12,26,27]. The coercive force delimiting the bistable region is artificially introduced based on measured values. Finally, we use the measured pulse profile for the current. The switching probability is then calculated in the macrospin approximation as a function of Ip, Bext, and p (Fig.3c-f). We first simulate the switching diagram as a function of Ip and Bext at constant pulse length p = 550ps (Fig. 3c). Remarkably, the computed diagram reproduces the main features of the experimental data shown in Fig. 3a: the switching range decreases while decreasing the pulse amplitude and increases with the in-plane field amplitude, reflecting the dependence of Tǁ on both the current density and the mx component of the magnetization. We find oscillations of the switching probability for low Bext and high current density. These are induced by T , which promotes precession of the magnetizationon time scale of ns. The critical current calculated in the simulations is larger compared to the experiment, which we attribute to the limitations of the macrospin model discussed later. To study the effects of thermal noise on the reversal mechanism, we simulated a random effective field that depends on temperature and acts on an effective macrospin volume of 4   

   

20x20x0.6 nm3 [28]. The temperature of the sample during the pulse injection is assumed to be 500 K for p = 380 ps based on the onset of stochastic switching events of the magnetization in the simulations, typical of thermal effects. The temperature is estimated to increase up to 520 K for p = 550 ps based on an analytical model of heat propagation as a function of Ip, p, and the bulk thermal conductance of Co, Pt, and SiO2 [29]. The diagram in Fig. 3e shows that thermal fluctuations randomly assist the reversal of the magnetization at lower currents, as expected. Further thermal effects, such as the decrease of the magnetic anisotropy energy with increasing temperature, can be included in the simulations, but do not change this qualitative trend. The switching diagrams computed as a function of pulse length and external field at constant pulse amplitude Ip = 7x108 A cm-2 (Fig. 3d,f), on the other hand, differ substantially from the experimental diagram of Fig. 3b. The simulations predict that the switching probability should not depend on Bext, i.e., on the initial tilt of the magnetization (Fig. 3d) and predict precessional switching at low Bext (induced by T ), i.e., oscillations of the switching probability depending on the length of p. These predictions hold also in the presence of thermal fluctuations (Fig. 3f). These inconsistencies cannot be resolved within a simple macrospin model. Such a model does not take into account the extended geometry of the sample, which favors the formation of magnetic domains. In fact, we find that the variation of the critical switching current is roughly proportional to p-1 at constant field (Fig. 2b), which is consistent with a nucleation-propagation mechanism. We suggest, therefore, that once a reverse domain nucleates due to Tǁ and T , switching is completed by the propagation of a domain wall through the dot. Since the domain wall velocity is proportional to the current density, the time required to travel throughout the sample will be inversely proportional. The large current-induced domain wall velocities reported in this system [26] are consistent with a fast reversal once a reversed domain has nucleated. Such a mechanism was found in micromagnetic simulations of Pt/Co/AlOx but considering much longer pulses and lower damping [30]. We note also that, contrary to the macrospin simulations, T in such a scenario can assist switching by favoring the nucleation of magnetic domains [14]. In conclusion, we have demonstrated robust and non-stochastic bipolar switching of perpendicular magnetization with sub-nanosecond pulses of in-plane current (down to 380 ps, limited by the power supply). For the current amplitudes and pulse lengths used in our experiment, we do not observe any evidence of precessional switching, contrary to switching induced by spin transfer torque. This makes the SOT-MRAM a promising candidate for ultra-fast writing applications such as cache memories. The linear dependence of the critical current with the pulse length cannot be captured by the macrospin model and suggests a nucleationpropagation scenario typical for the magnetization reversal of ultrathin films. Micromagnetic simulations may help to identify and optimize all the variables that determine the switching efficiency on sub-ns timescales.

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This work was supported by the European Commission under the Seventh Framework Program (Grant Agreement 318144, spOt project), the French government projects Agence Nationale de le Recherche (ANR-10-BLAN-1011-3 SPINHALL, ANR-11-BS10-0008 ESPERADO), and the European Research Council (StG 203239 NOMAD). The devices were fabricated at NanofabCNRS and the Plateforme de Technologie Amont (PTA) in Grenoble.

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Figure 1: Schematic of the experimental set-up. Inset: Current pulse measured at the output of the bias T (black curve) and in series with the device under test (DUT) (red curve).

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Figure 2: (a) Magnetization switching of a 135*135 nm2 dot induced by positive and negative current pulses with current density Ip = 3.1x108 A cm-2 for a pulse length of 380 ps. Note that Bext is swept only once from +0.75 to -0.75 T. (b) Dependence of the critical switching current on the inverse pulse width in the presence of an external field of 100 mT applied parallel to the current direction.

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