Abstract
Currentinduced magnetic domain wall motion is attractive for manipulating magnetization direction in spintronics devices, which open a new era of electronics. Up to now, in spite of a crucial significance to applications, investigation on a currentinduced domain wall depinning probability, especially in subnano to afewnanosecond range has been lacking. Here we report on the probability of the depinning in perpendicularly magnetized Co/Ni nanowires in this timescale. A high depinning probability was obtained even for 2ns pulses with a current density of less than 10^{12} A m^{−2}. A onedimensional LandauLifshitzGilbert calculation taking into account thermal fluctuations reproduces well the experimental results. We also calculate the depinning probability as functions of various parameters and found that parameters other than the coercive field do not affect the transition width of the probability. These findings will allow one to design highspeed and reliable magnetic devices based on the domain wall motion.
Introduction
Spintransfer torque (STT) is of great technological interest as a tool to manipulate the magnetization direction of nanoscopic magnets^{1,2} due to a potential to realize ultralowpower and highperformance integrated circuits^{3,4,5} and storages^{6}. There are mainly two ways to utilize the STT: one is currentinduced magnetization switching (CIMS) in magnetic nanopillars^{2,7,8,9,10,11,12}, and the other is currentinduced domain wall motion (CIDWM) in magnetic nanowires^{1,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27}. In particular, the latter is promising for applications in which a domain wall (DW) moves numerous times within several nanoseconds’ duration^{4}, or in which numerous DWs are shifted simultaneously^{6}. As the key ingredient of these devices is currentinduced displacement of DWs from one artificially prepared pinning site to another, it is very important to characterize the depinning properties, especially its probability and the relationship between the critical current and pulse duration in the nanosecond range. Whereas a theoretical model to address such issues for CIMS has been established^{11,12} and confirmed in several experiments^{7,9,10}, there has only been a few theoretical studies on CIDWM^{16,17}.
Here we show the DW depinning probability for various current pulse amplitudes and durations using an artificially prepared pinning site in magnetic nanowires. We observed DW depinning with a low error rate driven by nanosecondlong current pulses, suggesting suitability for the device application described above. Also, we elucidated a characteristic mechanism of the DW depinning, which is contrasting with the CIMS. Furthermore, we found that a theoretical model in which thermal fluctuations are taken into account showed good agreement with the experimental results. By using this model, we discuss the transition width of the depinning probability with various parameters.
Results
Sample structure and magnetic properties
For DW depinning experiments, a perpendicularly magnetized Co/Ni multilayer^{18} deposited onto a Si substrate was used. This is a promising material for application to DWmotion devices because of several features in terms of stable operation and retention characteristics^{21,22,23}, owing to its nature that the DW can be driven by an adiabatic STT^{22}. By measuring the magnetization curves of the blanket film along the inplane and outofplane directions, the saturation magnetization M_{s}, perpendicular anisotropy constant and coercive field were obtained to be 0.96 T, 6.1 × 10^{5} J m^{−3}, and 16.5 mT, respectively, which are consistent with the values reported in previous works^{21,22,23,24}. The deposited film was patterned into a crossshaped nanowire with a 160nm width, on which four Cr/Au electrodes (A–B, C, D, and E) were attached (Fig. 1). The magnetization direction around the Hall cross of the nanowire was detected by measuring the Hall resistance through the extraordinary Hall effect. To prepare a DW in the nanowire, after saturating the magnetization with a large outofplane magnetic field, a current pulse was applied to the Cr/Au electrode (from A to B) under an outofplane assist field H_{a}, where the current pulse generates a local Oersted field, which forms a reversal domain. All measurements were performed at room temperature. (See Fig. 1a for the experimental setup and Methods for detailed information about the sample preparation and measurement.)
Figure 2a,b show resistance versus outofplane magnetic field (R–H) curves for the nanowire without and with DW, respectively. The insets illustrate the corresponding magnetization configurations. The major R–H curve in Fig. 2a shows a square hysteresis, indicating a perpendicular easy axis of the present Co/Ni nanowire. The coercive field, which corresponds to a nucleation field for magnetic domains, is about 350 mT. Figure 2b displays 10 independent R–H curves measured after preparing DW. The R–H curves obtained with no H_{a} (labelled I) shows abrupt jumps at a sweeping field of about +20 and +35 mT. The former and latter fields correspond to a DW propagation field in the straight region and overpassing field to the right direction at the Hall cross, respectively^{18}. On the basis of this result, the H_{a} of +27 mT was used for initializing the DW at the left of the Hall cross, prior to all measurements of currentinduced DW depinning described below. The negative field sweeping starting in this initial state (labelled II) shows a resistance drop at a field of about −12 mT. This field corresponds to the magnitude of the pinning potential formed at the left side of the Hall cross originating from the geometrical effect (Supplementary Fig. S1) as well as pinning due to edge roughness and structural defects (see also Supplementary Note 1 in more detail); this field is different from the propagation field in the wire and coercive field of the blanket film.
Currentinduced DW depinning properties
To examine the currentinduced DW motion, the resistance versus current density (R–j) curve shown in Fig. 2c was measured by cycling through the following sequence: first, a DW was set at the initial site; second, a 10nslong current pulse with a variable amplitude and polarity was applied; third, the Hall resistance was measured. A positive (negative) current is defined as a current flowing from left (right) to right (left). The change in resistance without the DW preparation process was also measured as a reference (open symbols). In case that a DW was initialized, a positive (negative) current decreases (increases) the Hall resistance at an amplitude of more than ±7 × 10^{11} A m^{−2}, whereas the resistance remains in the lowest state in case of no DW preparation procedure. This indicates that the DW trapped at the Hall cross was displaced by the applied current pulse in the direction of electron flow, which is consistent with the DW motion induced by STT. We evaluated the temperature increase due to Joule heating and found that it was as low as 20 °C at the current density of 1 × 10^{12} A m^{−2} even in the dc case, suggesting that it is negligibly small in the present experiment using nanosecond pulses (see Supplementary Fig. S2 and Supplementary Note 2). Note also that the critical current density to move the DW to the right side becomes larger than that needed to move it to the left side as the pulse duration decreases to a few nanoseconds. This is due to a difference in pinning potential for the two DW motions, which will be discussed later. Hereafter, we will focus on DW depinning induced by a positive current injection, that is, depinning to the left direction from the Hall cross.
Experiment on DW depinning probability
Figure 3 shows the DW depinning probability P_{dep} for various current densities and pulse durations, where the rise and fall time of the pulse was less than 0.1 ns. To obtain P_{dep}, the measurement was repeated 100 times for each condition. When the applied current density exceeds a certain threshold by about 10%, DW depinning becomes highly deterministic, that is, 100 successes for 100 trials, for pulse durations of more than about 2 ns, whereas no depinning was observed when the current density is less than the threshold by about 10% (Fig. 3a). Also, the transition of P_{dep} from 0 to 1 with increasing current becomes steep when the pulse duration is longer than 2 ns (Fig. 3b). Interestingly, the critical current density with respect to the pulse duration is almost flat at durations longer than about 2 ns, whereas it significantly increases for durations less than 2 ns (Fig. 3c). We will discuss the relationship between the critical current density and pulse duration later. Figure 3d shows 1P_{dep}, that is, the depinning error rate, obtained by 10^{4} repetitions with the same measurement sequence for 2nslong current pulses with various amplitudes. A slight increase in current density above a certain threshold results in a steep decrease in the error rate. It is also notable that no error was observed in 10^{4} trials when the current density exceeds 8.5 × 10^{11} A m^{−2}.
Calculation of DW depinning probability
Let us consider the model describing the DW depinning probability shown in Fig. 3. In order to reproduce the stochastic nature of DW depinning, we took into consideration thermal fluctuations, which have also been considered for describing CIMS at a finite temperature^{11,12}. We used the onedimensional LandauLifshitzGilbert equation, in which the state of the DW is expressed by two collective coordinates (q, ϕ) representing the position and phase of the DW, and calculated the dynamics of a DW depinned from a parabolic pinning potential^{25}, (Fig. 4a), (see Supplementary Note 3). The effect of thermal fluctuations was included as a distribution of initial (q, ϕ) values (see Supplementary Note 4) and a random magnetic field that satisfies the fluctuationdissipation theorem^{28}, (see Supplementary Note 5). We set each parameter so as to match the present Co/Ni system (see Methods for details).
Figure 4b,c show the trajectories of (q, ϕ) after the current is turned on without and with thermal fluctuations, respectively. With the present set of parameters, the critical current density u_{c} is 36 m s^{−1} at 0 K (Fig. 4b). Figure 4c shows six independent calculation results where the current density and temperature T were 36 m s^{−1} and 300 K. Three of those are depinned while the others are not; this shows the stochastic nature of DW depinning. Also, looking carefully at the trajectories, we can see that both the initial distribution of (q, ϕ) and disturbance during the time evolution of (q, ϕ) affect whether the DW can be depinned. For example, DWs starting with a positive ϕ generally tend to be depinned, but the trajectory shown by the green curve shows no depinning, although its initial state is similar to the red curve, which shows depinning. Figure 4d–g are the calculation results corresponding to Fig. 3a–d, respectively. It is clear that the calculation well reproduces the tendencies observed in our experiment; for example, the critical current density is almost constant when the pulse duration is above 2 ns, and the error rate steeply decreases when the current density is slightly larger than a certain threshold value.
Discussion
First, we should note the good agreement between the experiment and the calculation. This allows us to evaluate the DW depinning properties at a finite temperature, as described below. Based on the above model, we calculated the DW depinning probability with various parameters (Supplementary Fig. S3). We found that size of the pinning potential q_{0} and intrinsic pinning field H_{c0} influence only the horizontal scale of the P_{dep} contour; for example, the pulse duration above which the critical current density is almost constant increases with q_{0}. On the other hand, the hardaxis anisotropy field H_{k} and DW width parameter Δ influence only the vertical scale, and the Gilbert damping constant α and nonadiabatic coefficient β have limited influence on the DW depinning properties. We also examined the transition width of P_{dep}. Figure 5 shows P_{dep} and its transition width with respect to the current density and pulse duration for various parameters calculated from 500 iterations. The transition width of P_{dep} is strongly affected by T (Fig. 5g). In addition, when H_{c0} is so small that the length scale of the motion of the DW in the potential well reaches q_{0}, the transition width with respect to the pulse duration significantly increases (Fig. 5b). The other parameters studied here do not prominently affect the transition width. Furthermore, the characteristics of the P_{dep} contour remain virtually unchanged even though these parameters vary by a factor of 2. Such features can be attributed to the DW depinning process being driven by an adiabatic spintransfer torque in the socalled intermediate regime, where the threshold of the depinning is determined by the hardaxis anisotropy^{19}. We also note that, because the numerically calculated μ_{0}H_{c0}, which gives the upper limit of the intermediate regime, is as large as 2 T, practically all realistic experimental conditions fall within this regime. Therefore, the present findings cover the entire experimentally accessible range and allow one to employ a wide variety of materials, because they do not affect the stable operation of the DWmotion devices.
Next, let us examine the relationship between the critical current density j_{c} and pulse duration τ_{p}, by comparing the experimental results with the calculation in more detail. Figure 6 shows the experimentally obtained j_{c} as a function of τ_{p} for three different devices with the same structure. Here, j_{c} is defined as the current density that gives P_{dep}=0.5. One can see that the dependence of j_{c} on τ_{p} is quite similar among the three devices. The fitting curve with the inverse relation, that is, j_{c}—j_{c0}1/τ_{p}, which is true for CIMS in nanopillars^{7,9,11}, is also shown in the figure (dotted line). This relation is based on magnetization reversal originating from the currentinduced torque through conservation of spin angular momentum^{7}. In addition, a result of a onedimensional LandauLifshitzGilbert calculation is shown (solid line). It is apparent that the inverse relation poorly fits the experimental results, whereas the numerical calculation agrees well with them. The discrepancy from the inverse relation can be accounted for in terms of the characteristic mechanism of DW depinning by adiabatic STT. In the adiabatic regime, a DW can be depinned when Walker breakdown occurs. Here, not only STT but also the effective field originating from the pinning potential has a significant role in inducing Walker breakdown^{19,20}. Consequently, the depinning time is not simply proportional to the inverse of the current (see Supplementary Fig. S4 and Supplementary Note 6 in more detail). This is why the critical current density does not increase as the pulse duration decreases down to about 2 ns, and a low error rate is possible even at a current density of less than 10^{12} A m^{−2}. The fact that j_{c} does not increase down to about 2 ns is favourable for DWmotion devices with an operation frequency of several hundred MHz. Similarly, the steep decrease in the error rate above the threshold is promising to achieve reliable devices operated at a high working rate. It should also be noted that these features are in stark contrast to the case of CIMS in nanopillars, where j_{c} is inversely proportional to τ_{p} and the logarithm of the switching error rate almost linearly decreases as the current density increases in afewnanosecond, that is, precessional, range.
In summary, we investigated the DW depinning probability both experimentally and theoretically. The experimental results indicate that highly deterministic DW depinning is possible with a nanosecondlong current pulse, suggesting a potential for device applications with highspeed driving of DWs at low error rates. Moreover, the theoretical model we used describes well the experimental results and gives us useful information for developing currentinduced DWmotion devices.
Methods
Film deposition
The Co/Ni multilayer was deposited on a highly resistive Si substrate with a natural oxidation layer at room temperature through dc magnetron sputtering. The stack structure was, from the substrate side, Ta(3.0)/Pt(2.0)/[Co(0.3)/Ni(0.6)]_{4}/Co(0.3)/Pt(1.5)/Ta(3.0) (in nanometres).
Device fabrication
The Co/Ni multilayer was patterned into crossshaped nanowires by using electron beam lithography and Ar ion beam milling. Cr(5)/Au(100) electrodes were formed onto the Co/Ni nanowire by using electron beam lithography and liftoff.
Hall measurement
The Hall resistance was evaluated by measuring the voltage between electrodes D and E upon application of a 50 μA dc current (4.2 × 10^{10} A m^{−2}) between electrodes A and C in Fig. 1b.
R–H curves measurement after DW injection
The two types of R–H curve shown in Fig. 2b were obtained through the following sequence: (1) apply a magnetic field of −500 mT to align all magnetizations in the downward direction; (2) inject a current pulse with an amplitude of about 150 mA and duration of 10 ns from electrodes A–B to nucleate an upward magnetic domain in the vicinity of the interconnection under an assist field H_{a}; (3) measure the Hall resistance by sweeping the magnetic field in the positive or negative direction.
Numerical calculation
The parameters used for the numerical calculation were set according to the present Co/Ni wire. The actual values are as follows: w=160 nm, t=3.9 nm, α=0.05 (ref. 29)^{29}, β=0.02^{26} (ref. 26), Δ=5.4 nm, which was derived from a twodimensional micromagnetic simulation, μ_{0}H_{k}=100 mT, which was also derived from a twodimensional micromagnetic simulation, μ_{0}H_{c0}=20 mT, which was determined from the experimental result shown in Fig. 2b with taking the effect of thermal fluctuation into account, and q_{0}=10 nm (see Supplementary Fig. S1 and Supplementary Note 1). T was 300 K. The actual form of the random magnetic field H_{rand} derived from the fluctuationdissipation theorem is , where the time step of the application of the random magnetic field dτ was set to be 1 ps (see Supplementary Note 5).
Additional information
How to cite this article: Fukami, S. et al. Depinning probability of a magnetic domain wall in nanowires by spinpolarized currents. Nat. Commun. 4:2293 doi: 10.1038/ncomms3293 (2013).
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Acknowledgements
We are grateful to S. Maekawa, J. Ieda, M. Hayashi, D. Chiba and T. Ono for their valuable discussion, and I. Morita, T. Hirata, H. Iwanuma, M. Murahata and C. Igarashi for their technical support. This research is granted by the Japan Society for the Promotion of Science (JSPS) through the ‘Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST Program),’ initiated by the Council for Science and Technology Policy (CSTP).
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Affiliations
Center for Spintronics Integrated Systems, Tohoku University, Sendai 9808577, Japan
 S. Fukami
 , M. Yamanouchi
 , S. Ikeda
 & H. Ohno
Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai 9808577, Japan
 M. Yamanouchi
 , S. Ikeda
 & H. Ohno
WPI Advanced Institute for Materials Research, Tohoku University, Sendai 9808577, Japan
 H. Ohno
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S.F. fabricated devices, collected and analysed data, and performed calculations. All authors discussed the results and wrote the manuscript.
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The authors declare no competing financial interests.
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Supplementary Information
Supplementary Figures S1S4, Supplementary Notes 16 and Supplementary References
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