Switching current density reduction in perpendicular

Switching current density reduction in perpendicular magnetic anisotropy spin transfer torque magnetic tunneling junctions Chun-Yeol You Citation: Journal of Applied Physics 115, 043914 (2014); doi: 10.1063/1.4862963 View online: http://dx.doi.org/10.1063/1.4862963 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/4?ver=pdfcov Published by the AIP Publishing

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JOURNAL OF APPLIED PHYSICS 115, 043914 (2014)

Switching current density reduction in perpendicular magnetic anisotropy spin transfer torque magnetic tunneling junctions Chun-Yeol You Department of Physics, Inha University, Incheon 402-751, South Korea

(Received 28 September 2013; accepted 9 January 2014; published online 27 January 2014) We investigate the switching current density reduction of perpendicular magnetic anisotropy spin transfer torque magnetic tunneling junctions using micromagnetic simulations. We find that the switching current density can be reduced with elongated lateral shapes of the magnetic tunnel junctions, and additional reduction can be achieved by using a noncollinear polarizer layer. The reduction is closely related to the details of spin configurations during switching processes with the C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862963] additional in-plane anisotropy. V

I. INTRODUCTION

In studies of spin transfer torque magnetic random access memory (STT-MRAM), the perpendicular magnetic anisotropy (PMA) free layer is a key issue for the realization of non-volatile memory devices.1 In order to overcome thermal stability problems and achieve a smaller switching current density, adapting a PMA free layer is the best solution.2 It is well known that the switching current density, Jc, of the PMA free layer can be smaller than that of the in-plane free layer because of the opposite role of demagnetization energy in the determination of the switching current density. In the in-plane free layer, the inevitable demagnetization energy acts as an energy barrier in the switching process. However, the demagnetization energy helps the switching of the PMA free layer (it causes a lower value of Jc, which has led to active studies of the PMA STT-MRAM).3,4 However, the value of Jc is still higher than the practical circuit requirement; therefore, lowering the value of Jc in the PMA free layer is still a challenging issue. Recently, we reported on a series of micromagnetic simulation results for the in-plane free layer of a STT-MRAM. We found that Jc can be noticeably reduced by introducing a “noncollinear” polarizer,5,6 and strong dependences of Jc on the exchange stiffness Aex and on the lateral shapes.7,8 Furthermore, if we introduce an artificial symmetry breaking structure (a small cutting of the one edge of the ellipse), a much lower value of Jc is found due to the more coherent spin switching.9 Such approaches cannot be explained with a simple macro-spin model.10–12 Only micromagnetic simulations can reveal such detailed spin dynamics during the switching process. Therefore, micromagnetic simulations are essential in order to obtain a deep insight into the switching processes. In the present study, we applied the aforementioned approaches to the PMA free layer case. We varied the shape of the magnetic tunneling junction (MTJ) from a circle to an ellipse by elongating one axis length. We found that the elongation of one axis resulted in a very different switching mode compared with the circular MTJ switching mode, and the value of Jc was reduced for larger aspect ratio ellipses. In addition, when we introduced the noncollinear polarizer,13,14 a noticeable reduction in Jc was observed. We will now 0021-8979/2014/115(4)/043914/6/$30.00

describe the detailed spin dynamics that occurred during the switching processes, and the physical reasons for the reduction in Jc for various cases. We used micromagnetic simulations based on the Object-Oriented MicroMagnetic Framework (OOMMF)15 software with the public STT extension module.16 II. MICROMAGNETIC SIMULATIONS

Figure 1 shows a typical PMA MTJ structure. In the figure, the “FFree,” “Insulator,” and “FPolarizer” layers represent the free, insulator, and polarizer layers, respectively. The thicknesses of the FFree, Insulator, and FPolarizer layers are 2, 1, and 1 nm, respectively. The saturation magnetizations of FFree and FPolarizer are 1.3  106 A/m. The interface perpendicular magnetic anisotropy energy Ks of the free and polarizer layers is 0.625 and 2.6  103 J/m2, respectively, without volume anisotropy energy. We note that the effective PMA energy, Kef f ¼ KtFs  12 l0 Ms2 , depends on the ferromagnetic layer thickness tF . The thinner polarizer layer is harder than the thicker free layer. The exchange stiffness energy (Aex ¼ 3.0  1011 J/m) and the Gilbert damping constant (a ¼ 0:02) were fixed in this study, even though the exchange stiffness energy affected the value of Jc.7 A unit cell size of 1  1  1 nm3 was used. The positive current is defined from the polarizer to the free layers. The positive current prefers anti-parallel (AP) configurations, as shown in Fig. 1. We applied a specific current density J for 10 ns from the stable spin configurations, and determined whether or not the switching occurred in order to determine Jc. More details of the micromagnetic simulations can be found in our previous report.16 For simplicity, we ignored the field-like term, and all simulations were done at 0 K, thermal activation spin motion was not considered. III. DEPENDENCES OF Jc ON THE LONG-AXIS LENGTH OF THE ELLIPSE

Our typical experiments for the PMA MTJ employed a circular shape,1 because the general belief is that in-plane anisotropy is not important in PMA free layer switching based on the simple macro-spin model. Figures 2(a)–2(d) show selected spin configuration snapshots of 40  40 nm circular

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FIG. 1. Schematics of the magnetic tunneling junction structure of the perpendicular magnetic anisotropy free layer. The coordinate system, the current direction, and short and long axes of the ellipse are shown.

free layer switching processes. Figure 2(b) shows that the in-plane components of spin form a counterclockwise rotation at 9.0 ns. Such a circular spin configuration persists for a while due to the circular rotational symmetry of the system. After 9.5 ns, the rotation symmetry broke down, as shown in Figs. 2(c) and 2(d). After breaking the circular symmetric spin motion, the switching occurred. We plotted the time-

FIG. 2. Snapshots of spin configurations of the free layer with a circular shape (a ¼ b ¼ 40 nm) at a specific time with a switching current density of Jc ¼ 2.87  1011 A/m2. (a) t ¼ 8.0, (b) t ¼ 9.0, (c) t ¼ 9.5, and (d) t ¼ 9.75 ns. In (a), points A–E indicate specific locations in the free layer.

J. Appl. Phys. 115, 043914 (2014)

dependent local Mz/Ms in Fig. 3 in order to better understand at the five specific points shown in Fig. 3 (see A–E in Fig. 2(a)). The figure clearly shows that points A and B (on the left side) oscillated together, and points D and E (on the right side) show out-of-phase oscillations with respect to A and B in the red box (9–10 ns). The motions of point C (center part of the free layer) are distinguishable from the others. Such anti-symmetry behaviors were discussed in our previous study for the in-plane symmetric system.6,9 We pointed out that the breaking of such asymmetric spin dynamics (out-of-phase spin motion) can introduce more coherent spin motion, and can reduce Jc. In order to break the circular symmetry of the system, we first elongated one axis length from 40 to 80 nm. Thus, the shapes of the MTJs are ellipses with different long axis lengths a. We depict the micromagnetic simulations results in Fig. 4 for AP-to-P and P-to-AP switching current density. We find that the elongated ellipse have smaller Jc for either cases. It implies that the in-plane anisotropy play an important role in the determination of Jc, despite irrelevant contributions of it in the macro-spin model. Here, the in-plane anisotropy occurred as a result of different demagnetization coefficients of the x- and y-axes. Even though the ellipticity is not large and the in-plane anisotropy is small, they were sufficient to change the detailed spin dynamics. When the spins precess, they preferred to tilt toward the x-axis, and the orbit of spin precession became an ellipse due to the in-plane anisotropy. We note that such reduction seems quite similar to the in-plane results.8 However, the underlying physics are totally different. For the in-plane cases, the dependence of junction size on the current density is unpredictable and complicated8 because it is closely related to the domain wall width and the junction size. In our previous study for the lateral shape dependent study for the in-plane cases, we found that the spin configurations are far from the single domain during the reversal processes. Excited spin waves with various wave vector form complex domain configurations

FIG. 3. Time-dependent Mz/Ms at points A–E of the circular shape free layer. Noticeable switching occurs during 9.0–10.0 ns within the red box. The spin dynamics of points A and B are the opposite of points D and E, and the center point C is different than the other points.

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FIG. 4. Dependence of Jc on the length of the free layer long axis a for P-to-AP and AP-to-P switching.

with domain wall, and the excitation are governed by the exchange stiffness, effective anisotropy energies. Here, the effective anisotropy included the shape anisotropy, which strongly depends on the lateral shape of the MTJ. Since the typical exchange length of the free layer magnetic material is comparable with the lateral junction size, the dependence is not a simple function. Therefore, there are no simple explanations. However, the dependence of a in PMA is very predictable, as shown in Fig. 4. Figures 5(a)–5(d) show selected snapshots for 80  40 nm2 cases at various times for the Jc. Figure 5(b) shows that the in-plane components of the left and right edge are opposite, which is quite different than the circular case (Fig. 2(b)). After 9.0 ns, the asymmetric spin motion became more clear (Figs. 5(c) and 5(d)), and the additional shape in-plane anisotropy plays an important role. Eventually, switching occurs in this case. However, the switching process

FIG. 5. Snapshots of spin configurations of the free layer with an elliptical shape (a ¼ 80, b ¼ 40 nm) at a specific time with a switching current density of Jc ¼ 2.25  1011 A/m2. (a) t ¼ 7.5, (b) t ¼ 8.0, (c) t ¼ 9.0, and (d) t ¼ 10.5 ns. In (a), points A–G indicate specific locations in the free layer.

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FIG. 6. Time-dependent Mz/Ms at points A–G of the ellipse shape free layer (a ¼ 80, b ¼ 40 nm). The spin dynamics of points A and G, E and F, and C and E are almost identical, whereas the spin motion of center point D is more abrupt and late.

details are not the same as in the circular case because of the additional in-plane anisotropy. The time-dependent local Mz/Ms for the 80  40 nm2 case is shown in Fig. 6. The figure shows that the time-dependent Mz/Ms at points A and G (the edges of the ellipse) is almost identical, whereas the motions of the in-plane component are the opposite, as shown in Figs. 5(b) and 5(c). And, they are already switched after 9.0 ns. Points B and F also showed the same time dependent and switched about 9.5 ns. Points C and E are similar, but switching occurs later. The important point is the behavior of center point D. This point was not switched until 11 ns, despite the switching was already occurred in the other parts of the free layer, and that the spin polarized current was already turned off at 10 ns. However, when the switching occurred at point D, it switched more abruptly. Similar behavior occurred

FIG. 7. Dependence of Jc on the tilt angle of the noncollinear polarizer layer, with various lengths of free layer long axis a for P-to-AP and AP-to-P switching

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J. Appl. Phys. 115, 043914 (2014)

FIG. 10. Snapshots of spin configurations of the free layer with an elliptical shape (a ¼ 80, b ¼ 40 nm) at a specific time. The switching current density (Jc ¼ 1.81  1011 A/m2) is applied, and the tilt angle of the noncollinear polarizer layer is 5 . (a) t ¼ 7.0, (b) t ¼ 7.4, (c) t ¼ 8.0, and (d) t ¼ 9.5 ns. In (a), points A–H indicate specific locations in the free layer.

FIG. 8. Snapshots of spin configurations of the free layer with the circular shape (a ¼ b ¼ 40 nm) at a specific time. The switching current density (Jc ¼ 1.875  1011 A/m2) is applied, and the tilt angle of the noncollinear polarizer layer is 5 . (a) t ¼ 8.0, (b) t ¼ 8.5, (c) t ¼ 9.5, and (d) t ¼ 10.0 ns. In (a), points A–E indicate specific locations in the free layer.

for the in-plane free layer case with the elliptical shapes in our previous studies.6 Based on our observations, we conclude that the additional in-plane anisotropy also plays an important role in the switching processes, which is in contrast to the simple macro-spin model. We note that the total current (Jc  area) did not decrease, while the value of Jc decreased for the elongated a-axis case. Therefore, there is no benefit when we consider real MTJ circuit design. However, we can appreciate the importance of small in-plane anisotropy in the switching process. In Sec. IV, we consider a noncollinear polarizer that leads to additional unidirectional in-plane anisotropy. IV. Jc WITH THE NONCOLLINEAR POLARIZER LAYER FOR A PMA FREE LAYER

Including our previous studies,5,6 it is known that a noncollinear polarizer17,18 leads to reductions in Jc for an

in-plane MTJ. For the PMA case, a noncollinear polarizer will reduce the value of Jc as shown in Fig. 7. The directions of the easy axis of the polarizer were varied from 0 to 20 with respect to the film normal in order to introduce the noncollinear polarizer. We emphasize that tilting of the easy axis was easily implemented in the micromagnetic simulations; however, it is not simple in the real experiments. For example, there is a report on a controllable tilting angle by using an exchange spring system.19 Nguyen et al.19 suggested that the combination of a PMA hard layer with an in-plane soft layer gives a continuous tilting angle as a function of the soft layer thickness. Since this is general phenomena, any combination of hard PMA and in-plane soft layers will work. With a finite tilting angle, the Jc is reduced for the circular and elliptical shape free layers, and they increase again after the tilt angle exceeds 5 . Such tendencies were discussed in our previous work5,6 for the in-plane anisotropy MTJ. In our previous studies, we revealed that the non-collinear polarizer layer enhanced the coherent spin rotation during the switching process. With the collinear polarizer layer, the switching process always involve complex spin configuration with incoherent spin dynamics for the in-plane case, which is similar to Figs. 2 and 5. However, the noncollinear polarizer breaks the symmetry of the system, and it enhanced the

FIG. 9. Time-dependent Mz/Ms at points A–E of the circular shape free layer. (a) The asymmetric spin dynamics of the left- and right-side edges disappear. (b) Before starting the large spin motion, the out-of-phase motions of both edges are still observed.

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J. Appl. Phys. 115, 043914 (2014)

FIG. 11. Time-dependent Mz/Ms at points A–H of the elliptical shape free layer (a ¼ 80, b ¼ 40 nm). The symmetric spin dynamics of the left and right side edges have disappeared.

coherent spin motions. It must be noted that our approaches differ from the work done by Beaujour et al.;20 they investigated the effect of the perpendicular polarizer layer with in-plane switching layer with additional in-plane reference layer in order to obtain Tunneling Magnetoresistance (TMR) signal. In this study, we introduce the noncollinear (tilted) PMA polarizer with PMA free layer. With a noncollinear polarizer, the additional in-plane anisotropies have unidirectional natures, while the elliptical shape free layers have additional uniaxial in-plane anisotropy. Such additional unidirectional in-plane anisotropy will lead to different spin dynamics over all free layer areas. Figures 8(a)–8(d) show very different spin motions compared with those shown in Figs. 2(a)–2(d). The spin motions shown in Fig. 2 hold circular symmetry. However, coherent spin motions were observed with the noncollinear polarizer layer shown in Fig. 8. The differences are clear when Figs. 2(b) and 8(b) are compared. To obtain a better understanding, the time-dependent Mz/Ms values at points A and E are shown in Figs. 9(a) and 9(b). In Fig. 9(a), the spin motions of points A–E are not strongly correlated after 9.5 ns, while they are strongly correlated before 8.5 ns, as shown in Fig. 9(b). The out-of-plane components, Mz/Ms, for the left and right edges show the opposite motion as in the collinear polarizer case shown in Fig. 6 for a small spin motion (Fig. 9(b)). However, when the spin motion is increasing, additional unidirectional in-plane anisotropy plays a more important role, and such correlations are broken after 9.2 ns (Fig. 9(a)). Next, we adapt the noncollinear polarizer for the elliptical free layers. Snapshots of the spin configuration of 80  40 nm2 are shown in Figs. 10(a)–10(d). At 7.0 and 7.4 ns (Figs. 10(a) and 10(b)), it appears that the spins are coherently rotated over the whole areas because the noncollinear polarizer breaks the symmetry of the system. However, when we consider the time-dependent Mz/Ms shown in Fig. 11(b), we note that the out-of-plane components of the spin motion are not very coherent. The center part (C, D, E) shows strong motion, and the right-hand edge (H) shows mostly weak spin motions before 8.0 ns. However, after 8.0 ns, the switching occurs from A to H (from the left edge to the right edge) because of the additional unidirectional in-plane anisotropy, as shown in Fig. 11(a). Points F, G, and H were switched after the current pulse was turned off at 10.0 ns.

We observed that the noncollinear polarizer layer reduced the TMR values. However, with a tilting angle of 5 , the TMR value was 99.6% of the collinear TMR. Therefore, the TMR reduction is not a serious disadvantage for the noncollinear polarizer system. V. CONCLUSION

We varied the aspect ratio of the free layers and introduced noncollinear polarizer layers for PMA MTJ structures. We found that an elongated one-axis length introduced additional uniaxial anisotropy, and the additional anisotropy broke the circular symmetry of the spin motion during switching processes. While the spin configuration of the circular shape PMA free layer holds circular symmetry during the switching processes (Fig. 2), the additional in-plane anisotropy promotes a laterally asymmetric spin motion (Fig. 5) as in the in-plane cases.6 With the additional in-plane anisotropy, the switching current density decreased as shown in Fig. 4. A further reduction can be achieved by employing the noncollinear polarizer layers. With a tilt angle of 5 , noticeable reductions can be established as shown in Fig. 7. The noncollinear polarizer layer causes unidirectional in-plane anisotropy, and also breaks the symmetry with respect to the spin dynamics (Figs. 8–11). It is clear that the breaking of the symmetric spin dynamics helps to reduce the switching current density, as we already described for the in-plane cases.9 Based on our micromagnetic simulations, we suggest that the switching current densities of PMA free layers depend on the lateral aspect ratio and the polarizer tilting angle. In other words, they depend on additional in-plane anisotropy. Therefore, a more detailed consideration of the additional in-plane anisotropy should be addressed in future examinations of the PMA STT-MRAM. ACKNOWLEDGMENTS

This work was supported by the Korean Research Foundation (NRF) (Grant Nos. 2013R1A1A2011936 and 2012M2A2A6004261). 1

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