Research Article

Blowing magnetic skyrmion bubbles

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Science  17 Jul 2015:
Vol. 349, Issue 6245, pp. 283-286
DOI: 10.1126/science.aaa1442
  • Fig. 1 Schematic of the transformation of stripe domains into magnetic skyrmion bubbles.

    (A) Infinitesimal section of a chiral DW in a ferromagnet (F)/heavy metal (HM) bilayer illustrating the relation between local magnetization vectors and the SOT-induced chiral DW motion of velocity Vdw in a device with a homogeneous electron current flow je along the +x axis. Blue corresponds to upward orientation of magnetization, whereas orange represents the downward orientation of magnetization. The bottom panel illustrates the magnetization directions inside of the Néel wall. (B) Top view of a trilayer device. The blue region is a stripe-shaped domain. Light blue arrows show the in-plane magnetization direction of the DW [as shown in the bottom panel of (A)] and indicate that the domain has left-handed chirality. The red arrows correspond to the current distribution. (C) Introducing a geometrical constriction into the device gives rise to an inhomogeneous current distribution, which generates a flow along the y axis (jy) around the narrow neck. This current distribution is spatially divergent to the right and convergent to the left of the constriction. The y component of the current distribution is highlighted in (D). This introduces an effective spin Hall force Embedded Image along the y axis that (E) locally expands the stripe domain on the right side. (F) Once the expansion approaches a critical point, the resultant restoring forces (Fres) associated with the surface tension of the DWs are no longer able to maintain the shape, and the stripe domains break into circular bubble domains, resulting in the formation of synthetic Néel skyrmions.

  • Fig. 2 Experimental generation of magnetic skyrmions.

    (A) Sparse irregular domain structures are observed at both sides of the device at a perpendicular magnetic field of Embedded Image mT. (B) Upon passing a current of je = +5 × 105 A/cm2 through the device, the left side of the device develops predominantly elongated stripe domains, whereas the right side converts into dense skyrmion bubbles. (C and D) By reversing the current direction to je = –5 × 105 A/cm2, the dynamically created skyrmions are forming at the left side of the device. (E and F) Changing the polarity of the external magnetic field reverses the internal and external magnetization of these skyrmions. (G) Phase diagram for skyrmion formation. The shaded area indicates field-current combinations that result in the persistent generation of skyrmions after each current pulse.

  • Fig. 3 Capturing the transformational dynamics from stripe domains to skyrmions and motion of skyrmions.

    (A to D) At a constant current density je = +6.4 × 104 A/cm2 and Embedded Image mT, the disordered stripe domains are forced to pass through the constriction and are eventually converted into skyrmions at the right side of the device. Red circles highlight the resultant newly formed skyrmions. (E) Illustration of the effective spin Hall field acting on these dynamically created skyrmions; the direction of motion follows the electron current. (F to I) Efficient motion of these skyrmions for a current density je = +3 × 104 A/cm2. (F) First, a 1-s-long single pulse je = +5 × 105 A/cm2 initializes the skyrmion state. (G to I) Subsequently, smaller currents (below the threshold current to avoid generating additional skyrmions through the constriction) are used to probe the current-velocity relation. These skyrmions are migrating stochastically and moving out of the field of view. See supplementary MOKE movies S1, S2, and S4 for the corresponding temporal dynamics. (J) The current-velocity dependence of skyrmions is acquired by studying ≈20 skyrmions via averaging their velocities by dividing the total displacement with the total time period.

  • Fig. 4 Absence of motion for the in-plane magnetic fields stabilized S = 0 magnetic bubbles.

    (A to E) (A) In-plane magnetic field–induced bubbles are created by first saturating at in-plane field Embedded Image mT and subsequently decreasing to Embedded Image mT. Depending on the direction of the current, these magnetic bubbles either shrink or expand. (A to E) Shrinking bubbles are observed upon increasing the current density from je = +5 × 104 to +2.5 × 105 A/cm2 in steps of 5 × 104 A/cm2. (F to J) Expansion of bubbles is revealed for currents from je = –0.5 × 105 to –2.5 × 105 A/cm2 in steps of 5 × 104 A/cm2. (K) These results are linked to the different spin textures (namely, S = 0 skyrmion bubbles) that were stabilized along the DW by the in-plane magnetic fields, which lead to different orientations of the effective spin Hall fields and different directions of DW motion, as illustrated.

Supplementary Materials

  • Blowing magnetic skyrmion bubbles

    Wanjun Jiang, Pramey Upadhyaya, Wei Zhang, Guoqiang Yu, M. Benjamin Jungfleisch, Frank Y. Fradin, John E. Pearson, Yaroslav Tserkovnyak, Kang L. Wang, Olle Heinonen, Suzanne G. E. te Velthuis, Axel Hoffmann

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S7
    • Captions for Movies S1 to S5
    • Full Reference List

    Images, Video, and Other Other Media

    Movie S1
    MOKE movie of the transformation from stripe domains into skyrmion
    This movie is the full version of the differential polar-MOKE imaging data presented in Figs. 3 (A) – (D). It was acquired while applying a continuous current density of je= +6.4×104 A/cm2 across the device, with B= +0.46 mT. The temporal resolution of the movie is 0.13 s. Generation of individual skyrmions can be observed.
    Movie S2
    MOKE movie for the persistent generation of skyrmions
    This movie consists of differential polar-MOKE images and was acquired with a continuous current density of je= +6.8×104 A/cm2, with B= +0.46 mT, and a temporal resolution of 0.08 s. This current density is slightly larger than for the data presented in Figs. 3 (A) – (D) and the previous movie, which results in transformational dynamics at a higher rate. It shows that a large population of skyrmions are continuously generated and subsequently move along the current direction. The generation of skyrmions occurs close to the constriction where the y-component of the current is the largest. Furthermore, the continuous skrymion production results in a lateral distribution of the skyrmions.
    Movie S3
    MOKE movie of skyrmion generation under large pulsed current
    This movie is the full version of the differential polar-MOKE imaging data presented in Fig. 2 in the main text and in Fig. S5. This MOKE movie was acquired in the presence of pulsed current je= +5×105 A/cm2 with B +0.5 mT. The duration of pulse is 1 s. Upon passing the current, it is observed that the images are getting blurry at the right side of the constriction, which can be attributed to the fast transformation dynamics that is beyond the temporal resolution of the imaging camera. It is also observed that the left side of stripe domains are compressed. Immediately after the pulse current, some stripe domains are present on both sides and eventually evolving into skyrmion bubbles on the right side.
    Movie S4
    MOKE movie of the motion of synthetic skyrmions
    This movie is the full version of the differential polar-MOKE imaging data presented in Figs. 3(F)–(G). With external magnetic field of B= -0.5 mT, skyrmions are first initialized by passing current density of je= +5×105 A/cm2. These synthetic skyrmions are stable at room temperature after switching off the large current density. After that, small pulse currents of various amplitudes and duration of 1 s were applied to avoid more skyrmions being generated for which would complicate the data analysis. This procedure enables the current-velocity characteristics of synthetic skyrmions to be investigated. By looking at the movie frame by frame (of time period 0.16 s) acquired at the current density je= +3×104 A/cm2, stochastic motion of these synthetic skyrmion under current stimulation can be resolved. By counting the number of skyrmions that moved out from the frame and their average time spent during travelling, the average velocity can be determined.
    Movie S5
    MOKE movie of the current response of the domains stabilized by in-plane field
    This movie consists of differential polar-MOKE images and was acquired by starting from zero field and ramping the field to B = 5 mT, with the magnetic field applied in the sample plane and parallel to the long axis of the device, and a temporal resolution of 0.15 s. Note that, while a small in-plane magnetic field is present, we still image the polar-MOKE contrast to visualize the out-of-plane component of the magnetization. By applying a pulse current of width 1s of amplitude je= +7×105 A/cm2 (which is higher than the threshold current that required to dynamically generated skyrmion for the out-of-plane magnetic fields), the migration of stripe domains through geometrical constriction, instead of transforming into skyrmion bubbles were observed.

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