Thermal nucleation and high-resolution imaging of submicrometer magnetic bubbles in thin thulium iron garnet films with perpendicular anisotropy

Ferrimagnetic iron garnets are promising materials for spintronics applications, characterized by ultralow damping and zero current shunting. It has recently been found that few nm-thick garnet films interfaced with a heavy metal can also exhibit sizable interfacial spin-orbit interactions, leading to the emergence, and efficient electrical control, of one-dimensional chiral domain walls. Two-dimensional bubbles, by contrast, have so far only been confirmed in micrometer-thick films. Here, we show by high resolution scanning transmission x-ray microscopy and photoemission electron microscopy that submicrometer bubbles can be nucleated and stabilized in $\sim 25$-nm-thick thulium iron garnet films via short heat pulses generated by electric current in an adjacent Pt strip, or by ultrafast laser illumination. We also find that quasistatic processes do not lead to the formation of a bubble state, suggesting that the thermodynamic path to reaching that state requires transient dynamics. X-ray imaging reveals that the bubbles have Bloch-type walls with random chirality and topology, indicating negligible chiral interactions at the garnet film thickness studied here. The robustness of thermal nucleation and the feasibility demonstrated here to image garnet-based devices by x-rays both in transmission geometry and with sensitivity to the domain wall chirality are critical steps to enabling the study of small spin textures and dynamics in perpendicularly magnetized thin-film garnets.

Figure 1

Properties of the as-grown, 26.5 nm thick TmIG film. (a) Symmetric $\theta -2\theta$ x-ray diffraction scan recorded with Cu $K_{\alpha }$ radiation ($\lambda =1.5406\AA$). Laue fringes are marked by arrows. The bulk (444) peak position of TmIG is at $2\theta ={51.339}^{\circ }$, as indicated by the vertical dashed line [21]. (b) In-plane ($x$) and out-of-plane ($z$) magnetic hysteresis loops.

Figure 2

Properties of TmIG (30 nm) on a back-polished GGG membrane substrate. (a) Membrane device geometry for scanning transmission x-ray microscopy (STXM). (b) STXM images of the domain states of a bare 30 nm thick garnet film (without Pt layer) at increasing out-of-plane magnetic field. The contrast indicates the out-of-plane magnetization. Field values are shown on the top right of each image. The field was applied by rotating permanent magnets. Field values and the out-of-plane field direction are only approximate.

Figure 3

Current-induced bubble nucleation and motion in 26.5 nm thick TmIG. (a) STXM image of a zero-field domain state in a TmIG film partly covered with a Pt track. The dashed line indicates the boundary of the Pt track. The image was taken immediately after inserting the sample into the instrument and the aligned orientation of the stripes (vertical in the top-view image) is possibly due to a previous exposure to an in-plane field. (b) STXM image after transmission of a single 100 ns current pulse of $8.2\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ amplitude in a pure out-of-plane field of 3.5 mT. (c)–(e) Images of bubble domains in (c) the initial state, (d) after application of a rightward-flowing current pulse, and (e) after subsequent application of a leftward-flowing current pulse, with 100 ns duration and $8.2\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ amplitude in an out-of-plane field of 4.5 mT. Solid circles show initial positions of the bubbles, from (c).

Figure 4

Laser-induced bubble nucleation in 26.5 nm TmIG. (a)–(c) PEEM images of domain state in a purely out-of-plane bias field of 2.1 mT, in the initial state (a), after one laser pulse (b) and after a second laser pulse (c). Light (dark) contrast corresponds to out-of-plane (into-the-plane) magnetization. Panel (d) shows a higher-magnification image of several bubbles in (c), where the light/dark contrast at the bubble perimeter is due to the in-plane orientation of the magnetization, marked as colored arrows. (e) Laser-induced bubble nucleation thresholds versus laser fluence and pulse number. Blue and tan regions indicate presence or absence of bubble nucleation for positive and negative laser helicity. The x-ray direction in all images was top-to-bottom and approximately perpendicular to the Pt strip.

Figure 5

Domain nucleation by quasistatic heating in 26.5 nm TmIG. (a) Out-of-plane hysteresis loops at temperature $T=300\mathrm{K}$ and $T=320\mathrm{K}$. The fields at which domain nucleation occurs on the increasing branch of the hysteresis loops are indicated by arrows. (b) PEEM image at $T=300\mathrm{K}$ after saturating the film and reducing the field to $B_z=2.3\mathrm{mT}$. (c)–(e) PEEM images at temperatures of 320 K (c), 330 K (d), and 340 K (e). $T$ was slowly increased ($\sim 1\mathrm{K}/\mathrm{s}$) and the purely out-of-plane field was kept constant at $B_z=2.3\mathrm{mT}$. (f) Calculated bubble energy as a function of its diameter in our TmIG material for three field values, as indicated. See Methods for parameters.