Skyrmions are topological spin textures that exhibit Brownian motion in solids. They have attracted increasing research interest in terms of realizing a device that utilizing stochastic behavior and investigating new physical phenomena. However, skyrmions that exhibit Brownian motion are sensitive to changes in magnetic properties and are easily affected by aging variation. For instance, although skyrmions appear in a sample immediately after fabrication, they sometimes disappear after few weeks. This characteristic prevents the reproducibility experiment and affects device stability. In this study, we demonstrated that aging variation can be suppressed by annealing in air for only 3 min, which is an easy and rapid method. We investigated the change in the magnetic properties by annealing and air exposure and found that the main mechanism of aging variation is oxidation of the sample surface. The magnetic properties of samples with Pt and thick SiO2 capping were analyzed, and we demonstrated that aging variation can be suppressed by avoiding surface oxidation. Our work accelerates the research of fundamental physics regarding skyrmion Brownian motion and of device applications utilizing stochastic system.

Magnetic skyrmions are topologically stable spin textures in solids.1,2 Skyrmions were initially observed at low temperatures,3 but have recently been observed near room temperature,4–6 thereby accelerating research in this concept. For skyrmion systems, various physical phenomena such as spin transfer torque driven,7,8 spin orbit torque driven,5 alternating magnetic field driven dynamics,9–11 voltage effect,12,13 and Brownian motion14,15 have been reported, and the applications of the racetrack memory,16 logic,17 and neuromorphic devices18 have been explored. The Brownian motion of skyrmions is promising for the basic technology of computing using probabilistic behavior, which has attracted significant interest in recent years.15,19–23 Also, in terms of the aspects of the fundamental physics, the Brownian motion of skyrmions is expected to be a useful experimental system for information thermodynamics24–26 because it is easy to control and detect in solids. Therefore, the control methods for skyrmions that exhibit Brownian motion (voltage effects14,27 and alternating magnetic fields28) and the implementation of circuit and cellular automata29–32 have been demonstrated experimentally.

In view of these backgrounds, the techniques for fabricating magnetic thin films that exhibit skyrmion Brownian motion are important. However, such the skyrmions are very sensitive to external factors, and even a slight change in the magnetic properties can significantly change their behavior. In particular, the magnetic properties of magnetic thin films that exhibit skyrmion Brownian motion change with time (aging variation), which limits the stability of the device and prevents the reproducibility experiment. Annealing in vacuum is generally an effective method for suppressing such changes over time; however, this method often requires several hours to anneal the device and cool it to room temperature, thereby limiting the efficiency of the experiment. Annealing in air does not require a long time, however, it has not been investigated whether annealing in air can suppress aging and how magnetic properties change.

In this study, we demonstrated that aging variation can be suppressed by annealing in air on a hot plate for a short period of time (3 min). In Sec. II, we explain the sample structures. Afterward, we discuss the measurement of the magnetic properties after such treatment using polar magneto-optical Kerr effect (MOKE) and the origin of the changes in the magnetic properties due to aging variation and annealing. Finally, we show the measurement results of the magnetic properties of samples with Pt and thick SiO2 capping to prevent oxidation from the sample surface.

In the experiment conducted in this study, three types of film stacks were prepared. The basic samples Ta(5.9)|Co16Fe64B20(1.2)|Ta(0.27)|MgO(1.6)|SiO2 (tSiO2 = 2.9) (described in nm) were deposited on thermally oxidized Si substrates using a magnetron sputtering system (Canon ANELVA, E-880S-M in Osaka University) at 20 °C. Co16Fe64B20, MgO, and SiO2 were deposited from compositional targets in Ar atmosphere. Then, we prepared the sample without and with annealing in air for 3 min at temperatures of 150 °C, 200 °C, and 250 °C. A hot plate (AS ONE, TH-900 in Osaka University) was used to anneal the sample, and the temperature indicates the set temperature of the heater surface. Figure 1 shows the images of the samples observed at room temperature using the polar MOKE microscope (NEOARK, BH-782P-OSU in Osaka University). Figures 1(a)1(d) show the magnetic images immediately after fabrication, and the magnetic domain structure was observed in the sample annealed at 200 °C or higher. This is because the easy axis of magnetization changed from in-plane to out-of-plane owing to annealing. Next, Figs. 1(e)1(h) show the results of exposing each sample to the air for more than two weeks. A maze domain appeared in the samples without annealing [Fig. 1(e)] and at an annealing temperature of 150 °C [Fig. 1(f)] compared with the sample immediately after fabrication [Figs. 1(a) and 1(b)]. At an annealing temperature of 200 °C [Fig. 1(g)], the maze domain width became smaller. These results indicate that samples annealed at temperatures below 200 °C exhibit aging variation. However, a maze domain similar to that of the sample immediately after fabrication [Fig. 1(d)] was observed in the sample annealed at 250 °C [Fig. 1(h)]. Therefore, annealing at 250 °C significantly suppressed aging variation. The change of domain width is caused by perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Here, the former increases domain width and the latter decreases it. When an out-of-plane magnetic field was applied to these samples, skyrmions appeared in all the films [shown by black dots in Figs. 1(i)1(l)]. These skyrmions exhibited Brownian motion (a movie of a sample annealed at 250 °C is shown in supplementary video). When the annealing temperature is less than 200 °C [Figs. 1(i)1(k)], the skyrmion size decreases and skyrmion density increases. This increases the in-plane component of magnetization, which prevents magnetization to direct along perpendicular; therefore, the magnetic field for skyrmion formation is high (0.4 mT). While at an annealing temperature of 250 °C [Fig. 1(l) (Multimedia view)], the skyrmion size increases and skyrmion density decreases. This decreases the in-plane component of magnetization, which assists magnetization to direct along perpendicular: therefore, the magnetic field for skyrmion formation is small (0.2 mT).

FIG. 1.

Magnetic domain pattern of the magnetic thin film observed using MOKE microscope. The rows represent different annealing conditions, namely no annealing, annealing temperatures of 150 °C, 200 °C, and 250 °C. The column represents aging variation. Polar MOKE microscope images of (a)–(d) Immediately after film deposition and annealing without magnetic field. (e)–(h) Exposure to air for more than two weeks without magnetic field. (i)–(l) Exposure to air for more than two weeks and saturation magnetic field is applied (Multimedia view).

FIG. 1.

Magnetic domain pattern of the magnetic thin film observed using MOKE microscope. The rows represent different annealing conditions, namely no annealing, annealing temperatures of 150 °C, 200 °C, and 250 °C. The column represents aging variation. Polar MOKE microscope images of (a)–(d) Immediately after film deposition and annealing without magnetic field. (e)–(h) Exposure to air for more than two weeks without magnetic field. (i)–(l) Exposure to air for more than two weeks and saturation magnetic field is applied (Multimedia view).

Close modal

To evaluate these magnetic properties, we measured the out-of-plane magnetic field dependence of the polar MOKE signal in samples with the same film structure design [Fig. 2(a)]. The saturation magnetic field for each annealing temperature was determined based on the initial magnetic permeability obtained from the experimental results [Fig. 2(b)]. We found that annealing at 225 °C or higher in air for 3 min reduces the saturation magnetic field. This result suggests that perpendicular magnetic anisotropy increases. We also found that the saturation magnetic field fluctuates below 200 °C and exhibits unstable behavior.

FIG. 2.

(a) Perpendicular magnetic field dependence of the normalized Polar MOKE signal. Color represents annealing temperature for 3 minutes in air. (b) Annealing temperature dependence of the saturation magnetic field calculated from the initial permeability.

FIG. 2.

(a) Perpendicular magnetic field dependence of the normalized Polar MOKE signal. Color represents annealing temperature for 3 minutes in air. (b) Annealing temperature dependence of the saturation magnetic field calculated from the initial permeability.

Close modal

Next, we measured the out-of-plane magnetic field dependence of the polar MOKE signal, including aging variation, using samples with the same film structure design [Fig. 3(a)]. The solid red and blue lines represent the results for samples without annealing and with annealing at 250 °C in air for 3 min, respectively. We found that annealing decreases the saturation magnetic field. The dotted line represents the magnetic field dependence of the polar MOKE signal after exposure to air for more than two weeks. Aging variation decreases the saturation magnetic field in the non-annealed sample compared with the results indicated by the solid line, and temporal change was suppressed in the annealed sample. This result suggests that annealing and aging in air increases perpendicular magnetic anisotropy. It is consistent with those shown in Figs. 1(a)1(h).

FIG. 3.

(a) Perpendicular magnetic field dependence of normalized polar MOKE signals for Ta (5.9)|Co16Fe64B20 (1.2)|Ta (0.27)|MgO (1.6)|SiO2 (2.9). The unit of the film thickness in parentheses is nm. Red and blue correspond to without and with annealing cases, respectively. The annealing temperature indicates a heater setting temperature of 250 °C. (b) Perpendicular magnetic field dependence of the normalized polar MOKE signal of the sample with Pt(3.0) deposited on the film of (a). (c) Perpendicular magnetic field dependence of the normalized polar MOKE signal for the sample with a SiO2 cap layer thickness of 10 nm for the film in (a).

FIG. 3.

(a) Perpendicular magnetic field dependence of normalized polar MOKE signals for Ta (5.9)|Co16Fe64B20 (1.2)|Ta (0.27)|MgO (1.6)|SiO2 (2.9). The unit of the film thickness in parentheses is nm. Red and blue correspond to without and with annealing cases, respectively. The annealing temperature indicates a heater setting temperature of 250 °C. (b) Perpendicular magnetic field dependence of the normalized polar MOKE signal of the sample with Pt(3.0) deposited on the film of (a). (c) Perpendicular magnetic field dependence of the normalized polar MOKE signal for the sample with a SiO2 cap layer thickness of 10 nm for the film in (a).

Close modal

Here, we analyze these results. Interfacial perpendicular magnetic anisotropy occurs at the interface between MgO and a ferromagnetic layer such as Co16Fe64B20 through hybridization of O 2p and Fe 3d orbitals.33–36 Further, the perpendicular magnetic anisotropy increases with the increase in the degree of surface oxidation of capping, as demonstrated in the Co-Fe|AlOx system.37 Our experimental results suggest that the penetration of oxygen from air through the sample surface increases the oxygen density at the ferromagnetic layer interface, resulting the increase in the perpendicular magnetic anisotropy through hybridization of the 3d orbitals of the ferromagnetic layer and 2p orbitals of oxygen. This suggests that oxygen passing through the thin SiO2, MgO, and Ta affected Co16Fe64B20 in our sample owing to atmospheric annealing and exposure to the air, which increases the perpendicular magnetic anisotropy.

This result is opposite tendency to that of annealing in vacuum. Ishikawa et al. and Miki et al. performed annealing at 200 °C in vacuum for 1 h.27,31 In these samples, annealing increases the saturation magnetic field. In contrast, annealing in air decreases the saturation magnetic field. This characteristic can be explained by the fact that annealing in vacuum diffuses oxygen from the interface between MgO and (Ta|) Co16Fe64B20 to SiO2, whereas annealing in air supplies oxygen from the SiO2 surface to the interface.

Because aging variation may be due to oxidation from the surface, Pt (3) was deposited on the top SiO2 capping layer to suppress this. The polar MOKE signal for this sample is shown in Fig. 3(b). We found that aging variation disappeared by Pt capping. Moreover, we found that the annealing decreases the saturation magnetic field. This result confirmed that aging variation is due to oxidation from the top SiO2 and can be suppressed by capping with Pt. In addition, annealing decreases the saturation magnetic field despite the suppression of surface oxidation. This is because the crystallization of Co16Fe64B20 by annealing enhances the perpendicular magnetic anisotropy.38,39

Another technique for reducing the effects of oxidation is by increasing the thickness of the SiO2 film. Figure 3(c) shows the experimental results of a sample with a cap layer thickness of 10 nm. A slight change in the saturation magnetic field was observed, but changes in the characteristics due to annealing and aging were suppressed. This is because the conditions are intermediate between annealing in air and vacuum. As mentioned above, when a sample with a thin SiO2 capping is annealed in air, oxygen penetrates from the SiO2 surface and the perpendicular magnetic anisotropy increases. While vacuum annealing decreases the perpendicular magnetic anisotropy by diffusing oxygen from the Co16Fe64B20 interface into SiO2. Annealing a thick SiO2 cap in air closes to the condition of vacuum annealing because the penetration of oxygen from the surface is suppressed, resulting cancelling out the changes in the magnetic anisotropy due to penetration and diffusion of oxygen. Therefore, a sample with SiO2 (10) capping can suppress changes in the characteristics due to aging and annealing.

The appropriate thickness SiO2 cap has the above advantages but the thick SiO2 cap is less sensitive to the structures above the cap layer. Jibiki et al. demonstrated that the surface sensitivity of a thin SiO2 cap can be used to design circuits by depositing an additional film on top of the cap layer.29 Some ingenuity is required when mounting such a circuit with a thick SiO2 cap. Ishikawa et al. deposited an additional layer of Pt (0.2) on the cap layer of thin SiO2 (3), and then deposited a thick SiO2 layer (50) on the entire sample.27 By creating such a structure, it is possible to implement circuits on the Pt (0.2) filmed portion and fabricate a stochastic skyrmion device that is resistant to changes in the characteristics due to aging and annealing.

In this study, we investigated the change in the magnetic properties of a skyrmion film with aging and annealing via experiments. We found that the change in the magnetic properties with aging and annealing is due to invasion of oxygen from the surface. We clarified that the aging variation can be suppressed by annealing at 250 °C or higher in air for 3 min. This method does not require a vacuum and can be implemented in a short time, which enhances the efficiency of the experiment. In addition, aging variation was suppressed in samples with a Pt cap or an SiO2 film thickness of 10 nm because oxidation from the surface was avoided. These results will accelerate the research of fundamental physics regarding skyrmion Brownian motion and of device applications utilizing stochastic system.

This work was supported by Toyota-Riken-Scholar, JSPS KAKENHI (S) Grant Numbers JP20H05666 and JST CREST Grant Number JPMJCR20C1, Japan.

The authors have no conflicts to disclose.

Minori Goto: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Ryo Ishikawa: Conceptualization (supporting); Investigation (supporting); Resources (supporting); Validation (supporting); Writing – review & editing (supporting). Hikaru Nomura: Software (supporting); Writing – review & editing (supporting). Yoshishige Suzuki: Conceptualization (supporting); Formal analysis (supporting); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (lead); Validation (supporting); Writing – review & editing (supporting).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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