Chapter 4: Electrical transport studies of Magnetic
4.3 Effects of circularly polarized light on the AHE of bilayer
cooling, the value of |Rxy| decreased with increasing fields in the large field limit, as shown in Figure 4.17(a), which is consistent with what we had observed before. Similar studies of the Rxx (B, T) data is shown in Figure 4.17 (b), which revealed how the Rxx (B, T) curve transitioned from a W-shape at low temperatures to a V-shape at high temperatures by considering horizontal cuts in Figure 4.17 (b).
It is interesting to note that the zero-field warming Rxy map (after field cooling) shown in Figure 4.17(c), the |Rxy| value became independent of the applied magnetic field for B > 1 T in the positive field response and for |B| > 0.5 T in the negative field response, which implies the saturation of the magnetization.
For the positive field response shown in Figure 4.17(c), Rxy evolved from negative to positive values between T = 7 K to 15 K, which corresponded to negative coercive fields, consistent with what we had observed from the hysteretic Rxy vs. B isotherms over this temperature range. In the case of negative field response, |Rxy| decreased monotonously with increasing temperature. However, the Rxy value after cooled in negative fields was found to be smaller than that after cooled in the positive fields for 10 K < T < 15 K. The asymmetric dependence of the AHE and spontaneous magnetization on positive and negative field responses is interesting, which may be associated with the asymmetric interfaces of both the TI and MTI layers, leading to Rashba-like splitting[87].
In the case of the Rxx (B, T) map shown in Figure 4.17(d), Rxx vs. B was independent of the pre-applied field for B > 0.1 T. This is not surprising because after field-cooled by sufficiently high magnetic fields, a saturation of magnetization completely suppressed disorder spin scattering and so Rxx obtained under zero-field warming would be only dependent on the temperature and would be independent of -the magnetic fields applied during field-cooling.
4.3 Effects of circularly polarized light on the AHE of bilayer ternary
Increasing the onset temperature of AHE in ferromagnetic materials is highly desirable for realistic spintronic applications. Given that the appearance of AHE requires long-range ferromagnetic order, increasing its onset temperature implies achieving long-range coupling of magnetic moments at a higher temperature. In the TI/MTI compounds of our study, the magnetic moments are associated with the Cr-dopants, and the onset temperature of AHE in these samples is always around 30 K, which we identified as the bulk Curie temperature πππΆπΆπππ’π’π£π£ππof the material. On the other hand, our STM studies revealed local surface gap opening at temperatures as high as ~ 200 K, suggesting that short-range ferromagnetic order could appear well above πππΆπΆπππ’π’π£π£ππ'. Additionally, our collaborators at UCLA had successfully fabricated AFM/MTI/AFM (AFM: antiferromagnet) heterostructures by MBE and demonstrated enhancement of πππΆπΆπππ’π’π£π£ππ up to 100 K by multilayers of AFM/MTI/AFM sandwich [88]. The mechanism for this enhancement is the result of exchange coupling between Dirac Fermions and the A-type AFM layer, which helps align the spins of Cr- dopants in the MTI. These results suggest that the relatively low πππΆπΆπππ’π’π£π£ππ in the ternary MTIs was due to disordered spins and so πππΆπΆπππ’π’π£π£ππ could be much enhanced by better spin alignments.
Indeed, our electrical transport studies of the ternary TI/MTI samples also revealed that with the help of an external magnetic field, the onset of AHE could appear at a temperature as high as 60 K, as shown in Figure 4.15(a), which further corroborated the notion that the onset temperature of AHE could be enhanced by whatever means feasible to align the spins of the Cr dopants.
Besides the application of high magnetic fields or the introduction of exchange coupling, it would be interesting to explore possible non-contact and non-destructive methods to enhance the alignment of spins of Cr-dopants in the MTIs. One such possibility could be the application of circularly polarized light.
Light-matter interaction can induce many interesting effects on the material surface, including the generation of photocurrents and quantum excitations that satisfy optical selection rules. For sufficiently high photon energy, the photon can excite an electron from the valence band to the conduction band. For 3D TIs, circularly polarized light can be shown
to generate a directional helicity-dependent photocurrent (HDPC) [89-91]. This photo- induced DC current is found to be dependent on the circular polarization of the light. Due to the spin-momentum lock-in on the TI surface states, when electrons are excited from the surface states to the conduction band, they still retain their spin helicity. Therefore, it is worth investigating whether we may populate one spin texture more than the other, thereby enhancing the overall spin alignment of Cr-dopants.
As described in Chapter 2, we constructed an optical probe for the PPMS system to conduct photo-assisted transport measurements. Given that we employed lock-in technique for the resistive measurements, the light-induced DC photocurrent may not be directly detected in our AC measurements. However, we may be able to observe changes in the Rxy signal because of its dependence on the sample magnetization, which motivated our investigations of the electrical transport properties under the excitations of circularly polarized light.
Adding light to the transport measurements requires substantial optical alignments and light source tuning to send light to the bottom of the PPMS successfully. Our homemade optical probe was designed, constructed, and calibrated by several groupmates, including Marcus Teague, Duxing Hao, and Adrian Llanos.
For the optical experiments, our broadband light source was a tunable lamp for wavelengths ππ = 200 to 2200 nm. The maximum available power for the light source was 10 mW. For our preliminary experiments, we used a wavelength ππ = 1750 ππππ (~ 0.7 eV), which corresponded to more than twice the bandgap of the ternary TI, and therefore it might also induce excitations of bulk electrons in addition to surface-state electrons.
4.3.1 Effect of circularly polarized light on (3+6)-10% ternary TI/MTI
We carried out Rxy vs. B measurements between B = β 0.1 T and B = 0.1 T for T = 2, 4, 7, 10, 12 and 14 K both with and without circularly polarized light, and the results for T = 2 K are shown in Figure 4.18(a). Circularly polarized light appeared to have enhanced the magnetization in one direction (B < 0) while nearly no effect in the other direction (B > 0), as shown from the coercive field of decreasing field sweep in Figure 4.18(a). This
asymmetric behavior is again consistent with what we have observed in the measurements of Rxy (T, B), which we attribute to possible Rashba effects. Additionally, from the coercive field vs. T measurements both with and without circularly polarized light, we found that Rxy with and without light deviates from each other below 7 K, as shown in Figure 4.17(b), which also corroborated the effect of circularly polarized light on the AHE.
Based on these promising preliminary measurements, we expect more interesting effects on the AHE of the bilayer ternary TI/MTI samples by the application of circularly polarized light with different wavelengths and power densities. It is a pity that we could not conduct more photo-assisted transport measurements due to the lockdown of the labs in the past few months. Future in-depth studies of these phenomena should yield very interesting results.
Figure 4.18: (a) Rxy vs. B with and without circularly polarized light. (b) Coercive field vs. T with and without circularly polarized light.
4.3.2 Effect of circularly polarized light on a monolayer (0+6)-10% MTI
We did the field sweep measurement on monolayer 6QL-10% ternary MTI at 2 K and 10 K.
At 2 K, Rxx vs. B shows a strong weak localization, which is consistent with (1+6)-10%
ternary TI/MTI. Rxy vs. B also shows a strong hysteresis loop. At 10 K, hysteresis nearly disappears for both Rxx and Rxy vs. B. However, it shows a very strong weak localization. In contrast, at charge neutrality point 13 K for (3+6)-10% ternary TI/MTI, Rxx vs. B shows weak
antilocalization. Further studies of field sweeps of (0+6)-10% MTI and (1+6)-10%
TI/MTI at higher temperatures to see if the hysteresis is indeed closed or only due to the charge neutrality. It is also interesting to see if weak antilocalization will show up at higher temperatures.
Figure 4.19 Circularly polarized light effect on 6-QL 10% ternary MTI. (a) Rxx vs. B at 2 K. (b) Rxy vs. B at 2 K. (c) Rxx vs. B at 10 K. (d) Rxy vs. B at 10 K
Unlike (3+6)-10% ternary TI/MTI, circularly polarized light seems to have a negative effect on a monolayer (0+6)-10% MTI at T = 2 K. Both Rxx and |Rxy| decrease when the circularly polarized light is applied. At 10 K, though, the light seems to enhance the Rxx vs. B a little.
The low-temperature decrease in Rxx under light may be attributed to light-induced excess
photo-carriers in the MTI, which were theoretically predicted to causes structural distortions that suppressed the FM coupling between neighboring Cr ions, leading to suppression in |Rxy| as observed experimentally. However, detailed studies of field sweeps at different temperatures both with and without light will be necessary to fully understand the interplay of photo-induced carrier densities and spin alignment by circularly polarized light.