The pump-modulated MOKE results (Fig. 1c-f) show the evolution of magnetism in a Cr-BST/CrSb bilayer sample from a hard, homogenous ferromagnetic state, into apparently one composed of two distinct magnetic components; one component exhibiting a relatively small coercive field and the other featuring both a prominent exchange bias and a large coercive field. We propose that when subjected to high pump intensities, the normally well-coupled surface and bulk magnetic order of the MTI decouple, where the magnetic switching that occurs at large coercive fields originates from the MTI/CrSb interface and switching observed at small-coercive fields corresponds to signals derived from the bulk of the MTI layer as explained in the following.
When radiated upon the material, the pump beam perturbs the magnetic order through several distinct channels. The shortest-lived effect is a temporary excitation of the electronic system into a highly non-thermal distribution. This effect occurs almost instantaneously and has the capacity to temporarily demagnetize the sample. Through a combination of electron-electron and electron-phonon processes, the electrons are believed to rapidly relax into an approximately thermal distribution over the first few hundred fs to 1 ps. [25–28] As the electronic system returns to a thermal distribution, it features an elevated density of photo-excited quasiparticles, that, due to a bottleneck in the density of states at the Dirac point in the MTI, may take hundreds of ps or longer to fully return to their equilibrium state. [27, 28] These photo-induced carriers in turn influence the magnetic properties of the MTI system. Finally, and most trivially, the pump beam will transfer energy to the crystal lattice. The energy imparted into the lattice will raise the temperature of the sample beyond that of the cryogenic bath. This final effect is sufficiently long-lived it is unlikely to recover on the time scales accessible by the time-delay stage (~ 1 ns).
In the Cr-(Bi,Sb)2Te3/CrSb bilayer system, the surface-state magnetization in the MTI layer is predominately mediated by the RKKY interaction between Dirac holes and impurity spins, while the bulk-state magnetization originates from the long-range Van Vleck p-d spin coupling among local moments. When the pump fluence is low, the surface RKKY exchange interaction is relatively weak, and the laser-induced thermal effect is small. Correspondingly, the magnetic moments at the surface and in the bulk are well coupled with each other. When the pump fluence is increased, the photo-induced carrier density and lattice heating both become more pronounced. While the elevated lattice temperature is inhospitable to magnetism of both RKKY and van Vleck origins, the impact of the photo-injected holes at the surface Dirac bands is more complex. van Vleck mediated bulk magnetism, being free-carrier agnostic, is insensitive to the increased density of excited carriers. At the surface, the elevated carrier density can strengthen the RKKY interaction and partially compensate for the deleterious effects of the elevated temperature. Macroscopically, the divergent photo-response of the RKKY (surface) and van Vleck (bulk) mediated magnetic moments (Fig. 3) leads to a mismatch in their respective coercive fields, with the bulk magnetic moments displaying a reduced coercivity compared with the surface. Figure 4 schematically shows a model of the photo-induced transient magnetic state in the MTI Cr0.26(Bi0.3Sb0.7)1.74Te3 under external field switching. The narrow component of the transient coercive field measured at the highest pump fluence is similar to what is observed in the 120-K MOKE loop collected with no pump (see Supplementary); suggesting that the film temperature may experience a temporary increase of nearly 40 K. This observation is consistent with the temperature rise we predict using the known heat capacity and optical absorption characteristics of these films (See supplementary).
To corroborate the understanding that the regions of large coercive fields correspond to the MTI surface magnetism, we now consider the photo-induced exchange-bias effect which is related to the exchange energy of the MTI/AFM interface. The onset of perpendicular exchange bias is a common feature of proximity coupled antiferromagnet/ferromagnet bilayers. As magnetic exchange bias is a purely interfacial phenomenon, the fact that the exchange bias is most pronounced in the region of the hysteresis loop featuring the large coercive field immediately suggests this signal indeed originates at the surface of the topological insulator. Meanwhile, the absence of exchange bias within the low-field switching feature indicates a decoupling of this interfacial magnetism from the bulk of the MTI. The increasing exchange bias observed at large pump intensities can be further understood using Meiklejohn and Bean’s exchange anisotropy model. [29, 30] Based upon this model, the macroscopic bias field Heb can be expressed with the Heisenberg-like interface exchange energy as where Jeb is the effective interfacial exchange energy, is the spin of an individual layer of CrSb (MTI) at the interface, θ is the angle between the applied field and perpendicular direction of the thin films, a2 is the unit cell area and tMTI is the effective thickness of MTI surface layers. The pump beam is unlikely to significantly modify intrinsic features of the bilayer system such as the Cr spin state, the exchange constant, or unit cell area. However, the pump-driven decoupling of the surface and bulk reduces the effective thickness and magnetization of the MTI surface layer, increasing the effective exchange-bias field.
Transient MOKE loops collected at different pump-probe delays provide further support for a pump-driven decoupling of the MTI surface and bulk magnetism, wherein the latter is uniformly suppressed through thermal effects and the former is sustained by optically excited carriers. As observed in Fig. 5, with increasing time delays the switching feature observed at large coercive fields becomes far less pronounced. Notably, rather than recover the square-shaped large coercivity switching observed at low pump intensities, with increasing time delays the coercivity of the transient features continues to decrease, moving further from, rather than closer to the expected equilibrium behavior. This counter-intuitive development indicates that the mechanism sustaining the large coercive field region of the two-step switching has a short lifetime on the order of hundreds of ps, while its small coercive field counterpart is comparatively long-lived. As discussed above, the timescales associated with these two switching behaviors notably correspond with the known characteristic times of carrier relaxation and thermal dissipation in these materials; indicating the large coercive field region is electronic in origin, whereas the small coercive field region is maintained by long-term thermal effects.
erg. While the magnitude of the exchange bias field observed here is comparable to what has been previously reported in the Cr2O3/CBST system,  the size of Jeb is three orders of magnitude smaller than typical perpendicularly exchange-biased FM/AFM systems such as [Pt/Co]/Cr2O3 and [Pt/Co]/IrMn, where the exchange couplings are believed to be short-range interactions. [32, 33]
In summary, we have studied the magnetization dynamics of Cr0.26(Bi0.3Sb0.7)1.74Te3/CrSb bilayer system using ultrafast magneto-optical techniques. We manipulate the magnetizations by pump laser irradiation and observe a photo-induced spin-valve-like magnetic structure from the pump-modulated MOKE results. We identify the two distinct photo-induced transient ferromagnetic phases as originating from the surface and bulk of the MTI. The soft ferromagnetic phase corresponds with the photo-induced bulk-magnetism while the resilient ferromagnetic phase relates to the photo-induced surface-magnetism. We then estimate the interfacial exchange energy and compare it with other exchange-biased FM/AFM systems. The results are relevant to other MTI-based bulk, surface, and interface systems with carrier-dependent and carrier-independent exchange coupling effects. Our findings facilitate the practical applications of MTI and MTI/AFM systems in laser-assisted magnetic recording, ultrafast optoelectronics, and spintronics.