Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers

The energy efficiency and speed of single-shot helicity-independent all-optical switching (AOS) hold great potential for future memory and storage applications.^1,2 Following the first observation of the phenomenon of AOS in GdFeCo alloys,³ much progress has been made toward implementing AOS in concrete memory applications. For instance, magnetic tunnel junctions (MTJs) have been shown to be all-optically switchable by replacing the free layer with TM-rare-earth (RE) systems, using Gd-based alloys,⁴ Co/Gd multilayers,⁵ as well as [Co/Tb] multilayers.⁶ Solutions regarding the design of photonic building blocks to address the spintronic devices in an integrated platform environment have also been demonstrated.^7–10 

In all-optically switchable materials, it is deemed essential that the transition metal (TM) sublattice (Fe and Co), and the RE Gd are exchange-coupled antiparallel and exhibit a strong disparity in demagnetization timescales to enable AOS.^1,2,11 However, for high density data storage applications, materials based on the TM-Gd system face significant challenges, largely due to the insufficient perpendicular magnetic anisotropy (PMA) of Gd and the TM system, required to stabilize competitively small domains.

In search for materials with higher magnetic anisotropy, compounds containing the RE element Tb have since gained much attention (see Refs. 12–20). Whereas this improved anisotropy is desirable from a storage density point of view, it was shown to lead to a much faster demagnetization of the 4f orbitals compared to Gd. Frietsch et al. explained this by a transfer of angular momentum from the 4f moments to the magnon and phonon system, which is only possible in the presence of spin–orbit coupling.²¹ 

While AOS in TM-Gd alloys and multilayers was explained by a strong difference in demagnetization timescales between the TM and the RE—not given for TM-Tb systems—the observation of single-shot AOS in [Co/Tb] multilayers^22,23 came as a surprise. However, the prerequisites for AOS and the exact mechanism behind the magnetization reversal are very different. In multilayers with Tb, AOS is only observed within a small window of compositions close to magnetic compensation, whereas Co/Gd-based multilayers can be engineered to show robust switching even in samples with no magnetic compensation point.^24,25 Another fundamental difference between the two material platforms is the timescale on which reversal occurs. In GdFeCo, it was found that the Fe magnetization crosses zero after only a few hundreds of fs,²⁶ which can be explained for instance by a phenomenological description,^27–29 exchange scattering,³⁰ or nonlocal angular momentum transfer.³¹ Reversal in [Co/Tb] on the other hand has recently been found to take place on a much longer timescale of $∼ 100$ ps,³² presumably driven by virtue of precessional switching around a transient in-plane (IP) anisotropy field, induced by ultrashort heating.³³ Furthermore, switching in [Co/Tb] was found to be independent of the duration of the laser stimulus in the range between 50 fs and 10 ps (not present in TM-Gd systems²⁸) clearly hinting toward slower switching dynamics.³³ 

Systems combining both Gd and Tb offer appealing alternatives, using Gd as the main driving force for AOS while exploiting the high magnetic anisotropy of Tb. Recently, AOS was found in systems where Gd makes up at least 20% of the total RE material.^34,35 A downside of alloys on the other hand are limited possibilities of (interfacial) thin film engineering compared to multilayer films.

In this work, we seek to combine the ultrafast and robust switching of Co/Gd bilayers with Tb to disentangle the roles of the two RE elements. To do this, we design multilayer stacks with Co, Gd, and Tb. We study for which RE layer thicknesses the system exhibits PMA and quantify effective anisotropy constants. Furthermore, we address the questions of the dependence of the critical fluence for AOS on magnetic compensation, which has been considered a critical parameter until now. Finally, we investigate the timescales on which this switching occurs.

Samples in this study are deposited by dc magnetron sputter deposition at room temperature on Si/SiO₂ and doped Si:B substrates for static and time-resolved experiments, respectively. For investigating static magnetic properties, the polar magneto–optic Kerr effect (MOKE) is used to record out-of-plane (OOP) hysteresis loops at room temperature. Furthermore, a superconducting quantum interference device vibrating sample magnetometer (SQUID-VSM) is employed to measure IP hysteresis curves at 300 K. Single-shot all-optical switching is triggered by single linearly polarized $∼ 100$ fs laser pulses at a central wavelength of 700 nm with variable energy at normal incidence. Optically reversed domains are imaged using Kerr microscopy in polar configuration. For the investigation of time-resolved dynamics, we employ time-resolved MOKE (TR-MOKE) using pump and probe pulses with a pulse duration of $< 50$ fs, central wavelengths of 800 and 400 nm, respectively, and a repetition rate of 5 kHz.

When engineering the synthetic ferrimagnetic sample stack, it is important to consider that the Curie temperatures (⁠ $T C$⁠) of the bulk Gd and Tb are 292 and 222 K, respectively, and, therefore, below room temperature. Their magnetization will only be stabilized when interfaced with a ferromagnetic material like Co through the magnetic proximity effect, pushing $T C$ above room temperature. Taking this into account, the choice is made to perform experiments on Ta(4)/Tb(⁠ $t Tb$⁠)/Co(0.8)/Gd(⁠ $t Gd$⁠)/Co(0.8)/Tb(⁠ $t Tb$⁠)/TaN_x(4) multilayer samples (thicknesses in nm). A sketch of the sample design is shown in Fig. 1(a). Comments on the reproducibility of the samples are provided in the supplementary material.

For the first sample, Gd and Tb thicknesses are spatially varied along perpendicular directions to study a wide composition range. In order to determine the composition range where PMA is present, we measure hysteresis loops at different locations along the sample using MOKE. As the MOKE measurement is sensitive to mostly the Co magnetization, a reversal of the sign of the Kerr angle is expected when scanning from Co to RE-dominated regions. We extract the saturation Kerr angle $θ s$ from our data and show the resulting phase diagram in Fig. 1(b).

Regions with no PMA are shown in white, Co and RE-dominated regions with PMA in blue and red, respectively. We find that PMA is absent in composition regions, where the Tb is thinner than a monolayer (⁠ $∼ 0.3$ nm). We attribute this observation to the onset of a discontinuous Co/Tb interface, which is detrimental to obtaining PMA. In regions with too thin Gd, we explain the lack of OOP anisotropy by the strong IP shape anisotropy of the joint two Co layers, overcoming interfacial anisotropy contributions. In the region with PMA, we observe a transition from Co to RE-dominated behavior. The compensation line separating these regions is indicated by a broken line in Fig. 1(b).

To gain quantitative information on the effective magnetic anisotropy constant $K eff$⁠, we record IP hysteresis loops using SQUID-VSM. For this purpose, separate samples with a fixed Gd thickness of 1 nm and varying Tb thicknesses are deposited. OOP MOKE and IP SQUID loops are presented in Figs. 2(a) and 2(b), respectively. We note that the average magnetization in b is calculated based on the assumption that the full volume of RE and Co layers contributes to the magnetic moment. Further elaboration on this is provided in the supplementary material. For the Co-dominated sample (⁠ $t Tb = 0.3$ nm), the step of the hysteresis loop in Fig. 2(a) is inverted as discussed above. The OOP loops show well-defined square shapes with decreasing coercive fields $H c$ as $t Tb$ increases [see Fig. 2(c)]. IP loops show typical hard axis reversal at low fields with close to zero remanence and a linear dependence as a function of field, until a plateau is reached. Samples with $t Tb < 0.5$ nm could not be saturated along their hard axis with the available field of 7 T, and corresponding loops are not shown. A peculiarity in all IP measurements is a second slow magnetization step-up to $∼ 4.5$ T. Such behavior has previously been ascribed to the so-called fanning cone in TM-Tb-based samples.^36,37

We now turn our attention toward the characterization of AOS in our samples. First, we investigate single-shot AOS at different spots in the double wedge sample, indicated in Fig. 1(b). To do so, the sample is first brought to saturation by a perpendicular magnetic field. Subsequently, single fs laser pulses are directed onto the sample surface. We image the irradiated regions using Kerr microscopy and show selected results in Figs. 1(c)–1(e). A more comprehensive set of Kerr images with further discussion is provided in the supplementary material. We find that in c and d, fully switched magnetic domains are created, which is confirmed by the difference in gray level of the saturated magnetic states. It is possible to switch reversed states back to their initial state upon irradiation with subsequent laser shots. The reversal in contrast from Figs. 1(c) to 1(d) is again caused by a transition from Co to RE dominated regions. However, for higher Gd and Tb thicknesses in Fig. 1(e), only a multi-domain state is present. This is likely due to the fact that in this region, the excess of RE content brings the system too far away from magnetic compensation, preventing AOS. As discussed in Ref. 40, other reasons as to why a multidomain state may arise include the phonon temperature temporarily exceeding $T C$⁠, or stray fields facilitating the nucleation of a multidomain state at elevated temperatures. Indeed, a multi-domain state is already visible in Fig. 1(d), although here it only occurs in the middle of the laser pulse, where the fluence is highest.

We note that the extended switching overview in the supplementary material suggests that for thin Gd, adding larger amounts of Tb even slightly reduces the efficiency of AOS. This notion does not contradict our findings here and could be explained by a deterioration of interface quality.

Finally, we want to address the question of the timescale on which magnetization reversal takes place by measuring the time-resolved response to a fs laser pulse with TR MOKE. To ensure that the magnetization returns to its initial state after each excitation, a variable external OOP field is applied during the experiments. We investigate samples with a Gd thickness of 1 nm and a Tb thickness of 0.3 nm. Time traces for different applied field strengths are shown in Fig. 4(a) at a pump fluence of 5.23 mJ/cm², which is greater than F₀. We find that the magnetization crosses zero approximately within the first ps after laser excitation, independently of the magnitude of the field. It is, therefore, not the slow precessional reversal found in [Co/Tb] multilayers, but the fast exchange-driven switching known to exist in Co/Gd systems that facilitates AOS in our samples. A full reversal to the antiparallel state is inhibited by the presence of the guiding field. As the magnetization slowly recovers over the following tens of ps, the magnetic state is reset by virtue of precessional switching, as found in Co/Gd.⁴² This process is clearly accelerated by increasing the guiding field, as the measurement recorded in the lowest field of 0.2 T takes the longest time to recover its magnetization.

Furthermore, we study the dynamics at different pump fluences in Fig. 4(b). As the fluence increases, the magnetization is quenched progressively until a zero crossing occurs at a fluence of 4.36 mJ/cm². This value agrees reasonably well with $F 0 = 4.44$ mJ/cm², determined from static experiments. When comparing F₀ with well-established AOS platforms, the values are comparable to those found in pure CoGd alloys,³⁴ but about twice as high as those in Pt/Co(1 nm)/Gd(3 nm) multilayers.⁴¹ This discrepancy could be attributed to a higher ratio of Co:Gd due to the different stack architecture.

In summary, we found PMA and robust and ultrafast AOS in Tb/Co/Gd/Co/Tb samples in a wide composition range. The zero crossing of the magnetization occurred approximately within the first ps after laser excitation, leading to the conclusion that the mechanism for reversal is identical as in Co/Gd. The fact that the addition of Tb does not result in a reduction of the critical fluence supports this notion. Our results show that hybrid structures based on Co/Gd and Co/Tb multilayers are promising candidates for magneto-photonic memory devices.