Photoinduced transient states of antiferromagnetic orderings in La_1/3Sr_2/3FeO₃ and SrFeO_3−δ thin films observed through time-resolved resonant soft x-ray scattering

In the last century, the challenges in material science have mainly included the observation and manipulation of charges in materials, for application in semiconductor electronics. In the past decades, the utilization of spins, i.e., spintronics, has attracted considerable interest. The faster and energy-efficient manipulation of spins is one of the main issues in spintronics. Optical control of spins with ultrashort laser pulses is an important research topic for the ultrafast control of magnetization [1]. The first reported ultrafast control involved photoinduced demagnetization in ferromagnetic Ni within 1 ps [2]. Subsequently, the photoinduced charge and spin dynamics have been studied extensively through diffraction and spectroscopy in ferro-, antiferro-, and ferrimagnetic materials [3–13].

Antiferromagnets are expected to be the key material for the photocontrol of magnetic orderings. In the case of ferromagnetic materials, the magnetic moments need to be transferred from the spin to the other degrees-of-freedom, such as the lattice, when the magnetic moments are changed. However, due to the quenched total magnetic moments of antiferromagnets, angular momentum transfer between the spin and other systems is not necessary in antiferromagnets, enabling ultrafast magnetization changes. This was clarified by comparing the ferro- and antiferromagnetic phases of Dy [14].

Several perovskite oxides with antiferromagnetic orderings have been discovered, whose properties can be controlled through elemental substitution and doping because of the strong interactions between the electrons, lattice, and spin degree-of-freedom [15]. Taking advantage of this feature, perovskite oxide materials suitable for the fast optical control of magnetism have been discovered [9, 10, 16, 17]. La_1/3Sr_2/3FeO₃ and SrFeO_3−δ perovskite thin films contain high valence Fe ions and oxygen holes. These features are clearly different from ordinary metals and insulators. In particular, La_1/3Sr_2/3FeO₃ has charge and spin ordered structure with three and six Fe ions period and ordered structures such as this are realized in a few perovskites. We focused on antiferromagnetic orderings of Fe perovskite oxides and surveyed the photoinduced dynamics of these orderings directly.

The equilibrium magnetic properties of La_1−x Sr_x FeO₃ perovskites can be controlled based on the La/Sr composition. La_1/3Sr_2/3FeO₃ thin films exhibit a charge disproportionation (CD) of 3Fe^3.67+ → Fe⁵⁺ + 2Fe³⁺ accompanying antiferromagnetic ordering below T_N = T_CD ≈ 190 K [18, 19]. The high valence state of Fe⁵⁺ is realized as ${\mathrm{F}\mathrm{e}}^{3+}{\underline{L}}^{2}$, where $\underline{L}$ denotes an oxygen 2p hole [20]. This CD phase results in a charge density wave with a period comprising three Fe ions as that of the crystal lattice, and a spin density wave of six Fe ion period along the 111 directions, as revealed by a neutron diffraction study (see figure 1(a)) [15]. SrFeO_3−δ includes the Fe⁴⁺ ion and shows a helimagnetic phase with incommensurate periodicity, which occurs because of the competition between the nearest neighbor ferromagnetic and the next-nearest neighbor antiferromagnetic interaction of the conducting 3d electrons, as indicated by neutron scattering results [21]. Fe⁴⁺ is realized as the ${\mathrm{F}\mathrm{e}}^{3+}\underline{L}$ in SrFeO₃ perovskites [20, 22]. T_N was reported to be 134 K [21] (bulk), $\approx 105$ K [23], $\approx 115$ K [24] (thin films). A multiple Q helimagnetic phase in SrFeO_3−δ was argued [23, 25], which could possibly host skyrmion crystals [26].

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Resonant soft x-ray scattering (RSXS) [27] is a powerful tool for revealing the ordered structures in solids, such as the magnetic, charge, and orbital ordering [27–33]. RSXS can be performed with the core level absorption process, and the interaction between x-rays and a specific element can be enhanced. Hence, RSXS can be applied to thin films with small sample volumes. X-ray polarization is sensitive to the orbitals of the valence electrons and RSXS can detect orbital and spin ordering, which are coupled through spin-orbital interaction. The magnetic ordering of irons in La_1/3Sr_2/3FeO₃ [33, 34] and SrFeO_3−δ [23] have been observed through RSXS at the Fe L_2,3 absorption edge (2p → 3d, $\approx 709$ eV).

In this study, we report the photoinduced magnetic dynamics of La_1/3Sr_2/3FeO₃ and SrFeO_3−δ thin films determined through time-resolved RSXS measurements. We ascertain the ultrafast melting of the La_1/3Sr_2/3FeO₃ magnetic ordering through charge transfer between Fe ions, which can be attributed to the strong coupling between the charge and spin in the system. Based on the comparison between La_1/3Sr_2/3FeO₃ and SrFeO_3−δ , the effect of the electronic properties on the dynamics of the photoinduced quenching of the magnetic orderings is examined.

La_1/3Sr_2/3FeO₃ thin films with ≈40 nm thicknesses were grown epitaxially on SrTiO₃ (111) substrates using the pulsed laser deposition method. The details of the synthesis of La_1/3Sr_2/3FeO₃ can be found in reference [35]. The magnetic ordering along [111] direction of La_1/3Sr_2/3FeO₃ was more stabilized than the other ⟨111⟩ direction owing to the rhombohedral distortion [34]. By oxidizing SrFeO_2.5 grown epitaxially on SrTiO₃ (100) substrates using the pulsed laser deposition method, SrFeO_3−δ thin films with $\approx 27$ nm thicknesses were fabricated on SrTiO₃(100). SrFeO_3−δ was obtained by annealing SrFeO_2.5 in an ozone atmosphere at 300 °C for 6 h with the sample exposed to UV light in a UV/O3 DRY CLEANER UV-1 (SAMCO Inc.). While, SrFeO₃ is metallic, SrFeO_3−δ is semiconductive and this introduced δ of our samples enables the comparison to the insulating La_1/3Sr_2/3FeO₃. In SrFeO_3−δ thin films, all ⟨111⟩ directions are equivalent and we observed one of four equivalent ⟨111⟩ directions.

In order to elucidate the photoinduced dynamics of the antiferromagnetic ordering in La_1/3Sr_2/3FeO₃ and SrFeO_3−δ , we performed time-resolved RSXS measurements using the pump-probe method at the slicing facility UE56/1-ZPM Helmholtz-Zentrum, Berlin [36]. The experimental setup is shown in figure 2. A Ti:sapphire laser (λ = 800 nm, 1.5 eV) was employed for photoexcitation, with a pulse duration of 50 fs. The spot size of the pump laser was $\sim 600\enspace \mu \mathrm{m}\times \sim 400\enspace \mu \mathrm{m}$, and that of the probe x-ray was $\sim 150\mu \mathrm{m}\times \sim 30\mu \mathrm{m}$ [36]. The separation angle of both beams was smaller than 2° and this separation angle had a negligible effect on the delay time error within the overall temporal resolution of the experiment. The laser and x-ray pulse frequencies were 3 and 6 kHz, respectively, and signals with and without pump laser excitation were obtained alternately. The signals without excitation were used for normalizing the pumped signals. The scattered x-rays were detected using an avalanche photodiode. The laser slicing facility at BESSY has two operation modes: one with 50 ps temporal resolution and high pulse energy and one with 130 fs temporal resolution and low pulse energy. We used 50 ps x-ray pulses to obtain the outline of the photo-induced dynamics of La_1/3Sr_2/3FeO₃ and SrFeO_3−δ and then investigated the detailed dynamics of La_1/3Sr_2/3FeO₃ using 100 fs x-ray pulses generated using the laser slicing technique [36]. The total time resolution was $\approx 130\enspace \mathrm{f}\mathrm{s}$. The scattering intensity from SrFeO_3−δ was too small for experiments with the weak 100 fs x-ray pulses. So we restricted ourselves to the lower temporal resolution of about 50 ps. SrFeO_3−δ was mounted on a wedge-shaped jig at an angle of 55 deg. in order to orient the [111] direction in the scattering plane, and the temperature was set to 35 K. We set the x-ray photon energy and polarization to the Fe L₃ edge ($\approx 710$ eV) and horizontal, respectively, for obtaining the magnetic signals. Static La_1/3Sr_2/3FeO₃ measurements were performed with a diffractometer at the soft x-ray beamline BL-16A, Photon Factory, KEK, Japan [37]. The experimental geometry and temperature were equivalent to the time-resolved measurement, and a silicon drift detector was used.

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Figure 3 depicts the Q = (q, q, q) antiferromagnetic ordering peaks of (a) La_1/3Sr_2/3FeO₃ and (b) SrFeO_3−δ . The diffraction peaks were scanned along the [111] direction. The temperature dependence of q and the peak intensities of La_1/3Sr_2/3FeO₃ and SrFeO_3−δ are displayed in figures 3(c) and (d) respectively. The temperature evolution of RSXS intensity reproduced results of the previous study Néel temperature T_N was determined via power-law fit $(\propto {\left\vert T-{T}_{\text{N}}\right\vert }^{\nu })$ of peak intensity, as shown in figures 3(c) and (d). T_N of La_1/3Sr_2/3FeO₃ and SrFeO_3−δ was estimated to be 187 K and 110 K, respectively. For the La_1/3Sr_2/3FeO₃ thin films, the peak position is fixed at (1/6, 1/6, 1/6) in the entire temperature range below T_N, as shown in figure 3(a). The SrFeO_3−δ diffraction peak appears below T_N ≈ 110 K, and the peak position shifts according to the temperature. Our SrFeO_3−δ thin films show the helimagnetic ordering below T_N = 110 K with q = 0.126 r.l.u. along with insulating feature, which implies that δ of our SrFeO_3−δ samples is $\sim 0.15$ by comparing it with the results of previous studies [23, 24, 38, 39]. q of the SrFeO_3−δ helimagnetic ordering was reported to be 0.128–0.112 (SrFeO_2.87) [21, 40], 0.13 (SrFeO_3−δ ) or 0.125 (Sr_0.99Co_0.01FeO₃) [23]; our observed q = 0.126 is in a similar range. In the heating and cooling cycle, thermal hysteresis was observed for the peak intensity and peak positions. The hysteresis has been reported to be related to the phase separation of paramagnetic and antiferromagnetic phases [41]. Similar phase separation was pointed out in La_1/3Sr_2/3FeO₃ [34]. According to previous resistivity measurement reports [23, 24], thermal hysteresis was observed at T = 46–71 K, and our observed thermal hysteresis reflects this phenomenon.

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We first discuss the La_1/3Sr_2/3FeO₃ dynamics. The time evolution of the magnetic peak intensity after laser pumping, revealed in the laser slicing mode, is shown in figure 4(a). Rapid reduction in the diffraction intensity is observed within a time resolution of 130 fs. The diffraction intensities decrease to 50% of the initial values with the fluence of 0.5 mJ cm⁻². This fluence corresponds to $\approx 0.03$ photons/unit cell assuming absorption coefficient of 1.7 × 10⁵ cm⁻¹ [17] and unit cell volume of (0.39 nm)³. We confirmed that the intensity of unpumped signal intensity remained unchanged due to the pump laser fluence, which implies that the base temperature was constant. The decreased diffraction intensity recovers 30 ps after laser excitation in the lower fluence regime; however, the recovery is slower at higher fluence. In order to discuss the results quantitatively, we fitted them using the function depicted by equation (1) for the decay and recovery processes. The fitting function is as follows:

$$\begin{equation}f(t)=1+\left[\left({R}_{\text{fast}}+R\right)\mathrm{exp}\left(\frac{-t}{\tau }\right)-{R}_{\text{fast}}\,\mathrm{exp}\left(\frac{-t}{{\tau }_{\text{fast}}}\right)-R\,\mathrm{exp}\left(\frac{-t}{{\tau }_{\text{decay}}}\right)\right]H(t)\ast \,\mathrm{exp}\left(-\frac{\mathrm{ln}(2)}{4}\frac{{t}^{2}}{{\tau }_{\text{resolution}}^{2}}\right),\end{equation} \tag{ 1 }$$

where τ_decay describes the time constant for photo-induced reduction, τ_fast and τ describe the time constants for the fast and slower components of recovery with an amplitude of R_fast and R. To consider time resolution, the fitting function was convoluted by a Gaussian with a full-width-at-half-maximum of τ_resolution = 130 fs. The parameters extracted from the experimental delay scans are summarized in table 1. The τ_decay time scale was estimated to be 0.1 ± 0.05 ps.

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For the nonthermal La_1/3Sr_2/3FeO₃ process with an ultrashort time scale of τ_decay ⩽ 0.1 ps, we consider that the ultrafast magnetization dynamics occurs because of the ultrafast photo-melting of the charge ordering, which destroys the magnetic ordering due to the strong coupling between the charge and spin. Demagnetization due to the photo-melting of the charge ordering has also been observed, for example, in NdNiO₃ thin films [9]. Figure 4(b) illustrates the origins of the ultrafast melting of the magnetic ordering. Due to the strong onsite Hund's rule coupling, excitation with linearly polarized light at 1.5 eV can result only in transitions between the ferromagnetically coupled sites [42]. Optical conductivity measurement has demonstrated that the optical gap of 0.15 eV has a strong interatomic d–d transition characteristic [42]. Therefore, the excited electron would be transferred only in the ferromagnetically coupled Fe³⁺–Fe⁵⁺–Fe³⁺ sites, and the observation in this study suggests that the excitation induces charge transfer in a unit of the charge-ordering site of Fe³⁺–Fe⁵⁺–Fe³⁺ into Fe³⁺–Fe⁴⁺–Fe⁴⁺, as shown in figure 4(b). It has been suggested that this charge transfer is a metastable state accompanying Fe–O bond-length change, in a previous report on the photoinduced transient states of La_1/3Sr_2/3FeO₃ [43]. Transfer in the charge ordering induces changes in the magnetic interactions. Fe³⁺–Fe⁴⁺ ions have ferromagnetic interactions, whereas the Fe⁴⁺–Fe⁴⁺ sites have antiferromagnetic ones, as shown in figure 4(b).

Furthermore, time-resolved RSXS measurements were performed for 35 and 80 K SrFeO_3−δ thin films as shown in figure 5(a) with a time resolution of τ_resolution ≈ 50 ps; the experimental results for La_1/3Sr_2/3FeO₃ with a similar time resolution are also shown in the right panel for comparison. Due to τ_resolution ≈ 50 ps, the fast decay process cannot be resolved in figure 5(a), and we focus on the changes in the magnetic ordering peak intensity. We fitted the results using equation (1) with setting the values of R_fast, τ_decay, and τ_fast to 0 because fast component cannot be observed due to the lower time resolution. The function for fitting is

$$\begin{equation}f(t)=\left[1-R\,\mathrm{exp}\left(\frac{-t}{\tau }\right)H(t)\right]\ast \,\mathrm{exp}\left(-\frac{\mathrm{ln}(2)}{4}\frac{{t}^{2}}{{\tau }_{\text{resolution}}^{2}}\right).\end{equation} \tag{ 2 }$$

The degrees of quenching R, defined by equation (2), of the La_1/3Sr_2/3FeO₃ and SrFeO_3−δ magnetic ordering are plotted as a function of the laser fluence in figure 5(b). For La_1/3Sr_2/3FeO₃, the quenching of the peak intensity reaches ≈70% at a fluence of approximately 0.75 mJ cm⁻²; however, the SrFeO_3−δ diffraction intensity decreases by less than 60% of the initial intensity below a fluence of 2.8 mJ cm⁻². This suggests that the antiferromagnetic ordering of La_1/3Sr_2/3FeO₃ is quenched with less excitation energy compared to SrFeO_3−δ . It should be noted that we observed ultrafast reduction of peak intensity of La_1/3Sr_2/3FeO₃ with the time resolution of 130 fs and we compared La_1/3Sr_2/3FeO₃ and SrFeO_3−δ results based on the experiments of which time resolution is 50 ps. The electron temperature increase is estimated to be about several hundred K, which is in the similar range of previous studies [2, 5]. The peak positions of the photoinduced transient states and the heating process are plotted as a function of the peak area intensity in figure 5(c). The diffraction peaks of the transient states are shown in figure 5(d). Area intensities are obtained as the integration of the diffraction peak scan. Both curves are similar, indicating that the observed photoinduced quenching of the SrFeO_3−δ helimagnetic ordering is triggered by the same mechanism as the temperature-induced change (table 2).

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SrFeO₃ is metallic and 1.5 eV laser excitation corresponds to the interband transition [44]. The photoinduced quenching of the helimagnetic ordering in SrFeO_3−δ can be interpreted as the result of the increase in the spin and electron temperatures, as depicted in figure 5(e). On the other hand, La_1/3Sr_2/3FeO₃ is insulating and its magnetic orderings disappear because of the charge transfer between Fe ions induced locally by photoexcitation, as discussed above. The photoinduced dynamics of the antiferromagnetic orderings reflect the versatile electronic feature of perovskite oxides. Owing to the Sr substitution dependence of the absorption coefficient [17], the absorption coefficient becomes larger, which indicates that the observed trend does not originate from the strong absorption.

In this study, we examined the photoinduced dynamics of the antiferromagnetic orderings in two perovskites, La_1/3Sr_2/3FeO₃ and SrFeO_3−δ , through time-resolved RSXS. The magnetic ordering in La_1/3Sr_2/3FeO₃ thin films was quenched within 130 fs. This ultrafast dynamics can be explained based on the photoinduced charge transfer between Fe ions, which induces a change in the strong magnetic interactions. The process is nonthermal. The spin moment can be canceled with the nearest neighbors in antiferromagnetic La_1/3Sr_2/3FeO₃ through this ultrafast process, leading to an ultrafast photoinduced change in the magnetic ordering. Compared to helimagnetic SrFeO_3−δ , the magnetic ordering of La_1/3Sr_2/3FeO₃ was destroyed at lower fluence. Spin manipulation in helical systems has been found to be more energy efficient than in ferromagnets. An even more energy-efficient process can be realized in La_1/3Sr_2/3FeO₃. Ultrafast changes in the magnetic ordering of antiferromagnetic perovskite thin films were discovered, and the photoinduced dynamics was shown to be controlled by tuning the electronic feature through perovskite doping. However, this comparison does leave aside the possibility that a strong reduction of the SrFeO_3−δ antiferromagnetic peak intensity was neglected due to the lower time resolution compared to La_1/3Sr_2/3FeO₃ case. Further investigations with better time resolution are needed in order to determine the time evolution of antiferromagnetic orderings completely. Changes of substrates or thickness affect the nature of thin films and such parameters characteristic of thin films are important to control the photo-induced magnetization dynamics. The photoinduced dynamics of the spin order in antiferromagnetic perovskite thin films is important for integrating functional oxides into spintronic device architectures.

We thank Yasuyuki Hirata for the productive discussions and HZB for the allocation of the synchrotron radiation beamtime, Karsten Holldack and Rolf Mitzner for experimental support. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2016PF-BL-19B, 2015G556, 2015S2-007, 2013G058, 2013G661). This work was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Nos. 19H05824, 19H01816, 19K23430, and 17K14334, and the MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant No. JPMXS0118068681. KY acknowledges the support from the ALPS program of the University of Tokyo.

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