Seminar: Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains and Dynamical fluctuations across the metal to insulator transition in rare earth nickelates

The talk will have two different parts. The first section will be on the role of domain curvature on the ultrafast response of magnetic domains upon laser excitation. The quest towards controlling magnetism at the femtosecond timescales is driven by the demand for energy efficient and fast magnetic storage devices [1]. Many studies have focused on switching the magnetization in different material, but few have discussed the role of the spatial evolution of the magnetization. Here, we study the ultrafast response of magnetic multilayers with domain patterns with different local symmetries [2]. Through time-resolved small angle X-ray scattering at the European XFEL and an accurate 2D fitting routing, we find a symmetry-dependent behavior of the ultrafast response. By observing an ultrafast shift in the scattering peak position for labyrinth domains, without translational symmetry, but no such shift stripe domains, with translational symmetry, we confirm the results of previous studies [3]. We also study regions of the sample where both labyrinth and stripe characters are present. By isolating the isotropic and anisotropic components of the scattering, we find that only the labyrinth domains exhibit an ultrafast shift in the isotropic diffraction peak position, even in a mixed domain pattern. In a subsequent experiment at the FERMI free electron laser, we found that the ultrafast distortion of the diffraction pattern showed markedly different timescales compared to the magnetization quenching [4]. The diffraction pattern distortion shows a threshold-dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain wall motion with pinning sites. Supported by simulations, we show that a speed of ≈ 66 km/s for highly curved domain walls can explain the experimental data. While our data agree with the prediction of extreme, non-equilibrium wall speeds locally, it differs from the details of the theory, suggesting that additional mechanisms are required to fully understand these effects.

The second part of the talk will be on the dynamical fluctuations across the metal to insulator transition in rare earth nickelates. Rare earth nickelates RNiO3 display an insulator to metal transition (MIT) which is accompanied by a magnetic transition, charge ordering, and a crystal structure change from orthorhombic low temperature to monoclinic high temperature state [5]. While this system has been widely studied, the nature of the fluctuation across the transition, and the associated time- and length-scales is not known. Spontaneous fluctuations are important components for stabilizing topological magnetic structures such as skyrmions in quantum materials. However, the dynamic susceptibility of the nickelates remains relatively unexplored, and the role played by nanoscale phenomena such as domain-wall formation and motion, local strain fields and phase separation in underlying pathways of MIT is not well-understood. In our work [5], we focus on understanding the role of nanoscale heterogeneities and their fluctuations in rare earth nickelates by employing x-ray photon correlation spectroscopy (XPCS). Our XPCS measurements on NdNiO3 thin films, show complex evolution of fluctuations dependent upon temperature and wavevector q. We also observe unexpected non-equilibrium dynamics and suggests a new approach to understanding these materials.

[1] A. Kirilyuk, A. V. Kimel, and Th. Rasing, Rev. Mod. Phys. 82, 2731-2784 (2010)

[2] N. Zhou Hagström et al., Phys. Rev. B 106, 224424 (2022)

[3] B. Pfau et al., Nature Communications 3, 1110 (2012); D. Zusin et al, Phys. Rev. B 106, 144422 (2022); M. Hennes et al., Phys. Rev. B 102, 174437 (2020)

[4] R. Jangid, N. Zhou Hagström et al., Phys. Rev. Lett. 131, 256702 (2023)

[5] S. Catalano et al., Reports on progress in Physics 81 046501 (2018)

[6] N. Zhou Hagström et al., to be submitted (2024)